Identification of heat-tolerant mungbean genotypes through morpho-physiological evaluation and key gene expression analysis

Abstract

Mungbean plays a significant role in global food and nutritional security. However, the recent drastic rise in atmospheric temperature has posed an imminent threat to mungbean cultivation. Therefore, this study investigates the growth and physiological changes of 87 mungbean germplasm lines under heat stress. Genotypes were examined using parameters including leaf area, chlorophyll content, membrane stability index (MSI), stomatal conductance, pollen viability, number of pods per cluster, number of pods per plant, number of seeds/pod, 100-seed weight and grain yield/plant under heat stress and control environments. A wide range of variation was observed for these traits among genotypes under heat stress and control environments. Genotypes were also identified with variable responses under both environments. The phenotypic expression of selected promising accessions was also validated in control environment conditions at the National Phytotron facility. The selected promising genotypes viz., IC76475, IC418452 and IC489062 validated their heat tolerance behavior for key candidate genes revealed by quantitative real-time PCR (qRT-PCR). These mungbean genotypes can act as potential resources in the mungbean improvement programs for heat stress tolerance. This study also provides a comprehensive understanding of the key mechanisms underlying heat tolerance in mungbean.

Keywords: abiotic stress tolerance; climate resilience; gene expression; green gram; physiological parameters.

FIGURE 1

Atmospheric temperature profile for the rainy season (RS), 2021 and summer season (SS), 2022 of the experiments. The average maximum day temperature for SS was significantly higher (40.3°C) than the RS (33.6°C).

FIGURE 2

Typical morphological symptoms in mungbean plants in response to heat stress for various mungbean plant parts such as severe flower drop in susceptible mungbean genotype (A); SML668, a check variety used in this experiment showed leaf margin burning (B) under SS condition, and no symptom of leaf burning (C) under RS conditions; effect of heat stress on heat-sensitive genotypes showing poor plant growth and leaf burning (D) under SS conditions. Photographs showing comparative higher pollen viability in IC76475, a tolerant mungbean genotype (E), and low pollen viability in IC616138, a susceptible genotype (F) under SS conditions.

FIGURE 3

Box plot chart distribution of morphological and physiological traits under RS and SS conditions. The distribution for each trait shows a significant change in the phenotypic expression of mungbean genotypes in response to heat stress.

FIGURE 4

Correlation Matrix of morpho-physiological traits of 87 mungbean genotypes under normal sown conditions (RS) and heat stress (SS) environments showing character correlation with each other [A positive correlations are shown in red and negative correlations are shown in blue color. The color intensity indicates the magnitude of correlation in either direction].

FIGURE 5

Principal Component Analysis (PCA) plots showing the multivariate variation among 87 mungbean genotypes in terms of environmental variables. The first two PCs explained 39.91% and 40.65% of the total variance under RS conditions and SS conditions. The variables with closer angles are more positively correlated, while variables in opposite directions are negatively correlated. A similar pattern is observed in both environments except for a few variables, which may be due to the partitioning of the variability in other PCs.

FIGURE 6

The agglomerative hierarchical clustering of 87 mungbean genotypes under RS and SS conditions. The promising accession identified as promising for heat stress tolerance based on their expression for multiple traits are circled.

FIGURE 7

Growth chamber view of mungbean genotypes used for qRT-PCR. Under the heat stress treatment (SS) vigorous canopy growth was observed in heat stress tolerant genotypes viz., IC76475, IC418452 and IC489062 (C), while the check varieties which are popular for summer season-grown mungbean crop viz., SML668 and IPM99-125 (B) showed poor growth and leaf burning at the elevated temperature of 40 °C. The heat-susceptible genotypes viz., IC616138 and IC548275 (A) also showed poor growth and leaf burning.

FIGURE 8

Effect of heat stress on the relative expression candidate genes [CP47, SUT, MTR_1g062190, MTR_7g092380, LOC101499292, VrLEA-55 (DHN)] involved in key physiological of heat stress tolerance. The analysis indicated a significantly higher level of expression for tolerant mungbean genotype (IC76475, IC418452, IC489062) in comparison to checks varieties (SML668, IPM99-125) and susceptible genotypes (IC616138, IC548275) used. The bars represent mean values of the relative normalized expression of genes ± SE.