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
] soyabean [3
] and cowpea [18
Among the genotypes assessed, Anet values ranged from 28.0 µmoles CO2
/s (Abhaya) to 35.9µmoles CO2
/s (ICGV 91114) in control where as it reduced to 7.4 µmoles CO2
/s (Narayani) to 11µmoles CO2
/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
O) under control treatment and reduced under stress conditions (2.0 µmoles CO2
O) 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.