Moisture-deficit-induced changes in leaf-water content, leaf carbon exchange rate and biomass production in groundnut cultivars differing in specific leaf area

Field experiments were conducted during two rainy seasons to study the effect of soil moisture deficit on total biomass, pod yield, harvest index (HI) and drought tolerance index (DTI) in groundnut (Arachis hypogaea L.) cultivars possessing a wide range of specific leaf area (SLA, 144-241 cm g). There were three soil moisture regimes: adequate irrigation (W), drought simulated under rain-out-shelter (W) and rain-fed (W). This experiment had two parts, in one, five cultivars were exposed to W, W and W, and in a second, seven cultivars were exposed to W and W. Using the same set of seven cultivars, pot-culture experiments were conducted to study relative water content (RWC), stomatal conductance (g) and single leaf carbon exchange rate (CER) during increasing moisture-deficit in two contrasting (rainy and summer) seasons. Variation in DTI was significant, and low SLA types had greater DTI under both W and W. The ranking of SLA among cultivars was consistent between experiments conducted during the two seasons. The rate of reduction in leaf RWC during the progressive moisture-deficit was related directly to SLA (r = 0.78, P < 0.01). The coefficient of determination of the slopes calculated between RWC and soil moisture during the experimental period was more in the summer (r = 0.82) than the rainy (r = 0.54) season. Under increasing moisture-deficit, the low SLA types were able to maintain higher RWC, CER and g in both seasons. The relationships between RWC and CER (r = 0.91, P < 0.01), and RWC and g (r = 0.65, P < 0.01) were significant. It is suggested that under water-limited conditions there is a significant inverse relationship between SLA and RWC. The low SLA types (water use efficient) were found to be drought tolerant in terms of total dry matter production in the field studies, and maintenance of higher RWC under drought like situations in pot-culture experiments. Thus the ability of the low SLA types (higher water use efficiency, WUE) to maintain higher RWC may form the basis for the differences in drought tolerance vis a vis WUE in groundnut cultivars differing in SLA. Suggestions are made to select parents for drought tolerance or WUE, and to initiate breeding to combine traits like high HI, and WUE in terms of lower SLA. Ultimately, selection for both WUE (measured in terms of SLA) and yield traits (HI) should result in cultivars with improved performance in rain-fed agriculture