Morpho-physio characterization of greengram (Vigna radiata L.) genotypes under water deficit stress

Introduction

Greengram (Vigna radiata (L.) Wilczek), a diploid member of the family Leguminosae (2n=2x=22), is a premier pulse crop with global economic significance, particularly in tropical and subtropical regions. In India, it is the third most vital grain legume, spanning approximately 3.44 million hectares (Dhunde et al., 2021) [5] . Often termed the "poor man's meat," it serves as a primary protein source (approx. 25%) for vegetarian populations, offering high concentrations of essential amino acids like lysine and phenylalanine (Rao et al., 2020) [17] . Its rapid growth cycle and wide adaptability make it a cornerstone of food security in the Indo-Burma-Thailand regions of Asia. Greengram (Vigna radiata L.) is the third most important grain legume after chickpea and pigeon pea, and is widely grown in the Asian countries. It has a wider adaptability and low input requirement and is grown mainly in rain-fed conditions at relatively high temperature (27-30 °C), with low humidity and moderate rainfall ranging from 60 to 80 cm (Bangar et al. 2019) [2] . It is a self-pollinating, fast-growing, short-duration crop that contributes to increased cropping intensity (Singh et al. 2016) [20] . It also improves soil fertility and productivity by fixing atmospheric nitrogen into the soil via symbiosis with rhizobium. Despite an economically important crop, the productivity of greengram is stagnant due to various biotic and abiotic stresses (Pratap et al. 2019) [14] . The global food supply is 71 million tons, with 79 million hectares dedicated to pulses (Anonymous, 2021). Greengram has a global area of roughly 7.3 million ha and a global production of approximately 5.3 million tons. Greengram is a well-known crop in Asian countries and India is the world's largest producer and consumer of pulses, accounting for 30% of global output. Greengram is an important short-duration pulse crop reaching 0.3 to 1.4 m in height, characterized by branched stems, trifoliate leaves, yellow flowers, and short hairy pods (Lee et al., 2021) [10] valued for its high nutritional and agronomic significance. It is a major source of easily digestible protein, especially for vegetarian populations. It is widely consumed as both whole and split pulse and forms a key component of cereal-based diets in Asia (Rao et al., 2020) [17] . Its nutritional quality improves when combined with cereals due to complementary amino acids. In addition to its dietary importance, greengram plays a crucial role in sustainable agriculture by fixing atmospheric nitrogen, improving soil fertility, and fitting well into crop rotation and intercropping systems, particularly under dryland conditions. Despite its economic importance, its productivity is significantly affected by various abiotic and biotic stresses. Among these, drought is the most critical factor, severely limiting plant growth and yield by disrupting water availability and increasing transpiration stress. With changing climate conditions and declining water resources, drought stress is expected to further reduce greengram productivity, making stress management essential for sustaining food production. Plant water deficits may occur as a consequence of a seasonal decline in soil water availability, developing in the long term or may result from drought spells. Plant strategies to control water status and resist drought are numerous. In general, genotypes native from climates with marked seasonality are able to acclimate to the fluctuating environmental conditions. In the case of slowly developing water deficits, plants may also escape dehydration by shortening their life cycles. In case of rapid dehydration, oxidative stress developing as a secondary effect is potentially very damaging to the photosynthetic machinery. The capacity for energy dissipation and metabolic protection (induced or constitutive) against the damaging effects of reactive oxygen species is a key element for the success of plants under drought (Chaves and Oliviera et al., 2004) [4] Drought is one of the most harmful abiotic stresses, particularly in the Mediterranean basin, where arid climate causes water scarcity and high evapotranspiration. Severe droughts may directly reduce or eliminate yields, leading to a decline in crop productivity by up to 50-70%, which may affect 40% of the 4 global population. Moreover, according to the World Health Organization, drought stress affects the livelihood of approximately 55 million people worldwide annually, and it is estimated that around 700 million people are at risk of displacement by 2030 because of droughts. Hence, better understanding the plants' responses to environmental stresses is a key prerequisite for the improvement of their drought tolerance and yield under water deficit. Drought causes an array of morphological, physiological, biochemical, and molecular changes in plants. Greengram being known as an essential tropical grain legume and an important cash crop with high protein content and high nitrogen fixing capability brings into focus the research it needs. In contrast, with erratic rainfall, the production of greengram is continuously threatened with the expanding water deficit stressed zones. This encourages researchers to know more about greengram water deficit stress tolerance which thereby necessitates a detailed screening of the morphological and physiological characterizations.

Materials and method

A total of twenty five genotypes of greengram were selected for their comparative analysis under drought stress including both tolerant and susceptible traits. The experiment is laid out in randomized block design (RBD). Plants were allowed to grow normally for 30 days after sowing, until flowering began in all genotypes. At the time of flowering drought stress was introduced by covering plot B with a white polythene sheet of 200 micron UV polythene sheet with side wall opening and supported by a temporary framework. (D.Chatti et al. 2017) The field experiment was conducted at Department of Agricultural Botany, Vasantrao Naik Marathwada Krishi Vidyapeeth (VNMKV), Parbhani (MH), during the kharif season of 2023-24.The field is situated at at a latitude of 19° 16' N, a longitude of 26° 47' E, and an altitude of 409 meters above sea level. The soil at the site was medium black with a moderate capacity for moisture retention. The experiment was conducted on land with a uniform topography. Sowing done by using the dibbling method at a depth of 3 cm. Two seeds were placed per hill, with a row spacing of 45 cm and a plant-to-plant distance of 10 cm. Germination counts were recorded 5 days after emergence, and plant populations were maintained through gap filling as needed. Drought stress was imposed during the flowering and podfilling stages by covering the plants with a white polythene sheet. The control field, which was not covered with the sheet, received rainfall and one irrigation during its growth cycle. Five plants per plot were randomly selected for recording biometric observations at respective stages. Data were collected on various morphological and physiological parameters in response to drought treatment.

Result and discussion

The statistical analysis of twenty-five greengram genotypes reveals significant genetic variability in field emergence, with values ranging from a minimum of 85% to a maximum of 98%. The highest performance was observed in a cluster of BWMCD genotypes, specifically BWMCD-6-10-1-1-5-1 and BWMCD-6-13-17-1-5-1, which performed at par with elite checks like PHULE CHETAK and BM-2002-1. While several lines maintained a stable and high profile around 95-96%, the genotypes BWMCD-6-53-1-1-3-1 and BWMCD-6-61-33-1-3-1 recorded the lowest emergence rates. This significant performance gap underscores that field emergence is a genotype-specific trait and a vital indicator of seed vigor. As emphasized by Paul D et al. (2022) [13] , high initial emergence is a critical prerequisite for drought tolerance; it facilitates rapid root development, allowing the plant to "escape" soil moisture depletion. Consequently, genotypes with superior emergence ensure successful crop establishment and resource competition, whereas susceptible lines struggle with yield stability under waterdeficit stress. [24] , and Baroowa et al. (2016) [3] , who concluded that drought-tolerant genotypes maintain greater height because moisture stress typically stunts growth by inhibiting cell elongation and disrupting mitosis. [9] , Farooq et al. (2009) [6] , Zhang et al. (2023) [7] , Mittler et al. (2002), Baroowa and Gogoi (2016) [3] , and Ranawake et al. (2011) [16] , who concluded that leaf count is a critical indicator of drought resilience, as water stress typically inhibits cell division and leaf expansion.

Significant difference recorded in genotypes among both conditions. Under control condition, minimum percentage recorded in genotype BWMCD-6-61-33-1-3-1 (75%) and maximum percentage was recorded in genotypes PHULE CHETAK (95%).Under water deficit condition, minimum plant stand was recorded in genotype BWMCD-7-4-1-1-1 (76%) and maximum plant stand was recorded in genotype PHULE VAIBHAV (88%), BWMCD-6-1-20-1-5-1 (85%) and this was found at par with genotypes BPMR145 (85%), BWMCD-6-53-5-1-3-1(86%), BWMCD-6-52-1-2-1 (83%),BWMCD-6-53-1-1-3-1 (81%), BM-4 (83%), BWMCD -6-58-5-1-4-1(85%), PHULE CHETAK (83%). [18] , Guo.Y. et al. (2023) [7] , Kumar et al. (2013) [26] , and Ashraf et al. (2012) [1] , who observed that drought-tolerant genotypes minimize the decline in RWC to maintain turgor pressure and essential metabolic functions. The findings suggest that high water retention in tolerant lines serves as a critical mechanism to mitigate oxidative stress and prevent the loss of photosynthetic efficiency during water scarcity. [32] , who concluded that drought-tolerant genotypes utilize higher chlorophyll density to maintain metabolic efficiency and pigment synthesis despite moisture limitations.

Relative Growth Rate (RGR) exhibited significant variation across genotypes, ranging from 0.11 to 0.57. BM-4 and AKM-4 emerged as the most resilient, maintaining the highest RGR under both control (0.42) and water deficit (0.39) conditions. Among breeding lines, BWMCD-6-170-1-4-1 and BWMCD-6-10-1-1-5-1 showed superior tolerance, whereas BWMCD-6-11-1-1-1-1 and BWMCD-6-5-8-1-3-1 recorded the lowest growth rates. These findings are supported by Singh et al. (2016) [20] , Kumar et al. (2014) [26] , and Nageswara Rao et al. (2020) [17] , who noted that while RGR naturally peaks under optimal conditions, drought-tolerant genotypes mitigate reductions caused by stomatal limitations and decreased carbon assimilation to maintain biomass production

The analysis of Absolute Growth Rate (AGR) revealed significant genetic diversity, with initial values ranging from 0.02 to 0.15. Under control conditions, BM-4 recorded the highest AGR (0.16), a performance it sustained under water deficit (0.13), indicating superior growth stability. Among breeding lines, BWMCD-6-53-5-1-3-1 and BWMCD-6-5-8-1-3-1 maintained relatively high AGR (0.11) under stress, while BWMCD-6-170-1-4-1 showed the most notable resilience with minimal decline. In contrast, BM-2002-1 and BWMCD-6-13-20-1-1-1 were identified as more sensitive to moisture limitations. These results emphasize that maintaining AGR under moisture stress is a genotypespecific trait, making consistent performers like BM-4 and specific resilient breeding lines critical candidates for drought prone cultivation. [17] , who noted that water deficit disrupts physiological processes and reproductive efficiency. The results suggest that genotypes capable of maintaining stable flowering and high grain retention under stress are critical for developing drought-resilient varieties. [25] , Sharma et al. (2015) [36] , Khazaei et al., Gowda et al, Farooq et al. (2009) [6] , Singh et al. (2016) [20] , Sivakumar et al, and Nageswara Rao et al., who concluded that moisture stress during the reproductive phase severely limits grain filling. The reduction is attributed to impaired pollination, reduced photosynthetic rates, and hormonal imbalances, marking high grain retention as a critical trait for drought resilient selection. (2016) [20] , Pandey and Shukla (2015) [25] , Kumar et al.

(2014) [26] , and Hamidou et al. (2013) [27] , who concluded that water deficit impairs photosynthesis and assimilate translocation. The results emphasize that maintaining pod weight through efficient grain filling despite moisture limitations is a critical indicator of drought tolerance in greengram genotypes. (2020) [29] , Sathi et al. (2022) [30] , and Linkemer et al.

(1998) [31] , who concluded that moisture stress reduces seed size by impairing translocation capacity during the critical pod-filling stage. The results suggest that the ability to maintain seed index under stress is a vital trait for identifying genotypes with efficient assimilate partitioning and yield stability. [32] , Pushpavalli et al. (2014) [33] , and Gholinezhad et al. (2009) [34] , who concluded that drought stress at the reproductive stage severely disrupts the allocation of carbohydrates to the grains. The results suggest that the ability to maintain a stable harvest index under moisture limitations is a key physiological indicator of drought tolerance and efficient dry matter partitioning. (2023) [37] , Nazran et al. (2019) [38] , and Nadeem et al.

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