Because of its relationship with water‐use efficiency (W), carbon isotope discrimination in leaves (Δ) was proposed to be useful for identifying genotypes with greater water‐use efficiency. In this study we examined the relationship between W and Δ in four peanut (Arachis hypogaea L.) genotypes. The genotypes were grown in and around mini‐lysimeters embedded in soil and were subjected to two drought regimes, intermittent and prolonged water deficit conditions, by varying the irrigation timing and amount. Automated rain‐out shelters prevented any rain from reaching the experimental plots during the treatment period. The mini‐lysimeters allowed accurate measurement of water use and total dry matter (including roots) in a canopy environment. Water‐use efficiency, which ranged from 1.81 to 3.15 g kg−1, was negatively correlated with Δ, which ranged from 19.1 to 21.8%. Tifton‐8 had the highest W (3.15 g kg−1) and Chico the lowest (1.81 g kg−1, representing a variation in W of 74% among genotypes. Variation in W arose mainly from genotypic differences in total dry matter production rather than from differences in water use. It is concluded that δ is a useful trait for selecting genotypes of peanut with improved W under drought conditions in the field. A strong negative relationship existed between W and specific leaf area (SLA, cm3 g−1) and between Δ and SLA, indicating that genotypes with thicker leaves had greater W. SLA could therefore be used as a rapid and inexpensive selection index for high W in peanut where mass spectrometry facilities are not available.

Chlorophyll stability during drought might be a promising criterion for selection for drought resistance in peanut. The study describes two field trials conducted at Khon Kaen University, Thailand which investigate genotype × drought interactions in a wide range of peanut germplasm in general and assess the relationship between chlorophyll stability and genotypic performance in particular, under drought. Two field experiments (during 2003/2004 and 2004/2005 dry seasons) were conducted in a split plot design with three water regimes [field capacity, 2/3 available water (AW) and 1/3 AW] as main, and 12 peanut genotypes as subtreatments, replicated four times. Observations on total dry matter (TDM), chlorophyll density (ChlD) (chlorophyll content per unit leaf area), chlorophyll content (chlorophyll content per plant) and SPAD chlorophyll meter readings (SCMR) were recorded at 30, 60 and 90 days after emergence. Transpiration (T) and transpiration efficiency (TE) were computed using the data on amount of water input and TDM. Drought stress significantly reduced TDM, T and chlorophyll content across genotypes but significantly increased TE and ChlD in peanut. However, there were significant differences among genotypes for TE and chlorophyll parameters. The genotype × drought interaction effects for chlorophyll characters (content and density) were not significant suggesting a strong genetic effect. The correlation coefficients between TDM and chlorophyll content (r = 0.51, P = 0.01 to r = 0.91, P = 0.01) and between TE and ChlD (r = 0.46, P = 0.05 to r = 0.77, P = 0.01) were positive and significant. These findings suggest that chlorophyll parameters are strongly linked with drought tolerance in peanut. There were highly significant and positive relationships between ChlD and SCMR (r = 0.67, P = 0.01 to r = 0.93, P = 0.01), between SCMR and TE (r = 0.41, P = 0.05 to r = 0.80, P = 0.01) suggesting that SCMR could be used as a tool for rapid assessment of relative chlorophyll status in peanut genotypes as well as for the indirect selection of drought tolerance in peanut.

The present study investigates the potential use of a hand‐held portable SPAD chlorophyll meter for rapid assessment of specific leaf area (SLA) and specific leaf nitrogen (SLN), which are surrogate measures of transpiration efficiency (TE) in peanut (Arachis hypogaea L.). The effects of sampling (leaf position, time of sampling and leaf water status) and climatic factors (solar radiation and vapour pressure deficit, VPD) on SLA and SPAD chlorophyll meter reading (SCMR) were studied in a range of peanut genotypes grown under field and greenhouse conditions. The correlation between SLA and SCMR was significant (r=−0.77, P < 0.01) for the second leaf from the apex but the correlation declined for leaves sampled from lower nodal positions. The diurnal fluctuation in SLA ranged from −20 % to +10 %, whereas SCMR was relatively unaffected by these diurnal changes. Solar radiation and VPD during the sampling period had a significant influence on the relationship between SLA and SCMR, largely through their effects on SLA. However, standardization of SLA for these environmental factors significantly improved the relationship between SLA and SCMR from −0.50 to −0.80 (P < 0.01), suggesting that, when protocols for leaf sampling and SLA measurements are followed, SCMR can be a surrogate measure of SLA. There were significant relationships between SLN and SCMR (r=0.84, P < 0.01) and SLN and SLA (r=−0.81, P < 0.01). These significant interrelationships amongst SLA, SLN and SCMR suggested that SCMR could be used as a reliable and rapid measure to identify genotypes with low SLA or high SLN (and hence high TE) in peanut.

The rainfed groundnut (Arachis hypogaea L.) crop suffers from moisture stress of varying intensity at different growth stages. The effect of drought on oil, protein and fatty acid contents were studied in 12 genotypes that differed in seed quality traits. The genotypes were subjected to mid-season and the end-of-season drought in field experiments at ICRISAT Asia Center (IAC), Patancheru, India, conducted during the 1991/92 and 1992/93 postrainy (November-April) seasons. Mid-season drought was imposed between 40 and 80 days after sowing (DAS). The crop received normal irrigation, 50 mm at 10 day intervals, before and after the imposition of mid-season drought until harvest. Using the line-source sprinkler technique, end-of-season drought of varying intensity was imposed from 80 DAS until harvest. Mid-season drought had no significant effect on the content of oil, protein and fatty acids other than eicosenoic fatty acid. End-of-season drought significantly reduced total oil, and linoleic and behenic fatty acid content, and significantly increased total protein and stearic and oleic fatty acid content. However, genotype by treatment interactions were found. In ICGVs 88369, 88371, 88381, 88382 and 88403, total oil content remained unaffected while oleic fatty acid content increased under end-of-season drought. These were identified as desirable parents for a breeding program to develop cultivars suitable for rainfed cultivation.

This review distills recent information on drought resistance characteristics of grain legumes with a view toward developing appropriate genetic enhancement strategies for water-limited environments. First, the possible adaptations that allow grain legumes to better cope with drought stress are summarized. It is suggested that there are considerable gains to be made in increasing yield and yield stability in environments characterized by terminal drought stress by further exploiting drought escape, by shortening crop duration. Many traits conferring dehydration avoidance and dehydration tolerance are available, but integrated traits, expressing at a higher level of organization, are suggested to be more useful in crop improvement programs. Possible genetic improvement strategies are outlined, ranging from empirical selection for yield in droughted environments to a physiological genetic approach. It is suggested that in view of recent advances in understanding drought resistance mechanisms, the latter strategy is becoming more feasible. It is concluded that use of this recently derived knowledge in a systematic manner can lead to significant gains in yield and yield stability of the world's major grain legumes, as they are mainly grown (and will continue to be grown) under rain-fed conditions.