CO₂ Enrichment in Protected Agriculture: A Bibliometric Review on Greenhouses, Controlled Environment Systems, and Vertical Farms—Part 1

Abstract

CO₂ enrichment in protected agriculture is a key strategy for enhancing crop productivity and quality, optimizing photosynthetic efficiency, and mitigating the impacts of climate change. In this study, a comprehensive bibliometric analysis of research on CO₂ enrichment is conducted by compiling and evaluating 171 relevant documents published between 1982 and 2024 in Scopus, utilizing R-Studio and VOSviewer for data processing. The analysis explores scientific output trends, predominant research methodologies, influencing factors, and emerging applications in controlled-environment agriculture. The findings reveal an exponential growth in scientific publications since 2015, with Asia and Europe leading the research landscape. The physiological and agronomic benefits of CO₂ enrichment in C3 crops, particularly tomatoes and lettuce, include enhanced photosynthesis, improved nitrogen assimilation, and reduced abiotic stress. Additionally, advancements in sustainable CO₂ capture and delivery technologies, such as industrial capture and fermentation-based systems, have been documented. However, significant challenges remain regarding the economic feasibility, accessibility for small-scale farmers, and environmental sustainability of CO₂ enrichment strategies. A network analysis of scientific collaboration highlights an increasing trend of international cooperation, with China, the United States, and Japan emerging as key contributors. The integration of plant physiology, agricultural engineering, and environmental sustainability reflects a transition toward multidisciplinary approaches aimed at optimizing CO₂ utilization in controlled environments. This study underscores the potential of CO₂ enrichment as a transformative tool in protected agriculture. However, its large-scale adoption necessitates international collaboration, rigorous research on socio-economic and environmental impacts, and the development of context-specific technologies. Strengthening global research networks and fostering applied innovation will be essential to ensuring the widespread and sustainable implementation of CO₂ enrichment strategies in protected agriculture.

1. Introduction

The cultivation of crops in protected environments represents a fundamental tool for strengthening food security and moving towards sustainable agriculture [1,2]. Unlike open field cultivation, this practice reduces the vulnerability of crops to adverse weather conditions [3,4]. In addition, depending on the type of technology implemented, microclimatic variables such as temperature, humidity, ventilation, light intensity, and CO₂ concentration can be precisely controlled or modified—essential factors that directly affect plant photosynthesis [5,6,7]. In this regard, protected cultivation has contributed significantly to the expansion of agricultural production, expanding both the geographical scope and seasonality of crops [4,8]. However, managing and maintaining suitable microclimatic conditions within agricultural infrastructures such as greenhouses or in controlled indoor environments involves high energy costs [9]. It is estimated that between 50% and 85% of the total energy consumption in protected cultivation systems corresponds to the control of microclimatic factors [10,11]. This has led to this type of agricultural production becoming one of the most demanding in terms of energy [12,13,14].

CO₂ constitutes the primary input for the photosynthesis process, and most plants do not reach their maximum photosynthetic capacity with the CO₂ concentrations present in the ambient air [15,16]. In this context, CO₂ enrichment, together with the control and conditioning of the other microclimate variables, has the potential to significantly increase photosynthetic activity and, as a result, biomass production in crops [17]. For this reason, CO₂ enrichment has been consolidated as a crucial technique within climate control strategies applied in protected agriculture [18,19]. Several studies over recent decades have documented the benefits of this practice, showing substantial improvements in the growth, yield, and quality of crops in greenhouses and plant factories, also known as vertical farming systems [12,17].

C3 crops, such as tomatoes, cucumbers, and lettuce, are particularly sensitive to changes in CO₂ concentrations due to the absence of efficient mechanisms to manage its scarcity. Lettuce, belonging to the Asteraceae family, is one of the most common leaf crops in plant factories, as its ability to grow in several layers allows the yield per unit area to be optimized [20]. In particular, red leaf lettuce has shown up to 30% yield increases under CO₂ conditions of 1000 μmol mol⁻¹, along with higher levels of phenolic compounds that enhance its nutritional value [21]. These results highlight the essential role of CO₂ enrichment in improving light use efficiency (LUE) and photosynthesis, thereby boosting crop yield and quality [22].

Recent studies reinforce the high responsiveness of C3 crops to elevated CO₂ environments, especially under controlled environment agriculture. For instance, Holley et al. [23] demonstrated that increasing CO₂ concentrations from 400 to 800 ppm significantly enhanced the fresh and dry weight of lettuce, with the greatest gains observed between these levels. Similarly, Noh and Jeong [24] reported that Ssamchoo and Romaine lettuce varieties exhibited improved growth under elevated CO₂ conditions compared to ambient levels. Furthermore, Singh et al. [25] found that basil, lettuce, and Swiss chard grown at 800 ppm CO₂ showed increased plant height, width, and biomass. These findings confirm the growing consensus on the strategic importance of CO₂ enrichment for optimizing productivity and resource efficiency in C3 crop systems under protected agriculture.

The optimal CO₂ concentration for enrichment varies depending on the growing environment. In greenhouses with this climate control technology, the typical level of CO₂ fertilization is set around 800 μmol mol⁻¹, while in closed systems such as plant factories, where crops are produced intensively, optimal levels range between 1000 and 1500 μmol mol⁻¹ [26,27]. Concentrations up to 1600 μmol mol⁻¹ have been explored for effects on productivity, which is approximately four times the environmental level. Such research is critical to identify the most effective and sustainable CO₂ thresholds, maximizing production without compromising plant health or the growing environment [20].

CO₂ enrichment technologies, such as atmospheric ventilation, compressed CO₂ supply, and the combustion of carbonaceous fuels, remain essential to maintain optimal CO₂ levels in greenhouses and plant factories [28,29,30,31]. However, reliance on conventional sources poses challenges related to cost and sustainability. The search for alternatives, such as CO₂ capture and reuse from industrial processes or compost fermentation, offers a promising avenue toward more efficient protected agriculture with a lower carbon footprint [32]. These strategies not only reduce carbon emissions, but also ensure a constant supply of CO₂, favoring the growth and quality of crops in controlled environments.

Despite significant advances in research on CO₂ enrichment, knowledge of its application in different protected agriculture systems remains fragmented. This study aims to analyze research trends, identify knowledge gaps, and highlight key scientific contributions related to CO₂ enrichment in greenhouses, controlled environment systems, and vertical farms. By consolidating the state of the art, this review provides a comprehensive overview that supports the optimization of CO₂ use, enhances the efficiency of protected agricultural systems, and contributes to the development of more sustainable and resilient agricultural practices [33,34].

Bibliometric analysis is a powerful quantitative tool that enables the systematic evaluation of scientific literature through citation analysis, co-authorship networks, bibliographic coupling, and thematic mapping. It facilitates the identification of emerging trends, influential research groups, and knowledge gaps, making it an essential methodology for understanding the evolution of scientific fields [35]. Over recent decades, bibliometric techniques have been widely applied to assess research impact and monitor the development of critical scientific domains [36,37]. Given its ability to provide data-driven insights, bibliometric analysis has become indispensable for tracking research dynamics and guiding future investigations in CO₂ enrichment and sustainable agricultural technologies [38,39,40,41].

In this vein, this study presents a bibliometric analysis of scientific articles focused on CO₂ enrichment in protected agriculture. Given the multidimensional nature of the topic, the following research questions are posed:

(1)

What is the current state of knowledge on CO₂ enrichment in greenhouses and other controlled agricultural systems?

(2)

What are the main trends, limitations, and future developments in this field?

(3)

Which specific knowledge gaps can be identified to guide future research?

This study seeks to consolidate a frame of reference that will drive new lines of inquiry, highlighting not only the agronomic benefits of CO₂ enrichment, but also its environmental, technological, and economic implications. Notably, the review identifies critical research gaps, such as the lack of cross-regional economic feasibility analyses for small-scale farmers, insufficient studies on CO₂ management under different climatic conditions, and the limited integration of CO₂ enrichment strategies with renewable energy sources and automation technologies. By addressing these gaps, this review aims to contribute to the development of innovative and sustainable solutions to optimize CO₂ use and enhance productivity in protected agricultural systems, especially in the face of global climate and food security challenges.

2. Materials and Methods

The bibliometric analysis conducted in this study departs from traditional review methodologies, such as literature reviews and scoping reviews [42,43,44]. While bibliometric analysis focuses on mapping and assessing the existing body of literature to identify research gaps and delineate the state of knowledge, the bibliometric analysis is also focused on the identification and evaluation of the existing body of literature [45]. In this work, we employed a systematic approach to data collection and evaluation in five stages, which allowed us to identify the most relevant studies, highlight key areas of research, and generate useful inputs to guide future work in this field. A flow chart illustrating the process followed in this bibliometric analysis is presented in Figure 1 [34].

To ensure methodological rigor and transparency, this study follows the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, which are widely recognized for structuring systematic reviews and bibliometric studies (Figure 1). The PRISMA framework facilitates the clear documentation of inclusion and exclusion criteria, systematic database searches, and structured data extraction processes, ensuring reproducibility and reliability in bibliometric research. By applying PRISMA-based selection criteria, we minimized potential biases and enhanced the robustness of our literature mapping [46,47,48].

2.1. Definition of the Search Equation

The search equation is designed to identify relevant scientific papers. The equation searches for articles investigating CO₂ enrichment in controlled agricultural environments and its impact on plant growth, photosynthesis, and agricultural productivity, excluding environmental or ecosystemic topics unrelated to protected agriculture. The structure of the equation is detailed below:

(TITLE-ABS-KEY ((“CO₂ enrichment” OR “carbon dioxide enrichment” OR “CO₂ injection” OR “carbon dioxide injection”) AND (“greenhouse” OR “vertical farm” OR “plant factory” OR “controlled environment agriculture” OR “CEA” OR “protected agriculture”) AND (“plant growth” OR “crop yield” OR “photosynthesis” OR “agricultural production”)) AND NOT TITLE-ABS-KEY (“climate change” OR “wild ecosystems” OR “ocean” OR “forests”))

(1)

2.2. Preliminary Results

Using the Scopus database and applying a search in the “title, abstract and keywords” fields, we exclusively collected and selected articles published in scientific journals, conference proceedings, books, short notes, and book chapters, according to the established search criteria [38]. A rigorous review of the titles and abstracts of the identified articles was performed, selecting those that potentially contained information relevant to the objectives of the study. The results were saved in “CSV” and “BibTeX” formats, ensuring the inclusion of all essential data, such as title, authors, affiliations, abstract, keywords, and bibliographic references.

2.3. Analysis of the Initial Database

To perform the bibliometric analysis, multiple filters were applied using criteria such as type of access, year of publication, author name, subject area, document type, source title, keywords, author affiliation, funding sponsors, country/territory, source type, and language. Initially, 212 documents were collected in Scopus (1982–2024) using a specific search equation. Duplicate documents or those with incomplete bibliographic information (n = 14), articles without access (n = 11), and those not related to protected agriculture (n = 16) were eliminated, leaving a final database of 171 documents for analysis. The selected documents were processed in R-Studio using the bibliometric package and the Biblioshiny application, as well as in VOSviewer to generate co-authorship maps, citations, and bibliographic linkage. These tools allowed a detailed analysis to be conducted of scientific performance, collaboration networks, knowledge transfer, and proposals for future research [34].

2.4. Performance Analysis

Performance analysis evaluates the contributions of the various research components within a specific field [49]. This descriptive approach is fundamental in bibliometric studies and is commonly found in scientific reviews, even those that do not employ scientific mapping [50]. Its purpose is to present the performance of authors, institutions, countries, and journals, providing an analytical view like the profiles presented in empirical research. Among the most used metrics are the number of publications and citations per year or per research component, where publications reflect productivity and citations measure impact and influence.

2.5. Scientific Mapping of the Field of Knowledge

Scientific mapping analyzes the relationships between the different components of the research, making it possible to understand the intellectual and structural connections that make up a field of study [51]. This analysis includes techniques such as citation analysis, bibliographic linkage, co-word analysis, and co-authorship. These methodologies, combined with network analysis, are essential to visualize both the bibliometric structure and the intellectual structure of a research field [52]. In the context of our work, these tools have been employed to map the impact and relationships of studies on protected agriculture and CO₂ enrichment, highlighting connections between authors, institutions, and keywords.

3. Results and Discussion

3.1. Number of Documents Published

Figure 2 shows the trend in the number of publications related to CO₂ enrichment in protected agriculture from 1982 to 2024. A remarkable increase in the number of publications has been observed in the last two decades, reflecting a growing scientific interest in the topic. This increase is particularly pronounced from 2015 onwards, coinciding with increased attention to agricultural sustainability and the development of protected agriculture technologies. The relative stability in publications during the 1990s and early 2000s can be interpreted as an initial phase of exploration, while the recent boom indicates significant progress in the application and understanding of the effects of CO₂ in controlled agricultural systems.

The increase observed in 2024 with 14 publications, the highest value in the period analyzed, can be associated with the expansion of research on crop optimization and climate change mitigation through sustainable agricultural systems. This trend suggests a growing relevance of the topic in the scientific and agricultural field, which reinforces the importance of developing innovative solutions to maximize productivity in protected environments. Furthermore, the data demonstrate a consolidation of interdisciplinary interest, integrating aspects of plant physiology, agricultural technology, and environmental impact, making this topic a key pillar for future research.

3.2. Types of Documents Published

Figure 3 shows the distribution of the 171 documents published, broken down into six main categories of publication type. The predominant category is articles, with 131 documents representing 76.6% of the total, reflecting a clear preference for this format in the dissemination of research results. Articles tend to be the primary means of communicating detailed scientific findings, which explains their high representation [53]. The second most important category is conference papers, with 28 documents, equivalent to 16.4%. This type of publication allows for the dissemination of research in progress or preliminary results in academic forums [54].

The other categories, such as reviews, book chapters, notes, and data papers, have a significantly lower representation. Reviews comprise five papers (2.9%), while book chapters total four (2.3%), indicating that there is less production of synthesis literature or contributions to collective works. Notes and data papers only reach 1.2% (two papers) and 0.6% (one paper), respectively. This distribution highlights the relevance of articles as a fundamental pillar in the dissemination of scientific knowledge in the area of study, reflecting a priority focus on the production of original and significant research [14].

3.3. Areas of Knowledge in Which the Published Papers Are Located

Figure 4 shows the distribution of documents published by subject area in relation to carbon fortification in protected agriculture. The predominant area is Agricultural and Biological Sciences, with 132 papers representing 50.4% of the total. This dominance is to be expected given that CO₂ enrichment is closely related to physiological processes, crop productivity, and the optimization of plant growth in controlled environments [55,56,57,58]. The study of these factors falls within biology and agricultural sciences, reflecting the importance of research to maximize efficiency in greenhouses and protected systems [59].

Other relevant areas include Biochemistry, Genetics, and Molecular Biology (30 papers, 11.5%) and Environmental Science (23 papers, 8.8%). Biochemistry and molecular biology provide a detailed understanding of the physiological and genetic mechanisms that respond to increased CO₂, allowing for the development of more resilient and efficient crops [60,61]. On the other hand, environmental science addresses the impacts of carbon fortification on the sustainability of agricultural systems, assessing the balance between productivity and ecological impact [12,62].

The field of Engineering also stands out with 22 papers (8.4%), highlighting the crucial role of technological innovations, such as CO₂ distribution systems, sensors, crop modeling, and computational modeling (CFD), in improving the efficiency of protected agricultural systems [15,63,64]. In addition, areas such as Earth and Planetary Sciences (10 papers) and Chemical Engineering (seven papers) indicate that CO₂ enrichment is not just an agricultural issue, but involves multidisciplinary applications in mass transfer, gas dynamics, and resource optimization [65,66,67].

The presence of categories such as Energy, Chemistry, and Pharmacology (each with five papers) reflects the exploration of new applications, such as the use of by-products of industrial processes or other methodologies for carbon enrichment in protected agricultural structures [68,69]. However, the low number of publications in areas such as Materials Science (one paper) suggests that there are research opportunities for the development of new materials that improve CO₂ capture and release efficiency in greenhouses and controlled environment growing systems.

3.4. Documents Published by Country

The analysis of the 171 published papers related to CO₂ enrichment in protected agriculture demonstrates a diverse geographic distribution in scientific production, with China leading with 33 publications, representing 19.3% of the total (Figure 5). This reflects China’s commitment to research in advanced technologies to optimize agricultural production in controlled environments, especially in the context of climate change and sustainability [70]. The United States, with 24 papers (14%), also stands out as a key player in the development of strategies to improve CO₂ use efficiency in protected crops, consolidating its leadership in technological innovation, including for international space station applications [71]. Japan, with 20 publications (11.7%), reinforces its role as a leader in advanced research, especially in practical applications integrating CO₂ enrichment with smart farming systems [12].

Canada, with 16 publications (9.3%), reflects a growing interest in this subject, particularly in the context of its climatic conditions, where greenhouses are essential to ensure agricultural production [72]. France and South Korea, with 10 papers each (5.8%), focus on innovative technologies to improve productivity under cover, highlighting their participation in research that combines enriched CO₂ with other variables such as light and temperature [73]. Germany and Spain, with nine publications each (5.3%), contribute significantly to technical knowledge, especially in microclimate optimization and carbon footprint reduction strategies [74,75]. The United Kingdom, with eight publications (4.7%), has made a notable contribution to the development of practical methodologies for its main types of producers [76]. Norway, with six documents (3.5%), shows interest in sustainability in protected agricultural systems, aligning with international climate objectives [77].

Australia, Belgium, Denmark, Egypt, and Mexico, with three publications each (1.7%), reflect an emerging approach to the subject, highlighting regional and emerging initiatives in the efficient use of CO₂ in crops under cover [65,78,79,80,81]. Brazil, Italy, and Saudi Arabia, each with two papers (1.2%), show initial but promising interest in CO₂ enrichment technologies adapted to their local agro-climatic conditions [82,83,84]. Finally, countries such as Algeria, India, Iran, and South Africa, with one publication (0.6%) each, demonstrate specific efforts that could be indicative of international collaborations or local approaches in protected agriculture [85,86].

In general terms, scientific production in CO₂ enrichment in protected agriculture is dominated by countries with advanced economies and consolidated research systems. However, the emerging participation of other countries suggests a growing interest in this technology as a key solution to improve agricultural productivity in a sustainable manner, which opens up opportunities for greater global collaboration and the development of innovations adapted to diverse climatic and economic realities.

3.5. Co-Authorship Network by Country

The analysis of the co-authorship network by country in the field of CO₂ enrichment in protected agriculture reveals clear patterns of international collaboration (Figure 6). Each node in the network represents a contributing country, and the size of the node reflects the number of joint publications [87]. The links connecting the nodes indicate international collaborations, while the link strength, expressed numerically, highlights the frequency of such collaborations [54]. China has the highest link strength with a value of 7, indicating strong cooperation with several countries.

The network shows that China maintains significant collaborations with the United States (liaison strength of 6), Canada (4), and Japan (3), reflecting the strategic role of these nations in research on protected agriculture and CO₂ enrichment. Japan also plays an important role, although its international links are more limited, showing primary connections with China and other Asian countries.

This analysis reveals that international collaboration is essential for the advancement of knowledge in protected agriculture [34]. However, it also evidences that certain countries, such as Australia with a liaison force of 2, could benefit from further integration into global research networks. The consolidation of new strategic alliances would allow these countries to strengthen their impact in the field of CO₂ enrichment, boosting technological and agricultural development at the international level.

3.6. Network of Citations by Country

The analysis of the citation network by country provides a comprehensive view of the dynamics of knowledge distribution in the field of CO₂ fortification in protected agriculture [88]. The network, generated from bibliometric data, reveals the main actors in terms of scientific production and their interconnectedness in the global academic ecosystem (Figure 7). Each node in the network represents a country that has contributed publications, while the size of the nodes reflects the volume of documents generated [88,89]. In this case, China, with its 33 publications, has generated a total of 744 citations, followed by the United States with 573 citations, and Japan with 172 citations. These data underscore the preponderant role of these nations in the development of advanced agricultural technologies and their capacity to influence the field of knowledge.

The links connecting the nodes symbolize the citations or collaborations between countries, and their thickness indicates the intensity or strength of these relationships [90]. The linkage strength, visible in the data table, shows that China has the highest level of connectivity (30), implying a strong network of international collaboration and a high level of influence. Japan follows with a linkage strength of 19, reflecting its active role in knowledge transfer and cross-border collaboration. The United States, despite having a high volume of publications and citations, has a relatively low linkage strength (4), which could indicate a more independent approach in terms of scientific output. Other countries such as Germany and France have 206 and 213 citations, respectively, standing out in the European network.

The network clearly visualizes the relationships between countries, where China emerges as the central node with robust connections to multiple regions, consolidating its position as the epicenter of research. The United States and Japan, although well positioned, show smaller nodes and less extensive connections. In addition, European countries such as Germany (206 citations), France (213 citations), and Spain (94 citations) demonstrate collaborative participation. Notable is the interaction of emerging countries such as Mexico (18 citations) and Egypt (eight citations), which show connections with China and other industry leaders, reflecting a trend towards diversification of research.

The analysis also highlights the participation of countries with a lower volume of publications but with the potential to expand through international collaborations. Norway, with six publications and 325 citations, stands out for having a high citation rate per paper, suggesting that its research has a significant impact. Similarly, Saudi Arabia and Australia, with two and three publications, respectively, maintain a modest but relevant presence. These countries could benefit from strategic alliances with leading nations to strengthen their capabilities in research and development in protected agriculture and CO₂ enrichment systems.

In conclusion, the analysis of the citation network by country evidences the existence of a hierarchical structure in knowledge generation, where a few countries lead the field with a high number of citations, while others play emerging roles. This structure highlights the importance of fostering international cooperation to address global challenges in sustainable agricultural production and CO₂ enrichment. Strengthening collaborative networks will not only broaden the scope of research, but also drive innovations that contribute to mitigate the effects of climate change and increase efficiency in crop production under protected conditions.

3.7. Bibliographic Linkage Between Countries

Analysis of the network of bibliographic coupling between countries allows the connections and collaborations in the field of research on CO₂ enrichment in protected agriculture to be visualized (Figure 8). In this network, each node represents a country that has contributed publications in the area, while the links between nodes indicate the existence of shared bibliographic references among the publications of these countries [91]. Link strength reflects the degree of bibliographic connection, implying that countries with stronger links tend to cite each other more frequently, suggesting greater collaboration or alignment in their lines of research [92].

China stands out as the most prominent node, with a total liaison strength of 1223, indicating high participation and an extensive bibliographic collaboration network. It is followed by the United States with a liaison strength of 688 and Japan with 605. These countries not only lead in terms of document production, but also in terms of influence and scholarly relationships in the field. The size of the nodes is proportional to the number of citations received, reflecting the relevance and impact of each country’s publications.

Analysis reveals that countries with the highest number of citations tend to be in the center of the network, such as China and the United States [88]. This suggests that these nations not only produce more academic literature, but are also widely referenced in other studies. In addition, the presence of multiple links with other countries, such as Canada and Germany, underscores the existence of significant collaborations that contribute to the advancement of knowledge in the area.

The network also shows the existence of regional or thematic subgroups, such as the one formed by European countries (Spain, France, Norway) and Asian countries (Japan, South Korea). This configuration highlights the importance of regional collaborations and their impact on scientific development. From this visualization, it can be inferred that strengthening international collaborations could further enhance knowledge exchange and accelerate progress in CO₂ fortification in protected agriculture.

3.8. Leading Institutions in the Production of Academic Documents

The analysis of the institutions with the highest academic production around CO₂ enrichment in protected agriculture reveals a strong participation of universities and research centers from Asia, Europe, and North America (

Table 1. Relevant institutions.

Institution	Number of Documents Published	Website
Kyushu University	7	https://www.kyushu-u.ac.jp/en/
China Agricultural University	7	https://en.cau.edu.cn/
Seoul National University	7	https://en.snu.ac.kr/index.html
Ministry of Education of the People’s Republic of China	6	http://en.moe.gov.cn/
Norges Miljø- og Biovitenskapelige Universitet	6	https://www.nord.no/
Kochi University	6	https://www.kochi-u.ac.jp/english/
National Agriculture and Food Research Organization, NARO	6	https://www.naro.go.jp/english/
INRAE	6	https://www.inrae.fr/en
Agriculture and Agri-Food Canada	5	https://agriculture.canada.ca/en
Ministry of Agriculture of the People’s Republic of China	5	http://english.moa.gov.cn/

). The institutions with the highest number of publications (seven papers each) include Kyushu University (Japan), China Agricultural University (China), and Seoul National University (South Korea). These universities are noted for their advances in the applied research and development of sustainable agricultural technologies. Their presence in this area underscores the importance that these countries attach to agricultural innovation as a key strategy for meeting the challenges of climate change and improving production efficiency under controlled conditions [70,93,94].

On the other hand, national governmental and research institutions also play a crucial role. The Ministry of Education of the People’s Republic of China and the National Agriculture and Food Research Organization (NARO) of Japan have six publications each, reflecting strong government investment in agricultural research [57,95]. In addition, Norges Miljø- og Biovitenskapelige Universitet (NMBU) of Norway and INRAE (France), both with six published papers. These institutions represent multidisciplinary approaches, integrating agricultural technology with environmental sciences, which strengthens the implementation of sustainable practices in protected agriculture [96,97].

The role of government agencies and ministries is equally significant, as demonstrated by the Ministry of Agriculture of the People’s Republic of China and Agriculture and Agri-Food Canada, both with five publications. The active participation of these entities reinforces the commitment of governments to develop public policies based on scientific research, promoting protected agriculture technologies and the optimization of resources through the enrichment of CO₂ [72,98]. This institutional collaboration also allows the transfer of knowledge to agricultural producers, facilitating the adoption of new technologies at the field level.

These institutions together represent a diverse and highly collaborative research ecosystem. The concentration of publications in Asia and Europe reflects the leadership of these regions in developing innovative solutions for protected agriculture. However, the presence of institutions such as Agriculture and Agri-Food Canada highlights the growing interest of North American countries in strengthening their capacity for agricultural production in controlled environments. This scenario underscores the importance of fostering greater inter-institutional collaborations at the global level to consolidate scientific and technological advances in protected agriculture, especially in the context of mitigating the effects of climate change.

3.9. Leading Authors in Academic Production

CO₂ enrichment in protected agriculture has attracted the attention of numerous world-renowned researchers. This analysis focuses on the ten authors with the greatest contribution to the field, evaluating the impact of their publications, the scope of their work, and their relevance to the advancement of scientific knowledge (

Table 2. Most relevant authors.

Author	Number of Documents	Total Citations	H-Index	Affiliation	Country
Kitano, Masaharu	7	1153	16	Kochi University	Japan
Yasutake, Daisuke	7	966	15	Kyushu University	Japan
Hidaka, Kota	4	552	14	Kyushu Okinawa Agricultural Research Center NARO	Japan
Mortensen, Leiv M.	4	2143	26	Bicotec As, Sandnes	Norway
Okayasu, Takashi	4	1106	19	Kyushu University	Japan
Son, Jung Eek	4	1866	22	Seoul National University	South Korea
Gosselin, André	3	3544	33	Université Laval	Canada
Hwang, Inha	3	128	7	Seoul National University	South Korea
Iwasaki, Yasunaga	3	339	10	Meiji University	Japan
Jung, Dae-ho	3	176	7	Yonam College	South Korea

). The metrics considered include the number of publications, H-index, and total citations, reflecting not only productivity, but also the influence of their research [99,100].

Kitano Masaharu of Kochi University (Japan) is a key researcher in the development of technologies to optimize the greenhouse microclimate and improve photosynthesis in crops. With 1153 citations and an H-index of 16, Kitano has led projects that integrate artificial intelligence (AI) models to estimate canopy photosynthesis in crops such as eggplant and strawberries [101,102]. His work highlights the simulation of photosynthetic and annual yields of greenhouse-grown tomatoes using diffuse canopy and CO₂ enrichment techniques [15]. Kitano has explored how the spatial distribution of CO₂ directly affects photosynthesis and energy use efficiency, developing sustainable and localized strategies to enrich CO₂ in commercial greenhouses [12]. In addition, his research in phenotyping systems with Pan Tilt Zoom cameras has been essential for accurately monitoring crop growth and yields [103].

Yasutake Daisuke, affiliated with Kyushu University, shares a similar line of research with Kitano, accumulating 966 citations and an H-index of 15. Yasutake is recognized for his contributions to the simulation of greenhouse crop growth by modeling photosynthesis [104]. This author has worked extensively on the dynamics of carbohydrate accumulation and growth in strawberries, which has led to a better understanding of the balance between vegetative and reproductive growth in greenhouse crops [105,106,107]. His work on canopy photosynthesis models has been instrumental in predicting yield and adjusting agricultural practices, especially in tomato cropping systems with high plant density structures [15]. Yasutake has also investigated the role of starch as a regulator of carbon flux in response to photosynthetic activity, providing new insights into crop metabolism under CO₂ enrichment [108].

Hidaka Kota, with 552 citations and an H-index of 14, is affiliated with the Kyushu Okinawa Agricultural Research Center NARO. This author has focused his research on photosynthate distribution and photosynthesis optimization in strawberries and other horticultural crops [106,109,110]. Hidaka has led studies on the growth dynamics of strawberries in protected agriculture systems, highlighting the importance of temperature control on photosynthate distribution [111,112]. This work has shown how increasing temperature in the root zone directly affects carbon allocation in asparagus plants [109]. Hidaka has pioneered the use of ¹¹C carbon imaging to track photosynthate transport, thus facilitating a better understanding of carbon flux in greenhouse crops [111].

Mortensen Leiv M., of Bicotec AS in Norway, stands out with 2143 citations and an H-index of 26, positioning him as one of the most influential authors. His work has revolutionized the way photosynthesis is measured in semi-enclosed greenhouses, providing a basis for the development of intelligent climate control technologies [113]. Mortensen has investigated how temperature fluctuations affect the diurnal photosynthetic rhythms of Kalanchoe and other crops [114,115]. In addition, he has explored the use of algae as a mechanism for the remediation of nutrient-rich waters, integrating aquaculture and protected agriculture concepts. His studies on the growth of Chlorella and Chlamydomonas under different CO₂ and oxygen concentrations have contributed to the development of carbon sequestration strategies in agricultural systems [116,117,118].

Okayasu Takashi, with 1106 citations and an H-index of 19, also from Kyushu University, has worked extensively on the development of phenotyping systems and microclimate control in greenhouses. Okayasu has developed low-cost 3D reconstruction systems for crop phenotyping, which has enabled accurate models of light and CO₂ distribution within greenhouses [119]. His research on vapor pressure deficit (VPD) control has proven to be crucial for improving microgreen growth in plant factories [120]. In addition, this author has explored how localized CO₂ enrichment affects strawberry yield, providing evidence of how the spatial distribution of CO₂ can be optimized to maximize photosynthetic efficiency [95,121].

Son Jung Eek, Seoul National University (South Korea), accumulates 1866 citations with an H-index of 22. Son has made important contributions in the integration of metabolomics and photosynthetic analysis in hydroponic crops. He has led studies on how nutrient flux affects lignin biosynthesis and other metabolic pathways in crops such as lettuce [122]. His work on the 3D modeling of photosynthesis in mango and other fruit trees has provided tools for estimating light and CO₂ requirements in greenhouses [123]. In addition, LED inter-illumination systems have been developed to optimize the efficiency of light and water use in sweet peppers [124,125].

Gosselin André, from Laval University in Canada, leads in terms of impact with 3544 citations and an H-index of 33. His work focuses on the optimization of photosynthesis processes and the extraction of bioactive compounds. He is recognized for his work on the development of sustainable protocols for the extraction of bioactive compounds, such as lutein and chlorophyll, from agricultural by-products [126]. Gosselin has investigated the effect of CO₂ and LED lighting on the growth of Nicotiana benthamiana, as well as the optimization of photosynthesis processes for commercial greenhouse crops [127,128].

Hwang Inha, also from Seoul National University, has 128 citations and an H-index of 7. Hwang has worked on the analysis of the light environment and the production of lettuce in greenhouses with flexible solar cells [129]. Hwang has explored how inter-illuminated light and temperature affect the production of peppers and other vegetables [130,131]. Iwasaki Yasunaga, with 339 citations and an H-index of 10, from Meiji University in Japan, has contributed studies on tomato growth under different temperature and elevated CO₂ conditions [132]. Iwasaki has developed alternative methods for analyzing lettuce growth and designed CO₂ enrichment systems for greenhouses [69,133]. Finally, Jung Dae-ho, from Yonam University in South Korea, with 176 citations and an H-index of 7, has focused his research on the growth of peppers, tomatoes, and other crops of commercial interest under different CO₂ concentrations and temperatures [123,134,135].

3.10. Co-Authorship Network Between Authors

The co-authorship network presented in Figure 9 illustrates the collaborative relationships between researchers working in the field of CO₂ enrichment in protected agriculture. In this network, each node represents an author, while the links (lines connecting the nodes) indicate the existence of direct collaborations between them, as reflected in joint publications [136]. Link strength does not simply represent the number of shared papers, but measures the intensity of the connection between authors, considering factors such as the frequency of collaboration and the relevance of the jointly published papers [38,90].

This analysis is essential to understand the structure of scientific communities and the dynamics of collaboration in protected agriculture research [54,137]. The authors with the strongest linking strengths tend to act as central nodes within the network, facilitating the dissemination of knowledge and promoting the development of new lines of research. In addition, the distribution of these strengths allows the identification of possible areas where collaborations could be strengthened or expanded, promoting synergy among researchers and increasing the impact of their contributions in the field of CO₂ enrichment.

For example, Masaharu Kitano shows a link strength of 24, implying that his collaborations with other authors are closer and possibly more influential in terms of academic impact. Similarly, Daisuke Yasutake shows a link strength of 23, indicating a strong integration in the collaborative network. Other authors such as Takashi Okayasu and Kota Hidaka have a link strength of 18, suggesting an intermediate level of active collaboration within the same field. Zhang Yue and Tomoyoshi Hirota, with link strengths of 15 and 11, respectively, may have more specific or limited collaborations compared to the above mentioned authors.

3.11. Citation Network Between Authors

Analysis of the inter-author citation network reveals significant patterns of collaboration and recognition in the field of CO₂ enrichment in protected agriculture (Figure 10). In the graph presented, each node represents an author, while the links between them reflect citation relationships, indicating how many times an author is cited or has been cited by another author [138]. Link strength, on the other hand, measures the intensity of these relationships, implying that higher link strength indicates a higher frequency of reciprocal or indirect citations [34]. This analysis makes it possible to identify the authors with the greatest impact and visibility in the academic community.

The authors Kitano, Masaharu and Yasutake, Daisuke stand out with a link strength of 42, the highest in this network, suggesting that their work is highly cited and recognized by other researchers. Both authors have seven published papers and 86 citations, reflecting consistent production and considerable influence. Their collaboration is evident in connection with other key authors, such as Hidaka, Kota and Okayasu, Takashi, who have a linkage strength of 30, highlighting their role in consolidating new methodologies and technological advances around photosynthesis and microclimates in greenhouses.

Zhang Yue, with three papers and 30 citations, also shows a strong connection with Kitano and Yasutake, with a linkage strength of 18. This author actively contributes to the development of innovative techniques in the optimization of CO₂ distribution and photosynthesis in protected crops. On the other hand, Kimura Kensuke exhibits a linkage strength of 25 with three published papers, indicating a significant level of collaboration and cross-citation with authors such as Kitano and Hidaka.

Finally, it is important to highlight that authors such as Son, Jung Eek and Hwang, Inha, despite having a lower linkage strength (12 and 11, respectively), present important contributions to the field, with solid academic production focused on the use of CO₂ and light to maximize crop production. This network underscores the importance of interdisciplinary collaboration and the existence of well-established working groups that drive innovation and continued development in protected agriculture.

3.12. Network of Bibliographic Coupling Among Authors

The analysis of the bibliographic coupling network between authors makes it possible to identify connections based on the similarity of their cited references [139]. Unlike co-authorship networks, which reflect direct collaborations, bibliographic coupling indicates how closely related the works of different authors are through the sources they use in common [140]. In Figure 11, each node represents an author, while the links between nodes reflect the degree of coincidence in their bibliographic citations [88].

The strength of bibliographic coupling shows the intensity of the connections between authors [141]. Kitano, Masaharu and Yasutake, Daisuke have the highest coupling strength with a value of 1546, suggesting high convergence in the cited sources. Other authors such as Hidaka, Kota and Okayasu, Takashi have a strength of 1063, reflecting considerable overlap in their references. This pattern indicates that while their research may have been conducted independently, it is deeply anchored in a common body of literature.

The network also reveals the existence of clearly distinct clusters. For example, Kitano, Yasutake, Hidaka and Zhang form a closely connected cluster, implying significant thematic affinity in their publications. On the other hand, authors such as Son, Jung Eek and Iwasaki, Yasunaga appear in a distinct cluster, with a lower linkage strength (547 and 71, respectively), which could reflect a difference in focus areas or choice of reference literature.

3.13. Journals Selected by Authors for Publication of Papers

The analysis of

Table 3. Most frequent journal or source of publication.

Journal	No. Documents	Publisher	SJR Ranking	H-Index	Country
Acta Horticulturae	24	International Society for Horticultural Science	Q4	71	Belgium
Scientia Horticulturae	11	Elsevier B.V.	Q1	145	Netherlands
Horticulturae	4	Multidisciplinary Digital Publishing Institute (MDPI)	Q1	36	Switzerland
Horticulture Environment and Biotechnology	4	Springer Science + Business Media	Q1	40	South Korea
Journal of the American Society for Horticultural Science	4	American Society for Horticultural Science	Q2	94	United States
Biosystems Engineering	3	Elsevier B.V.	Q1	132	United States
Frontiers in Plant Science	3	Frontiers Media SA	Q1	216	Switzerland
Global Change Biology	3	Wiley-Blackwell Publishing Ltd.	Q1	313	United Kingdom
Journal of Herbs, Spices and Medicinal Plants	3	Taylor and Francis Ltd.	Q3	34	United States
Tree Physiology	3	Oxford University Press	Q1	147	United Kingdom

reveals a diverse distribution of scientific journals that contribute significantly to research in horticulture and related sciences, with a particular focus on CO₂ enrichment in protected agriculture; in total, 93 journals were used to publish 171 papers. Acta Horticulturae leads in terms of volume of publications (24 papers), consolidating itself as an essential reference within the International Society for Horticultural Science in Belgium. However, its SJR ranking in quartile 4 (Q4) indicates a need for improvement in terms of impact and visibility, despite its H-index of 71, suggesting an established track record in the discipline.

On the other hand, Scientia Horticulturae, published by Elsevier B.V. in the Netherlands, presents a combination of high scientific production (11 papers) and academic prestige, reflected in its ranking in quartile 1 (Q1) and an H-index of 145. This journal is positioned as one of the main publication sources for innovative research, ranging from plant physiology to advanced technologies for microclimate control in greenhouses. Its influence in the field underscores its central role in the dissemination of studies related to the optimization of photosynthesis and the development of new CO₂ enrichment strategies.

Journals such as Horticulturae and Horticulture Environment and Biotechnology, both ranked Q1, also play a prominent role, although with a smaller number of publications (4 papers each). Horticulturae, published by MDPI in Switzerland, shows an H-index of 36, while Horticulture Environment and Biotechnology, under Springer Publishing in South Korea, has an H-index of 40. The presence of these journals in the top quartile reflects their continued contribution to cutting-edge research in agricultural technologies, energy efficiency, and microclimate modeling, key elements for improving productivity in protected agricultural systems.

Finally, high-global-impact journals such as Frontiers in Plant Science (H-index 216) and Global Change Biology (H-index 313), both ranked Q1, stand out for their influence on the understanding of plant response to climate change and environmental variability. Although their direct contribution to the area of horticulture is minor (three papers each), their relevance in plant biology and global change studies strengthens cross-cutting research that seeks to improve sustainability and efficiency in greenhouses through CO₂ enrichment and emerging technologies.

3.14. Citation Network Between Journals

An analysis of the citation network between journals reveals key interactions in the field of the subject area of studies, with a particular focus on CO₂ enrichment in protected agriculture (Figure 12). Each node represents a specialized journal in areas such as horticulture, agricultural systems engineering, and plant physiology, while links between them indicate connections through shared citations in relevant research. The size of the node reflects the cumulative citation level, which provides a clear view on which publications are the most influential in disseminating knowledge related to greenhouse microclimate control, photosynthesis optimization, and improvements in agricultural productivity [88].

In this network, Acta Horticulturae stands out with 24 papers and 143 citations, consolidating it as one of the most influential journals in studies on CO₂ enrichment techniques and the development of new technologies to improve greenhouse efficiency. Its link strength of 4 suggests a moderate connection with other journals, reflecting its role as a bridge between more specialized research. On the other hand, Scientia Horticulturae, with 11 papers and 392 citations, shows high connectivity (link strength of 6), positioning itself as a fundamental pillar in the publication of innovative research on crop physiology, microclimate optimization, and sustainable practices in protected agriculture.

The journal Biosystems Engineering, with three papers and 127 citations, also plays a crucial role in the integration of agricultural technologies and the design of efficient CO₂ enrichment systems. Its linkage strength of 6 indicates that its publications are cited by multiple journals, suggesting that advances in airflow modeling, CO₂ distribution, and energy efficiency have cross-cutting applications in different areas of protected agriculture. This connection highlights the importance of interdisciplinary approaches to address the challenges of climate change and agricultural sustainability.

The network reflects how research on CO₂ enrichment in greenhouses spreads through journals such as Frontiers in Plant Science and Horticulturae, which, despite presenting fewer papers and citations, maintain relevant links to leading publications. This underlines the role of these journals as niche platforms where experimental studies and technological innovation are published, which are subsequently absorbed by higher-impact publications. These dynamics highlights the importance of fostering collaborations between disciplines and continuing to generate applied knowledge to optimize agricultural productivity in protected agriculture systems.

3.15. Citation Network Between Publication Sources

The bibliographic coupling network between journals reflects the relationship and thematic affinity between publications according to the citations they share (Figure 13) [142]. Each node represents a journal, while the links indicate the strength of the connection between them. Link strength reflects how many bibliographic references they have in common, providing a measure of thematic closeness and indirect collaboration [38,143]. In this analysis, Acta Horticulturae and Scientia Horticulturae stand out as the top journals in terms of linkage, with a total of 24 and 11 papers, respectively, and a high linkage strength (183 and 208, respectively).

The journal Acta Horticulturae emerges as the most influential node in the network, suggesting that it plays a central role in the publication of studies related to horticulture and related areas. Its high link strength of 183 indicates extensive connection with other journals, facilitating knowledge dissemination and academic exchange. This journal has high relevance in the scientific community dedicated to protected horticulture, CO₂ enrichment, and related technologies. Scientia Horticulturae, with a linking strength of 208, complements the domain of Acta Horticulturae. The presence of journals such as Biosystems Engineering and Frontiers in Plant Science in the network indicates the involvement of interconnected disciplines such as agricultural engineering and plant biotechnology, underlining the interdisciplinarity of research in protected agriculture and CO₂ enrichment systems.

The structure of the network shows well-defined clusters, where journals such as Horticulturae, Horticulture Environment, and Biotechnology and Tree Physiology maintain significant links with the main journals, reinforcing academic collaboration. The inclusion of high-impact journals such as Global Change Biology and the Journal of Experimental Botany highlights the importance of the study of plant physiology and climate change in the improvement of horticultural systems under controlled conditions. This network provides a comprehensive view of the connections and strengths between journals, highlighting the key role of certain publications in the consolidation and advancement of knowledge in the agricultural sector.

3.16. Top Ten Most Relevant Articles

A common practice in bibliometric studies is to identify the most influential articles in the field of knowledge (

Table 4. Top 10 most cited articles.

Document	Title	Citations	Reference	Journal
1	Review: CO2 enrichment in greenhouses. Crop responses	236	[97]	Scientia Horticulturae
2	Interactive effects of increased pCO2, temperature and irradiance on the marine coccolithophore Emiliania huxleyi (Prymnesiophyceae)	225	[144]	European Journal of Phycology
3	Growth in elevated CO2 protects photosynthesis against high-temperature damage	133	[145]	Plant Cell & Enviroment
4	Nutrient relations in calcareous grassland under elevated CO2	83	[146]	Oecologia
5	CO2 enrichment can produce high red leaf lettuce yield while increasing most flavonoid glycoside and some caffeic acid derivative concentration	74	[21]	Food Chemistry
6	Automatic carbon dioxide enrichment strategies in the greenhouse: A review	70	[93]	Biosystems Engineering
7	Tight coupling of root-associated nitrogen fixation and plant photosynthesis in the salt marsh grass Spartina alterniflora and carbon dioxide enhancement of nitrogenase activity	63	[147]	Applied and Environmental Microbiology
8	Photosynthetic acclimation to elevated atmospheric carbon dioxide and UV irradiation in Pinus banksiana	56	[148]	Physiologia Plantarum
9	Modelling CO2-enrichment effects using an interactive vegetation SVAT scheme	50	[149]	Agricultural and Forest Meteorology
10	Effects of CO2 enrichment on photosynthesis and growth in Gerbera jamesonii	49	[150]	Scientia Horticulturae

), generally selecting the 10 articles with the most citations [37]. CO₂ enrichment in greenhouses has proven to be a fundamental strategy for increasing crop productivity and improving plant quality. Over time, this interest has fluctuated, but recent years have seen a significant increase, driven by a deeper understanding of the physiological mechanisms underlying the impact of CO₂ on plants. Increased photosynthetic rate and plant growth are attributed to the decreased inhibition of photosynthesis by oxygen, especially under high irradiance conditions. This process allows plants to use light and nutrients more efficiently, resulting in higher yields. The review by Mortensen [97] highlights how high CO₂ concentrations between 700 and 900 µL L⁻¹ represent the optimal range for yield improvement in potted plants, cut flowers, vegetables, and some forestry plants. This work has 236 citations and is consolidated as one of the most referenced sources in the field and offers a comprehensive view on the benefits and limits of carbon enrichment in protected agricultural systems, covering a wide range of crops.

The interaction of CO₂ enrichment with environmental factors such as temperature and irradiance has also been the subject of relevant studies. The research carried out by Feng et al. [144], which has 225 citations, provides evidence of the synergistic effect between increased CO₂, temperature, and light on marine organisms, specifically on the coccolithophore Emiliania huxleyi. The results suggest that the growth of Emiliania huxleyi under these conditions is significantly accelerated, highlighting the importance of addressing climate change with holistic approaches that incorporate multiple environmental variables, which could be extrapolated to protected agriculture, particularly in hydroponic crops sensitive to changes in light and temperature.

CO₂ enrichment also contributes to the protection of photosynthesis against thermal damage, allowing plants to maintain high rates of carbon assimilation under high-temperature conditions. The article written by Taub et al. [145], which has 133 citations, presents results obtained in field and laboratory facilities, indicating that photosystem II (PSII) thermotolerance is markedly increased under elevated CO₂ conditions in crops such as cucumber (Cucumis sativus). This finding is crucial, as photosynthesis is one of the physiological processes most sensitive to heat stress. The study demonstrates that plants grown under elevated CO₂ maintain higher photosynthetic efficiency at higher temperatures, suggesting that this practice could be key to the adaptation of horticultural crops in the face of global warming.

In the context of grasslands, CO₂ enrichment not only affects plant growth, but also nutrient ratios. The publication developed by Nikalus et al. [146], which has 83 citations, reveals that, after four years of exposure to elevated CO₂ levels, the C/N ratio in plant tissues increased significantly. This finding suggests that increases in biomass are achieved at the expense of nutrient dilution, which could influence the nutritional quality of forage crops and grasses. In addition, elevated CO₂ was found to promote nitrogen storage in soil microbial reservoirs, which could have implications for long-term soil fertility, benefiting grass and legume crops.

A relevant aspect of CO₂ enrichment is its ability to improve not only yield, but also the biochemical composition of crops. Research conducted by Becker and Peter Kläring [21], which currently has 74 citations, demonstrates that increased CO₂ increases the concentrations of flavonoids and other beneficial compounds in red leaf lettuce crops. This result is particularly relevant for vegetable and leafy vegetable crops, where nutritional quality and antioxidant concentration are differentiating factors in the marketplace.

Automatic CO₂ enrichment in greenhouses is a fundamental strategy to optimize crop yields, as reviewed in the 70-citation article by Li et al. [93]. The review addresses the most recent theoretical and applied studies on CO₂ enrichment, highlighting the advantages and limitations of available methods. The five main sources of CO₂ used in greenhouses, monitoring, and data processing systems to regulate CO₂ concentrations and methods to automate and control enrichment efficiently are discussed in detail. The review highlights that, although CO₂ enrichment can significantly increase crop growth and quality, maintaining consistently optimal concentrations is challenging due to the dynamic interaction with other factors such as temperature, humidity, and light intensity. From a technological approach, the paper emphasizes the need for advanced sensor and automation systems to accurately manage CO₂ levels, thus avoiding economic losses due to leakage or suboptimal concentrations. The ability to integrate these systems allows photosynthetic efficiency to be maximized and energy costs to be minimized, while contributing to reducing the greenhouse’s carbon footprint.

The relationship between nitrogen fixation and photosynthesis under CO₂ enrichment has also been studied. The work carried out by Whiting et al. [147], which has 63 citations, shows that increased CO₂ enhances root nitrogenase activity, facilitating greater nutrient uptake and increasing plant biomass in salt marsh ecosystems (Spartina alterniflora). This close coupling between nitrogen fixation and photosynthesis suggests that CO₂ not only acts directly on plant growth, but also modulates nutrient assimilation processes, benefiting coastal grassland species and perennial crops.

In forestry, CO₂ enrichment affects photosynthetic acclimation to UV radiation. Research conducted by Stewart and Hoddinott [148], which has 56 citations, highlights that, although exposure to high UV-B inhibits photosynthesis at normal CO₂ levels, this effect is mitigated when CO₂ concentrations are elevated. This result suggests that CO₂ may play a protective role against environmental stressors, contributing to the resilience of forest species such as Pinus banksiana under changing conditions.

Modeling also plays a crucial role in understanding the effects of CO₂ on ecosystems. The study, which has 50 citations and was written by Calvet and Soussana [149], involved the use of computational simulations to predict the effects of CO₂ enrichment on the energy and water balance of ryegrass (Lolium perenne) crops. This approach allows plant responses to be anticipated under different climatic scenarios and agricultural management strategies to be designed based on predictive data.

Finally, the effects of CO₂ on ornamental crops have also been explored. The study conducted by Xu et al. [150] demonstrates that continuous morning CO₂ enrichment significantly increases the photosynthetic rate and growth of Gerbera jamesonii, extending the longevity of flowers and improving their aesthetic quality. This research has 49 citations. These results underline the potential of CO₂ enrichment as a key tool for floriculture, enabling higher productivity and competitiveness in the ornamental sector.

3.17. Top Ten Recently Published Articles

Carbon dioxide (CO₂) enrichment in greenhouses has been identified as an effective strategy for enhancing crop growth, productivity, and quality under controlled environments. Various studies have demonstrated that increasing CO₂ concentrations in greenhouses can boost photosynthesis, improve water use efficiency, and optimize agricultural production. However, its implementation must consider factors such as ventilation, temperature, relative humidity, and interactions with other technologies, such as supplemental lighting and automated climate control [58].

Recent studies have evaluated the impact of CO₂ enrichment combined with supplemental lighting on cucumber production. Findings indicate that the combination of these strategies significantly increases the number of female flowers, crop yield, and economic profitability. An experiment conducted in a plastic greenhouse showed that CO₂ enrichment led to a 35% increase in cucumber production compared to the control group. Furthermore, economic analysis revealed a higher income rate for CO₂-enriched treatments, suggesting its feasibility as a productive strategy [55]. Tomato cultivation in greenhouses has also benefited from CO₂ enrichment. Photosynthesis simulation models have estimated that doubling the CO₂ concentration from 400 ppm to 800 ppm can increase annual crop yield by 30%. This improvement results from an increased carbon assimilation rate, which enhances canopy growth. The implementation of additional strategies, such as diffuse covers and high-wire cultivation structures, could further amplify the benefits of CO₂ enrichment [15].

The use of CO₂ in urban greenhouses has also proven to be a promising strategy for improving agricultural sustainability. A study on rooftop greenhouses highlighted how CO₂ generated by building heating systems can be repurposed for growing vegetables and ornamental plants. The research emphasized the case of the Thanksgiving cactus, a CAM species that can utilize nighttime CO₂ to enhance growth. Thus, urban greenhouses can contribute to reducing the carbon footprint and improving resource efficiency in urban environments [151]. In controlled strawberry F1 hybrid production, CO₂ enrichment has demonstrated significant improvements in growth and fruit quality. A study conducted in Saudi Arabia found that increasing CO₂ concentration from 400 ppm to 600 ppm enhanced the photosynthetic rate by 129.7% and total fruit yield by 42.2%. However, a reduction in the uptake of essential nutrients such as nitrogen, phosphorus, and potassium was observed, suggesting the need to adjust fertilization strategies in crops exposed to high CO₂ levels [84].

The development of predictive climate control systems has optimized CO₂ management in rooftop-integrated greenhouses. Through advanced nonlinear model predictive control (NMPC) frameworks, energy consumption has been reduced while maintaining stable microclimatic conditions. A case study in Brooklyn, New York, demonstrated a 15.2% reduction in energy consumption by optimizing CO₂ utilization alongside other environmental parameters, highlighting the importance of automation in efficient CO₂ management in urban greenhouses [152].

To assess the efficiency of CO₂ utilization in greenhouses, analytical methods based on the fossil-derived carbon ratio (FDCR) in plants have been implemented. A study found that tomatoes cultivated in CO₂-enriched greenhouses used approximately 60% of the injected CO₂, while 40% originated from ambient air. This analysis is crucial for improving enrichment efficiency and minimizing CO₂ loss through ventilation, which could optimize its application in intensive agricultural production systems [153]. An innovative approach to CO₂ capture and reuse in greenhouses involves the use of biomass flue gases. A CO₂ enrichment system based on activated carbon adsorption successfully increased tomato crop productivity by 18%, maintaining optimal CO₂ levels within the greenhouse. This approach not only improves CO₂ utilization efficiency, but also contributes to emission reduction by integrating renewable energy sources into agricultural production [74].

CO₂ has also been explored as a versatile resource across multiple industries, including agriculture. A systematic review highlighted its potential applications in concrete production, biodegradable plastics manufacturing, and synthetic fuel synthesis. In the agricultural sector, CO₂ enrichment has demonstrated yield improvements and resource-use efficiency gains. However, for widespread adoption, challenges related to the economic feasibility and scalability of CO₂ capture and reuse technologies must be addressed [154].

In greenhouse grape production, the optimal CO₂ concentration for maximizing photosynthesis and yield has been determined to be 700 ppm. At this concentration, increases in carboxylation efficiency, electron transport rates, and key carbon metabolism enzyme activities were observed. However, CO₂ levels exceeding 1000 ppm led to adverse effects, reducing photosynthesis and yield, highlighting the necessity of establishing optimal CO₂ thresholds to avoid negative impacts on crop growth [56].

This body of research confirms the fundamental role of CO₂ enrichment in protected agriculture. However, its implementation must consider multiple factors, including crop-specific physiological responses and integration with advanced climate control technologies. As these strategies are refined and optimized, CO₂ enrichment emerges as a key tool for sustainably and efficiently enhancing agricultural productivity in controlled environments.

3.18. Most Used Keywords

The word cloud generated from the 100 most used keywords visually reflects the main themes and focuses of the current article on CO₂ enrichment in greenhouses and controlled environments, with their respective impact on plant growth (Figure 14). The largest words, such as “carbon dioxide” (84 occurrences), “photosynthesis” (44 occurrences), “greenhouses” (54 occurrences), and “carbon dioxide enrichment” (32 occurrences), indicate a strong concentration of studies analyzing the role of carbon dioxide in photosynthetic processes and agricultural productivity under controlled environments [124,155,156]. The prominence of these terms reinforces the importance of CO₂ as a determining factor in crop efficiency, and how its manipulation through enrichment technologies in greenhouses or controlled agricultural production spaces can maximize yield.

In addition to the main terms, the word cloud highlights concepts such as “plant growth” (14 occurrences), “growth rate” (10 occurrences), “chlorophyll” (9 occurrences), “biomass” (7 occurrences) and “crop yield” (6 occurrences), suggesting a comprehensive approach to the measurement of plant growth and development [155,157]. The presence of words related to “gas exchange” (seven occurrences), “stomatal conductance” (four occurrences) and “photosynthetic rate” (five occurrences) highlights the interest in understanding the physiological processes that directly affect CO₂ assimilation and photosynthetic efficiency [60,75,158]. The inclusion of specific terms such as “Lycopersicon esculentum” (10 occurrences) and “capsicum annuum” (4 occurrences) suggests that certain species have been recurring models in research, reflecting interest in high-value cash crops under CO₂ enrichment conditions [76,159,160].

On the other hand, terms such as “air pollution” (six occurrences), “irrigation” (four occurrences), “ventilation” (four occurrences) and “temperature effect” (three occurrences) indicate that the analysis is not limited to plant growth, but also encompasses environmental factors and their interaction with CO₂ [161,162]. This multidisciplinary approach highlights the importance of studying the entire greenhouse ecosystem, considering aspects such as water use efficiency and temperature control [155,163]. The use of terms such as “forecasting” (three occurrences), “gene expression profiling” (three occurrences) and “ecosystem” (three occurrences) suggests that the studies go beyond plant physiology to encompass predictive modeling and genetic analysis to optimize controlled agricultural systems [164,165].

3.19. Keyword Co-Occurrence Network

The keyword co-occurrence network presented in Figure 15 reflects the main lines of research on CO₂ enrichment in protected agriculture. The figure was generated with VOSviewer. This visualization allows identifying the frequency and relationships between relevant terms in scientific publications. The nodes represent keywords, whose size indicates their recurrence, while the connections show how many times these words appear together in the same documents. The intensity of the lines suggests the strength of the relationship between terms, while the different colors group words that tend to co-occur, revealing specific subtopics within this field [38,166].

Red Cluster—Carbon and Photosynthesis—Fundamentals of CO₂ Enrichment: This cluster is centered on the words “carbon dioxide” and “photosynthesis” and highlights the fundamental role of CO₂ in photosynthesis and crop production. This cluster reflects research aimed at understanding how increased CO₂ directly influences crop yields as well as physiological processes critical to agricultural production. Terms such as “crop production” and “ventilation” suggest that the control of ventilation conditions is essential to regulate CO₂ levels and maximize photosynthetic efficiency, thus contributing to productivity optimization. Likewise, terms such as “energy” and “Cucumis sativus” (cucumber) indicate that this group of studies addresses the production of specific crops and their relationship with energy consumption in protected agriculture systems.

Green Cluster—Growth Response and Biochemical Composition Under CO₂ Enrichment: This cluster addresses plant responses to CO₂ enrichment and its impact on biochemical composition. Keywords such as “carbon dioxide enrichment”, “growth response”, and “carbohydrate” reflect studies focused on the accumulation of carbohydrates and other biochemical compounds resulting from increased CO₂ concentrations. The presence of specific crops like Capsicum annuum (bell pepper) indicates that certain species have been used as models in these experiments, highlighting the importance of analyzing the impact of CO₂ enrichment at the level of specific species and varieties.

Blue Cluster—Environmental Factors and Plant Growth Under CO₂ Enrichment: This cluster delves into plant growth and its relationship with environmental factors under enriched CO₂ conditions. Concepts such as “plant growth” and “nitrogen fixation” suggest a line of research analyzing how CO₂ interacts with light, nitrogen, and other essential elements for plant development. This cluster emphasizes the importance of physiological processes like “morphogenesis”, indicating that studies not only focus on crop yields, but also on structural and anatomical changes occurring during growth under CO₂-enriched conditions.

Purple Cluster—Controlled Agriculture and Cultivation Environment Optimization: This cluster focuses on controlled agriculture systems and their relationship with environmental factors. Terms like “controlled environment agriculture”, “plants (botany)”, and “environmental conditions” indicate that these studies are directed toward production in enclosed environments, such as greenhouses and plant factories. The inclusion of “forecasting” and “fruits” highlights a trend toward predicting outcomes and optimizing fruit crop production under these conditions. This cluster reflects the growing interest in developing technologies to precisely control environmental factors and maximize crop productivity in protected spaces.

Yellow Cluster—Light Intensity and Hydroponic Crops Under CO₂ Enrichment: This cluster explores the relationship between light and CO₂ enrichment in protected agriculture. Terms like “light intensity”, “hydroponics”, and “tomato” suggest that much of the research in this field focuses on hydroponic crops, where the interaction between light and CO₂ is a critical factor for successful production. Tomatoes appear as one of the most studied crops, reflecting their economic importance and responsiveness to CO₂ enrichment in controlled environments.

Light Blue Cluster—Physiological and Biochemical Responses of Plants Under CO₂ Enrichment: This cluster focuses on the physiological and biochemical changes experienced by plants in controlled environments with high levels of carbon dioxide (CO₂). Keywords such as “plant leaves”, “chlorophyll”, “plant leaf”, “sugar”, “starch”, and “controlled study” suggest that studies analyze in detail the behavior of leaves, chlorophyll content, and the accumulation of compounds like sugars and starch, essential for plant growth. Leaves, as the primary photosynthetic organs, exhibit the most notable changes under CO₂-enriched conditions, allowing for the direct evaluation of its impact on photosynthesis and biomass production. Increased levels of sugars and starch indicate that CO₂ not only accelerates growth, but also enhances plants’ energy storage capacity, positively influencing crop quality and yield. The mention of “controlled study” highlights that these experiments are conducted in standardized environments, such as greenhouses or protected agricultural systems, where environmental variables are manipulated to ensure precise and replicable results.

3.20. Multiple Correspondence Analysis: Knowledge Gaps and Future Directions in CO₂ Enrichment for Protected Agriculture

Figure 16 presents a Multiple Correspondence Analysis (MCA) plot, a multivariate technique used to visualize relationships and patterns among different categorical variables. In this case, the plot represents the distribution of key terms related to CO₂ enrichment, photosynthesis, and agricultural production in greenhouses. The terms are grouped into clusters distinguished by colors, suggesting specific thematic associations. The axes Dim 1 and Dim 2 collectively explain 73.49% of the data variability, indicating that most of the relationships are well represented in this two-dimensional plane. These dimensions correspond to the principal components derived from correspondence analysis (MCA), a statistical technique used to visualize associations between key terms in research on CO₂ enrichment in protected agriculture. In this study, Dim 1 (52.93%) represents the main variability in the data, differentiating concepts related to productivity and efficiency of CO₂ in crops from those focused on environmental and methodological aspects. Dim 2 (20.05%), on the other hand, captures the second source of variability, allowing the identification of groupings of terms that highlight the different applications and approaches within the field of study [167].

The green cluster is associated with terms such as “carbon”, “chemistry”, “controlled study”, and “agriculture”, reflecting a research focus on the effects of CO₂ and its relationship with controlled studies and the chemistry of plant growth. On the other hand, the blue cluster groups concepts like “crop yield”, “Lycopersicon esculentum”, and “growth rate”, suggesting a more applied focus on specific crops, yield, and growth under CO₂-enriched conditions. The red cluster, with terms such as “greenhouses”, “ventilation”, and “photosynthetic rate”, denotes research centered on the physical environment of greenhouses and its influence on photosynthesis. Finally, the purple cluster addresses topics such as “net photosynthetic rate” and “atmospheric composition”, highlighting studies related to plant physiology and gas dynamics in the cultivation environment.

From this figure, significant research gaps emerge that require greater attention in future investigations. One major gap is the limited representation of terms associated with advanced technological innovations, such as artificial intelligence (AI), the Internet of Things (IoT), and intelligent sensor systems applied to CO₂ enrichment in greenhouses. The absence of these terms suggests that, while studies have broadly addressed the impact of CO₂ on plant growth, the integration of real-time monitoring tools and predictive algorithms for resource optimization remains underexplored. This gap presents a crucial opportunity to develop greenhouse models that not only maximize agricultural yield, but also automate the control of environmental variables, reducing energy consumption and minimizing losses. Future research should focus on the synergy between emerging technologies and crop biology, evaluating how these tools can enhance precision and efficiency in managing CO₂ and other environmental factors.

Another critical gap lies in the absence of terms related to sustainability, the circular economy, and the mitigation of environmental impacts. The lack of concepts such as “climate change”, “carbon footprint”, “carbon capture”, and “water efficiency” in the main clusters reveals that much of the research has focused on immediate productivity without fully addressing the long-term implications of CO₂ enrichment on agricultural ecosystems and the environment. Since CO₂ enrichment may influence atmospheric balance and crop structure, future studies must adopt a holistic approach that incorporates life cycle assessments (LCAs) and sustainability analyses. Research exploring the reuse of residual CO₂ from industrial processes or combining enrichment practices with renewable energy sources could open new avenues for reducing environmental impact and promoting regenerative agriculture.

Additionally, there is an underrepresentation of studies analyzing the socioeconomic effects of CO₂ enrichment, such as the impact on production costs, smallholder access to these technologies, and large-scale economic feasibility. Implementing these strategies could disproportionately benefit large producers, sidelining smaller farmers. Future research should consider regional case studies and inclusive economic models to analyze the adoption of CO₂ enrichment technologies across diverse agroclimatic and socioeconomic contexts. This approach would enable the design of technology transfer strategies that promote equity in protected agriculture, expanding access to sustainable technologies and strengthening the agricultural sector’s resilience to global challenges.

4. Conclusions and Recommendations

CO₂ enrichment represents a critical strategy in protected agriculture, significantly enhancing crop productivity, quality, and resource-use efficiency. The reviewed literature underscores advances not only in photosynthesis and nitrogen assimilation, but also in automation, stress mitigation, and climate resilience. These findings affirm the relevance of CO₂ enrichment as a key technological pillar to support climate-smart agriculture and the long-term competitiveness of the sector.

The bibliometric analysis revealed a steady increase in scientific output over the past decade, particularly led by China, the United States, and Japan. It also highlighted the emergence of interdisciplinary integration between plant physiology, controlled environment engineering, and sustainability sciences. The mapping of co-authorship and bibliographic coupling networks suggests that fostering targeted international collaborations can accelerate innovation and reduce technological asymmetries between regions.

Rather than general appeals for cooperation, this study specifically recommends the development of structured research alliances between Asia and Latin America, aiming to transfer and adapt CO₂ enrichment technologies to diverse agroecological and socioeconomic contexts. These alliances should prioritize joint research programs, shared experimental platforms, and open-access datasets, particularly focusing on smallholder and resource-limited systems.

To support evidence-based decision-making, future studies should incorporate cross-regional economic feasibility analyses, environmental impact assessments, and policy simulations under different climate scenarios. Additionally, we encourage the inclusion of CO₂ enrichment strategies in national agricultural innovation agendas, particularly in policies that promote climate change adaptation, technology transfer, and greenhouse gas mitigation.

5. Methodological Innovation and Limitations

This review integrates a systematic PRISMA-based selection protocol with bibliometric mapping techniques, offering a reproducible and transparent approach to analyzing global trends and knowledge gaps in CO₂ enrichment research. The combination of these methodologies enhances the analytical depth and contributes a structured framework that can be replicated in related agricultural technology domains.

This study is limited by its exclusive reliance on the Scopus database, which, despite its comprehensive indexing, may exclude relevant studies indexed in other databases such as Web of Science or regional repositories. Future reviews could address this limitation through multi-database triangulation to ensure broader coverage.