Morpho-physiological Variability and Yield Determinants for Drought Tolerance in Blackgram [Vigna mungo (L.) Hepper]

The environmental changes can influence crop growth and thereby yield in nature due to abiotic and biotic stresses. These abiotic and biotic stresses bring changes in yield due to physiological and biochemical parameters from a mild to the larger extent (Baroowa and Gogoi, 2013). Among the different stresses, water deficit stress occurs in over 1.2 billion hectares of rainfed agricultural land, reducing crop yield worldwide (Boyer, 1982). More than 87% of the area under pulses is presently rainfed and moisture stress is the main reason for crop failure or for low yield realization. Water deficit stress can lead to various physiological peculiarities in plants, getting in the way of their growth and production (Banerjee et al., 2021). In fact, this abiotic stress appearing at the reproductive phase of plants may pose serious threats to the maturing of leguminous crops by means of leaf area and chlorophyll content ultimately culminating into poor production (Banerjee et al., 2021). Improved varieties of different pulse crops hold promise to increase productivity by 20-25% (Ali and Gupta, 2012). An understanding of genotypic differences to water deficit stress can help in identifying genotypes that can tolerate drought with reasonable seed yield. The present experiment was conducted to assess the physiological and yield-related parameters of ten black gram genotypes under water deficit stress environment.

Two pot experiments were conducted with ten black gram genotypes during Kharif 2015-2016 in the net house of the Department of Plant Breeding and Genetics, Assam Agricultural University, Assam. The pot were filled with a mixture of 4 parts finely powdered upland field soils of sandy loam texture acidic in reaction (pH 6.5) and 1 part vermicompost supplemented with N: P: K @ 15:35:10 kg ha^-1 and a soil pesticide Carbofuran 3G @ 30 kg ha^-1 calculated on surface area basis. The ten black gram genotypes - AKU 10-6, MU 44, COBG 10-06, VBG 11-31, SBC 47, SBC 40, PU 11-14, NDUK 13-4, MU 06 and AKU 11-8 were laid out in completely randomized design (CRD) with three replications in each experiment. One day ahead of sowing, the pots were wetted up to field capacity (~20% moisture content). The non-stress pots were maintained by watering at weekly intervals while water deficit stress was imposed at the vegetative stage by withholding irrigation till appearance of wilting symptoms in 80% of the pots, which coincided with flower bud initiation (33 DAS) and thereafter stress was released by irrigating the pots once in the same way as done in the non-stress experiment. The soil moisture content of the pots from a depth of 15 cm was estimated by gravimetric method once at 33 DAS in both the experiments (Dastane, 1972). The average soil moisture contents in the water deficit stress and non-stress condition were 3.98±0.25 and 14.35±0.24%, respectively.  The plants were observed for leaf area (cm²) (LA), chlorophyll content (mg g^-1FW) (CHL), proline content (μ mg g^-1) (PC), relative leaf water content (%) (RLWC), days to flower bud initiation (DFBI), plant height (cm) (PH), number of branches per plant (BR), clusters per plant (CL), pods per plant (PP), pod length (cm) (PL), number of seeds per pod (SP), 100-seed weights (g) (HSW) and seed yield per plant (g) (SY). The data generated were subjected to environment wise and pooled analysis of variance (ANOVA) as per CRD taking replications, genotypes and environments as fixed effects in Windostat v.9.2.

Analysis of variance

The environment wise and pooled ANOVA revealed significant genotypic differences for all the traits except days to flower bud initiation (Table 1). The mean sum of squares due to G ×E interaction was significant for all the characters except for chlorophyll content, days to first bud initiation and number of branches per plant suggesting a differential behaviour of the genotypes to stress and non-stress environment for majority of the traits.



Mean performance of physiological traits of the genotypes

The water deficit stress significantly reduced all the physiological traits of the black gram genotypes (Table 2a). The genotype MU 06 was found to be adapted to the water-stress environment as indicated by the lowest pooled mean leaf area (22.0 cm²). The resulting smaller leaf area transpires less water and hence this can be considered as the first line of defense against drought (Gurumurthy et al., 2019). The chlorophyll content of all the genotypes reduced significantly under the water deficit stress as compared to the non-stress environment. The mean chlorophyll content under non-stress and water deficit stress environment varied from 2.8 mg/g (MU 06) to 4.4 mg/g (SBC 40) and 1.3 mg/g (MU 06) to 2.2 mg/g (AKU 10-6), respectively. The decrease in chlorophyll content due to moisture deficit stress was also reported by Rao et al., (2015) in black gram. The proline content ranged from 6.7 (MU 44) to 10.8 μ mole g^-1 (AKU 10-6) under water stress and 3.9 (MU 06) to 10.6 μ mole g^-1 (SBC 40) under the non-stress environment. A significant increase in proline was observed in response to water stress in black gram. Proline is a multifunctional amino acid and accumulates in many plant species in response to an array of abiotic stresses (Gurumurthy et al., 2019). The trait relative leaf water content (RLWC) decreased from 73.8% under non-stress to 49.7% under water deficit stress. The presence of higher RLWC in tolerant genotypes indicates their more ability to retain water, possibly due to better osmo-regulation (Gambao et al., 1991). The genotype VBG 11-31 with the highest RLWC of 64.0% under stress environment suggested its ability to retain more water among the ten genotypes selected for the present study.
 
Mean performance of yield and its components of the genotypes
 
The yield and its components traits decreased when water deficit stress was imposed during the flowering stage but there was significant variation among the genotypes (Table 2a and 2b). The water deficit stress significantly reduced plant height varying from 26.6 (MU 06) to 38.8 (VBG 11-31) as against the range from 39.1 (COBG10-06) to 48.0 (VBG11-31) under non-stress condition. However, the water stress during the vegetative stage, in particular, is the most detrimental in terms of height (Baroowa et al., 2016). Under the non-stress environment, the branches per plant ranged from 7.4 (AKU 11-8) to 10.8 (COBG 10-06) whereas under water stress it varied from 4.6 (AKU 11-8) to 6.4 (COBG 10-06. The mean number of clusters per plant was 2.6 under stress as against 4.7 under non-stress environment. The pods per plant ranged from 7.0 (MU 06) to 17.9 (SBC 40) under the well watered environment and 3.6 (MU 06) to 11.7 (COBG 10-06) under water deficit stress. The genotypes SBC 40 and COBG 10-06 recorded the highest number of pods per plant in well watered and water deficit stress, respectively. Anitha et al., (2015) also reported that pod number decreased under water deficit stress in black gram with significant variation among the genotypes. The seeds yield per plant ranged from 2.02 g (MU 06) to 3.93 g (SBC 40) in non-stress environment and from 0.67 (MU 06) to 1.55 (AKU 10-6) under water deficit stress. The genotype AKU 10-6 performed the best under both the environments with the lowest mean difference. According to Baroowa and Gogoi (2016) yield loss caused by drought was the highest in plants receiving stress during the early reproductive stage.





Based on the grain yield efficiency index, the ten genotypes were classified as drought efficient (>1.00), intermediate (0.50-1.00) and inefficient (<0.50) (Table 3). The drought efficient genotypes viz., AKU 10-6, COBG 10-06 and NDUK 13-4 exhibited mean grain yield performance at par with each other, the former being the best under moisture deficit stress condition.

 
 
Coefficients of variability
 
The genotypic coefficient of variability for the traits ranged from 5.6 for seeds per pod to 32.4% for proline content under the non-stress environment and from 3.4 for pod length to 34.6% for RLWC under stress environment (Table 4). The traits like chlorophyll content, RLWC, plant height, seeds per pod, 100-seed weights and grain yield per plant had greater GCV estimates under stress than the non-stress environment. These results provided the evidence that these traits showed more variation under stress environment, suggesting scope for selection for these traits under stress environment. These observations are in agreement with the findings of Manggoel et al., (2012).



The higher PCV values than their corresponding GCV estimates by narrow margins for all the traits in both the environments indicated that selection for improvement of such characters based on phenotypes would be rewarding to improve the present black gram genotypes. Similar results have been reported by Atta et al., (2008).
 
Heritability and genetic advance
 
In the present experiment, high heritability estimates were recorded for chlorophyll content, proline content, relative leaf water content, pods per plant and 100-seed weights in both non-stress and stress environments. The results are in agreement with Noor et al., (2003) who also reported the high estimates of heritability for these traits. Thus, a direct selection for these traits would be effective in breeding for high-yielding black gram varieties adapted to stress and non-stress environments.

In the present investigation, the high heritability estimates in conjunction with high genetic advance were observed for 100-seed weights, proline content, pods per plant, relative leaf water content, chlorophyll content and seed yield per plant under both the environments. These results are in agreement with Riaz and Chwodhry (2003). In our study, the heritability estimate for seeds per pods was low, coupled with the low genetic advance under the non-stress environment whereas a high heritability with high genetic advance observed under stress environment for this trait. Collaku (1994) reported a differential expression of heritability with the genetic advance in drought stress and the normal environment in many traits in chickpea which was attributed to different sets of alleles and possibly different loci expressed under different environments.

Differential behaviour of the genotypes to stress and non-stress environment for the majority of the traits would provide scope for formulating efficient selection criteria for drought tolerance in black gram. The genotype AKU 10-6 would be suitable for moisture stress condition because it is early flowering and has the highest performance for seed yield, proline and relative leaf water content.