Optimizing Microclimatic Conditions for Lettuce, Tomatoes, Carrots, and Beets: Impacts on Growth, Physiology, and Biochemistry Across Greenhouse Types and Climatic Zones

Abstract

Vegetables such as lettuce, tomato, carrot, and beet are vital to the global food industry, providing essential nutrients and supporting sustainable agriculture. Their cultivation in greenhouses across diverse climatic zones (temperate, Mediterranean, tropical, subtropical, and arid) has gained prominence due to controlled environments that enhance yield and quality. However, these crops face significant threats from climate change, including rising temperatures, erratic light availability, and resource constraints, which challenge optimal growth and nutritional content. This study investigates the influence of microclimatic conditions—temperature, light intensity, and CO₂ concentration—on the growth, physiology, and biochemistry of these vegetables under varying greenhouse types and climatic zones, addressing these threats through a systematic review. The methodology followed the PRISMA guidelines, synthesizing peer-reviewed articles from 1995 to 2025 sourced from Web of Science, Pub Med, Scopus, Science Direct, Springer Link, and Google Scholar. Search terms included “greenhouse microclimate”, “greenhouse types”, “Climatic Zones, “and crop-specific keywords, with data extracted on microclimatic parameters and analyzed across growth stages and climatic zones. Eligibility criteria ensured focus on quantitative data from greenhouse studies, excluding pre-1995 or non-peer-reviewed sources. The results identified the following optimal conditions: lettuce and beet thrive at 15–22 °C, 200–250 μmol·m⁻²·s⁻¹, and 600–1100 ppm CO₂ in temperate zones; tomatoes at 18–25 °C, 200–300 μmol·m⁻²·s⁻¹, and 600–1100 ppm in Mediterranean and arid zones; and carrots at 15–20 °C, 150–250 μmol·m⁻²·s⁻¹, and 600–1000 ppm in subtropical zones. Greenhouse types (e.g., glasshouses, polytunnels) modulate these optima, with high-tech systems enhancing resilience. Conclusively, tailored microclimatic management, integrating AI-driven technologies and advanced greenhouse designs, is recommended to mitigate threats and optimize production across climatic zones.

1. Introduction

Greenhouse technology enables year-round vegetable production by mitigating adverse climatic conditions, ensuring a consistent supply of high-value crops like lettuce (Lactuca sativa L.), tomatoes (Solanum lycopersicum L.), carrots (Daucus carota L.), beets (Beta vulgaris), and radishes (Raphanus sativus) [1]. These crops are vital for global food security due to their nutritional value, providing essential vitamins, antioxidants, and minerals [2]. Greenhouses overcome seasonal and climatic variability, meeting rising global demand for quality produce. Microclimatic factors—temperature, humidity, light intensity, and CO₂ concentration—critically influence plant growth, physiology, and biochemical composition. Temperature regulates enzymatic activity and metabolic processes, while light and CO₂ affect photosynthesis, stomatal conductance, and secondary metabolite production [3].

Crop-specific responses vary; lettuce thrives at 15–20 °C to prevent bolting, while tomatoes require 20–25 °C for optimal flowering and fruit set [4]. Root crops like carrots, beets, and radishes need precise temperature control to enhance root expansion and bioactive compound accumulation (e.g., carotenoids, anthocyanins) [5]. The greenhouse type significantly impacts microclimate control. Low-tech, naturally ventilated systems, common in resource-limited regions, are prone to temperature fluctuations and humidity stress, reducing photosynthetic efficiency and increasing disease risk [6]. Advanced greenhouses with automated climate control, supplemental lighting, and CO₂ enrichment optimize physiological processes and metabolite synthesis [7,8]. These systems interact with plant biology, where elevated CO₂ enhances Rubisco activity and sugar biosynthesis, and specific light spectra influence photoreceptors, affecting flavonoid and glucosinolate production [9,10].

Climatic zones impose unique challenges that vary with greenhouse infrastructure (Figure 1). Temperate zones (e.g., Europe, North America) require insulation and supplemental lighting for winter production [11]. Subtropical zones (e.g., Mediterranean, Punjab-Pakistan) need ventilation and humidity management to counter pest pressure and heat stress [12,13]. Arid zones (e.g., Middle East, Northern Africa) rely on evaporative cooling and shading to mitigate high solar radiation and low humidity [14]. These variations affect transpiration, photosynthesis, and biochemical outputs, with suboptimal conditions leading to oxidative stress or reduced nutrient content [8,10].

Despite advances, comprehensive studies comparing microclimatic effects across crops and climatic zones are scarce, limiting tailored greenhouse management strategies [4]. Various advanced approaches and technology-based equipment have revolutionized greenhouse production by enhancing environmental control, resource efficiency, and crop productivity (Figure 1). This review evaluates optimal microclimatic conditions for lettuce, tomatoes, carrots, beets, and radishes across temperate, subtropical, and arid zones. By analyzing growth, physiological responses (e.g., photosynthesis, transpiration), and biochemical profiles (e.g., antioxidants, sugars), it provides evidence-based recommendations for sustainable greenhouse practices [15]. These insights aim to enhance crop yield, quality, and resource efficiency, supporting global food security and agricultural resilience amid climate change. Future research should focus on integrated, multi-crop studies to develop adaptive technologies, benefiting growers and strengthening the food economy.

This review aims to evaluate the optimal microclimatic conditions for lettuce, tomatoes, carrots, beets, and radishes in greenhouse systems across temperate, subtropical, and arid zones. By synthesizing data on growth parameters, physiological responses (e.g., photosynthesis, transpiration, stomatal conductance), and biochemical profiles (e.g., antioxidants, sugars, pigments), this review provides evidence-based recommendations for adaptive and sustainable greenhouse management. These insights will contribute to enhancing crop yield, quality, and input-use efficiency, thereby strengthening global food security and the resilience of agricultural systems [13].

2. Materials and Methods

2.1. Methodological Framework

This study employed a systematic review and meta-analysis approach following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines to synthesize evidence on microclimatic conditions for greenhouse cultivation of lettuce (Lactuca sativa L.), tomato (Solanum lycopersicum L.), carrot (Daucus carota L.), and beet (Beta vulgaris L.). The methodology was designed to ensure a comprehensive and rigorous evaluation of peer-reviewed research and experimental data.

2.1.1. Search Strategies

The search process began with a comprehensive exploration of peer-reviewed articles published between 1995 and 2025, accessed through databases including Web of Science, PubMed, and Google Scholar. Search terms were strategically combined, encompassing “greenhouse microclimate”, “greenhouse types”, “lettuce”, “tomatoes”, “carrots”, “beets”, “temperature”, “light intensity”, “photoperiod”, “CO₂ concentration”, and “climatic zones”. The initial focus was on studies reporting quantitative data on growth, physiology (e.g., photosynthesis, stomatal conductance), or biochemical responses (e.g., nutrient content, antioxidants) for the specified crops under controlled greenhouse environments. To expand the dataset, cited references and additional relevant articles from Google Scholar were included, ensuring a diverse and robust literature base.

2.1.2. Eligibility and Exclusion Criteria

To refine the scope and maintain relevance, specific eligibility and exclusion criteria were applied. Studies were included if they (1) focused on greenhouse cultivation of lettuce, tomato, carrot, or beet; (2) provided quantitative data on microclimatic parameters (temperature, light, CO₂); and (3) were published between 1995 and 2025. Exclusion criteria comprised (1) studies prior to 1995 due to limited historical data on greenhouse microclimates; (2) non-peer-reviewed sources (e.g., theses, unpublished reports); (3) research on crops other than the target species; (4) studies lacking quantitative microclimatic data; (5) analyses beyond greenhouse-level (e.g., regional studies); and (6) reviews or meta-analyses, to avoid redundancy.

2.1.3. Data Extraction and Analysis

Data extraction targeted the following key microclimatic variables: air and root-zone temperature, light intensity and photoperiod, CO₂ concentration, and nutrient levels (e.g., nitrogen, phosphorus, potassium). Results were organized by crop, growth stage (vegetative, reproductive, root development), physiological and biochemical responses, greenhouse type (e.g., glasshouses, polytunnels, shade-net houses), and climatic zone (temperate, Mediterranean, tropical, subtropical, arid). Extracted data were compiled into tables for comparative analysis of optimal ranges. Statistical insights from original studies (e.g., ANOVA, regression models) were referenced to assess significant effects, with trends analyzed to identify patterns influencing crop performance across diverse conditions.

3. Results

3.1. Microclimatic Conditions

3.1.1. Temperature

The present review study identified a wide range of temperature conditions across different climatic zones and greenhouse settings, highlighting the significant variability in how crops such as lettuce, tomatoes, carrots and beetroot respond to temperature fluctuations (Figure 2). These differences emphasize the importance of region-specific temperature management strategies to optimize growth, yield, and quality in controlled environments.

Lettuce (Lactuca sativa L.) Response

Lettuce (Lactuca sativa L.), a cool-season crop, demonstrates a highly temperature-sensitive growth response, with vegetative development, biochemical stability, and final yield significantly influenced by greenhouse type and prevailing climatic zone [16,17]. The intricate interplay between environmental temperature, greenhouse structure, and external climate underscores the need for precise thermal management strategies. In temperate U.S. climates, greenhouse cultivation at optimal temperatures of 15–20 °C significantly enhances vegetative growth, boosting leaf number by 20%, fresh weight by 15%, and vitamin C content by 10 mg/100 g. However, temperatures above 24 °C can trigger premature bolting and increase bitterness, negatively affecting crop quality [18]. In Turkey, the Mediterranean climate combined with unheated plastic tunnels significantly enhances lettuce performance, boosting head weight by 28%, improving water-use efficiency by 40%, and increasing chlorophyll content by 15% at optimal temperatures of 15–18 °C [19]. Subtropical shade-net houses in Taiwan optimize lettuce fresh yield (+30%), stomatal conductance (+25%), and antioxidant capacity (+18%) at 20–25 °C [20]. In tropical regions like Malaysia, advanced hydroponic systems using the nutrient film technique (NFT) with root-zone cooling have harnessed ambient temperatures of 25–30 °C while maintaining roots at a cool 20 °C [21]. The NFT system significantly boosts plant growth, increasing shoot dry weight by 50%, photosynthetic rates by 40%, and phenolic compound levels by 20%, enhancing both yield and nutritional quality [22,23]. In controlled environments, such as those in Romania, NFT hydroponics paired with LED lighting optimizes plant performance further. By maintaining precise day/night temperatures of 22/18 °C and stabilizing root zones at 20 °C, these systems achieve remarkable improvements; photosynthetic efficiency rises by 20–25%, the quantum yield of Photosystem II increases by 25%, and overall yields surge by 15–30% [24]. Additionally, these conditions elevate vitamin C content by 30%, antioxidant levels by 35%, and varietal phenolic compounds by 20–40%, delivering nutrient-rich produce [25]. Even in space agriculture simulations, such as NASA’s Veggie Growth Chamber, maintaining stable air and root-zone temperatures at 22 °C ensures biomass production comparable to ground-based controls while preserving gas exchange and vitamin content, paving the way for sustainable food production in extraterrestrial environments [26].

Biochemical responses remain critically temperature-dependent; while the 15–20 °C range in temperate systems boosts antioxidants like vitamin C, excessive heat (>24 °C) elevates stress-induced phenolic compounds. Conversely, optimized CEA regimes (e.g., 22/18 °C with LEDs) simultaneously enhance both nutritional quality and physiological efficiency. Collectively, these findings confirm that lettuce’s functional thermal range spans 15–22 °C in actively managed environments, with specific targets varying by climate and technology—notably requiring root-zone cooling (20 °C) for productivity in high-ambient temperatures (e.g., 25–30 °C in tropics).

Tomatoes (Solanum lycopersicum L.)

The growth, physiology, and fruit quality of tomato (Solanum lycopersicum L.) crucially depends on temperature, greenhouse design, and climatic conditions, with complex interactions dictating yield outcomes. Optimal temperatures vary by developmental stage; vegetative growth peaks at 20–25 °C, while fruit set and reproductive development are maximized at 18–24 °C [27].

In temperate climates (e.g., Romania), greenhouses maintaining stable temperatures of 20–24 °C enhance fruit number, plant height, and biomass by 15–20% by mitigating heat stress and improving inflorescence development [28]. Conversely, in arid regions (e.g., Saudi Arabia), advanced greenhouses with evaporative cooling sustaining 20–24 °C boost yields by up to 40%, counteracting heat-induced pollen sterility and fruit-set losses that can reach 25% at 30 °C [1]. Temperatures exceeding 30 °C consistently reduce pollen viability and fruit set across Mediterranean and tropical zones [29]. The detrimental effects of elevated temperatures on reproductive success are magnified in humid tropical regions—where high humidity exacerbates flower abortion—and in arid zones—where low humidity increases evapotranspirative stress on reproductive tissues [30]. Controlled greenhouse environments effectively mitigate these impacts [31,32]. For cherry tomatoes, sustaining 20–24 °C in greenhouses optimizes photosynthesis and nutrient uptake, elevating yield and biochemical quality (e.g., lycopene, antioxidants) [33]. Strategic interventions like shading and ventilation can lower temperatures by 1–5 °C, enhancing fruit quality globally [34]. While high-tech cooling is essential in arid zones, temperate regions often achieve resilience through simpler shading systems [35].

Figure 2. Tomato cultivation in a technology-equipped greenhouse compartment: a showcase of integrated climate and irrigation control systems.

Figure 2. Tomato cultivation in a technology-equipped greenhouse compartment: a showcase of integrated climate and irrigation control systems.

Carrots (Daucus carota L.)

Temperature exerts a profound influence on carrot (Daucus carota L.) growth, yield, and quality, with optimal ranges varying by climate and greenhouse systems. In temperate regions like Canada, greenhouse temperatures of 15–20 °C enhance root biomass by 25% and reduce deformities by 20% compared to 25 °C, where heat stress increases root cracking by 15% [36]. This temperature range also boosts transpiration and water-use efficiency by 25%, driving a 30% increase in photosynthesis and CO₂ assimilation [36]. However, maintaining 15–18 °C slows growth but elevates sugar content, while temperatures of 28–30 °C reduce biomass due to heightened respiration and suppressed photosynthesis. In Mediterranean climates, fan ventilation proves superior to evaporative cooling, increasing transpiration by 60% and promoting robust root development at 15–20 °C [37]. Above 30 °C, vegetative growth accelerates, but root quality declines due to lignification and reduced sugar content, though cooling systems help mitigate photosynthetic losses [37].

In tropical climates like Egypt, shaded polytunnels maintaining 15–18 °C improve various vegetable parameters, including increasing carrot root length by 20% and yield by 15% [38]. Temperatures exceeding 30 °C trigger reactive oxygen species accumulation, reducing chlorophyll and photosynthesis by 18% [39]. Techniques such as misting, drip irrigation, and heat-tolerant cultivars effectively alleviate heat stress [37]. Temperature also interacts dynamically with CO₂ levels to shape carrot physiology. In temperate zones, at 600 ppm CO₂ and 15–20 °C, increased leaf area enhances photosynthesis and early growth, but temperatures above 28 °C reduce yields through feedback inhibition [40,41]. In tropical greenhouses, temperatures of 30–35 °C under elevated CO₂ temporarily boost flavonoid and phenolic content in carrots, followed by a biochemical decline [42]. At high altitudes, cooler temperatures of 5–15 °C promote sucrose accumulation by reducing respiration but limit carotenoid and polyphenol synthesis [43].

The biochemical quality of carrots is closely tied to temperature. In Canada and Egypt, temperatures of 15–20 °C increase β-carotene and sugar content by 15–20% [36,37], while temperatures above 25 °C induce oxidative stress, reducing β-carotene by 10% [36]. It has been reported that optimal carotenoid and sugar synthesis occurs at 20–25 °C, with diminished biosynthesis below 15 °C or above 30 °C [44]. Chlorogenic acid peaks at 28 °C but declines above 32 °C due to enzymatic degradation, while 15–20 °C favors the retention of glucosinolates and anthocyanins, which diminish at higher temperatures.

Beet (Beta vulgaris L.) Response

A controlled study on temperature effects in sugar beets reported that the optimum root-zone temperature for dry-matter production in foliage decreased from ~26.3 °C at 6 weeks after emergence to ~23 °C at 13 weeks [45]. However, temperatures reaching 25 °C can induce bolting, reducing marketable roots by 25% [45]. A study conducted in a tropical region (East Java, Indonesia) indicates that beetroot yields tend to decline when ambient daytime temperatures exceed approximately 25–30 °C. This suggests that cooler growing conditions, such as those maintained at 15–20 °C in controlled environments, could potentially enhance beetroot performance [46]. Modeling for Castilla y León (Spain) under future warming scenarios (RCP4.5, mid-century) suggests a projected 9% decrease in sugar beet yield and a ~6% drop in CO₂ assimilation under current management—unless adaptation and improved irrigation are applied, which could yield up to ~17% gains and 9–13% more CO₂ storage [47]. A comprehensive review described how evaporative cooling systems (e.g., fan-and-pad, roof film) in hot and arid regions can reduce greenhouse temperatures by up to ~6 °C, achieving daytime temperatures around 20 °C and nighttime temperatures around 18 °C—consistent with the 18–24 °C target range [48]. In cold temperate regions like China, passive solar greenhouses at 15–20 °C increase beet root size by 10%, improve enzyme stability, and enhance sucrose accumulation by 10%, though temperatures below 10 °C impair chlorophyll function [45]. Similarly, Mediterranean ventilated tunnels in Greece at 18–20 °C maximize beet sucrose yield, but temperatures above 25 °C reduce CO₂ uptake by 15% and lower sugar content [48]. Globally, controlled environments at 20–25 °C optimize beet root growth and sugar content, promoting peak photosynthesis, minimizing respiration, and increasing invertase and sucrose synthase activity. These optimized greenhouse conditions highlight significant variability in temperature responses for beets (Beta vulgaris L.), alongside other crops like lettuce, tomatoes, and carrots, as illustrated in Figure 3.

3.1.2. Light Intensity (μmol·m⁻²·s⁻¹) and Photoperiod (hours)

The present study has also found a wide range of light intensities (μmol·m⁻²·s⁻¹) across different climatic zones and greenhouse settings, highlighting the significant variability in how crops such as lettuce, tomatoes, carrots, and beetroot respond to temperature fluctuations. These differences emphasize the importance of region-specific temperature management strategies to optimize growth, yield, and quality in controlled environments (Figure 4).

Lettuce (Lactuca sativa L.) Response

Lettuce (Lactuca sativa L.), a C3 crop, thrives under precise environmental conditions, with its growth heavily influenced by light intensity (μmol·m⁻²·s⁻¹), photoperiod (hours), greenhouse type, and climatic zone [49]. These factors shape lettuce’s photosynthesis, biomass, and nutrient uptake, significantly boosting its yield and quality when optimized. In temperate glass greenhouses, such as those in The Netherlands, vertical farms, using LED lights, achieve higher LUEinc through controlled environmental conditions, nearing maximum theoretical efficiency. At 200 μmol·m⁻² s⁻¹ PPFD and a 16 h photoperiod, the highest LUEinc based on shoot fresh weight was 44 g mol⁻¹. With continuous 500 μmol·m⁻² s⁻¹ light, vertical farms can produce up to 700 kg of lettuce per m² annually per layer [50]. Research shows lettuce’s fresh weight increases by 30% at 23 °C with 200 μmol·m⁻²·s⁻¹ and a 16 h photoperiod [51]. Intensities of 200, 400, and 600 μmol·m⁻² s⁻¹ improve efficiency and yield, with 200 μmol·m⁻² s⁻¹ being most efficient and 600 μmol·m⁻² s⁻¹ yielding the highest. The 400–600 μmol·m⁻² s⁻¹ range is optimal, with 400 μmol·m⁻² s⁻¹ suited for winter greenhouse lighting and 600 μmol·m⁻² s⁻¹ for shading in late spring/early autumn [52]. In tropical polycarbonate greenhouses, such as in Malaysia, lettuce prospers at 18–22 °C with 150 μmol·m⁻²·s⁻¹, 14 h photoperiods (DLI: 7.56 mol·m⁻²·d⁻¹), and shading, increasing its leaf area by 20% while reducing heat stress [53]. However, temperatures above 30 °C or PPFD exceeding 300 μmol·m⁻²·s⁻¹ cause photooxidative stress in lettuce, reducing its protein content by 10–15% [54].

Greenhouse types play a critical role in regulating lettuce’s environment. Glass greenhouses maintain a stable 20–24 °C, ideal for lettuce’s photosynthesis [55]. In the tropical climate of Thailand, this study found that among various temperature control systems used in greenhouses, the fan-pad evaporative cooling system significantly improved green romaine lettuce yield and quality [56]. In a Mediterranean region (Italy), a study on lettuce has reported that PPFD of 215 μmol m⁻² s⁻¹ with 16 h daylength supported up to a 2-fold yield at a red/blue ratio of 3. This light intensity enhanced chlorophyll, flavonoid levels, and nutrient uptake, improving resource use efficiency [57]. Climatic zones further refine lettuce’s needs; in Mediterranean climates, 20–23 °C with red/blue LEDs (4:1 ratio) and 480 PPFD enhances chlorophyll in cultivars like Lollo Rosso [24,57]. Red/blue LEDs at 210–240 μmol·m⁻²·s⁻¹ and 23/18 °C (day/night) improve lettuce’s flavonoid content and yield [58]. Far-red light at 22–24 °C promotes lettuce’s canopy growth but may dilute its nutrient density if overused [59]. Elevated CO₂ and airflow at 23 °C further enhance lettuce’s photosynthetic efficiency [60]. A study examining three temperature regimes—cool (~15 °C), optimal (~23 °C), and high (~30 °C)—under varying light intensities found that the maximum net photosynthetic rate (Pₙₘₐₓ), chlorophyll content, light saturation point, and overall yield were all significantly higher at ~23 °C, indicating that both cooler and warmer extremes (~15 °C and ~30 °C) adversely affect lettuce’s physiological performance [61,62].

Tomatoes (Solanum lycopersicum L.)

Tomatoes (Solanum lycopersicum L.) are highly photosensitive crops, with their growth and development closely regulated by the light spectrum, intensity—measured as photosynthetic photon flux density (PPFD)—and photoperiod. These light-related factors directly influence photosynthesis, plant morphology, and the synthesis of secondary metabolites. For optimal growth, tomatoes require daily light integrals (DLIs) ranging from 20 to 30 mol/m²/day. Specific wavelengths play distinct roles; red and blue light spectra are particularly effective in enhancing photosynthesis, while far-red light is critical in triggering shade-avoidance responses. Studies conducted across different climatic zones underscore how tomatoes respond to variable light and environmental conditions. In the warm temperate region of Spain, tomatoes (Cultivar Siranzo) were cultivated in a building-integrated rooftop greenhouse. The application of supplemental red and blue LED lighting for 16 h per day at an intensity of approximately 170 µmol·m⁻²·s⁻¹ resulted in a significant increase in yield (~17%) and enhanced fruit ripening parameters [63]. In environments with elevated air temperatures (30–36 °C), lowering root-zone temperature (RZT) to around 24 °C significantly reduced stress in temperate leafy greens such as lettuce and spinach, enabling them to be grown successfully in tropical greenhouses [64]. Additionally, a recent study confirms that supplementing such red/blue LED lighting (at similar background PPFD ~250 µmol·m⁻²·s⁻¹) with far-red radiation significantly increases lettuce canopy light interception, leading to notable enhancements in fresh and dry weight—despite some reduction in leaf-level photosynthesis. The net result is an increase in whole-plant yield and greater light-use efficiency [65,66].

In Romania’s continental zone, research on soilless cultivation system under greenhouse and solar-heated conditions has examined yield and physiological performance of tomatoes. For instance, research on tomatoes grown in perlite substrate mattresses showed enhanced yield and fruit quality compared to soil-based cultivation [67]. A DSSAT-CROPGRO-Tomato model simulation, using 14 years of weather data from Immokalee, South Florida, revealed that rising air temperatures above ambient conditions reduced tomato yields by 52–85%, mainly due to heat-induced flower drop and reduced pollen viability at temperatures approaching or exceeding 35 °C [68]. A study from Uruguay found that the greenhouse microclimate influences the biochemical responses of tomatoes, particularly phenolic compounds and defense enzymes [69]. Finally, in the arid conditions of Saudi Arabia, climate-controlled greenhouses that optimized both light and temperature conditions resulted in a 40% increase in yield and a 15% boost in antioxidant levels [70]. These diverse climatic studies collectively highlight the importance of tailoring light quality and thermal conditions to maximize tomato productivity and fruit quality in different environmental settings.

Carrot (Daucus carota L.) Response

Carrots (Daucus carota L.), a biennial root crop, exhibit distinct growth responses across climatic zones, greenhouse types, and microclimatic light conditions, impacting yield, physiology, and biochemical quality. In temperate climates, such as Canada or Germany, CO₂-enriched glasshouses at 15–25 °C illuminated with red LED light (~200 µmol·m⁻²·s⁻¹, 12 h photoperiod) notably increase root biomass and chlorophyll content, while enhancing β-carotene accumulation and reducing oxidative stress [71]. Similarly, controlled studies demonstrating red + blue lighting (6:4 ratio) at 200 µmol·m⁻²·s⁻¹ also report improvements in both shoot and root fresh and dry weight, linked to better carotenoid levels [72]. In Mediterranean zones (Spain, Greece), combining red + blue LED (e.g., R:B = 70:30) at 200–250 µmol·m⁻²·s⁻¹ with longer photoperiods (16 h) improves biomass, CO₂ assimilation, antioxidant capacity, and sugar content, while suppressing bolting—a trend consistent with findings that moderate blue light add-ins increase yield and physiological efficiency [73,74]. Under subtropical conditions (like parts of Australia), polycarbonate greenhouse systems operated at 15–20 °C using red + blue LEDs (~70:30, 200 µmol·m⁻²·s⁻¹ with ~14 h photoperiod) have been linked to 20% gains in root yield, net photosynthesis increases of ~18%, carotenoid levels up by ~20%, and soluble sugars by ~10%—consistent with controlled lighting trials enhancing biochemical quality without altering carotenoid levels while boosting sucrose accumulation [74].

Carrots are a root crop where light primarily affects foliage photosynthesis, which in turn influences root development, sugar accumulation, and overall yield. Studies show that net photosynthesis (P_N) increases with PPFD, but saturation points vary by cultivar and conditions. Optimal PPFD for growth is typically 400–800 μmol m⁻² s⁻¹, with diminishing returns above 700–1000 μmol m⁻² s⁻¹ Increasing PPFD (100–1000 μmol m⁻² s⁻¹) enhanced carrot photosynthesis (P Nmax: 16.40–19.79 μmol CO₂ m⁻² s⁻¹), with no saturation at 1000 μmol m⁻² s⁻¹. Compensation irradiance ranged from 25 to 54 μmol m⁻² s⁻¹, quantum yields were 0.033–0.057 mol CO₂ mol⁻¹ PAR, and water use efficiency peaked at 600–800 μmol m⁻² s⁻¹ [75]. Finally, in tropical regions (e.g., Malaysia), heated greenhouse environments with misting can help maintain temperatures in the ideal 15–20 °C range, improving photosynthetic efficiency and limiting heat stress, whereas temperatures above ~30 °C are known to reduce chlorophyll, photosynthesis, and sugars and increase oxidative stress, as shown by hydroponic studies where elevated root zone temperature suppresses growth while increasing soluble solids and stress markers [76,77,78].

Beet (Beta vulgaris L.) Response

Several beets (Beta vulgaris) respond significantly to variations in light intensity, photoperiod, and spectral quality, which directly influence their growth and biochemical properties. Moderate light intensities between 150 and 250 µmol·m⁻²·s⁻¹ and photoperiods of 12–14 h are generally effective in balancing biomass production with desirable quality traits [79]. Increasing light intensity and photoperiod duration can enhance the synthesis of antioxidants and betalains, although this often comes at the expense of yield, highlighting a trade-off between quality and quantity [79].

Controlled environment studies, particularly in vertical farming setups, have shown that light intensities around 200 µmol·m⁻²·s⁻¹ combined with extended photoperiods, such as 14 h, can improve sugar content, betacyanin levels, and stomatal conductance [80,81]. However, exposure to light intensities exceeding 300 µmol·m⁻²·s⁻¹ may induce photo-oxidative stress, reducing photosynthetic efficiency and negatively affecting plant performance. Spectral quality also plays a crucial role. Red light tends to stimulate stem elongation and overall biomass, while blue light enhances pigment synthesis and photosynthetic activity [82]. Red/blue LED combinations are particularly beneficial, improving both physiological and biochemical attributes. Red-dominant spectra often increase sugar and phenolic compounds, whereas blue light contributes to higher pigment content and nitrogen uptake, although it may also lead to increased nitrate accumulation. Interestingly, higher overall light intensities can help lower nitrate levels in beet leaves, thereby enhancing nutritional value [83].

The climatic zones and greenhouse settings highlight significant variability in light intensity (μmol·m⁻²·s⁻¹) responses among beetroot (Beta vulgaris L.) and other crops such as lettuce (Lactuca sativa L.), tomatoes (Solanum lycopersicum L.), and carrots (Daucus carota L.) (Figure 5).

3.1.3. CO₂ Concentration

Lettuce (Lactuca sativa L.) Response

Elevated CO₂ concentrations enhance lettuce growth across various climatic zones and greenhouse types, boosting biomass, photosynthesis, and water-use efficiency (WUE), but often at the cost of nutritional quality (Table 1). Elevated CO₂ promotes photosynthesis, reduces stomatal conductance, increases WUE, and can reduce protein and mineral concentrations in C₃ vegetables such as lettuce [41,84]. In temperate regions (USA and Europe) using glasshouses, CO₂ levels of ~900–1100 ppm increase fresh weight by 10–19% and enhance photosynthesis and carbohydrate storage but reduce nitrate and protein content [85]. Meta-analyses and controlled greenhouse studies report an ~18% increased yield in lettuce at 800–900 ppm CO₂, along with ~9% lower protein and ~18% lower nitrate concentrations [84]. In solar-greenhouse trials (e.g., China/Japan), 700–900 ppm CO₂ yields ~20–25% more biomass, yet lower transpiration increases tip burn risk due to reduced calcium transport and leaf nitrogen. For example, studies in solar greenhouses show enhanced photosynthesis and growth and increased vitamin C, but nitrate decreases under elevated CO₂ (~700–900 ppm), which can limit calcium transport and raise tip burn risk [86]. Meta-analyses covering various vegetables, including lettuce—though mostly conducted under non-tropical conditions—have shown that CO₂ enrichment to approximately 800–900 ppm can significantly enhance crop productivity. Specifically, yield increases of around 18–19% have been reported for lettuce, carrot, and parsley grown in greenhouse environments with elevated CO₂ levels (700–900 ppm) [87]. Chamber experiments show that fresh and dry weight rise up to ~1200 ppm, after which yields plateau; WUE is highest at ~1200 ppm under strong light (~300 PPFD); and anthocyanins, some pigments, and mineral/nutrient content decline beyond 800–1200 ppm [85]. At 1400–1500 ppm in controlled settings, shoot biomass rises by ~30–36%, but anthocyanin and vitamin C levels drop significantly. High-CO₂ (≥1200–1600 ppm) studies report modest additional biomass gains but clear reductions in anthocyanins, violaxanthin, vitamin content, and nutrient [88]. CO₂ enrichment to ~1000–1200 ppm optimizes yield and WUE across systems but requires strategies to address nutritional losses and physiological issues like tip burn.

Tomatoes (Solanum lycopersicum L.)

The effectiveness of CO₂ enrichment for tomato production depends critically on the interplay of climatic zone, greenhouse type, and microclimate. In tropical high-tech greenhouses at 25–28 °C, 1100 ppm CO₂ boosts yield by 30%, enhances photosynthesis and water-use efficiency (WUE), and increases fruit sucrose by 12–18% and lycopene by =10% [89]. Conversely, A study in southern Japan showed that maintaining CO₂ at 800 ppm during non-ventilation and ~400 ppm during ventilation resulted in ~25 % higher marketable fruit yield and modest increases in fruit Brix compared to constant 400 ppm CO₂ conditions [89,90]. Arid sealed greenhouse trials show that 800 ppm CO₂ improves yield under low VPD (24–27 °C), sustaining photosynthesis during drought—but may slightly reduce lycopene and modestly increase sugar content (meta-meta-analyses; carotenoid responses vary with CO₂ levels) [86,90]. In subtropical winter greenhouses (18–22 °C), enrichment around 500 ppm above ambient promotes leaf expansion and vegetative growth but induces minimal change in sugar or nutrient content (typical yield response of ~18% at 550–650 ppm, with modest sugar increase [91]. Temperate glasshouses (Denmark) with 800 ppm CO₂ enhance yield, water-use efficiency, and stress resilience, with genotype-dependent photosynthetic responses peaking at 800–1000 ppm (a systematic review reporting 30–36% yield increases in European greenhouse trials at ~800 ppm) [92,93]. Controlled-chamber studies demonstrate benefits climbing steeply up to 1000–1200 ppm CO₂ at ~25 °C—with theoretical Rubisco-driven yield gains up to 80%—but these benefits plateau or decline above 1200 ppm or above 30 °C [90]. Excessive soil/ambient CO₂ (1500 ppm) can disrupt stomatal function, reducing biomass, fruit weight, lycopene and vitamin C content, and altering sugar–acid balance; even moderate CO₂ (550 ppm above ambient) may dilute nutrient density via carbohydrate dilution effects [93,94]. Thus, optimal CO₂ enrichment must be tailored to specific environmental contexts—including climate zone, greenhouse type, VPD, ventilation, and cultivar—to maximize yield and quality while avoiding trade-offs.

Carrots (Daucus carota L.)

The effectiveness of CO₂ enrichment in carrot (Daucus carota L.) production depends strongly on environmental conditions, greenhouse type, and cultivar physiology. Studies indicate that under optimal temperature ranges (22–28 °C), moderate enrichment (~700–800 ppm CO₂) enhances photosynthetic efficiency and biomass accumulation. For instance, in controlled environments, elevated CO₂ has been shown to promote vegetative growth, increase root mass, and improve water-use efficiency—particularly under a low-vapor-pressure deficit (VPD) [91,95].

Carrots cultivated with bio stimulants and optimal fertilization exhibit enhanced shoot and root development under CO₂ enrichment, demonstrating the effectiveness of this approach [96]. However, excessive enrichment (>1200 ppm) may lead to nutrient dilution effects due to carbohydrate accumulation, potentially lowering root quality [90].

While much of the CO₂ enrichment literature is based on tomato models, parallels in physiological response—such as increased sucrose accumulation, photosynthetic upregulation, and WUE—suggest similar mechanisms apply to carrots, albeit with root-specific outcomes [89,93]. Moreover, free-air CO₂ enrichment (FACE) experiments have emphasized that long-term CO₂ elevation benefits are often constrained by nutrient availability and acclimation effects, underscoring the need for integrated nutrient and CO₂ management strategies [95].

Finally, theoretical modeling suggests that Rubisco-limited photosynthesis in C₃ crops like carrots could support yield increases up to 80% under elevated CO₂—if coupled with optimized nutrient supply and controlled temperature [97]. Therefore, tailoring CO₂ enrichment in carrots requires careful consideration of temperature, fertilization, biostimulants, and cultivar sensitivity to ensure sustainable yield and quality improvements.

Beet (Beta vulgaris L.) Response

Beets (Beta vulgaris L.) exhibit significant growth and bio-chemical responses to elevated CO₂, especially under controlled environments and Mediterranean-type climates. Under CO₂ concentrations around 1000 ppm in closed-chamber systems, beets demonstrate a notable increase in biomass accumulation, with dry weight per plant reaching up to 28–31 g and a harvest index exceeding 90%, indicating a high allocation of biomass to edible storage roots [98,99]. Water-use efficiency (WUE) is markedly improved, with values as high as 0.003 mol C per mol H₂O observed under enriched CO₂, suggesting enhanced carbon assimilation per unit of water lost through transpiration [98]. Moreover, elevated CO₂ enhances photosynthetic rates while promoting antioxidant defense responses, likely mitigating oxidative stress under drought conditions. Although specific studies coupling elevated CO₂ with drought in beets are limited, data from wild beet accessions (B. maritima) suggest that CO₂ enrichment may further bolster drought resilience by improving leaf succulence, osmotic regulation, and roots [100]. However, temperature plays a counterbalancing role; increases of +4 °C can reduce both root and total biomass, potentially offsetting CO₂-induced gains, especially in nutrient-limited scenarios. Furthermore, exposure to elevated CO₂ often results in nutrient dilution effects—declines in tissue nitrogen and potassium—implying trade-offs between yield and nutritional quality. Beets, with their high edible biomass and efficient harvest index (~94%), show strong potential for bioregenerative life support systems. Under controlled environments with elevated CO₂ and optimal light, staged beet stands yielded up to 31.4 g dry weight per plant and contributed approximately 2% of daily air revitalization needs per square meter [100]. Despite these constraints, beets show a convex response to CO₂ concentration, with optimal biomass and pigment (e.g., betanin) accumulation occurring around 600–800 ppm, beyond which benefits plateau or decline. These physiological patterns position Beta vulgaris L. as a promising crop for climate-smart agriculture, provided nutrient management and temperature regulation are carefully integrated.

Table 1. Optimal CO₂ ranges (ppm) for vegetable production across different climatic zones.

Vegetable	Climatic Zone	CO2 Range (ppm)	Growth Impact	Physiological Impact	Biochemical Impact	Reference(s)
Lettuce	Temperate (USA)	900–1100	+19% fresh weight	↑ photosynthesis, ↑ WUE, ↑ carbohydrate storage	↓ nitrate, ↓ protein	[101]
Lettuce	Temperate (Japan)	700–900	+20–25% biomass	↑ photosynthesis, ↓ transpiration, ↑ tipburn risk	↓ leaf nitrogen, ↓ calcium transport	[102]
Lettuce	Temperate (Europe)	900–1100	~10% more biomass	photosynthetic saturation near 1000 ppm	stable nutrient content	[103]
Lettuce	General (Controlled)	1100–1300	+20–30% biomass	↑ photosynthesis, ↑ carbohydrate accumulation	↓ anthocyanin, ↓ antioxidants, ↓ calcium transport, ↑ tipburn risk	[104,105]
Lettuce	General (Controlled)	1400–1500	+30–36% shoot biomass	↑ photosynthesis (maximized), saturation at 1200 ppm	↓ anthocyanin, ↓ vitamin C	[82]
Tomato	Temperate (Asia—Japan)	700–800	↑ leaf area, ↑ dry matter	↑ photosynthesis, reduced by ventilation	not specified	[88]
Tomato	Temperate (Europe—Denmark)	800	↑ yield, ↑ stress tolerance	↑ photosynthesis, genotype-specific responses	↑ stress resilience, stable nutrient content	[25]
Tomato	Arid (China)	800	↑ yield (low VPD)	sustained photosynthesis under drought	↓ lycopene, ↑ sugar (slight)	[106]
Tomato	Subtropical (Middle East)	500	↑ growth, ↑ leaf expansion	minimal sugar change	stable sugar, limited nutritional changes	[107]
Tomato	Tropical (High-Tech)	1100	↑ yield by 30%	↑ photosynthesis, ↑ WUE	↑ sucrose (12–18%), ↑ lycopene (10%)	[108]
Tomato	General (Controlled)	1000–1200	↑ yield by 80% via Rubisco activity	Photosynthesis plateaus after 1200 ppm	Not specified	[40]
Tomato	General (Controlled)	1500 (soil CO2)	↓ biomass, ↓ fruit weight	Disrupted stomatal conductance, ↓ photosynthesis	↓ fruit quality, ↓ sugar-acid balance, ↓ lycopene, ↓ vitamin C	[109]
Tomato	General (Canada)	800	↑ flowering, ↑ plant height	↓ pigment concentration	Not specified	[110]
Tomato	Global (Meta-study)	550	Not specified	Not specified	↓ nutrient density in fruits (nutrient dilution)	[111]
Carrot	Temperate (USA)	600–800	+15–20% root biomass	↑ photosynthesis (10–15%), ↑ WUE, optimal root:shoot ratio	↑ sucrose (8–10%), ↑ β-carotene (5–7%), ↑ ascorbic acid (15–25%)	[112,113]
Carrot	Temperate (USA)	800–1000	+25–40% biomass	↓ stomatal conductance, ↓ leaf N, photosynthetic acclimation	↓ protein, ↓ calcium, ↓ magnesium, nutrient dilution risk	[111,114]
Carrot	Temperate (USA)	1000–1300	+20–40% biomass, ↓ root yield > 1000 ppm	Excessive shoot growth, ↓ RuBisCO activity	↓ zinc, ↓ iron, ↓ protein	[115]
Carrot	General (Controlled)	600–1000	+25–60% biomass	↑ photosynthesis, ↓ transpiration, downregulation > 1000 ppm	↓ nitrogen, ↓ potassium, yield-quality trade-off	[116,117]
Carrot	General (Meta-analysis)	700–1000	+10–20% yield (C3 crops)	↑ photosynthesis, ↓ WUE	↓ protein, ↓ Zn, ↓ Fe, dietary concerns	[111]
Beet	Mediterranean	600–800	+18–32% root biomass, +10% root diameter	↑ photosynthesis, ↑ WUE, ↓ stomatal conductance	↑ sucrose (8–20%), ↑ betanin (7–10%)	[118]
Beet	Temperate (Europe)	550–700	+28–30% root yield (with nitrogen), ↓ at high temps	↑ Rubisco activity, ↓ leaf nitrogen, ↑ photosynthesis	↑ sucrose, ↓ nitrogen, ↓ potassium	[97]
Beet	Temperate (Europe)	600	+18% biomass (under drought)	↑ antioxidant responses, ↑ WUE	↑ sucrose maintained, ↓ protein	[119]
Beet	Temperate (Austria)	1000	+40% root biomass, ↓ 8–12% root dry weight	↑ carbon fixation, ↓ stomatal conductance	↑ betalain pigments, ↓ protein	[120]

Note: ↑ indicates increase; ↓ indicates decrease.

Table 1. Optimal CO₂ ranges (ppm) for vegetable production across different climatic zones.

Vegetable	Climatic Zone	CO2 Range (ppm)	Growth Impact	Physiological Impact	Biochemical Impact	Reference(s)
Lettuce	Temperate (USA)	900–1100	+19% fresh weight	↑ photosynthesis, ↑ WUE, ↑ carbohydrate storage	↓ nitrate, ↓ protein	[101]
Lettuce	Temperate (Japan)	700–900	+20–25% biomass	↑ photosynthesis, ↓ transpiration, ↑ tipburn risk	↓ leaf nitrogen, ↓ calcium transport	[102]
Lettuce	Temperate (Europe)	900–1100	~10% more biomass	photosynthetic saturation near 1000 ppm	stable nutrient content	[103]
Lettuce	General (Controlled)	1100–1300	+20–30% biomass	↑ photosynthesis, ↑ carbohydrate accumulation	↓ anthocyanin, ↓ antioxidants, ↓ calcium transport, ↑ tipburn risk	[104,105]
Lettuce	General (Controlled)	1400–1500	+30–36% shoot biomass	↑ photosynthesis (maximized), saturation at 1200 ppm	↓ anthocyanin, ↓ vitamin C	[82]
Tomato	Temperate (Asia—Japan)	700–800	↑ leaf area, ↑ dry matter	↑ photosynthesis, reduced by ventilation	not specified	[88]
Tomato	Temperate (Europe—Denmark)	800	↑ yield, ↑ stress tolerance	↑ photosynthesis, genotype-specific responses	↑ stress resilience, stable nutrient content	[25]
Tomato	Arid (China)	800	↑ yield (low VPD)	sustained photosynthesis under drought	↓ lycopene, ↑ sugar (slight)	[106]
Tomato	Subtropical (Middle East)	500	↑ growth, ↑ leaf expansion	minimal sugar change	stable sugar, limited nutritional changes	[107]
Tomato	Tropical (High-Tech)	1100	↑ yield by 30%	↑ photosynthesis, ↑ WUE	↑ sucrose (12–18%), ↑ lycopene (10%)	[108]
Tomato	General (Controlled)	1000–1200	↑ yield by 80% via Rubisco activity	Photosynthesis plateaus after 1200 ppm	Not specified	[40]
Tomato	General (Controlled)	1500 (soil CO2)	↓ biomass, ↓ fruit weight	Disrupted stomatal conductance, ↓ photosynthesis	↓ fruit quality, ↓ sugar-acid balance, ↓ lycopene, ↓ vitamin C	[109]
Tomato	General (Canada)	800	↑ flowering, ↑ plant height	↓ pigment concentration	Not specified	[110]
Tomato	Global (Meta-study)	550	Not specified	Not specified	↓ nutrient density in fruits (nutrient dilution)	[111]
Carrot	Temperate (USA)	600–800	+15–20% root biomass	↑ photosynthesis (10–15%), ↑ WUE, optimal root:shoot ratio	↑ sucrose (8–10%), ↑ β-carotene (5–7%), ↑ ascorbic acid (15–25%)	[112,113]
Carrot	Temperate (USA)	800–1000	+25–40% biomass	↓ stomatal conductance, ↓ leaf N, photosynthetic acclimation	↓ protein, ↓ calcium, ↓ magnesium, nutrient dilution risk	[111,114]
Carrot	Temperate (USA)	1000–1300	+20–40% biomass, ↓ root yield > 1000 ppm	Excessive shoot growth, ↓ RuBisCO activity	↓ zinc, ↓ iron, ↓ protein	[115]
Carrot	General (Controlled)	600–1000	+25–60% biomass	↑ photosynthesis, ↓ transpiration, downregulation > 1000 ppm	↓ nitrogen, ↓ potassium, yield-quality trade-off	[116,117]
Carrot	General (Meta-analysis)	700–1000	+10–20% yield (C3 crops)	↑ photosynthesis, ↓ WUE	↓ protein, ↓ Zn, ↓ Fe, dietary concerns	[111]
Beet	Mediterranean	600–800	+18–32% root biomass, +10% root diameter	↑ photosynthesis, ↑ WUE, ↓ stomatal conductance	↑ sucrose (8–20%), ↑ betanin (7–10%)	[118]
Beet	Temperate (Europe)	550–700	+28–30% root yield (with nitrogen), ↓ at high temps	↑ Rubisco activity, ↓ leaf nitrogen, ↑ photosynthesis	↑ sucrose, ↓ nitrogen, ↓ potassium	[97]
Beet	Temperate (Europe)	600	+18% biomass (under drought)	↑ antioxidant responses, ↑ WUE	↑ sucrose maintained, ↓ protein	[119]
Beet	Temperate (Austria)	1000	+40% root biomass, ↓ 8–12% root dry weight	↑ carbon fixation, ↓ stomatal conductance	↑ betalain pigments, ↓ protein	[120]

Note: ↑ indicates increase; ↓ indicates decrease.

4. Discussion

The optimization of microclimatic conditions—temperature (15–25 °C), light intensity (150–300 μmol·m⁻²·s⁻¹, 12–18 h photoperiod), and CO₂ concentration (600–1300 ppm)—for lettuce, tomato, carrot, and beet across temperate, Mediterranean, tropical, subtropical, and arid climatic zones is critical for enhancing plant growth, physiology, and biochemistry [120,121]. These factors intricately regulate fundamental biological processes, including photosynthesis, enzyme activity, and secondary metabolite synthesis, which drive crop productivity and quality [122,123]. Climatic zones define the baseline conditions and challenges for maintaining these optima, while external threats such as extreme weather and resource constraints can disrupt them [124]. Greenhouse types play a pivotal role in modulating these conditions, with advanced systems enabling precise control to mitigate variability. This discussion elucidates the biological interplay between microclimatic factors and plant responses, the influence of climatic zones on optimal ranges, potential threats to these conditions, and the critical role of greenhouse infrastructure, providing growers with a clear framework to maximize production.

At the biological level, temperature governs enzymatic processes, particularly Rubisco activity, which catalyzes CO₂ fixation in photosynthesis, the primary driver of biomass accumulation [121]. For lettuce and beets, temperatures of 15–22 °C optimize Rubisco kinetics and stomatal conductance, enhancing carbohydrate synthesis and leaf expansion, while temperatures above 25 °C increase respiration, reducing net photosynthesis and inducing stress-related phenolic accumulation, which can alter flavor profiles [124]. Tomatoes require 18–25 °C to support reproductive development, as warmer conditions enhance pollen viability and fruit set through optimized hormonal signaling, including auxin and gibberellin pathways [1]. Carrots benefit from 15 to 20 °C, which balances respiration and photosynthesis, promoting root biomass and sucrose accumulation via sucrose synthase activity [125]. Light intensity (150–300 μmol·m⁻²·s⁻¹) and spectral quality (red-blue LEDs) regulate chlorophyll synthesis and photosynthetic electron transport, critical for energy capture [126]. Blue light enhances stomatal opening and pigment production (e.g., anthocyanins in lettuce, lycopene in tomatoes), while red light promotes carbohydrate partitioning and stem elongation in beets and carrots [127]. Excessive light (>300 μmol·m⁻²·s⁻¹) induces photooxidative stress, generating reactive oxygen species (ROS) that damage chloroplasts and reduce Fv/Fm, a measure of photosynthetic efficiency [128]. CO₂ enrichment (600–1300 ppm) boosts photosynthesis by increasing Rubisco substrate availability, enhancing carbohydrate storage across all crops, but levels above 1000 ppm can cause stomatal closure, reducing transpiration and nutrient uptake (e.g., nitrogen, calcium), leading to physiological disorders like tip burn in lettuce or nutrient dilution in carrots and beets [129]. These factors interact synergistically; optimal temperatures amplify light-driven photosynthesis, while elevated CO₂ enhances carbon assimilation, provided light and temperature remain within suitable ranges. For example, lettuce at 20 °C with 200 μmol·m⁻²·s⁻¹ and 900 ppm CO₂ maximizes biomass by coordinating enzyme activity and stomatal function, but deviations (e.g., 30 °C or 1500 ppm CO₂) disrupt this balance, increasing ROS and reducing quality [88,130,131].

Climatic zones establish the environmental context for these optimal conditions and introduce specific challenges [92]. Temperate zones (e.g., The Netherlands, Canada) offer moderate temperatures (10–25 °C) and stable light regimes, facilitating precise control of 15–22 °C, 200–250 μmol·m⁻²·s⁻¹, and 800–1000 ppm CO₂ for lettuce and beets, with tomatoes requiring 18–24 °C [22,25,92,132]. Mediterranean climates (e.g., Spain, Greece) experience temperature fluctuations (15–30 °C), necessitating ventilation to prevent heat stress, which can elevate respiration and reduce sugar content in carrots and beets [133]. Tropical zones (e.g., Malaysia, Singapore) face high temperatures (25–35 °C) and humidity, requiring cooling systems to maintain 18–22 °C for lettuce and 20–25 °C for tomatoes, while intense sunlight risks photoinhibition [100,134]. Subtropical zones (e.g., Taiwan, Australia) balance moderate temperatures (15–28 °C) but require shading to manage light intensity (150–250 μmol·m⁻²·s⁻¹) for carrots and beets [135,136]. Arid zones (e.g., Saudi Arabia, Egypt) contend with extreme heat (>30 °C) and low humidity, increasing evapotranspiration and necessitating evaporative cooling to sustain 15–24 °C [137,138]. Threats to optimal conditions include heatwaves in Mediterranean and tropical zones, which elevate temperatures above 30 °C, reducing pollen viability in tomatoes and inducing bolting in lettuce and beets [139]. Cold snaps in temperate zones (<10 °C) impair chlorophyll synthesis in carrots and beets, while erratic light in cloudy temperate or high-altitude subtropical regions reduces photosynthetic efficiency [100,140]. CO₂ enrichment is challenging in resource-scarce tropical or arid zones, where maintaining 600–1000 ppm requires costly infrastructure [141]. Climate change intensifies these threats, with rising temperatures and extreme weather events disrupting microclimatic stability, particularly in passive systems [142].

Greenhouse types and design are instrumental in maintaining optimal microclimatic conditions and mitigating these threats (Figure 6) [143]. High-tech Venlo glasshouses in temperate zones provide precise control over temperature, light, and CO₂ through advanced cooling, tunable LED lighting, and CO₂ enrichment systems, ensuring consistent conditions (15–24 °C, 200–300 μmol·m⁻²·s⁻¹, 600–1100 ppm) for all crops [143]. These systems excel for lettuce and tomato production by optimizing photosynthesis and minimizing stress. Mediterranean plastic polytunnels and Parral-type greenhouses rely on ventilation and shading to manage temperatures above 25 °C but struggle with CO₂ consistency, impacting biochemical quality in beets and carrots [32,34,144,145]. Tropical shade-net houses use passive cooling to maintain 18–25 °C but are less effective at controlling light and CO₂, limiting yields in high-humidity environments [146]. Arid high-tech greenhouses with evaporative cooling and sealed designs maintain optimal temperatures and CO₂ (700–1000 ppm), excelling for tomatoes and carrots [48,147]. Sunken solar greenhouses in semi-humid regions stabilize temperatures (17–20 °C) for beets and carrots but lack advanced light control [148]. Vertical farms and plant factories with tunable LEDs and CO₂ systems offer unparalleled precision for lettuce and tomatoes, though space constraints limit their use for root crops [149]. Innovative technologies, such as AI-driven microclimate control and dynamic LED systems, enable real-time adjustments to prevent photooxidative stress in lettuce or optimize lycopene synthesis in tomatoes, enhancing resilience across climatic zones [150].

For growers, understanding the biological interplay of temperature, light, and CO₂, alongside the influence of climatic zones and greenhouse types, is crucial for maximizing production. Maintaining temperatures of 15–22 °C for lettuce and beets, 18–24 °C for tomatoes, and 15–20 °C for carrots, with light intensities of 150–300 μmol·m⁻²·s⁻¹ and CO₂ levels of 600–1100 ppm, optimizes photosynthesis, biomass, and nutritional quality (

Table 2. Crop-specific optimal microclimates and key responses.

Crop	Temp Optimum (°C)	Key Temp Response	Key Light Response	Key CO2 Response
Lettuce	15–22	>25 °C: ↑ Respiration, ↓ Pn, ↑ Phenolics, Bolting	Blue: ↑ Anthocyanins, Stomatal Opening; Excess: ROS	Enrichment Boosts Pn; >1000 ppm: Tipburn Risk
Tomato	18–25	Optimizes Pollen Viability, Fruit Set, Hormonal Signaling	Blue: ↑ Lycopene; Excess: ROS	Enrichment Boosts Pn; >1000 ppm: Stomatal Closure Risk
Carrot	15–20	Balances Respiration/Pn, ↑ Root Biomass, Sucrose Synthase	Red: Promotes Partitioning/Elongation; Shading often needed	Enrichment Boosts Storage; >1000 ppm: Nutrient Dilution Risk
Beet	15–22	>25 °C: ↑ Respiration, ↓ Pn, ↑ Phenolics, Bolting	Red: Promotes Partitioning/Elongation; Shading often needed	Enrichment Boosts Storage; >1000 ppm: Nutrient Dilution Risk

Note: ↑ indicates increase; ↓ indicates decrease.

and

Table 3. Greenhouse suitability by climate zone and key features.

Climate Zone	Suitable Greenhouse Types	Primary Control Methods	Key Limitations	Best Suited Crops
Temperate	High-Tech Venlo Glasshouses	Advanced Cooling, Tunable LEDs, CO2 Enrichment	High Capital Cost	Lettuce, Tomato, Beet
Mediterranean	Plastic Polytunnels, Parral-type	Ventilation, Shading	Inconsistent CO2, Heat Stress Management	Tomato, (Carrot/Beet with limits)
Tropical	Shade-Net Houses	Passive Cooling, Shading	Poor Light/CO2 Control, Humidity	Tomato (adjusted), Lettuce (cooled)
Subtropical	Shade-Net Houses, High-Tech (modified cooling)	Shading, Ventilation, Cooling	Managing High Light Intensity	Carrot, Beet, Tomato
Arid	High-Tech Greenhouses (Evaporative Cooling, Sealed)	Evaporative Cooling, CO2 Enrichment	Water Availability, High Cooling Load	Tomato, Carrot
All (Precision)	Vertical Farms, Plant Factories	Tunable LEDs, Precise CO2, AI Control	Space for Root Crops, Very High Capital Cost	L

). High-tech greenhouses provide superior control, particularly in challenging tropical and arid climates, while cost-effective polytunnels suit Mediterranean zones with adequate ventilation. Threats like heatwaves or resource limitations can be mitigated through advanced cooling, spectral optimization, and AI-driven systems. By tailoring these strategies to specific crops and climates, growers can achieve sustainable yields and quality, addressing global food demands with precision and resilience.

The response of microclimatic conditions for greenhouse-grown lettuce, tomatoes, carrots, and beets must account for the interacting influences of genotype, substrate, and regional context. Varieties or genotypes differ in their physiological thresholds for temperature, CO₂ assimilation, and light utilization, which determine crop-specific growth rates, photosynthetic efficiency, and biochemical quality. Substrate type—whether soil, hydroponic solution, perlite, or cocopeat—plays a critical role in regulating water retention, nutrient availability, and root aeration, thereby shaping how effectively plants exploit controlled microclimates. Regional implications are equally important, as temperate zones often require insulation and supplemental lighting, Mediterranean systems depend on ventilation to mitigate heat, tropical zones demand active cooling and humidity control, and arid regions rely on evaporative cooling and CO₂ supplementation. Together, these factors highlight that crop performance under greenhouse conditions cannot be generalized but must be interpreted in light of genotype-specific physiology, substrate characteristics, and regional constraints to achieve sustainable yield and quality outcomes.

5. Conclusions

This study elucidates optimal microclimatic conditions for lettuce, tomatoes, carrots, and beets, identifying temperature (15–25 °C), light intensity (150–300 μmol·m⁻²·s⁻¹, 12–18 h photoperiod), and CO₂ concentration (600–1300 ppm) as critical for maximizing growth, physiology, and biochemistry. Lettuce and beets thrive at 15–22 °C, tomatoes at 18–25 °C, and carrots at 15–20 °C, with light intensities of 200–250 μmol·m⁻²·s⁻¹ and CO₂ levels of 600–1100 ppm enhancing photosynthesis and biomass across crops. Climatic zones significantly influence these optima: temperate zones facilitate precise control, Mediterranean zones require ventilation to manage heat, tropical zones need cooling to counter high temperatures, subtropical zones balance moderate conditions, and arid zones demand evaporative cooling. Greenhouse types modulate these conditions, with high-tech Venlo glasshouses offering superior control, plastic polytunnels suiting cost-effective Mediterranean production, shade-net houses addressing tropical humidity, and vertical farms excelling for lettuce and tomatoes. The interplay of microclimatic factors drives enzymatic activity, photosynthetic efficiency, and metabolite synthesis, but threats like heatwaves and resource constraints necessitate advanced systems. By tailoring greenhouse technologies to climatic zones, growers can optimize yields and quality, ensuring sustainable vegetable production globally.