Variability in Drought Stress Induced Responses of Groundnut (Arachis hypogaea L.) Genotypes

The response of Anet, gs, Tr were highly significant (p<0.01) for genotypes, drought stress and interaction of drought stress×genotype. The ANOVA and mean performance of these parameters are presented in Table 1-2.

Drought stress reduced all the physiological parameters of all six groundnut genotypes and the reduction was high with Anet (70%) and gs (78%) as compared with Tr (59%), hence the reduction was less for intrinsic WUE (24%). However, the magnitude of reduction was different with genotype. Similar genotypic variability in response to drought stress was reported in groundnut [16,17] soyabean [3] and cowpea [18].

Among the genotypes assessed, Anet values ranged from 28.0 µmoles CO₂/m²/s (Abhaya) to 35.9µmoles CO₂/m²/s (ICGV 91114) in control where as it reduced to 7.4 µmoles CO₂/m²/s (Narayani) to 11µmoles CO₂/m²/s (Abhaya and Dharani) under drought stress conditions. It was interesting to observe that genotype ICGV 91114 with highest Anet under control condition recorded highest reduction with drought stress whereas genotype Abhaya with lowest Anet under control condition recorded highest Anet with lowest reduction under drought stress.

The results agrees with the reports of Kalariya et al. [19] that there is a decrease in the Anet of groundnut genotypes with different magnitude due to drought stress. The reduction of Anet due to drought stress may be due to several coordinated events, such as stomatal closure which reduces CO₂ availability in the leaves. This in turn inhibits carbon fixation and reduced activity of photosynthetic enzymes [5] such as rubisco. The genotypes with higher Anet and low reduction with drought stress is expected to perform better and in the present investigation, the genotypes Abhaya and Dharani can perform better under drought stress conditions as they recorded higher Anet.

First response to drought stress is closing of stomata which prevents the rate of water loss with reduced gs, and Tr. Higher gs was recorded in Greeshma (0.64 mmol/m²/s) under control condition and lowest in Narayani (0.32 mmol/m²/s), whereas highest gs was in JL-24 (0.127 mmol/m²/s) and lowest in Narayani (0.078 mmol/m²/s) under stress conditions. There are previous reports that there was a genotypic variation and also reduction of gs in groundnut in response to drought stress [17]. Tr also reduced under drought stress from 39.13% (Abhaya) to 73.52% (ICGV 91114) to prevent the rate of water loss. With high transpiration rates under control treatment, the genotypes Greeshma (13.4 mmol/m²/s) and Dharani (12.8 mmol/m²/s) showed moderate Tr (4.7 mmol/m²/s) whereas under stress conditions ICGV 91114 recorded lowest Tr (3.3 mmol/m²/s) under stress conditions with highest reduction (73.52%) from its control values. Earlier reports on groundnut also showed similar trend and variation within genotypes for Tr in response to drought stress [19]. In the present study, it is evident that in groundnut, the reduction of gs was higher than Tr clearly indicating that gs was more impacted by drought stress than Tr [20].

Intrinsic water use efficiency (WUE) decreased under drought stress conditions and it was non-significant for genotypes and highly significant (P<0.01) for drought stress and significant (p<0.05) for drought stress×genotype interaction (Table 2). Among the genotypes, Abhaya recorded highest WUE (3.0 µmoles CO₂/mmol H₂O) under control treatment and reduced under stress conditions (2.0 µmoles CO₂/mmol H₂O) due to higher reduction of Anet than Tr. The genotypes ICGV 91114 and Dharani could maintain better WUE under stress treatment though registered moderate WUE under control conditions (Table 1). It was interesting to observe that genotype Dharani was able to maintain better Anet under stress as higher reduction of Tr resulted better WUE whereas the genotype ICGV 91114 recorded higher reduction of Anet and Tr. This was also in line with the pattern of WUE observed by Songsri et al. [16] in groundnut genotypes due to drought stress.

A major impact of drought stress is usually on cellular membrane modification, which results in total dysfunction and it is generally accepted that the maintenance of integrity and stability of membranes under drought stress is a major component of drought tolerance in plants. In the present study, MSI was highly significant for genotypes, drought stress and drought stress × genotype interactions. MSI was ranged from 83% (ICGV 91114) to 86.6% (Abhaya and Narayani) in controls and whereas under stress it was 77% (Greeshma) to 84.4% (Dharani). Under drought stress conditions higher MSI was recorded in Dharani (84.4%) and Abhaya (83.5 %) with less reduction 1.2% and 3.4% respectively over controls (Figure 2). Earlier records on groundnut also showed significant effect of drought stress on MSI, the groundnut varieties WEST- 44, showed higher MSI with low decrease in MSI over control under drought stress conditions [9]. The genotypes with higher MSI have better adaptations under drought stress [21] so in the present investigation, genotypes Dharani and Abhaya with better MSI are likely to have better adaptations under drought stress conditions.

Drought stress induced higher amount of proline, free amino acids, soluble protein in all genotypes and they are highly significant (p<0.01) for genotypes, drought stress and drought stress × genotype interaction; however the degree of increase varied with genotypes. Plants respond to different types of stresses by accumulating certain specific metabolites such as amino acids, proteins in general and proline in particular [6]. Proline content is used as an indicator of drought tolerance capacity of plant tissue as accumulation of proline helps in osmotic balance during drought stress. In the present study proline content of all genotypes increased as a result of drought stress. The genotypes Dharani and Abhaya recorded higher increase in proline content as well as registered higher content under drought stress (Figure 3).

There are similar reports that there is an increase of proline content in cultivars of blackgram, green gram [4] and groundnut [22] due to drought stress. Genotypes Dharani and Abhaya in the present study with higher increment and proline content under drought stress are expected to have better stress tolerance capacity.

With drought stress the content of free amino acids (FAA) increased in all the genotypes and this increase was pronounced in Dharani (32%) with higher FAA (3.4 mg g^-1 FW) whereas lower in Greeshma with lower FAA (2.6 mg g^-1 FW) (Figure 3). Genotypic variability and an increase in the activity of FAA were also observed in pearl millet [6] and groundnut [5]. All the selected groundnut genotypes recorded increased amount of total soluble proteins (TSP) due to drought stress conditions. The increase was more conspicuous in Dharani and Abhaya as these two genotypes maintained higher protein (79 and 73 mg g^-1 FW) whereas Greeshma had lower protein (59 mg g^-1 FW) (Figure 4). These results are in tune with previous results of blackgram [23] and groundnut [5] genotypes. Usually under drought stress, increase of total soluble proteins was as a result of high amino acids contents as plants accumulate small molecular mass proteins as there may be increase of de novo synthesis or inhibition of amino acid degradation [5] and higher protein content might impart better drought tolerance [23] as it helps in osmotic balance. Among the genotypes, Dharani and Abhaya have higher protein content when exposed to drought stress and this may alleviate the stress impact.

Reduction of yield recorded due to drought stress in all groundnut genotypes that were studied and it was highly significant (p<0.01) for genotypes, drought stress and their interaction (Table 2). The genotypes Dharani and Abhaya performed better in terms of per se seed weight under control (29 g plant^-1, 26.5 g plant^-1) as well as stress conditions (22 g plant^-1, 17 g plant^-1) (Figure 5). Under stress conditions lower reduction of yield was recorded in Dharani (25%) and ICGV 91114 (6%), and but the per se seed yield was lower in ICGV 91114 (13.7 g plant^-1) and higher in Dharani (22 g plant^-1). According to Ratnakumar and Vadez, [10] the seed weight reduced significantly under drought stress in groundnut genotypes and tolerant genotypes were able to maintain better yield.