Possible Impacts of Elevated CO₂ and Temperature on Growth and Development of Grain Legumes

Highlights

• This review presents a summary of the literature on the effects of elevated CO₂ and temperature on grain legumes
• Fertilization effect of elevated CO₂ has a positive effect on photosynthesis and biomass growth
• Grain nutrition content of grain legumes can decrease
• Pollinators’ visits to flowering plants can be higher, enhancing pollination

Abstract

Carbon dioxide (CO₂) is the most abundant greenhouse gas (GHG) in the atmosphere and the substrate for the photosynthetic fixation of carbohydrates in plants. Increasing GHGs from anthropogenic emissions is warming the Earth’s atmospheric system at an alarming rate and changing its climate, which can affect photosynthesis and other biochemical reactions in crop plants favorably or unfavorably, depending on plant species. For the substrate role in plant carbon reduction reactions, CO₂ concentration ([CO₂]) in air potentially enhances photosynthesis. However, N uptake and availability for protein synthesis can be a potential limiting factor in enhanced biomass synthesis under enriched [CO₂] conditions across species. Legumes are C₃ plants and symbiotic N fixers and are expected to benefit from enhanced [CO₂] in the air. However, the concurrent increase in air temperatures with enhanced [CO₂] demands more detailed investigations on the effects of [CO₂] enhancement on grain legume growth and yield. In this article, we critically reviewed and presented the online literature on growth, phenology, photosynthetic rate, stomatal conductance, productivity, soil health, and insect behavior under elevated [CO₂] and temperature conditions. The review revealed that specific leaf weight, pod weight, and nodule number and weight increased significantly under elevated [CO₂] of up to 750 ppm. Under elevated [CO₂], two mechanisms that were affected were the photosynthesis rate (increased) and stomatal conductivity (decreased), which helped enhance water use efficiency in the C₃ legume plants to achieve higher yields. Exposure of legumes to elevated levels of [CO₂] when water stressed resulted in an increase of 58% in [CO₂] uptake, 73% in transpiration efficiency, and 41% in rubisco carboxylation and decreased stomatal conductance by 15–30%. The elevated [CO₂] enhanced the yields of soybean by 10–101%, peanut by 28–39%, mung bean by 20–28%, chickpea by 26–31%, and pigeon pea by 31–38% over ambient [CO₂]. However, seed nutritional qualities like protein, Zn, and Ca were significantly decreased. Increased soil temperatures stimulate microbial activity, spiking organic matter decomposition rates and nutrient release into the soil system. Elevated temperatures impact insect behavior through higher plant feeding rates, posing an enhanced risk of invasive pest attacks in legumes. However, further investigations on the potential interaction effects of elevated [CO₂] and temperatures and extreme climate events on growth, seed yields and nutritional qualities, soil health, and insect behavior are required to develop climate-resilient management practices through the development of novel genotypes, irrigation technologies, and fertilizer management for sustainable legume production systems.

1. Introduction

The primary greenhouse gases (GHGs) that warm the Earth’s atmospheric system are CO₂, methane (CH₄), and nitrous oxide (N₂O). GHGs emitted from anthropogenic activities absorb and retain more solar heat energy in the atmosphere, warming the earth’s surface more than usual and causing undesirable consequences to the global climate. Termed ‘climate change’, such changes can have both beneficial and detrimental effects on crop and food production systems worldwide from associated enhanced air temperatures and other extreme weather events [1,2]. Atmospheric [CO₂] is expected to increase significantly from 421 ppm in 2024 to about 550–700 ppm by 2050 and 670–940 ppm by 2100 [2,3]. Changes in plant stomatal behavior and photosynthetic efficiency are the main factors influencing how rising atmospheric CO₂ concentrations affect the metabolism of carbon and water in plants. Due to the well-known [CO₂] fertilization effects, elevated [CO₂] is frequently linked to higher biomass and crop yield. However, geographically, air temperatures during the crop growing season near the equator are already at, near, or above the upper optimum levels conducive for crop growth compared to places that are North and South of the equator. As such, areas closer to the equator face a decrease in production, and regions away from the equator are expected to increase production due to a further rise in temperatures [4,5]. The enhanced carbohydrates produced from the [CO₂] fertilization effect significantly impact the nutritional composition and overall quality of food crops [6]. As such, enhanced [CO₂] and air temperatures pose another major threat to global food security by adding uncertainty to the nutritional quality of plant-based food. In legumes, with elevated [CO₂], higher rates of photosynthesis and enhanced production in soybeans and peanuts have been reported [7,8]. Research shows that the increase in photosynthetic rate also increases the translocation rate from source to sink, resulting in higher grain yield returns [9]. However, a drop in tissue nitrogen (N) levels and grain protein concentration were also observed in legumes exposed to enhanced [CO₂], impacting seed qualities [10].

The impact of elevated [CO₂] on crops varies with different photosynthetic pathways such as C₃ vs. C₄ or CAM (crassulacean acid metabolism), plant varieties and cultivars, geographical locations, soil fertility, and climates. In general, C₃ crops stand to benefit from the [CO₂] fertilization effect in the Rubisco-catalyzed carbon fixation reactions in the Calvin–Benson–Bassam photosynthesis cycle. In contrast, the impact of an air [CO₂] enhancement mechanism mediated by the phosphoenolpyruvate enzyme before air enters the photosynthetic site in ] bundle sheath cells in C₄ plants hinders their chances to reap an equal benefit from the enhanced atmospheric [CO₂] like the C₃ species. Reported results, in general, show that the C₃ crops can have yield improvements between 20–35%, while the C₄ crop yield improvement can be between 0–15% [11]. Therefore, there is a growing need to further understand the impact of climate change factors, specifically the rising [CO₂] and air temperatures, on photosynthetic carbon assimilation and crop production. This understanding is crucial for predicting how these factors affect the growth and productivity of emerging crop cultivars and varieties for enhanced nutritional value and sustainable production.

Approximately 28 million km² of land is classified as dry regions, and 46 million km² is classified as semi-arid regions [12,13]. The air temperatures associated with GHG build-up in the atmosphere significantly impact water scarcity in those climates characterized by frequent droughts [14,15,16]. In those climates, although the enhanced stomatal resistance with [CO₂] can improve consumptive water demands of the crops grown, increased air temperatures combined with the existing drought stress have often led to uncertain crop responses [14,17,18,19,20]. Legumes, in general, are significant food crops because of their crucial role in enriching soil through the symbiotic fixation of atmospheric N [21,22,23]. Globally, grain legumes cultivated in rainfed agroecology are often water-limited and are more susceptible to the negative impacts of drought compared to cereal crops, especially with the associated rise in temperatures [5]. Under drought, legumes have an advantage over non-leguminous plants with elevated [CO₂] conditions because they have the potential to fix atmospheric N commensurate with increasing demands on the Calvin cycle [24,25,26,27,28].

In general, increased levels of [CO₂] have been found to enhance growth by increasing the rate of photosynthesis across crop species. However, the distribution of the additional biomass produced across different plant organs vary within and between species [29,30]. Studies conducted in open-top chambers and a free-air [CO₂] enrichment (FACE) experiment reported that the adverse effects of higher temperatures on crop growth could be offset by the fertilization effect of elevated [CO₂] [31,32]. The response of legume crops to elevated [CO₂] and temperatures, as reported in the literature, is illustrated and summarized in Figure 1. Elevated [CO₂] affects crop physiology and yield and impacts soil as a significant component of the soil–plant–atmospheric continuum.

Soil is a spatially heterogeneous body and constitutes minerals, organic matter, air, microorganisms, and water [33,34,35]. Temperature and elevated [CO₂] will directly and indirectly affect soil microbial activity, diversity, proliferation, nutrient cycle, and physical qualities, all critical for soil health and productivity. Consequently, increased soil temperatures will stimulate microbial activity, spiking organic matter decomposition rates and nutrient release into the soil system and atmosphere. For example, this can increase soil nitrification rates [36] and induce higher soil organic carbon (SOC) loss [37], negatively affecting the soil’s biological, chemical, and physical properties [38]. Likewise, climate change can impact pest and disease occurrences, host–pathogen interactions, distribution and ecology of insects, natural enemy populations, insect migration, and insect feeding habits, which may significantly setback agricultural production [39]. However, there is little or no evidence in the literature about soil health effects such as soil’s physical, chemical, and biological activities, pest resurgence, and insect feeding behavior on legumes under elevated [CO₂] and temperature. In the past, studies have reported a lack of investigations focused on elucidating how legume plants physiologically, in terms of growth and development, respond to elevated carbon dioxide levels and temperature in different environments with varied water, temperature, and humidity conditions [40,41]. Studies on the combined effects of increased [CO₂] and associated temperature on seed germination, phenology, and yield in legumes were also reported as lacking [42,43]. In this context, this review summarizes the available literature on the impacts of increased [CO₂] and temperatures on legume stomatal conductance, photosynthetic rate, biomass growth, phenology, productivity, seed nutritional qualities, soil health, and insect behavior. Therefore, the objective of this review article was to determine how future climates with elevated temperatures and [CO₂] in combination will affect grain legumes. The work was conducted based on a web of science collection of peer-reviewed papers, reports, and reviews using the search terms ‘legumes’, ‘elevated CO₂’, ‘temperature’, ‘growth chamber’, ‘photosynthetic rate’, ‘stomatal conductivity’, ‘seed nutrient content’, ‘soil health’, ‘soil nutrient status’, ‘insect behavior’, and ‘pollinator response’. Based on the search items, 167 research and review articles were found, and evidence from 142 articles was utilized in this review article. The review can help researchers develop agronomic strategies to alleviate the impacts of climate change while also assisting breeders in developing novel genotypes that are more adaptable to the effects of climate change.

2. Mechanisms of Plant Responses to Elevated [CO₂] and Temperature Stresses

Higher temperatures and increased CO₂ concentrations can affect plant growth, metabolism, and crop productivity. Increased formation of reactive oxygen species (ROS) such as superoxide (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (OH⁻) can result from elevated temperatures. These ROS can oxidatively damage cellular structures in plants [44]. Similarly, heat stress affects photosystem II (PSII) more than photosystem I (PSI). The oxygen-evolving complex (OEC) involved in water splitting and oxygen evolution in PSII can be harmed by high temperatures [45]. Thus, reduced photosynthetic rates and a decline in electron transport efficiency are the outcomes of this damage. However, plants use adaptive mechanisms through physiological, biochemical, and molecular defenses, such as osmotic adjustments, protein accumulation, gene expression alterations, and antioxidant defense, to deal with these challenges. These responses can vary depending on crop type, species, stress intensity, and duration of exposure. Therefore, the primary response to heat stress and photodamage is the repair of the PSII reaction center, particularly the D1 protein. High temperatures often lead to the degradation of the D1 protein in PSII, causing photoinhibition. Upon recovery, plants increase the turnover of the D1 protein to restore PSII activity [46]. Similarly, plants synthesize heat shock proteins (HSPs) in response to elevated temperatures, which help in protein refolding, stabilization, and repair [47]. However, these defense mechanisms have been well-studied under high-temperature stress in plants. The combined effects of elevated [CO₂] and temperature on mechanisms of plant responses need to be explored.

Plants use non-enzymatic antioxidants like ascorbate and glutathione as well as enzymatic antioxidants like superoxide dismutase (SOD), catalase (CAT), and peroxidases (POD) to lessen the harmful effects of ROS [48]. These systems support cellular integrity preservation and ROS detoxification [49]. Likewise, some genes that code for proteins are frequently upregulated under heat-stress conditions. Under such conditions, plants are shielded by producing HSPs [50,51]. It is well known that proline, an amino acid, builds up in response to several environmental stressors, such as heat and high [CO₂]. By stabilizing proteins and cellular membranes, proline acts as an osmo-protectant, shielding plants from osmotic stress [52,53]. Thus, proline buildup may be induced by increased [CO₂], which could serve as an adaptive response to lessen heat stress. Elevated [CO₂] typically stimulates carbohydrate production, and in combination with temperature stress, plants may accumulate sugars and polyols, which serve as osmolytes and protect cells from dehydration [54]. These reactions, however, might be complicated by the interplay between high [CO₂] and temperature stress. These mechanisms were well established when the plants were exposed to single-factor stresses like heat or [CO₂] stress. Furthermore, these mechanisms need to be established with future elevated [CO₂] and high temperature studies in grain legumes.

3. Elevated [CO₂] and Temperature Effects on Crop Growth and Development

3.1. Crop Phenology

Legumes have previously been reported to respond to elevated [CO₂] levels depending on their growth stage. Studies by Srivastava et al. [55] and Das et al. [56] reported more pronounced effects during the early growth stages (0–20 days after germination) compared to the maturity stages. In general, legume plants were consistently exposed to elevated [CO₂] until they reached maturity, with enhanced plant growth due to increased photosynthesis. This was attributed to decreased dark respiration during the early crop growth stages, but the mechanisms involved were not explained. Elevated [CO₂] in chickpeas resulted in an accelerated process of reproductive development and advanced the initiation of flowering by 3–4 days [57]. However, when the elevated [CO₂] was combined with increased levels of ozone (O₃), a decrease of 2–3 days in the pod initiation stage was observed, attributed to enhanced translocation to leaves.

The impact of [CO₂] and temperature on the phenology of legumes, as reported in the literature, was inconsistent. Some studies reported that plants exposed to 700 ppm [CO₂] levels did not have any effect on Faba bean [58] and cowpea phenology [59]. However, other studies reported that increased [CO₂] accelerated the time to flowering in soybeans and decreased the time to pod development [60,61,62]. Consequently, legume plants that exhibited delayed phenology also experienced a reduced crop growth rate when exposed to higher [CO₂].

A rise in temperature from 29 °C/23 °C to 32 °C/29 °C (day–night cycle) decreased the length of the vegetative period by seven days in different soybean genotypes [20]. Renato et al. [63] reported a decrease of 11 days in the growth cycle of field beans when air temperature alone was increased by 5 °C. Similarly, greater temperature levels decreased cowpea growth by 14 and 23 days [20,63]. To summarize, responses of legumes to increased [CO₂] and temperature in reported studies varied across different locations and types of legumes. The exact mechanisms that lead to the observed differences in crop duration in response to [CO₂] at various stages of plant development remain unclear.

3.2. Photosynthesis

Elevated [CO₂] significantly impacts plant growth and productivity, particularly in crop species that use the C₃ photosynthetic pathway, by reducing rates of photorespiration in the photosynthetic carbon reduction reactions of the Calvin cycle [6,20,64,65]. Legume crops, in general, are known to be C₃ plants. Such photosynthetic [CO₂] acclimation response in C₃ crops is attributed to the increased carboxylation and decreased oxylation efficiency of the RuBP carboxylase oxygenase enzyme from enhanced [CO₂] levels in the air [66,67]. In this context, legumes such as peanuts, pigeon peas, soybeans, mung beans, and cowpeas were found to be highly sensitive to elevated [CO₂] [14,18,28,62,68,69,70,71,72]. Typically, a significant quantity of chlorophyll a and b absorbs energy from photosynthetically active radiation (PAR) to be used in photosynthesis carbon reduction (PCR) activities [73]. During this process, carotenoids serve to shield the reaction center from excessive light and aid in capturing PAR by directing it towards auxiliary pigments of chlorophyll a. Hence, alterations in the pigment concentration in leaves are closely associated with the physiological condition and, consequently, the production of crops [74]. Since most research has been conducted in controlled conditions, the effects of elevated [CO₂] on plant growth and development may not be the same as those experienced in field situations [75,76]. For instance, limited root growth constrains the plant’s ability to respond to elevated [CO₂]. Another primary mechanism frequently used to explain the increased yield under elevated [CO₂] is the reduction in stomatal conductance, which reduces transpiration water loss during drought [41,73,77,78,79].

The combined effects of enhanced [CO₂] and temperature on stomatal conductance and photosynthetic rates under optimal light conditions were also reported to be more evident during drought stress [14]. They found that long-term exposure of peanut plants to elevated levels of [CO₂] when water stressed resulted in an increase of 58% in [CO₂] uptake, 73% in transpiration efficiency, and 41% in rubisco carboxylation (Table 1). Grain legumes respond differently to [CO₂] fertilization depending on soil type, nutrient availability, water availability, and interactions with other environmental stresses. Elevated CO² concentrations can accelerate photosynthesis, resulting in heightened biomass generation and possible yield increases but lowering vital nutrients such as vitamins, minerals, and proteins. When the physiological and biochemical responses of crops to elevated [CO₂] are varied depending on their photosynthetic pathway, leaf photosynthetic pigments and antioxidative system are also synchronized [17,80,81]. In addition to reduced stomatal conductance, plants undergo physiological adaptations to improve their ability to withstand drought by generating isoflavones at vegetative and reproductive stages, as observed in soybeans under elevated [CO₂] [6]. The production of isoflavones, which follow the phenylpropanoid pathway, uses a substantial amount of carbon [82]. As such, through this mechanism in legumes, it may be possible to sequester larger quantities of atmospheric carbon under future elevated [CO₂] levels [83,84].

Increasing [CO₂] levels and higher temperatures may have a cumulative effect in intensifying heat stress, partially due to decreased transpirational cooling from enhanced stomatal resistance [31,85]. Enhanced [CO₂] at 740 ppm was shown to decrease stomatal conductance by 13–30%, causing a 2 °C increase in leaf temperature [73]. High temperatures can directly harm the photosynthetic apparatus, particularly PS-II [80]. Several locations within the chloroplast membranes susceptible to heat-induced damage have been identified [86,87,88]. When exposed to greater temperatures, plants redistribute their resources from shoots to roots [89,90,91]. The adverse impacts of elevated temperatures on crops vary depending on their optimum temperatures for the growth and development processes. Legume growth and development impacts from heat stress produced by the combined effects of enhanced [CO₂] and temperature need further investigation.

Photosystem II (PSII) and photosystem I (PSI) are pigment–protein mega complexes embedded in the thylakoid membranes that drive photosynthesis, which is a crucial process for plant growth. High temperatures and high [CO₂] levels can influence PSII and PSI functions in complex ways. To increase plant resistance to changing climatic conditions, it is essential to comprehend how these two photosystems react to such challenges. PSII is especially vulnerable to heat stress due to the breakdown of the OEC [45]. Similarly, the D1 protein, a crucial part of PSII, is extremely heat-sensitive and may break down more quickly at higher temperatures, which could impair PSII function and cause photoinhibition [92]. To fix PSII, plants respond by increasing the turnover rate of D1, but excessive heat can affect this repair system. Even though PSI is a little more stable than PSII, it can still be less effective in heat stress. Thus, PSI’s overall efficiency may be decreased by high temperatures that affect the electron transport chain [93]. Even though heat-induced damage to PSI is usually not as bad as it is to PSII, it can still reduce total photosynthetic efficiency, especially when exposed to heat for an extended period. Because more CO₂ is available for carbon fixation, photosynthesis typically becomes more efficient under higher [CO₂] conditions. Higher [CO₂] usually increases PSI activity by increasing photosynthesis’s overall efficiency because more electrons are directed through PSI to reduce NADP⁺ [94]. This is especially advantageous for C₃ plants, as photosynthesis is frequently hampered by [CO₂] limitation in ambient conditions [95]. However, if high temperatures are prolonged, the recovery of photosynthetic efficiency might take longer, particularly in species sensitive to heat [96]. Therefore, the detrimental effects of heat can offset the beneficial effects of [CO₂] enrichment on PSI function in extreme situations where high [CO₂] and temperature stresses co-occur. Further, these mechanisms need to be established with a combination of future elevated [CO₂] and high-temperature studies in grain legumes.

3.3. Growth and Productivity

Under elevated [CO₂] conditions, seed vigor and germination rates have been reported to increase, decrease, or remain unchanged [42,43,97]. In these studies, impacts on various aspects of plant growth in peanuts [14,28,68,72], mung beans [31,45,71,98,99], peas [19,24], soybeans [6,79], pigeon peas [70], faba beans [16], cowpeas [20,69], chickpeas [46], and black gram [100] were investigated, measuring crop growth parameters such as leaf area, leaf size, number of nodules, specific leaf weight, shoot weight, number of pods, and seed yield. Increased levels of [CO₂] in general have been reported to have beneficial effects on legume crop growth, development, and productivity (Table 1). Madhu and Hatfield [62] noticed a reduction in the number of soybean leaves by 14–23% and leaf area by 9.7–11.7% under increased [CO₂] alone. Moreover, higher temperatures reduced photosynthesis and stomatal conductance, producing smaller leaves in plants [89,101,102]. In legumes, nodule growth is a significant process that fixes atmospheric nitrogen. However, the nitrogen fixation process in nodules is restricted by many environmental factors that control the flow of carbon into them [24]. Plants exposed to elevated [CO₂] showed a significant increase in the number of root nodules by 114% in soybeans [62], 22% in faba beans [16], and 38% in pigeon peas [1]. Concurrently, the nodules of peas under high [CO₂] conditions had a 38% greater leghemoglobin content [24]. The potential beneficial impacts of elevated [CO₂] fertilization on grain legume yields may be attributed to improved soil health indicators with better nutrient cycling, as discussed in Section 3.6. Nevertheless, the influence of increased carbon dioxide levels on the productivity of grain legumes, including the concurrent effects of drought and soil quality, remains unclear. Heat stress can have detrimental effects on plants at all stages of growth and development. During the vegetative stage, it can hinder the growth of shoots and roots. During the reproductive stage, it can lead to pollen sterility and flower shedding [103]. Heat stress in beans, peas, and soybeans was studied, and these plants at the seedling stage exposed to elevated temperature of 28 °C for eight days resulted in 50–88% and 36–38% reduced germination percent/dead seeds in soybeans and beans, respectively [104]. Likewise, mung bean seeds treated for 10, 20, and 30 min at 50 °C significantly dropped seed germination [105]. According to Chakraborty and Pradhan [106], lentil seeds exposed to 35–40 °C for four hours showed less germination and slower seedling growth. Similarly, heat stress decreases the number of flowering branches and flowers in peanuts [107]. Therefore, due to the drastic reduction in germination, reduced growth and development under elevated temperatures by 2–3 °C significantly affect seed yield in peanuts [107], mung beans [108], lentils [109,110], and chickpeas [111].

Table 1. Effect of elevated CO₂ concentration ([CO₂]) and temperature in the atmosphere on crop yield and photosynthetic characters in different crops.

Crop	Method	[CO2] Level	Temperature Level and Response	Elevated CO2 Response on Yield	Elevated CO2 Response on Photosynthesis	Reference
Peanut (Arachis hypogaea L.)	Canopy Evapotranspiration And Assimilation Chamber	AC: 400 ppm EC: 650 ppm	None	Increased pod yield by 39% under water stress.	Increased 41% Rubisco efficiency, reduced 16% leaf N.	[14]
Peanut (Arachis hypogaea L.)	Open Top Chambers (OTC)	AC: 380 ppm EC: 550 ppm EC: 700 ppm	None	Decreased kernel yield by 32% at 550 ppm and 28% at 700 ppm. Pod yield unaffected.	-	[72]
Peanut (Arachis hypogaea L.)	OTC	AC: 375 ppm EC: 548 ppm EC: 730 ppm	None	Increased pod number by 16%.	Increased net photosynthesis by 23%. Reduced stomatal conductance by 42%.	[68]
Mung bean (Vigna radiata L.)	Free-Air Carbon Dioxide Enrichment (FACE)	AC: 400 ppm EC: 550 ppm	None	Increased BM by 34%, seed yield by 34–50%.	-	[28]
Mung bean (Vigna radiata L.)	FACE: Open Air Condition	AC: 400 ppm EC: 550 ± 17 ppm	None	Increased BM 12%, seed yield 14%.	Increased net photosynthesis by 7–19%, Chl and total Chl content by 11–12%	[71]
Mung bean (Vigna radiata L.)	Growth Chamber (Model ATC26, Winnipeg, MB).	AC: 400 ppm EC: 700 ppm	Day/night: 6/22 °C, 32/28 °C,	Higher stem and leaf biomass, higher transpiration.	-	[31]
Mung bean (Vigna radiata L.)	OTC	EC: 600 ± 50 ppm	None	[CO2] exposure of 0–20 days caused 28–35% higher shoot growth.	Decreases in Chl a by 10% and 18% at 15 and 35 days after germination.	[56]
Mung bean (Vigna radiata L.)	OTC	AC: Field experiment EC: 700 ppm	None	Increased seed number and weight by 34.6% and 25%.	It increased photosynthesis by 25–29%, chloride by 30–39%, and carotenoid production by 8–15%.	[98]
Mung bean (Vigna radiata L.)	FACE	AC: 411 ± 15 ppm EC: 550 ± 19 ppm	None	Increase in pod BM by 26%, seed yield by 26%, and plant BM by 17%.	-	[99]
Pea (Pisum sativum)	Australian Grains Free-Air [CO2] Enrichment (AGFACE)	AC: 400 ppm EC: 550 ppm	None	Increased 33% number of nodules	Reduced stomatal conductance by 44% with lower amino acid.	[19]
Pea (Pisum sativum)	OTC	AC: 360 ppm EC: 700 ppm EC: 1000 ppm	None	Increase BM by 40% at 1000 ppm and leghemoglobin by 38%.	No response.	[24]
Faba bean (Vicia faba L.)	Australian Grains Free-Air [CO2] Enrichment	AC: 400 ppm EC: 550 ppm	None	Increased seed yield by 58% under irrigated and 23% under drought.	-	[16]
Pigeon pea (Cajanus cajan L.)	OTC	AC: 380 ppm EC: 580 ppm	None	Increased yield and pods by 12% and 76% (Pusa-992 genotype). Decrease seed yield by 33% (PS-2009 genotype).	-	[70]
Cowpea (Vigna Unguiculata)	Growth Chambers	AC: 370 ppm EC: 550 ppm	day/night: 26/20 °C, 29/23 °C, 32/29 °C	Increased seed number and weight by 21 and 23%.	Decreased pod number and weight by 23 and 24% under 32/29 °C and 29/23 °C.	[20]
Soybean (Glycine max (L.) Merr.)	Rhizotron Chambers	AC: 380 ppm EC: 800 ppm	None	Decreased number of leaves by 14–23%, leaf area by 10–12%. Increased number of pods by 55%.	-	[62]
Soybean (Glycine max (L.) Merr.)	OTC	AC: 390 ± 30 ppm EC: 550 ± 30 ppm	None	Reduced seed protein by 2–6% in different genotypes.	-	[6]
Soybean (Glycine max (L.) Merr.)	OTC	AC: 410 ppm EC: 610 ppm	None	Seed yield with increases of 101% and 65% in high-WUE and low-WUE, respectively.	Increased canopy photosynthesis by 37% (high-WUE genotype) and 76.3% (low-WUE genotype).	[79]
Chickpea (Cicer arietinum L.)	FAOCE (Free-Air Ozone [CO2] Enrichment) Chambers	AC: ambient. EC: 550 ± 25 ppm	None	Increased seed yield by 26–31%	Increased net photosynthetic rate by 11–17%.	[57]
Black gram (Vigna mungo)	OTC	AC: 365 ppm EC: 550 ppm and 700 ppm	None	Higher BM by 65% at 700 ppm and 39% at 550 ppm. Higher HI by 39% at 550 ppm and 40% at 700 ppm.	-	[100]

Note: OTC: open top chambers; FACE: free-air [CO₂] enrichment; AC: ambient [CO₂]; EC: elevated [CO₂]; HI: harvest index; BM: biomass; Chl: chlorophyll; WUE: water use efficient.

Table 1. Effect of elevated CO₂ concentration ([CO₂]) and temperature in the atmosphere on crop yield and photosynthetic characters in different crops.

Crop	Method	[CO2] Level	Temperature Level and Response	Elevated CO2 Response on Yield	Elevated CO2 Response on Photosynthesis	Reference
Peanut (Arachis hypogaea L.)	Canopy Evapotranspiration And Assimilation Chamber	AC: 400 ppm EC: 650 ppm	None	Increased pod yield by 39% under water stress.	Increased 41% Rubisco efficiency, reduced 16% leaf N.	[14]
Peanut (Arachis hypogaea L.)	Open Top Chambers (OTC)	AC: 380 ppm EC: 550 ppm EC: 700 ppm	None	Decreased kernel yield by 32% at 550 ppm and 28% at 700 ppm. Pod yield unaffected.	-	[72]
Peanut (Arachis hypogaea L.)	OTC	AC: 375 ppm EC: 548 ppm EC: 730 ppm	None	Increased pod number by 16%.	Increased net photosynthesis by 23%. Reduced stomatal conductance by 42%.	[68]
Mung bean (Vigna radiata L.)	Free-Air Carbon Dioxide Enrichment (FACE)	AC: 400 ppm EC: 550 ppm	None	Increased BM by 34%, seed yield by 34–50%.	-	[28]
Mung bean (Vigna radiata L.)	FACE: Open Air Condition	AC: 400 ppm EC: 550 ± 17 ppm	None	Increased BM 12%, seed yield 14%.	Increased net photosynthesis by 7–19%, Chl and total Chl content by 11–12%	[71]
Mung bean (Vigna radiata L.)	Growth Chamber (Model ATC26, Winnipeg, MB).	AC: 400 ppm EC: 700 ppm	Day/night: 6/22 °C, 32/28 °C,	Higher stem and leaf biomass, higher transpiration.	-	[31]
Mung bean (Vigna radiata L.)	OTC	EC: 600 ± 50 ppm	None	[CO2] exposure of 0–20 days caused 28–35% higher shoot growth.	Decreases in Chl a by 10% and 18% at 15 and 35 days after germination.	[56]
Mung bean (Vigna radiata L.)	OTC	AC: Field experiment EC: 700 ppm	None	Increased seed number and weight by 34.6% and 25%.	It increased photosynthesis by 25–29%, chloride by 30–39%, and carotenoid production by 8–15%.	[98]
Mung bean (Vigna radiata L.)	FACE	AC: 411 ± 15 ppm EC: 550 ± 19 ppm	None	Increase in pod BM by 26%, seed yield by 26%, and plant BM by 17%.	-	[99]
Pea (Pisum sativum)	Australian Grains Free-Air [CO2] Enrichment (AGFACE)	AC: 400 ppm EC: 550 ppm	None	Increased 33% number of nodules	Reduced stomatal conductance by 44% with lower amino acid.	[19]
Pea (Pisum sativum)	OTC	AC: 360 ppm EC: 700 ppm EC: 1000 ppm	None	Increase BM by 40% at 1000 ppm and leghemoglobin by 38%.	No response.	[24]
Faba bean (Vicia faba L.)	Australian Grains Free-Air [CO2] Enrichment	AC: 400 ppm EC: 550 ppm	None	Increased seed yield by 58% under irrigated and 23% under drought.	-	[16]
Pigeon pea (Cajanus cajan L.)	OTC	AC: 380 ppm EC: 580 ppm	None	Increased yield and pods by 12% and 76% (Pusa-992 genotype). Decrease seed yield by 33% (PS-2009 genotype).	-	[70]
Cowpea (Vigna Unguiculata)	Growth Chambers	AC: 370 ppm EC: 550 ppm	day/night: 26/20 °C, 29/23 °C, 32/29 °C	Increased seed number and weight by 21 and 23%.	Decreased pod number and weight by 23 and 24% under 32/29 °C and 29/23 °C.	[20]
Soybean (Glycine max (L.) Merr.)	Rhizotron Chambers	AC: 380 ppm EC: 800 ppm	None	Decreased number of leaves by 14–23%, leaf area by 10–12%. Increased number of pods by 55%.	-	[62]
Soybean (Glycine max (L.) Merr.)	OTC	AC: 390 ± 30 ppm EC: 550 ± 30 ppm	None	Reduced seed protein by 2–6% in different genotypes.	-	[6]
Soybean (Glycine max (L.) Merr.)	OTC	AC: 410 ppm EC: 610 ppm	None	Seed yield with increases of 101% and 65% in high-WUE and low-WUE, respectively.	Increased canopy photosynthesis by 37% (high-WUE genotype) and 76.3% (low-WUE genotype).	[79]
Chickpea (Cicer arietinum L.)	FAOCE (Free-Air Ozone [CO2] Enrichment) Chambers	AC: ambient. EC: 550 ± 25 ppm	None	Increased seed yield by 26–31%	Increased net photosynthetic rate by 11–17%.	[57]
Black gram (Vigna mungo)	OTC	AC: 365 ppm EC: 550 ppm and 700 ppm	None	Higher BM by 65% at 700 ppm and 39% at 550 ppm. Higher HI by 39% at 550 ppm and 40% at 700 ppm.	-	[100]

Note: OTC: open top chambers; FACE: free-air [CO₂] enrichment; AC: ambient [CO₂]; EC: elevated [CO₂]; HI: harvest index; BM: biomass; Chl: chlorophyll; WUE: water use efficient.

Soybean seed yield responded significantly to elevated [CO₂] and an associated temperature rise, ranging from 10–95% [112,113] to 101% [79]. Laza et al. [14] reported a 39% increase in peanut pod production and a 58% increase in above-ground biomass when grown under enriched [CO₂] in field-based canopy evapotranspiration and assimilation (CETA) chambers (Table 1). However, Bagudam et al. [72] reported a 32% and 28% decrease in peanut kernel yield in response to 550 and 700 ppm [CO₂], respectively, in open-top chambers (Table 1). They also reported a decrease in kernel oil by 6.54% and 2.98% and protein content by 7.07% and 4.56%, respectively. Ji et al. [71] demonstrated that increased [CO₂] had a beneficial impact on mung bean yield; however, the photosynthetic capability of the plants was significantly inhibited. Pigeon pea cultivars exhibited 52.3% more radiation use efficiency (RUE) under enhanced [CO₂] levels [70]. At the stage of pod development, increased levels of [CO₂] resulted in a reduction of the leaf nitrogen (N) concentration by 4.4–13.9%, and increased phosphorus (P) and potassium (K) uptake by 13.2% and 22.4%, respectively [99]. Enzymes such as sucrose synthase, pyrophosphorylase, and phosphoenolpyruvate carboxylase substantially increased, ranging from 33% to 50% [24]. Nevertheless, no notable disparity was observed in the activity of enzymes such as alkaline invertase and malate dehydrogenase. The overall responses to elevated [CO₂] and temperature in legume crops and its associated impacts on plant growth and development attributes are depicted in Figure 2.

3.4. Grain Nutritional Content

The impacts of elevated [CO₂] levels and temperatures on the nutritional content of seeds in legume crops have not been investigated in depth. In general, the reported studies showed a decline in seed protein content under elevated [CO₂] levels, unlike cereals, due to an imbalance in N fixation, which results in lower N concentrations [10,114]. Studies conducted under elevated [CO₂] conditions alone also showed decreased N concentration in seeds [6,19,71,115,116]. The decrease in N levels in plant organs may be attributed to increased carbohydrate buildup incommensurate with the N fixation levels under elevated [CO₂] [115,117]. Weigel and Manderscheid [118] found that the decrease in protein content under elevated [CO₂] could not be mitigated by providing more N fertilizer.

Burkey et al. [68] and Li et al. [6] reported that elevated [CO₂] significantly enhanced soybean oil quality by increasing oleic acid and decreasing linoleic acid contents. Soba et al. [79] found decreased concentrations of calcium (Ca) and boron (B) in high-water-use efficient genotypes of soybeans and reduced levels of phosphorus (P) and zinc (Zn) in low-water-use efficient genotypes. Kennedy [119] documented a decrease in the levels of iron and zinc in soybeans and field peas, causing a considerable reduction in the overall concentration of free amino acids [6]. The iron and manganese content of peanut kernels were not impacted. In contrast, the copper content decreased by 13.93% and 26.19%, and the calcium content decreased by 24.33% and 8.20% at elevated [CO₂] levels of 550 and 700 ppm, respectively [72]. Mishra and Agrawal [98] reported reductions in soluble protein content by 9.9% and sugars by 8.9 to 9.4%. An increase in total soluble sugars (9.3–15.1%) and starch content (15.5%) was also observed. Therefore, dietary nutrient deficiencies can become more apparent under elevated [CO₂] levels in the future. Research conducted by Martel and Qaderi [120] demonstrated that elevated temperatures have a detrimental impact on the nitrogen balance index (NBI) in pea plants (Pisum sativum).

3.5. Soil Moisture or Drought Stress

Increased levels of [CO₂] when soil moisture levels were nonlimiting resulted in a significant increase in the number of pods (54.8% to 122.4%) and weight of dried pods (29.8% to 56.6%) in soybeans [62]. Soil moisture availability and vapor pressure deficit often significantly impact photosynthesis and the stomatal conductance in plants grown under enhanced [CO₂] [14,73,121,122,123]. Increases in [CO₂] levels enhance the water status of legume plants by decreasing stomatal conductance, which helps increase WUE. This helps mitigate the negative impacts of drought stress on plant growth and other physiological processes [73,77,79,124,125,126]. However, Saha et al. [70] reported 10.7% more cumulative soil moisture depletions in plants grown under enhanced [CO₂] levels. This suggests that plants under elevated [CO₂] consume more soil water for enhanced photosynthesis rates. Further investigation can further elucidate the relationship between leaf area, stomatal conductance, and crop usage efficiency [73].

3.6. Soil Health

The microbial activity, diversity, proliferation, nutrient cycle, and the soil’s physical characteristics are directly and indirectly impacted by temperature and rising [CO₂] levels [126,127]. Increased soil temperatures stimulate microbial activity, spiking organic matter decomposition rates and nutrient release into the soil system and atmosphere. For example, increased soil nitrification rates [36] and higher soil organic carbon (SOC) loss [37] negatively affect the soil’s biological, chemical, and physical properties [38]. Net accumulation of soil organic matter positively affects soil physical properties through soil particle aggregation, the process upon which soil structure develops [128,129]. As the frequency of extreme drought and flooding scenarios increases due to climate change, nitrification and denitrification processes will become important, as flooding creates anaerobic conditions conducive to denitrification, losing N to the atmosphere [36,130,131]. In general, enhanced plant development and root exudation typically result in larger soil organic carbon inputs when atmospheric [CO₂] levels are higher [132]. This additional carbon can stimulate microbial activity by expanding the substrate available for breakdown. Similarly, increasing temperatures directly affect the microbiological activity of soil, generally accelerating the rates of decomposition and nutrient cycling. Temperature-induced improvements in microbial metabolism result in faster organic matter decomposition and better nutrient mineralization [133]. In addition to increasing soil respiration and enhancing plant nutrient availability, this may cause soils to release more CO₂ into the atmosphere. For example, increased decomposition may lead to increased nutrient availability; but, if soil moisture is insufficient or if severe temperatures negatively affect microbial populations, nutrient uptake may be limited [134]. Studies on understanding the direct impact of elevated [CO₂] and temperatures on soil-plant-microbiome interactions and other soil physical, chemical, and biological processes in legumes are lacking in the literature.

3.7. Legume Insect Pests

Insect herbivores’ interactions with their host plants may be impacted by elevated [CO₂] levels [135,136]. The insect species Spodoptera litura exhibited increased feeding rates on mung bean leaves due to their low nutrient content under elevated [CO₂] [137]. Likewise, increased temperature and [CO₂] significantly impacted lepidopteran pest behavior [138,139]. However, such impacts on insect growth were species-specific. With elevated [CO₂], soluble amino acids and protein in Phaseolus vulgaris and other legumes increased, which also improved the physiology of thrips and aphids during their larval stages [140,141]. The size of the aphid population on soybean plants increased significantly at 550 ppm [CO₂], leading to a substantial loss in yield [142]. The parasitism rate of Aphidius avenae decreased under elevated [CO₂], which impaired its ability to produce parasitoids [143]. Since insects are cold-blooded organisms, high temperatures profoundly impact insect behavior, longevity, and reproduction. Incidence of Helicoverpa armigera and Maruca vitrata in pigeon peas increased with rising temperatures [144]. Spodoptera litura’s peanut leaf-feeding rate increased with elevated temperature and [CO₂] [145]. At higher temperatures, Popillia japonica (Japanese beetle) caused severe leaf damage in soybeans [146]. In summary, an increased risk of invasive pests attacking legume crops is expected under elevated temperatures and [CO₂].

For efficient pollination, legumes like beans, peas, and lentils frequently depend on pollinators. Pollinators help in fertilization and seed formation by moving pollen from one flower to another. Reduced pollination efficiency may result from a drop in pollinator populations. In crops like soybeans and chickpeas, a decline in pollinator population can lead to a reduction in total yield and fewer seeds per pod [147]. Ensuring high agricultural output and optimizing seed set depend heavily on pollination efficiency. By encouraging cross-pollination, which expands the genetic pool of legume crops, pollinators support genetic variety. Plants with greater genetic diversity are more likely to be able to tolerate environmental challenges, diseases, and pests [148]. Similarly, elevated temperature and [CO₂] levels would potentially affect pollinators’ behavior in leguminous crops. Honeybee pollinator visits to flowers increased under elevated [CO₂] and temperature, leading to higher field bean pollination and seed set [149]. Heat stress from elevated air temperatures can also significantly affect the interaction between pollinators and legumes. Otieno et al. [149] observed a significant increase in pollination rate in Vicia faba beans under elevated temperature and [CO₂] conditions. Further research is required to understand how the elevation in [CO₂] and temperature influences mutualistic relationships of insect pests in legumes.

4. Gaps in Existing Research

Inconsistencies in phenological responses were observed in C₃ legumes. For instance, some studies report accelerated flowering and pod development in legumes under elevated CO₂ (e.g., chickpeas), while others find negligible effects or even delays (e.g., faba beans, cowpeas). A significant gap exists in photosynthetic processes between controlled environment studies and real-world field conditions. Similarly, research shows variable effects of elevated [CO₂] on seed vigor, gemination, crop yield, and quality. The inconsistent results indicate a lack of knowledge about the responses of different genotypes and species of legumes to increased [CO₂] at different phases of growth. Legumes’ reactions to increased [CO₂] in controlled environments, however, might not fully translate to field conditions where additional factors (such as soil type, and bug and pest pressure) are involved. Simultaneously, less is known about the combined effects of increasing [CO₂] and temperature on soil microbial communities and their functions in the cycling of nutrients, legume yield, and quality. Long-term research is required to evaluate the effects of persistently high [CO₂] and temperature on soil fertility and health over time.

5. Conclusions

The reviewed literature generally shows a negative impact of rising temperatures caused by elevated atmospheric GHGs on agricultural productivity worldwide. However, while the fertilization effect of increasing [CO₂] levels on crop growth is evident, the combined impact of rising temperatures and [CO₂] levels on legume crops was less investigated. Under elevated [CO₂] alone, two mechanisms affected were photosynthesis rate (increased) and stomatal conductivity (decreased), which helped enhance legume productivity and water use efficiency. Only a few studies have focused on investigating the nutritional status of legumes under elevated [CO₂] and temperature conditions. Seed nutritional qualities like protein, Zn, and Ca were significantly decreased under elevated [CO₂]. Increased soil temperatures enhance microbial activity, resulting in faster organic matter decomposition and nutrient release into the soil system while negatively altering the soil’s biological, chemical, and physical properties. However, less evidence about soil health indices under high [CO₂] is available. Enhanced [CO₂] and temperature significantly impact herbivore survival and natural enemy populations in legumes. Understanding the pest biology and trophic-level interactions under changing climate conditions is essential to develop integrated pest management strategies against major legume pests. Breeding genotypes appropriate for elevated [CO₂] and temperature are required for adaptation. Further studies on the growth and development of legumes exposed to the combined effects of enhanced [CO₂] and temperature under varying water availability and nutrient supply scenarios are required to adapt to the changing climates and lessen the impacts on production.