Visual Analysis of Research Progress on the Impact of Cadmium Stress on Horticultural Plants over 25 Years

Abstract

In recent years, there has been a significant growth in scholarly attention to the effects of Cd stress on horticultural plants, as reflected by the abundance of research articles on this issue in academic publications. Therefore, it is necessary to conduct a review of current research and provide a comprehensive perspective to quickly grasp the latest developments and future trends in the research field of “horticultural plants-Cd responses”. By utilizing a visualizing bibliometric analysis software CiteSpace, this study integrated and analyzed a total of 4318 relevant research records—2311 from the Web of Science (WOS) database and 2007 from the China National Knowledge Infrastructure (CNKI) database—related to “horticultural plants-Cd responses”, covering the period from 1999 to 2024. A visual analysis was conducted in the form of knowledge mappings, including the current research status of “horticultural plants-Cd responses”, as well as the differences in publications’ temporal distribution, spatial distribution (cooperation networks) and intellectual base between China and foreign countries, precisely uncovering the core aspects of research topics related to the field. The results indicated the following: (1) Scientific research on “horticultural plants-Cd responses” has experienced a significant increase in publication volume and has entered a phase of rapid development. Globally, there has been an annual average increase of 217 articles in the WOS since 2019, while in China, the annual average increase has been 134 articles in the CNKI since 2015. (2) China is the most productive country in terms of publication volume (1165 articles, 52.79%), engaging in active partnerships with other countries worldwide. Chinese scholars (Lin L. and Liao M.) are leading researchers in both domestic and international research fields of “horticultural plants-Cd responses”. The network of collaborations among authors and institutions in the WOS database seemed denser compared to that in the CNKI database. (3) International research hotspots have focused on accumulation, tolerance and oxidative stress, while domestically, the focus has been on antioxidant enzymes, growth and seed germination. Phytoremediation, subcellular distribution and the transcriptome are the world’s emerging topics, while in China, growth and physiological characteristics are still emerging topics. (4) In comparison, China exhibited a lagging development trend, which is reflected in the fact that it began to focus on gene expression and transcriptome research only after the global frontier shifted towards biochar and cadmium co-stress and yield response. Based on these, this study provides a systematic theoretical basis for subsequent research on “horticultural plants-Cd responses”, aiding scholars in their efforts to understand the dynamic frontiers and address the challenges in this field.

1. Introduction

The contamination of heavy metals (HMs) has become one of the most significant environmental issues worldwide because of several anthropogenic activities (such as industrials melting and electroplating, agricultural fertilization and improper disposal of used batteries) [1,2,3,4]. The HM contamination can result in altering the natural chemical properties of soil, affecting crop yields and food safety and disrupting the balance of ecosystems [5,6,7,8]. Cadmium (Cd) is regarded as a highly toxic HM element, which has those features of difficulty of degradation, easy migration and high carcinogenicity [9,10,11]. Cd can not only inhibit the growth of plants (such as reducing seed germination rates and affecting root system formation) and disrupt the physiological activities of plants (such as reducing antioxidant enzyme activities and inhibiting photosynthesis) but also severely affect animal and human health through respiratory intake by atmospheric deposition and esophageal ingestion by the food chain [12,13,14,15,16]. Cd stress is bound to cause intricate environmental challenges and heighten ecological threats [17,18,19]. Therefore, research on the effects of Cd contamination has become a global hot topic.

Horticultural plants cultivated for human consumption or ornamental purposes, generally include plants such as vegetables, fruit trees, flowers and medicinal plants [20,21,22,23,24,25]. The resources of horticultural plants around the world are extremely abundant and incredibly diverse. Horticultural plants can be found everywhere in our daily lives, and they are also one of the earliest varieties of plants to be cultivated by human beings [26,27,28]. The relationship between horticultural plants and human beings is extremely close [29,30]. Cd contamination resulting from anthropogenic activities will inevitably have an effect on horticultural plants, and greater focus is needed on attaching importance to this problem [31,32,33].

In recent years, there has been a significant growth in scholarly attention to the effects of Cd stress on horticultural plants, as reflected by the abundance of research articles on this issue in academic publications. As scholarly interest grows in the effects of Cd stress on horticultural plants, numerous studies have been undertaken by scientists to explore this research field such as those on the response process of seed germination, plant growth, photosynthesis, antioxidant systems and gene expression [34,35,36,37,38,39,40]. These studies are often presented as independent research to indicate the changes in certain indicators or functions of plants under environmental stress, having less information on providing a comprehensive perspective to quickly grasp the latest developments in the research field. Therefore, it is necessary to conduct a review of current research, which can not only integrate and summarize a wide range of information from diverse studies but also point out unresolved issues and future directions in the research field, as a guiding significance for researchers in selecting research topics and designing experiments.

Despite many years of research exploring the effects of Cd stress on horticultural plants, to date, there is still a lack of systematic analysis and comprehensive understanding of how horticultural plants respond to Cd stress on a global scale, including within China. CiteSpace, as a visualizing bibliometric analysis software, has garnered widespread attention in recent years [41,42,43,44,45]. It provides data support for quantitative analysis, aiding in the evaluation of future research directions by examining the expansion of articles in specific areas of research [46,47]. The software of CiteSpace can transform complex bibliometric analyses into intuitive visual outcomes, helping researchers understand and explore data from the scientific literature more effectively [48,49,50]. This technology offers researchers a novel approach to grasp the most recent development trends and future research directions of China and the world in various research fields [51]. It is a precious instrument for obtaining insights and staying updated with the newest breakthroughs [52,53,54]. Therefore, to obtain more comprehensive research findings, this study integrates and analyzes data from the Web of Science (WOS) and the China National Knowledge Infrastructure (CNKI) databases, covering the period from 1999 to 2024, using CiteSpace 6.3.R1 software. The objectives of this study are to conduct a visual analysis in the form of knowledge mappings, including the current research status of “horticultural plants-Cd responses”, as well as the differences in publications’ temporal distribution, spatial distribution (cooperation networks) and intellectual bases between China and foreign countries. It will be helpful for scholars to gain a quantitative and intuitive grasp of the current state of research and to track the research hotspots and dynamic frontiers in the field of “horticultural plants-Cd responses”. This study also provides a systematic theoretical basis for subsequent research on “horticultural plants-Cd responses”, aiding scholars in their efforts to understand and address the challenges in this field.

2. Materials and Methods

2.1. Data Sources

All data in this study were derived from the databases of Web of Science (WOS) and China National Knowledge Infrastructure (CNKI), which cover a wide range of high-quality academic research studies from around the world, ensuring the comprehensiveness and authority of the data sources. The search period extended from January 1999 to September 2024. On 6 September 2024, research studies related to “horticultural plants and cadmium” were retrieved from the databases of WOS and CNKI. In the database of WOS, the retrieval formula was set as follows: (TS = “horticultural plant” OR “horticultural flower” OR “vegetable” OR “fruit tree” OR “fruit” OR “horticultural flower” OR “ornamental plant” OR “ornamental flower” OR “garden flower” OR “flower plant” OR “flowering plant” OR “landscape greening plant” OR “landscape plant” OR “greening plant” + TS = “cadmium” OR “Cd”). In the database of CNKI, the retrieval formula was set as follows: (subject = “horticultural plant” OR “horticultural flower” OR “vegetable” OR “fruit tree” OR “horticultural flower” OR “ornamental plant” OR “ornamental flower” OR “garden flower” OR “flower plant” OR “flowering plant” OR “landscape greening plant” OR “landscape plant” OR “greening plant” + TS = “cadmium” OR “Cd”). Through careful reading and screening, 1598 unrelated or duplicate research records were excluded, leaving a total of 4318 relevant research records–2311 from the WOS database and 2007 from the CNKI database–related to “horticultural plants-Cd responses”. These retained records served as the foundational data for analysis.

2.2. Analysis Method

CiteSpace is a sophisticated tool for visualizing and analyzing scholarly literature, which generates graphical representations derived from quantitative analytical methods [55,56]. It was initially developed by Dr. Chen and his research group in 2004 and has been progressively refined and disseminated ever since [57,58]. Currently, CiteSpace is widely applied in research fields such as environmental pollution and plant science [59,60,61,62]. In the present study, CiteSpace 6.3.R1 (https://citespace.podia.com/) was used to conduct a visualizing bibliometric analysis based on those retained research records from the databases of WOS and CNKI. In the analysis of literature growth trends and predominant publication outlets, we utilized data that had been processed through CiteSpace. Subsequently, we refined and arranged these data to create visual graphs for bibliometric examination. CiteSpace was also employed to graphically represent the distribution of authors, countries, research institutions and keywords since 1999. CiteSpace emphasizes the use of connecting lines to illustrate the prominence of each subject area and dissects the clustering connections among various nodes [63,64,65,66]. As a result, the software is capable of precisely uncovering the core aspects of research topics related to “horticultural plants-Cd responses”. Additionally, an analysis of the general publication trends was carried out using Microsoft Office Excel 2020 and Origin 2022 to create the corresponding charts.

3. Results and Discussion

3.1. Temporal Distribution of Publications on “Horticultural Plants-Cd Responses”

Research literature analysis is the process of examining the volume, trajectory and composition of scholarly publications within a specific academic domain. This type of analysis enables researchers to understand the annual publication volume, cumulative publication volume and annual average increase and to forecast future trends in scholarly output within their field of study [67]. The study analyzed a total of 4318 relevant research records—2311 from the Web of Science (WOS) database and 2007 from the China National Knowledge Infrastructure (CNKI) database—related to “horticultural plants-Cd responses”. A statistic analysis of research studies on “horticultural plants-Cd responses” by year is shown in

Table 1. Statistical chart of research studies on “horticultural plants-Cd responses” by year.

Years	1999	2000	2001	2002	2003	2004	2005	2006	2007
WOS	7	4	7	17	15	21	27	27	31
CNKI	1	1	6	2	9	9	17	25	56
Years	2008	2009	2010	2011	2012	2013	2014	2015	2016
WOS	52	64	61	77	55	74	78	107	108
CNKI	71	91	82	71	77	76	82	121	121
Years	2017	2018	2019	2020	2021	2022	2023	2024
WOS	112	125	159	203	181	258	258	183
CNKI	114	132	168	147	141	162	133	92

. In the WOS database, there are a greater number of studies related to “horticultural plants-Cd responses” compared to the CNKI database; however, overall, there is an upward trend, indicating that scholars from both domestic and international backgrounds have a significant interest in this field of research.

The academic interest in the effects of Cd on plants can be traced back to the early 1990s. One of the earliest studies on the effects of cadmium on plants is the 1990 study by Assche F.V. and Clijsters H., titled “Effects of metals on enzyme activity in plants” published in the journal “Plant, Cell & Environment” [68]. This study explored the impact of metals on the activity of plant enzymes, laying the foundation for subsequent research on the effects of Cd on plant growth. In addition, the literature reviewed by Wang W. in 1991, titled “Literature review on higher plants for toxicity testing”, published in the journal “Water, Air, & Soil Pollution”, also involved the study of Cd in plant toxicity testing [69]. Nonetheless, research on “horticultural plants-Cd responses” started relatively late. As shown in Table 1, seven articles were published in the WOS database, while only one article was published in the CNKI database.

As shown in Figure 1, the global research on “horticultural plants-Cd responses” in the WOS database exhibits an overall publication trend that divides the study into three distinct research stages: StageI, the initial exploration stage (1999–2007); StageII, the steady growth stage (2008–2018); and StageIII, the rapid development stage (2019–2024). In comparison, from the overall publication trend in the CNKI database, the study related to “horticultural plants-Cd responses” is also divided into the three distinct research stages: StageI, the initial exploration stage (1999–2006); StageII, the steady growth stage (2007–2014); and StageIII, the rapid development stage (2015–2024). Over time, the focus on “horticultural plants-Cd responses” has varied, with the annual publication count in a particular field serving as an indicator of the research’s developmental progress [70]. Scientific research on “horticultural plants-Cd responses” has experienced a significant increase in publication volume and has entered a phase of rapid development. Globally, there has been an annual average increase of 216.60 articles in the WOS since 2019, while in China, the annual average increase has been 134.44 articles in the CNKI since 2015.

As shown in Figure 2 and Table 1, the annual publication volume on “horticultural plants-Cd responses” exhibited a trend of fluctuating growth in both domestic and international research contexts. In terms of annual publication volume, from 1999 to 2006, the international research studies on “horticultural plants-Cd responses” were published more than that in China; however, from 2007 to 2019, China’s publication volume of research studies began to surpass the international publication volume. From 2020 to the present, the international publication volume on “horticultural plants-Cd responses” has been significantly higher than that of China. This reflects the trend observed in various research domains, where the global scientific community has consistently maintained a higher output of publications compared to China during this period. It is important to note that publication volumes can be influenced by numerous factors, including research funding, academic collaborations and the focus of research interests at a global level. As shown in Figure 2, compared to China, the international cumulative publication volume has increased by 15.15%, indicating a year-by-year rise in the number of publications and a continuous increase in global scientific attention to “horticultural plants-Cd responses”.

3.2. Spatial Distribution of Publications on “Horticultural Plants-Cd Responses”

3.2.1. Countries’ Cooperation Networks

The progression of research on “horticultural plants-Cd responses” varies across nations. Examining the geographic spread of scholarly efforts and collaborative initiatives provides insight into the research trends within this specific domain [67,71,72]. As shown in Figure 3, the number of nodes in the country cooperation networks from the database of WOS is N = 93, E = 512 and D = 0.1197. “N = 93” stands for the number of nodes, which represent 93 distinct countries in the country cooperation networks. “E = 512” stands for the number of edges, which represent 512 connections or relationships between nodes in the country cooperation networks. “D = 0.1197” stands for “density,” which refers to the ratio of actual edges (connections) to the maximum possible connections in the country cooperation networks [73]. Therefore, the co-occurrence knowledge graph in the realm of “horticultural plants-Cd responses” revealed a network comprising 93 distinct countries interconnected by 512 collaborative ties, with a network density of 11.97%, which demonstrated a certain level of collaborative activity between distinct countries, being neither too extensive to manage nor too limited to lack diversity. The cooperation among these countries is relatively frequent but not overly dense, with numerous potential opportunities for collaboration yet to be tapped into, allowing for further strengthening of partnerships among multiple nations. The top 10 productive countries from the database of WOS are shown in

Table 2. Top 10 productive countries from the database of WOS.

Rank	Countries	Publication Number	Percentage (%)	First Published Year
1	PEOPLES R CHINA	1165	52.79	2002
2	INDIA	200	9.06	1999
3	PAKISTAN	176	7.97	2008
4	USA	144	6.52	2001
5	SAUDI ARABIA	120	5.44	2011
6	IRAN	105	4.76	2002
7	POLAND	88	3.99	2001
8	EGYPT	73	3.31	2009
9	SPAIN	72	3.26	1999
10	TUNISIA	64	2.90	2003

. In terms of publication volume, the five most productive countries are the People’s Republic of China (1165 articles), India (200 articles), Pakistan (176 articles), the USA (144 articles), and Saudi Arabia (120 articles). Their respective proportions are 52.79%, 9.06%, 7.97%, 6.52%, and 5.44%, indicating that China is the most productive country, actively engaging in partnerships with other countries worldwide.

3.2.2. Authors’ Cooperation Networks

As shown in Figure 4, there are N = 274 nodes in the authors’ cooperation network, E = 342 connections, and a network density (D) of 0.0091 within the WOS database. The number of nodes (N = 274) represents a broad base of 274 researchers contributing to the study field of “horticultural plants-Cd responses”, and the number of edges (E = 342) shows the 342 connections between these authors through a research collaboration or co-authorship on a published paper. A density of 0.0091, or 0.91%, is quite low, suggesting that while there is collaboration, it is not widespread across the entire network. Most authors likely collaborate with only a small fraction of the total number of authors in the network. The authors’ collaboration network within WOS shows a significant number of authors and collaborations, but with a low overall density, indicating a decentralized structure with opportunities for further collaboration and network growth.

Comparatively, there are N = 278 nodes in the authors’ cooperation network, E = 244 connections, and a network density (D) of 0.0063 within the CNKI database. The number of nodes (N = 278) represents 278 distinct authors or research entities involved in the network, suggesting a broad engagement in collaborative research within the field of “horticultural plants-Cd responses”. The number of edges (E = 244) shows a moderate level of collaboration through co-authorship on a publication, indicating that there is an active exchange of ideas and research efforts. A density of 0.0063, or 0.63%, is considered sparse, which means that while there are collaborations, they represent only a small fraction of all possible connections among the authors. The authors’ collaboration network within CNKI shows a moderate level of collaboration with low-density rate, indicating the network is not fully connected but has considerable potential for expansion and strengthening of collaborative ties. Further analysis could reveal key authors or keyword clustering that could be targeted for interventions to enhance collaboration. In summary, the network of collaborations among authors in the WOS database seemed denser compared to that in the CNKI database.

In the authors’ cooperation network, the circles (nodes) represent individuals such as authors or researchers in the network, and the number and thickness of the lines (edges) reflect the collaborative relationships and tensity between these individuals (authors or researchers) [74]. As shown in Figure 4, whether it is the WOS database or the CNKI database, it is evident that among the research teams in the field of “horticultural plants-Cd responses”, the team centered around Lin L. is the largest. Additionally, the teams centered around Liao M. are also gradually growing. The top 10 productive authors from the databases of WOS and CNKI (1999–2024) are summarized in

Table 3. Top 10 productive authors from the databases of WOS and CNKI (1999–2024).

WOS				CNKI
Rank	Author	Quantity	Year	Rank	Author	Quantity	Year
1	Lin L	75	2015	1	Lin L	35	2013
2	Liao M	43	2014	2	Liao M	25	2013
3	Wang J	38	2016	3	Liu S	14	2013
4	Tang Y	37	2016	4	Guan P	10	2014
5	Xia H	34	2016	5	Shi J	8	2015
6	Liang D	34	2016	6	Ding J	8	2013
7	Wang X	33	2017	7	Liu P	7	2008
8	Lv X	25	2016	8	Wang L	6	2013
9	Cuypers A	12	2010	9	Pan Z	6	2013
10	Yasin NA	12	2020	10	Chen B	6	2020

. In the WOS database, Lin L. has a total of 75 research articles published, followed by Liao M. with 43, Wang J. with 38, Tang Y. with 37, and Xia H. with 34. In the CNKI database, Lin L. has a total of 35 research articles published, followed by Liao M. with 25, Liu S. with 14, Guan P. with 10, and Shi J. with 8. It is observed that Chinese scholars (Lin L. and Liao M.) are leading researchers in both domestic and international research fields for “horticultural plants-Cd responses”. In recent years, the research teams led by Lin L. and Liao M. from Sichuan Agricultural University have maintained active research interests in the field of “horticultural plants-Cd responses”. They have been focusing particularly on the effects of intercropping, straw application, and exogenous substances on horticultural plants under Cd stress [75,76,77,78].

3.2.3. Institution Cooperation Networks

Analyzing the institutions and collaborations behind publications related to “horticultural plants-Cd responses” can provide an accurate understanding of the distribution of research efforts in this field [79,80]. As shown in Figure 5, there are N = 291 nodes in the institutions’ cooperation network, E = 718 connections, and a network density (D) of 0.017 within the WOS database. Nevertheless, there are N = 254 nodes in the institutions’ cooperation network, E = 87 connections, and a network density (D) of 0.0027 within the CNKI database. Comparatively, the collaboration network in the WOS database is larger than that in the CNKI database, which may reflect differences in the research fields and regions covered by the different databases. The number of connections in WOS is much higher than in CNKI, indicating that collaborations recorded in WOS are more frequent. Both databases have low network densities, but the network density of WOS is higher than that of CNKI, meaning that the collaborative ties in WOS are relatively more intensive compared to the maximum possible number of connections. The low density of both networks indicates a substantial opportunity to increase the number of collaborative relationships, especially in the CNKI database, where the potential for collaboration may be even greater. These data can help us understand the current state and potential for collaboration between different research fields and regions, providing a reference for promoting future collaborations.

The top 10 productive institutions from the databases of WOS and CNKI (1999–2024) are summarized in

Table 4. Top 10 productive institutions from the databases of WOS and CNKI (1999–2024).

Database	Rank	Research Institute	Count	Year
WOS	1	Chinese Academy of Sciences	159	2004
	2	Sichuan Agricultural University	139	2014
	3	Zhejiang University	81	2002
	4	King Saud University	72	2011
	5	Egyptian Knowledge Bank (EKB)	71	2011
	6	Nanjing Agricultural University	70	2008
	7	Ministry of Agriculture & Rural Affairs	65	2009
	8	Chinese Academy of Agricultural Sciences	51	2009
	9	University of Chinese Academy of Sciences	46	2005
	10	University of Agriculture Faisalabad	46	2015
CNKI	1	College of Horticulture, Sichuan Agricultural University	37	2013
	2	Institute of Fruits and Vegetables, Sichuan Agricultural University	31	2015
	3	College of Landscape Architecture, Sichuan Agricultural University	23	2013
	4	College of Life Sciences, Guizhou University	16	2014
	5	Graduate School of Chinese Academy of Sciences	14	2009
	6	College of Life Science and Technology, Xinxiang University	13	2016
	7	College of Resources and Environment, Yunnan Agricultural University	12	2012
	8	College of Horticulture and Landscape Architecture, Henan University of Science and Technology	11	2020
	9	Shenyang Institute of Applied Ecology, Chinese Academy of Sciences	10	2007
	10	College of Life Sciences, Nanjing Agricultural University	9	2003

. In terms of publication volume from the database of WOS, the five most productive institutions are Chinese Academy of Sciences (159 articles), Sichuan Agricultural University (139 articles), Zhejiang University (81 articles), King Saud University (72 articles), and Egyptian Knowledge Bank (71 articles). Chinese Academy of Sciences, as a leading research institution, has a substantial output (159 articles), reflecting its broad research capabilities and extensive resources. Sichuan Agricultural University (139 articles), with a high number of publications, is particularly active in the field of “horticultural plants-Cd responses”, indicating a focused research effort and potentially strong faculty in this area. Zhejiang University’s significant publication volume (81 articles) suggests a strong commitment to research on horticultural plants’ responses to Cd, indicating well-funded programs and collaborative efforts. The productivity of King Saud University (72 articles) indicates a substantial contribution to this field, suggesting a research focus that aligns with environmental and agricultural priorities. The significant number of publications from Egyptian Knowledge Bank (71 articles) reflects a dedication to advancing knowledge in the agricultural sciences, which is crucial for a country where agriculture is a vital sector.

Comparatively, in terms of publication volume from the database of CNKI, the five most productive institutions are College of Horticulture, Sichuan Agricultural University (37 articles), Institute of Fruits and Vegetables, Sichuan Agricultural University (31 articles), College of Landscape Architecture, Sichuan Agricultural University (23 articles), College of Life Sciences, Guizhou University (16 articles), and Graduate School of Chinese Academy of Sciences (14 articles). College of Horticulture, Sichuan Agricultural University (37 articles) focusing on horticulture suggests specialized research in plant cultivation and responses to environmental stressors like Cd. The focus of Institute of Fruits and Vegetables, Sichuan Agricultural University (31 articles) signifies research aimed at understanding and mitigating the impact of Cd on these essential food crops. It is observed that Sichuan Agricultural University is highly productive across both databases, indicating a strong focus on horticultural research related to Cd. The prominence of Sichuan Agricultural University in both databases suggests that this region of China is a hub for research on “horticultural plants-Cd responses”. Zhejiang University and the Chinese Academy of Sciences also show consistent productivity. The higher publication volumes in WOS suggest a wider scope of research or more extensive international collaborations. The CNKI data show a more concentrated effort within specific colleges and universities in China.

3.3. Visual Analysis of Intellectual Base on “Horticultural Plants-Cd Responses”

The intellectual base, which constitutes a comprehensive collection of prior references in a given field, primarily focuses on analyzing the most frequently cited topics, their interconnectedness, and the relationships among these central topics [81]. The top 10 highest cited articles from the database of WOS are shown in

Table 5. Top 10 highest cited articles from the database of WOS.

Title	Author	Year	Journal	Article Types	Institutions	Journal Impact Factor (5 Years)	Citation	Reference
The effects of cadmium toxicity	Genchi, G. et al.	2020	International Journal of Environmental Research and Public Health	Review	Università della Calabria	4.4	1124	[82]
Unravelling cadmium toxicity and tolerance in plants: Insight into regulatory mechanisms	Gallego, SM. et al.	2012	Environmental and Experimental Botany	Review	Universidad de Buenos Aires	5.2	891	[83]
Cadmium in soils and groundwater: A review	Kubier, A. et al.	2019	Applied Geochemistry	Review	University of Bremen	3.4	634	[84]
Cadmium toxicity in plants: Impacts and remediation strategies	Haider, FU. et al.	2021	Ecotoxicology and Environmental Safety	Review	Gansu Agricultural University	6.3	618	[85]
Cadmium in plants: uptake, toxicity, and its interactions with selenium fertilizers	Ismael, MA. et al.	2019	Metallomics	Critical Review	Huazhong Agricultural University	3.7	386	[86]
Cadmium bioavailability, uptake, toxicity and detoxification in soil-plant system	Shahid, M. et al.	2017	Reviews of Environmental Contamination and Toxicology	Review	COMSATS Institute of Information Technology	7.1	369	[87]
A critical review on effects, tolerance mechanisms and management of cadmium in vegetables	Rizwan, M. et al.	2017	Chemosphere	Critical Review	Government College University	7.7	341	[88]
Cadmium stress in plants: A critical review of the effects, mechanisms, and tolerance strategies	El Rasafi, T. et al.	2020	Critical Reviews in Environmental Science and Technology	Critical Review	University Mohammed 6 Polytechnic	14.5	243	[89]
Morphological and physiological responses of plants to cadmium yoxicity: A review	He, SY. et al.	2017	Pedosphere	Review	Zhejiang Gongshang University	5.3	238	[90]
Toxicity of cadmium and its competition with mineral nutrients for uptake by plants: A review	Qin, SY. et al.	2020	Pedosphere	Review	Henan Agricultural University	5.3	237	[91]

; these have been influential in shaping the discourse and have significantly contributed to the intellectual base, which encompasses the foundational theories, concepts, and empirical evidence that form the core of the scholarly discipline or field of “horticultural plants-Cd responses”.

Among these 10 articles, the most highly cited was “The Effects of Cadmium Toxicity”, authored by Genchi G. et al., with 1124 citations. It was published in the “International Journal of Environmental Research and Public Health” (IF = 4.4) in 2020 [82]. The article pointed out the main sources, routes of transmission, and half-life of Cd. The study also demonstrated that Cd can induce a variety of epigenetic changes in mammalian cells both in vivo and in vitro, leading to pathological risks and the development of various types of cancers. The research also proposed that plants such as sunflowers (Helianthus annuus L.) and Indian mustard (Brassica juncea) possess the ability to remove cadmium from contaminated soil and water. The second most highly cited was “Unravelling cadmium toxicity and tolerance in plants: Insight into regulatory mechanisms”, authored by Gallego S.M. et al., with 891 citations. It was published in “Environmental and Experimental Botany” (IF = 5.2) in 2012 [83]. This article provided a comprehensive review of the mechanisms by which plants absorb, transport, and accumulate Cd, as well as an overview of the detoxification processes that plants employ to combat Cd. Additionally, it discussed the oxidative stress caused by Cd and the antioxidant defenses plants activate in response. Furthermore, the article elucidated the impact of reactive oxygen and nitrogen species on Cd-induced toxicity in plants. The third most highly cited was “Cadmium in soils and groundwater: A review”, authored by Kubier A. et al., with 634 citations. It was published in “Applied Geochemistry” (IF = 3.4) in 2019 [84]. This article examined the concentration of Cd in rocks, sediments, and soils and discussed anthropogenic sources of Cd. It also explored the hydrochemical behavior of Cd, including its solubility, complexation, and sorption. Additionally, the article presented case studies of elevated Cd concentrations in soil and groundwater. The fourth most highly cited was “Cadmium toxicity in plants: Impacts and remediation strategies”, authored by Haider F.U. et al., with 618 citations. It was published in “Ecotoxicology and Environmental Safety” (IF = 6.3) in 2021 [85]. This article reviewed the sources of Cd contamination in the environment, the soil factors that affect Cd uptake, and the dynamics of Cd in the soil rhizosphere. It also discussed the mechanisms of Cd uptake, translocation, and toxicity in plants. The article highlighted how Cd toxicity in horticultural plants such as tobacco, rapeseed, pea and Thlaspi caerulescens L. can reduce the uptake and translocation of nutrients and water, increase oxidative damage, disrupt plant metabolism, and inhibit plant growth and development. Furthermore, the article explored plant defense mechanisms against Cd toxicity and potential remediation strategies. These strategies include the use of biochar, mineral nutrients, compost, organic manure, growth regulators, and hormones, as well as the application of phytoremediation, bioremediation, and chemical methods. This article, published in 2021, has garnered 618 citations, indicating widespread attention and recognition within the international academic community.

In general, these 10 highest cited papers theoretically analyzed the main sources of Cd, critical thresholds, pollution pathways, and toxicity characteristics. They systematically dissected the stress responses and detoxification mechanisms of plants to Cd, and in practice, they proposed targeted suggestions for the management methods and remediation technologies for Cd. Additionally, as shown in Table 5, these 10 highly cited papers are all review articles. The reason might be multifaceted. On one hand, the topics covered by these reviews could be at the forefront of research, attracting attention from a broad spectrum of scholars and consequently leading to frequent citations. On the other hand, cadmium pollution and its impact on plants and the environment are of interest across various disciplines, including environmental science, toxicology, plant biology, and public health. These reviews might serve as interdisciplinary bridges, appealing to a wide academic audience.

3.4. Visual Analysis of Research Hotspots on”Horticultural Plants-Cd Responses”

3.4.1. Keyword Co-Occurrence Network Analysis

Keywords serve to encapsulate the principal content of a research article, extracting essential details like objectives, methodologies, and perspectives [92]. The frequency analysis of these keywords is vital for pinpointing trending issues and evolutionary trends within a specific domain [67,93,94]. As shown in Figure 6, in keyword co-occurrence network analysis within the WOS database, there are 260 nodes (N = 260), 1254 edges (E = 1254), and a network density of 0.0372. In contrast, within the CNKI database, there are 285 nodes (N = 285), 751 connections (E = 751), and a network density of 0.0186. Keywords with higher centrality are highlighted in pink circles on the map. For instance, in the WOS database, keywords such as “plants”, ‘‘phytoremediation”, and “glutathione” are emphasized. Similarly, in the CNKI database, keywords like “cadmium stress”, “antioxidant enzymes”, “cadmium contamination”, “growth”, “phytoremediation”, “rapeseed”, “heavy metal”, “photosynthesis” and “Cd stress” stand out. This visual representation helps researchers quickly identify key themes and important concepts in their research field. The size and position of the pink circles can intuitively show the importance of the keywords, that is, their frequency of occurrence in the literature and the degree of association with other keywords. In the WOS co-occurrence network, the plant-related terms were Phaseolus vulgaris [95] and Brassica juncea [96]. Additionally, “bean Phaseolus vulgaris” is also mentioned. In the CNKI co-occurrence network, the plant-related terms included 油菜 (rapeseed), 小白菜 (pakchoi), 紫花苜蓿 (Medicago sativa L.), 龙葵 (Solanum nigrum L.), 金银花 (Lonicera Japonica Thunb.), among others. The plants mentioned are likely to be the focus of significant research within the respective databases, indicating areas of interest for scientists and scholars. Despite having fewer nodes (N = 260) compared with the CNKI, WOS has more connections within the field of “horticultural plants-Cd responses”, indicating a higher degree of interlinkage between keywords. This could mean that the keywords in WOS are more frequently co-occurring in the research articles, suggesting a denser network of research topics in the field of “horticultural plants-Cd responses”. This could imply a more interdisciplinary research environment or more consistent use of certain combinations of keywords. The density of the network in WOS (Density = 0.0372) is almost double that of CNKI (Density = 0.0186), indicating a relatively more interconnected network of keywords in WOS. A higher density suggests a more closely knit group of topics that frequently appear together in the context of the research articles indexed. The sparser network in CNKI might suggest opportunities for researchers to explore new intersections between existing areas of research. Conversely, the denser network in WOS could indicate a more established body of research where connections between topics are well explored. The comparison of keyword co-occurrence networks between WOS and CNKI databases highlighted differences in the structure and interconnectivity of research topics. WOS showed a denser network with more connections, suggesting a more interconnected research landscape. CNKI, with a higher number of nodes and a sparser network, might indicate a broader range of distinct research areas with potential for further integration. Understanding these differences can help researchers identify trends, opportunities for collaboration, and areas ripe for interdisciplinary exploration.

Analyzing a knowledge graph allows for the identification of research topics and trends within the field of “horticultural plants-Cd responses” by examining the significance and connections of keywords. It is recognized that in Citespace, centrality is a measure of how much a node affects the shortest paths between other nodes; a higher centrality value suggests a greater level of influence [23]. A node is considered to be a key node when its centrality value exceeds 0.1. The top 10 high-frequency keywords from the databases of WOS and CNKI are shown in

Table 6. Top 10 high-frequency keywords from the databases of WOS and CNKI.

Database	Rank	Keyword	Count	Centrality	Year
WOS	1	accumulation	752	0.08	1999
	2	tolerance	587	0.08	2002
	3	heavy metals	562	0.05	1999
	4	oxidative stress	505	0.04	2001
	5	toxicity	496	0.05	2006
	6	growth	493	0.05	2002
	7	responses	270	0.05	2005
	8	Cd	269	0.03	2002
	9	cadmium stress	232	0.08	2005
	10	phytoremediation	222	0.11	2004
CNKI	1	cadmium stress	625	0.43	2001
	2	antioxidant enzymes	115	0.16	2004
	3	cadmium contamination	104	0.13	2000
	4	growth	91	0.16	2007
	5	seed germination	90	0.06	2007
	6	phytoremediation	85	0.14	2001
	7	rapeseed	83	0.18	2001
	8	physiological characteristics	79	0.08	2006
	9	heavy metal	78	0.08	2005
	10	photosynthesis	63	0.09	2007

. International research findings from the databases of WOS showed that the top 10 high-frequency keywords are accumulation (752), tolerance (587), heavy metals (562), oxidative stress (505), toxicity (496), growth (493), responses (270), Cd (269), cadmium stress (232), and phytoremediation (222); moreover, the centrality value of phytoremediation is above 0.1, indicating that globally, research hotspots have focused on topics such as accumulation, tolerance, and oxidative stress, as well as the formation of core keywords like phytoremediation. Domestic research findings from the databases of CNKI showed that the top 10 high-frequency keywords are cadmium stress (625), antioxidant enzymes (115), cadmium contamination (104), growth (91), seed germination (90), phytoremediation (85), rapeseed (83), physiological characteristics (79), heavy metal (78) and photosynthesis (63); moreover, the centrality values of cadmium stress, antioxidant enzymes, cadmium contamination, growth, phytoremediation, and rapeseed all exceed 0.1, indicating that in China, research hotspots have focused on topics such as antioxidant enzymes, growth, and seed germination, as well as the formation of multiple core keywords including antioxidant enzymes, growth and seed germination.

3.4.2. Keyword Clustering Analysis

The CiteSpace software evaluates the effectiveness of a knowledge map by considering two key metrics: the Q value and the S value [97]. The Q value, also known as modularity, ranges from 0 to 1, with a score above 0.3 denoting that the divided community structure is significant. The S value, also known as weighted mean silhouette, which stands for the silhouette score, measures the average clarity of the clusters [98]. An S value above 0.5 suggests that the clustering outcomes are reasonable, while an S value above 0.7 indicates that the clustering outcomes are reasonable and persuasive [67,93]. As shown in Figure 7, in keyword clustering analysis within the WOS database, Q = 0.3598, and S = 0.6751. The Q value (0.3598) is slightly above the threshold of 0.3, indicating that the community structure identified in the clustering analysis is statistically significant, meaning that the nodes (keywords) within these communities are more closely connected than they would be if distributed randomly. The S value (0.6751) exceeds the threshold of 0.5 but is below 0.7. This suggests that the clustering results are reasonable and the clustering effect is good, but there may still be room for improvement, and the optimal clustering effect has not yet been achieved. The international clusters from the WOS database are #0 cadmium stress, #1 phytoremediation, #2 heavy metals, #3 subcellular distribution, #4 phaseolus vulgaris, #5 transcriptome, #6 chlorophyll fluorescence.

Comparatively, in keyword clustering analysis within the CNKI database, we have Q = 0.4425, and S = 0.7693. The Q value (0.4425) being greater than the threshold of 0.3 indicates that the community structure identified in the clustering analysis is significant, meaning that the nodes (keywords) within these communities are more closely connected than they would be if distributed randomly, demonstrating a good clustering effect. An S value (0.7693) exceeding the threshold of 0.7 suggests that the clustering results are persuasive, meaning that the clustering effect is not only reasonable but also has good discrimination and internal consistency. The domestic clusters from the CNKI database are #0 油菜(rapeseed), #1 叶绿素 (chlorophyll), #2 抗氧化酶 (antioxidant enzymes), #3 生理指标 (physiological indicators), #4 生长 (growth), #5 镉含量 (cadmium content), #6 镉污染 (cadmium contamination), and #7 镉胁迫 (cadmium stress). By observing the connections between different clusters, potential interdisciplinary cross-points can be discovered. The results of clustering analysis revealed that clusters like phytoremediation, subcellular distribution, transcriptome, and chlorophyll fluorescence are emerging topics globally. Meanwhile, in China, research on growth and physiological characteristics, including chlorophyll and antioxidant enzymes, continues to be an area of emerging interest.

Through keyword clustering, it is possible to identify research trends and hotspots within a specific field, as well as help researchers discover areas or topics that have not been fully explored [99,100]. As shown in

Table 7. The comparison of keyword clustering from the databases of WOS and CNKI (1999–2024).

Database	Label	Node	S Value	Mean (Year)	Keywords
WOS	#0	65	0.622	2011	cadmium stress (48.72, 1.0 × 10−4; nitric oxide (37.96, 1.0 × 10−4); antioxidant enzymes (33.25, 1.0 × 10−4); gene expression (24.24, 1.0 × 10−4); phytoremediation (20.33, 1.0 × 10−4)
	#1	62	0.658	2012	phytoremediation (65.32, 1.0 × 10−4); cadmium stress (41.58, 1.0 × 10−4); oxidative stress (35.32, 1.0 × 10−4); hyperaccumulator (33.78, 1.0 × 10−4); ornamental plant (32.78, 1.0 × 10−4)
	#2	35	0.798	2005	heavy metals (20.15, 1.0 × 10−4); nutrients (14.44, 0.001); yield (12.66, 0.001); root morphology (12.03, 0.001); nutrient uptake (12.03, 0.001)
	#3	32	0.689	2017	subcellular distribution (24.82, 1.0 × 10−4); antioxidant activity (17.15, 1.0 × 10−4); piriformospora indica (9.58, 0.005); pectin (9.24, 0.005); heavy metals accumulation (9.24, 0.005)
	#4	28	0.654	2005	phaseolus vulgaris (10.17, 0.005); nitrogen metabolism (9.49, 0.005); senescence (9.18, 0.005); glutamine synthetase (9.18, 0.005); ammonium (9.18, 0.005)
	#5	27	0.625	2015	transcriptome (31.09, 1.0 × 10−4); cd stress (21.63, 1.0 × 10−4); qrt-pcr (19.03, 1.0 × 10−4); genes (14.27, 0.001); expression pattern (14.27, 0.001)
	#6	9	0.865	2010	chlorophyll fluorescence (31.64, 1.0 × 10−4); ascorbate-glutathione cycle (20.61, 1.0 × 10−4); photosynthesis (14.08, 0.001); stomatal conductance (12.75, 0.001); spectral reflectance (12.75, 0.001)
CNKI	#0	33	0.809	2010	rapeseed (81.78, 1.0 × 10−4); phytoremediation (76.45, 1.0 × 10−4); cadmium (34.73, 1.0 × 10−4); cadmium stress (29.26, 1.0 × 10−4); enrichment (28.24, 1.0 × 10−4)
	#1	31	0.798	2013	chlorophyll (51.5, 1.0 × 10−4); stress (42.96, 1.0 × 10−4); malondialdehyde (34.65, 1.0 × 10−4); heavy metal (33.31, 1.0 × 10−4); seed germination (31.06, 1.0 × 10−4)
	#2	31	0.794	2010	antioxidant enzymes (39.92, 1.0 × 10−4); physiological characteristics (39.56, 1.0 × 10−4); Cd stress (38.7, 1.0 × 10−4); reactive oxygen species (28.2, 1.0 × 10−4); seedling growth (24.37, 1.0 × 10−4)
	#3	24	0.684	2016	physiological indicators (58.05, 1.0 × 10−4); pakchoi (50.45, 1.0 × 10−4); nutrient elements (16.25, 1.0 × 10−4); seedling (13.61, 0.001); cotton (12.54, 0.001)
	#4	23	0.836	2011	growth (66.1, 1.0 × 10−4); photosynthetic properties (29.97, 1.0 × 10−4); photosynthesis (28.68, 1.0 × 10−4); enrichment coefficient (24.04, 1.0 × 10−4); physiological responses (18.38, 1.0 × 10−4)
	#5	22	0.753	2015	cadmium content (23.78, 1.0 × 10−4); biochar (22.5, 1.0 × 10−4); yield (20.38, 1.0 × 10−4); soil (18.49, 1.0 × 10−4); resistance (17.07, 1.0 × 10−4)
	#6	22	0.83	2013	cadmium contamination (78.99, 1.0 × 10−4); biomass (25.74, 1.0 × 10−4); physiology and biochemistry (18.42, 1.0 × 10−4); transport factor (17.39, 1.0 × 10−4); cadmium (15.88, 1.0 × 10−4)
	#7	21	0.605	2014	cadmium stress (151, 1.0 × 10−4); cadmium (54.98, 1.0 × 10−4); cadmium contamination (24.91, 1.0 × 10−4); heavy metal (17.4, 1.0 × 10−4); Cd stress (12.6, 0.001)

, the symbol “#” denotes a cluster, while “note” indicates the count of citation references within each cluster “#”. The mean (year) of publications within a cluster “#” acts as a simple but informative metric, highlighting whether the articles are relatively recent or dated [101]. A summary of the international keyword clustering from the databases of WOS showed the following: #0 (cadmium stress), with 65 nodes; #1 (phytoremediation), with 62 nodes; #2 (heavy metals), with 35 nodes; #3 (subcellular distribution), with 32 nodes; #4 (phaseolus vulgaris), with 28 nodes; #5 (transcriptome), with 27 nodes; #6 (chlorophyll fluorescence), with 9 nodes. In contrast, domestic keyword clustering summary from the databases of CNKI showed the following: #0 油菜 (rapeseed), with 33 nodes; #1 叶绿素 (chlorophyll), with 31 nodes; #2 抗氧化酶 (antioxidant enzymes), with 31 nodes; #3 生理指标 (physiological indicators), with 24 nodes; #4 生长 (growth), with 23 nodes; #5 镉含量 (cadmium content), with 22 nodes; #6 镉污染 (cadmium contamination), with 22 nodes; and #7 镉胁迫 (cadmium stress), with 21 nodes. The largest cluster from the databases of WOS was “cadmium stress”, labeled 2011, consisting of 65 keywords, including primary terms such as cadmium stress, nitric oxide, antioxidant enzymes, gene expression, and phytoremediation. In contrast, the largest cluster from the databases of CNKI was “rapeseed”, labeled 2010, consisting of 33 keywords, including rapeseed, phytoremediation, cadmium, cadmium stress, and enrichment. Keywords were extracted from each cluster based on a weighting algorithm. The five most significant keywords in each cluster are displayed from left to right, arranged according to their level of importance.

3.5. Visual Analysis of Dynamic Frontier

3.5.1. Evolution Trends

Through CiteSpace analysis, a timeline mapping of keywords regarding the field of “horticultural plants-Cd responses” was generated, revealing the temporal scope and evolution trends of emerging research themes. The timeline mapping of keywords from the databases of WOS and CNKI progresses from left to right, with the circular nodes representing keywords (Figure 8). The size of each node corresponds to the occurrence rate of its respective keyword throughout the timeline, and a higher occurrence rate implies an area of intense research interest. The year a node appears on the timeline marks the initial emergence of the keyword, with a more extended presence indicating a more enduring research focus [102]. The international timeline settings were defined in the following way: time span = 1999–2024, slice length = 1, and g-index (k = 8). There were 260 nodes and 1254 connection lines in the network mapping. The node with the biggest size, labeled 1999, was “accumulation”, subsequently joined by increasingly prominent high-frequency keywords such as “tolerance” (labeled 2002), “heavy metals” (labeled 1999), “oxidative stress” (labeled 2001), “toxicity” (labeled 2006), and “growth” (labeled 2002). The current research emphasis is centered on rhizosphere, subcellular distribution, biochar, and yield.

Comparatively, in timeline mapping of keywords from the CNKI database, there were 238 nodes and 652 connection lines in the network mapping. The node with the biggest size, labeled 2001, was “cadmium stress”, subsequently joined by increasingly prominent high-frequency keywords such as “antioxidant enzymes” (labeled 2004), “cadmium contamination” (labeled 2000), “growth” (labeled 2007), “seed germination” (labeled 2007), and “phytoremediation” (labeled 2001). The current research emphasis is centered on transport factor, cadmium absorption, physiological responses, gene expression, and transcriptome. The results above showed that keywords in CNKI tended to describe specific biochemical processes and molecular mechanisms, while those in WOS focused more on plant physiology and the responses of the entire organism. Both databases demonstrated a sustained interest in phytoremediation and heavy metal pollution research, but CNKI might concentrate more on molecular and genetic mechanisms, while WOS might place more emphasis on physiological and applied studies. This evolutionary process, in terms of timeline mapping of keywords, intuitively showcases the gradual developmental trajectory of international and domestic research in “horticultural plants-Cd responses”.

3.5.2. Research Frontiers

CiteSpace’s burst detection feature was utilized to quantify the prevalence and chronological emergence of keywords within scholarly texts, subsequently establishing the dynamic frontiers and prospective orientation of the field’s research on “horticultural plants-Cd responses” [103,104]. Keywords that emerge and continue to appear for at least two years often represent a trend, which can assist in predicting future research hotspots and trajectories [67].

Table 8. Mapping of outburst keywords from the databases of WOS (A) and CNKI (B).

Top 20 Keywords with the Strongest Citation Bursts (A)
Keywords	Year	Strength	Begin	End	1999–2024
leaves	1999	28.2	1999	2012	▃▃▃▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂
zinc	1999	11.13	1999	2012	▃▃▃▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂
bean phaseolus vulgaris	1999	8.69	1999	2010	▃▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂▂
copper	2003	15.34	2003	2015	▂▂▂▂▃▃▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂
chlorophyll	2003	7.56	2003	2013	▂▂▂▂▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂
thlaspi caerulescens	2004	12.27	2004	2014	▂▂▂▂▂▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂
lipid peroxidation	2004	9.99	2004	2014	▂▂▂▂▂▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂
superoxide dismutase	2004	8.98	2007	2011	▂▂▂▂▂▂▂▂▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂
metabolism	2007	8.22	2007	2011	▂▂▂▂▂▂▂▂▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂
hydrogen peroxide	2004	8.17	2007	2012	▂▂▂▂▂▂▂▂▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂
nickel	2008	6.74	2008	2016	▂▂▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂
hyperaccumulator	2010	7.49	2010	2015	▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂
metal accumulation	2010	7.88	2016	2019	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▂▂▂▂▂
induced oxidative stress	2009	7.19	2016	2018	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▂▂▂▂▂▂
brassica napus	2009	6.84	2017	2020	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▂▂▂▂
rhizosphere	2018	8.07	2018	2020	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▂▂▂▂
subcellular distribution	2017	7.07	2019	2021	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▂▂▂
oilseed rape	2020	6.78	2020	2021	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▂▂▂
biochar	2021	7.62	2021	2024	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃
yield	2022	7.89	2022	2024	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃
Top 20 Keywords with the Strongest Citation Bursts (B)
Keywords	Year	Strength	Begin	End	1999–2024
rapeseed	2001	7.23	2001	2011	▂▂▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂
protective enzyme	2001	5.36	2001	2011	▂▂▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂
vegetable	2005	4.66	2005	2009	▂▂▂▂▂▂▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂
chlorophyll	2006	7.08	2006	2017	▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂
accumulation	2007	3.87	2007	2015	▂▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂
cabbage	2007	3.64	2007	2011	▂▂▂▂▂▂▂▂▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂
malondialdehyde	2007	5.7	2008	2016	▂▂▂▂▂▂▂▂▂▃▃▃▃▃▃▃▃▃▂▂▂▂▂▂▂▂
growth	2007	4.69	2008	2012	▂▂▂▂▂▂▂▂▂▃▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂
germination	2009	4.89	2009	2012	▂▂▂▂▂▂▂▂▂▂▃▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂
quality	2010	3.65	2010	2015	▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▃▂▂▂▂▂▂▂▂▂
phytoremediation	2001	3.82	2015	2018	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▂▂▂▂▂▂
Solanum nigrum L.	2015	3.68	2015	2017	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▂▂▂▂▂▂▂
cadmium content	2016	3.49	2016	2018	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▂▂▂▂▂▂
cadmium accumulation	2011	3.66	2018	2022	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▂▂
transport factor	2019	4.94	2019	2024	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃▃▃
cadmium absorption	2019	4.82	2019	2020	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▂▂▂▂
chili pepper	2020	4.48	2020	2021	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▂▂▂
physiological responses	2009	4.18	2020	2021	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▂▂▂
gene expression	2021	5.52	2021	2024	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃
transcriptome	2021	4.6	2021	2024	▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▃▃▃▃

lists the top 20 keywords with the strongest citation bursts from the 1999–2024 co-occurrence networks in both the WOS and CNKI databases, which provides a clear view of the timing and magnitude of keyword outbursts. The term “Year” indicates the initial appearance of a term in our dataset, “Strength” measures the magnitude of its mutation, “Begin” marks the start of its mutation period, and “End” denotes its termination. International keywords with strong citation bursts, arranged in descending order of strength, from the databases of WOS were “leaves” (28.2), “copper” (15.34), “thlaspi caerulescens” (12.27), “zinc” (11.13), “lipid peroxidation” (9.99), “superoxide dismutase” (8.98), “bean phaseolus vulgaris” (8.69), “metabolism” (8.22), “hydrogen peroxide” (8.17), “rhizosphere” (8.07), etc., indicating that the listed keywords are at the forefront of research, garnering increased attention from the global scientific community. Emerging topics emphasized the importance of the temporal scope [26]. In light of this, the temporal scope of keyword bursts on the research of “horticultural plants-Cd responses” in WOS showed a sustained span: “leaves” (13 years), “zinc” (13 years), “copper” (12 years),”bean phaseolus vulgaris” (11 years), “chlorophyll” (10 years), “thlaspi caerulescens” (10 years), “lipid peroxidation” (10 years), “nickel” (8 years), etc. This analysis indicates that in the research on “horticultural plants-Cd responses”, certain themes and indicators have received long-term and continuous attention within the international research community. The enduring research interest in these keywords may be related to their importance in plant physiology, nutrient cycling, and environmental pollution remediation. Researchers are likely exploring the tolerance, accumulation mechanisms, and potential impacts on human health of horticultural plants under Cd stress through these keywords [14, 105,106,107,108].

In comparison, domestic keywords with strong citation bursts, arranged in descending order of strength, from the databases of CNKI were as follows: “rapeseed” (7.23), “chlorophyll” (7.08), “malondialdehyde” (5.7), “gene expression” (5.52), “protective enzyme” (5.36), “transport factor” (4.94), “germination” (4.89), “cadmium absorption” (4.82), “growth” (4.69), “vegetable” (4.66), etc. The analysis suggests that these keywords are correlated with the research interests of domestic scientists. The temporal scope of keyword bursts on the research of “horticultural plants-Cd responses” in CNKI showed a sustained span: “chlorophyll” (11 years), “rapeseed” (10 years), “protective enzyme” (10 years), “accumulation” (8 years), “malondialdehyde” (8 years), etc. The data suggest that in the research on “horticultural plants-Cd responses” within the CNKI database, certain keywords have maintained a consistent focus over the years. These continuous research interests could be attributed to the desire to understand the physiological and biochemical changes in plants under Cd stress, develop Cd-tolerant or low-Cd-accumulating crop varieties, and ensure food safety by reducing the uptake of toxic elements in edible parts of plants.

The burst strength indicators for the terms “biochar” and “yield” in the WOS database, which were 7.62 and 7.89, respectively, suggest that these topics have gained notable research attention since 2021 and 2022, respectively. Their sustained interest up to the present (2024) indicate that they are emerging as new frontiers in international scientific inquiry. In the CNKI database, “gene expression” and “transcriptome” show burst strengths of 5.52 and 4.6, respectively. These topics, whose notable research interest began in 2021, persist to date (2024), indicating that they have emerged as new domestic research frontiers in China. The comparison above clearly demonstrates the research interest and trends of different topics in the two databases, while also revealing the differences in research focus between international and domestic scientific communities. On the whole, China has exhibited a lagging development trend, which is reflected in the fact that it began to focus on gene expression and transcriptome research only after the global frontier shifted towards biochar and cadmium co-stress and yield response.

4. Conclusions and Outlook

As a highly carcinogenic environmental contaminant, the persistence of Cd in the environmental medium and its toxic effects on horticultural plants have garnered widespread attention from researchers around the world in recent years, as reflected by the abundance of research articles on this issue in academic publications. By utilizing a visualizing bibliometric analysis software, CiteSpace, this study integrated and analyzed a total of 4318 relevant research records—2311 from the WOS database and 2007 from the CNKI database—related to “horticultural plants-Cd responses”, covering the period from 1999 to 2024. From the temporal distribution of publications, domestically and internationally, research on “horticultural plants-Cd responses” has experienced three distinct phases: StageI, the initial exploration stage; StageII, the steady growth stage; and StageIII, the rapid development stage. The annual publication volume on “horticultural plants-Cd responses” has exhibited a trend of fluctuating growth in both domestic and international research contexts, while, compared to China, the international cumulative publication volume has increased by 15.15%, indicating a year-by-year rise in the number of publications and a continuous increase in global scientific attention to “horticultural plants-Cd responses”. From countries’, authors’ and institutions’ cooperation networks, in terms of publication volume, the five most productive countries are the People’s Republic of China (1165 articles), India (200 articles), Pakistan (176 articles), the USA (144 articles), and Saudi Arabia (120 articles). Their respective proportions of the total global output are 52.79%, 9.06%, 7.97%, 6.52%, and 5.44%, indicating that China is the most productive country and is actively engaging in partnerships with other countries worldwide. Chinese scholars (Lin L. and Liao M.) are leading researchers in both domestic and international research fields for “horticultural plants-Cd responses”. The five most productive institutions, with their respective article counts, are Chinese Academy of Sciences (159 articles), Sichuan Agricultural University (139 articles), Zhejiang University (81 articles), King Saud University (72 articles), and Egyptian Knowledge Bank (71 articles). This distribution reflects the broad research capabilities and extensive resources of these institutions, with the Chinese Academy of Sciences leading in research output. The analyses above underscore the importance of research output from countries, authors, and institutions as an indicator of the field’s development and the focus areas within the field of “horticultural plants-Cd responses”. They also highlight the value of monitoring the temporal distribution trends of publications to understand research priorities and achievements at the national, author, and institutional levels. The highest cited articles have been influential in shaping the discourse and have significantly contributed to the intellectual base, which encompasses the foundational theories, concepts, and empirical evidence that form the core of the scholarly discipline or field of “horticultural plants-Cd responses”. From keyword co-occurrence network and clustering analysis, international research hotspots have focused on accumulation, tolerance, and oxidative stress, while in China, the focus has been on antioxidant enzymes, growth, and seed germination. Phytoremediation, subcellular distribution, and the transcriptome are the world’s emerging topics, while in China, growth and physiological characteristics are still emerging topics. The analysis of timeline mapping and outburst keywords revealed the temporal scope and evolution trends and research frontiers of emerging research themes. On comparison, it is seen that China exhibited a lagging development trend, which is reflected in the fact that it began to focus on gene expression and transcriptome research only after the global frontier shifted towards biochar and cadmium co-stress and yield response.

To enhance future research on “horticultural plants-Cd responses” in China, the following suggestions could be considered:

(1)

Emphasize interdisciplinary research: Collaboration should be encouraged between different disciplines such as biology, soil science, environmental science, and public health to develop a comprehensive understanding of Cd stress and its impacts on agriculture and the environment.

(2)

Focus on biochar and co-stress studies: Since the global research frontier has shifted towards biochar and co-stress studies, Chinese researchers should focus more on how biochar can mitigate Cd stress and how plants respond to multiple stresses such as Cd and drought or exogenous substances.

(3)

Promote phytoremediation research: Research should be supported on phytoremediation techniques using horticultural plants that can hyperaccumulate Cd, such as Thlaspi caerulescens, Lonicera japonica Thunb., to remediate contaminated soils.

(4)

Invest in plant breeding and genetic engineering: Cd-resistant horticultural plant varieties should be developed through traditional breeding and genetic engineering techniques to reduce the impact of Cd on agriculture and food safety.

(5)

Leverage International Collaboration: International research institutions should collaborate to share knowledge, resources, and expertise and to stay abreast of the latest advancements in cadmium research.

(6)

Foster innovation in nanotechnology: The use of nanotechnology should be explored in addressing Cd stress, e.g., nanomaterials for soil remediation or nanosensors for monitoring Cd levels in horticultural plants and soil.

By implementing these strategies, China can improve its research capabilities in the area of “horticultural plants-Cd responses” and contribute to global efforts in managing this significant environmental and health challenge.