Mungbean, also referred to as green gram, is an ancient pulse crop extensively grown in diverse agro-ecological environments across India, primarily during the Kharif and summer seasons (Kumar et al., 2024). It is a diploid species with a specific chromosome number, belonging to the Leguminosae family and the Papilionaceae sub-family and is botanically classified as [Vigna radiata (L.) Wilczek] (Anita et al., 2022). Originating in South Asia, Vigna radiata var. sublobata is considered the potential progenitor of mungbean. This crop exhibits a predominantly self-pollinating nature (Singh et al., 2015).
       
Mungbean is a significant pulse crop grown for its high protein content and nitrogen-fixing ability, making it a crucial component of sustainable cropping systems. However, its productivity is often constrained by varying environmental conditions, particularly water stress, which affects physiological traits such as photosynthesis and water use efficiency (WUE). Photosynthesis is the key physiological process influencing crop yield. The net photosynthetic rate (Pn) represents the plant’s ability to assimilate carbon, while transpiration rate (E) indicates water loss via stomata. The balance between these two parameters defines WUE, a critical trait for drought tolerance. Several studies have reported that selecting genotypes with high WUE and stable Pn can improve productivity under water-limited environments (Islam et al., 2018; Rahman et al., 2023).
       
Water scarcity, a prevalent issue in arid and semi-arid areas, is one of the primary limitations to agricultural production. Drought stress affects key physiological processes in plants, including photosynthesis, stomatal conductance and transpiration (Farooq et al., 2009). In mungbean, drought conditions can lead to reduced growth, lower yields and poor seed quality (Singh et al., 2017). As climate change exacerbates these conditions, developing drought-tolerant mungbean genotypes that can maintain productivity under water-limited environments has become essential (Gupta et al., 2020).
       
The use of infra-red gas analyzers (IRGAs) provides a precise method for evaluating plant responses to water stress by measuring CO₂ exchange rates, photosynthesis, stomatal conductance and water-use efficiency (WUE). IRGA technology has been widely employed in plant physiology research to assess the impact of drought and other environmental stresses on gas exchange processes (Flexas et al., 2014). By identifying genotypes that can sustain higher photosynthetic rates and improved WUE under drought conditions, IRGAs enable researchers to screen large numbers of plants efficiently (Farquhar and Sharkey, 1982). The identification of such genotypes is crucial for ensuring food security and sustainability in regions increasingly affected by climate variability and water scarcity (Chaves et al., 2009).
               
This study was aimed to evaluate the photosynthetic performance, water use efficiency and seed yield of 35 diverse mungbean genotypes under four different environmental conditions to identify high-performing genotypes for sustainable mungbean production.

Thirty-five diverse mungbean genotypes were obtained from multiple research institutions, including the National Bureau of Plant Genetic Resources’ Regional Station in Jodhpur and the Rajasthan Agricultural Research Institute in Durgapura, Jaipur. Field experiments were conducted at the Experimental Farm, College of Agriculture, under the auspices of Swami Keshwanand Rajasthan Agricultural University, Bikaner, in two consecutive cropping seasons: summer 2019 and Kharif 2019-20. In randomized block design, the experimental materials, consisting of 35 mungbean genotypes, underwent evaluation with three replications. Each replication comprised two rows of 3 meter length at a spacing of 30 cm, thereby creating four distinct environmental conditions through varying dates of sowing: (i) Summer season: Early sowing (06 March) and late sowing (20 March); (ii) Kharif season: Early sowing (06 July) and late sowing (20 July). Compliance with recommended agricultural practices was ensured to ensure optimal crop growth. Subsequently, seed yield observations were recorded on a per-plant basis for five randomly selected individuals from each genotype of each replication. Remaining observations were recorded for traits viz., net photosynthetic rate (Pn), photosynthetic water use efficiency (WUE) and transpiration rate (E) using Infra-Red Gas Analyzer (Model CI-340). Mean values for various genotypes were analysed using Analysis of Variance to ascertain the statistical significance of differences between genotypes, employing both singular environmental and pooled datasets.

Principle of infrared gas analyzer
 
The infrared gas analyzer (IRGA) utilizes the principle of infrared (IR) light absorption to quantify hetero-atomic trace gases. The absorption of IR radiation is unique to molecules composed of disparate atoms (hetero-atomic gas molecules), such as CO₂, H₂O and NH₃, whereas monatomic gas molecules, consisting of a single atom (e.g. O₂, N₂), exhibit minimal or no IR radiation absorption. Carbon dioxide, specifically, effectively absorbs intermediate IR wavelengths. By measuring the reduction in IR transmission caused by the presence of a gas within the radiation source and detector, IRGA determines the gas concentration. This technique has been employed to assimilate carbon dioxide and water concentrations, as well as photosynthetic processes. The two primary types of IRGA devices, dispersive and non-dispersive, diverge in their respective measures according to the specificity of the measured gas species. Dispersive analysers concurrently apply mono-chromatic radiation to determine the concentration of various gas types in a complex gas mixture, whereas non-dispersive analysers quantify a singular gas species by employing broad-spectrum IR radiation selectively filtered for the targeted analyte. Non-dispersive IRGAs are commonly utilised for photosynthesis measurements. However, the presence of water vapour in the air intake can induce cross-sensitivity in CO₂ detectors, necessitating the application of corrective measures such as the use of filters to reduce interference or the elimination of water vapour through condensation or chemical removal.
       
The CI-340 hand-held photosynthesis system represents a technologically advanced infrared gas analyser for field and laboratory photosynthesis measurements, distinguished by its compact design, exceptional accuracy and rapid measurement capabilities. Utilizing a solid-state design concept, the entire analytical system is encapsulated within a single, ruggedized, hand-held casing, thereby ensuring a remarkably lightweight device. The incorporation of a direct analytical pathway minimises sample degradation through admittance to photo-respiratory gases, facilitated by the close proximity of a CO₂/H₂O differential gas analyser and leaf chamber. This design enables facile measurements of photosynthesis and transpiration rates, stomatal conductance and intracellular CO₂ concentrations, uncompromised by water vapour or temperature fluctuations. Additionally, the system is designed to accommodate simultaneous determination of absolute and differential CO₂ concentrations in leaves or plants. Operational simplicity facilitates user calibration, ensuring the attainment of high-quality measurement data. Data transfer to a computing device is also facilitated via an integrated USB connector. Data were compiled from observations of noted subjects.
 
Net photosynthetic rate (Pn)
 
Photosynthetic rates were measured on the underside of the third fully expanded leaf positioned from the uppermost of the plant, at 45 days after sowing, between 9.00 AM and 11.00 AM. The measurement was conducted using a portable photosynthesis system (CI-340) equipped with an infra-red gas analyser and a data logger, in accordance with the protocols outlined by Kubota and Hamid (1992). The assimilation chamber’s dimensions were kept constant at 6.25 cm² and illuminated with a 150 W metal halide lamp, generating photosynthetically active radiation of 1600 µmol m^-2s^-1. Airflow through the chamber was maintained at a rate of 400 ml min^-1, with an air relative humidity of 50% and a controlled temperature of 30±1^oC. The resulting data consisted of net photosynthesis rate, transpiration rate, stomatal conductance and other related parameters, which were subsequently obtained from the system’s computer output.
 
Parameters                                               Units
 
Rate of photosynthesis                               µmole CO₂ m^-2 s^-1
Rate of transpiration                                   µmole H₂O m^-2 s^-1
Stomatal conductance                               µmole CO₂ m^-2 s^-1
 
Transpiration rate (E)
 
The transpiration rate of the third fully extended leaf from the top was measured on the abaxial surface at 45 days after sowing, specifically from 9:00 to 11:00 AM, utilizing a movable photosynthesis system coupled with an infra-red gas analyzer.
 
Photosynthetic water use efficiency (WUE)
 
It was estimated on the basis of observations of net photosynthesis rate and transpiration rate which was recorded using fully expanded leaf (the third from the top) with help of ‘Infra-red gas analyzer’ (Model CI-340) at 45 days after sowing during 9.00 to 11.00 AM. The formula for estimation of photosynthetic water use efficiency as under:

 Photosynthetic Water Use Efficiency = Net photosynthesis rate (Pn) /Transpiration rate (E)

       
Statistical analysis was conducted using analysis of variance methodology and complemented with the application of software package Statistics 10. Comparisons were facilitated using a least significant difference test with an alpha level of 5%. The coefficient of variation was calculated using the standard deviation and mean, specifically as the standard deviation divided by the mean.

Net photosynthetic rate (Pn)
 
Significant variations in Pn were observed across genotypes and environments (p<0.01). The highest Pn was recorded in genotype SML-668 (57.62 µmol m^-2 s^-1), while MUM-2 had the lowest (22.56 µmol m^-2 s^-1) (Table 1). Environment-C exhibited the highest mean Pn (43.28 µmolm^-2 s^-1), suggesting favourable conditions for photosynthesis in this environment. These results align with earlier findings where genotypes with higher Pn demonstrated better yield stability under diverse conditions (Sharma et al., 2022).



Transpiration rate (E)
 
The transpiration rate varied significantly across environments (p<0.05). The mean E across all genotypes and environments was 4.21 mmol H₂Om^-2s^-1, with a range from 1.62 mmol H₂O m^-2 s^-1 (MH-421) to 5.95 mmol H₂O m^-2 s^-1 (IC-103244). Environment-B exhibited the highest mean E (4.41 mmol H₂O m^-2 s^-1), indicating increased water loss under these conditions. Higher transpiration rates under stress conditions indicate greater water loss and potential yield reduction (Rahman et al., 2023).
 
Photosynthetic water use efficiency (WUE)
 
Water use efficiency (WUE) is a critical trait in drought-prone environments. The highest WUE was recorded in genotype MH-421 (18.07 µmol CO‚  mol^-1 H₂O), while MUM-2 exhibited the lowest (5.50 µmol CO₂ mol^-1 H₂O) presented in Table 2. Genotypes with high WUE maintained efficient photosynthesis with lower water loss, making them suitable for drought-prone regions (Islam et al., 2018).


 
Seed yield (SY) and genotypic performance
 
The highest SY was recorded in GM-4 (13.14 g/plant), followed by SML-832 (11.13 g/plant). A strong positive correlation (r = 0.68, p<0.01) was found between Pn and SY, indicating that genotypes with higher photosynthetic capacity produced more yield (Table 3).


 
Genotype performance across environments
 
Genotype IC-52087 consistently exhibited high Pn across all environments, peaking at 65.20 µmolm^-2 s^-1 in Environment-D (Table 1). Similarly, SML-668 maintained high Pn values, with a maximum of 65.37 µmolm^-2 s^-1 in Environment-C. These genotypes also demonstrated stable seed yield across environments, suggesting their potential for cultivation in diverse conditions.
 
Environmental influence
 
Environmental conditions significantly affected all measured traits. Environment-C provided the most favourable conditions for photosynthesis and yield, while Environment-A was the least favourable. These variations underscore the importance of selecting adaptable genotypes for specific environmental conditions.

This study demonstrated significant genotypic and environmental variations in photosynthetic traits and seed yield of mungbean. Genotypes SML-668, IC-52087 and GM-4 emerged as high performers across multiple environments, making them potential candidates for mungbean breeding programs. High WUE genotypes like MH-421 could be valuable for drought-prone areas. Future research should focus on molecular characterization of these traits for further improvement in mungbean productivity.

The researchers gratefully acknowledges the invaluable support and provision of research materials received from prominent university administrators.
 
Disclaimers
 
The views expressed in this article are those of the authors and may diverge from the positions of their affiliated institutions. The authors are solely responsible for the accuracy and comprehensiveness of the information presented, although they disclaim any liability for potential outcomes arising from the utilisation of this content.
 

The authors declare that they hold no conflicts of interest that may have influenced the publication of this article. None of the study’s design, data collection, analysis, publication decision, or manuscript preparation was compromised by financial assistance or sponsorship.