The ability of a plant to modify its root distribution to exploit deeper stored soil water may be an important mechanism to avoid drought. This study aimed at assessing root distributions, variations in root length density (RLD) and percentage of root distribution, and the relevance of root traits for yield of drought‐resistant peanut genotypes under different available soil water levels. The experiment was conducted in the dry season during the years 2003/04 and 2004/05. Eleven peanut genotypes (ICGV 98300, ICGV 98303, ICGV 98305, ICGV 98308, ICGV 98324, ICGV 98330, ICGV 98348, ICGV 98353, Tainan 9, KK 60‐3 and Tifton‐8) and three soil moisture levels [field capacity (FC), 2/3 available soil water (AW) and 1/3 AW] were laid out in a split‐plot design with four replications. Roots were sampled by a core sampler at 37, 67 and 97 days after sowing (DAS). Root length was determined by a scanner and the WINRHIZO Pro 2004a software. RLD was calculated as the ratio of root length (cm) and soil volume (cm3). Graphical illustration of root distribution was constructed by merging RLD in the first and second soil layers (0–40 cm) as upper roots and pooling RLD at the third, fourth and fifth layers (40–100 cm) as lower roots. Pod yield, biomass and harvest index (HI) were recorded at harvest. A drought tolerance index (DTI) was calculated for each parameter as the ratio of the parameter under stress treatment to that under well‐watered conditions. Variations in RLD in 40 to 100 cm layer (RLD40 to 100 cm) were found under well‐watered conditions, and the peanut genotypes could be readily identified as high, intermediate and low for this trait. Changes in RLD in the 40 to 100 cm soil layer were found at 2/3 AW and were more evident at 1/3 AW. ICGV 98300, ICGV 98303, ICGV 98305, ICGV 98308 and KK 60‐3 were classified as drought responsive as they increased RLD in the deeper subsoil level in response to drought. In general, RLD under drought conditions was not related to biomass production. The ability to maintain the percentage of RLD (DTI for %RLD) was related to pod yield, DTI for pod yield and DTI for HI. ICGV 98300, ICGV 98303, ICGV 98305 exhibited high DTI (RLD40 to 100 cm) which may explain their high pod yield, DTI (PY) and DTI (HI). Based on these observations we classified them as drought‐avoiding genotypes.

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.

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D rought is the major abiotic constraint aff ecting peanut (Arachis hypogaea L.) productivity and quality worldwide. Twothirds of the global production occurs in rain-fed regions of the semi-arid tropics where rainfall is generally erratic and insuffi cient, causing unpredictable drought stress, the most important constraint for peanut production (Wright and Nageswara Rao, 1994; Reddy et al., 2003). Even peanut grown under irrigation may experience drought because of limited water supply or because irrigation water is applied in amounts at frequencies less than optimal for plant growth. Improving water access and management are practically diffi cult since water is a scarce resource. Therefore, breeding for drought resistance is an important strategy in alleviating the problem and off ers the best long-term solution. Selection of segregating populations under stress conditions has been a standard approach for developing cultivars with improved stress tolerance. While direct selection for yield under stressed conditions can be eff ective, the limitations of this approach are high resource investment and poor repeatability of the results due to the large genotype ×

The relationship between biomass production and N2 fixation under drought‐stress conditions in peanut genotypes with different levels of drought resistance is not well understood. The objective of this study was to determine the effect of drought on biomass production and N2 fixation by evaluating the relative values of these two traits under well watered and water‐stress conditions. Twelve peanut genotypes were tested under field conditions in the dry seasons of 2003/2004 and 2004/2005 in north‐east Thailand. A split‐plot design with four replications was used. Main‐plot treatments were three water regimes [field capacity (FC), 2/3 available soil water (AW) and 1/3 AW], and sub‐plot treatments were 12 peanut lines. Data were recorded on biomass production and N2 fixation under well watered and water‐stress conditions. Genotypic variations in biomass production and N2 fixation were found at all water regimes. Biomass production and N2 fixation decreased with increasing levels of drought stress. Genotypes did not significantly differ in reductions for biomass production, but did differ for reductions in N2 fixation. High biomass production under both mild and severe drought‐stress conditions was due largely to high potential biomass production under well‐watered conditions and, to a lesser extent, the ability to maintain high biomass production under drought‐stress conditions. High N2 fixation under drought stress also was due largely to high N2 fixation under well‐watered conditions with significant but lower contributions from the ability to maintain high nitrogen fixation under drought stress. N2 fixation at FC was not correlated with the reduction in N2 fixation at 2/3 AW and 1/3 AW. Positive relationships between N2 fixed and biomass production of the tested peanut genotypes were found at both levels of drought stress, and the relationship was stronger the more severe the drought stress. These results suggested that the ability to maintain high N2 fixation under drought stress could aid peanut genotypes in maintaining high yield under water‐limited conditions.

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Jerusalem artichoke could be an alternative feedstock for bioenergy during times when there are shortages of other raw materials for the ethanol industry. However, insufficient water under rainfed conditions is a major cause of Jerusalem artichoke losses. Genetic variation for drought tolerance is an essential prerequisite for the development of Jerusalem artichoke cultivars with improved drought tolerance. The objectives of this study were to determine the effects of drought stress on tuber dry weight and biomass and to investigate the genotypic variability in Jerusalem artichoke germplasm. The line-source sprinkler technique was used to compare moisture responses of a range of 40 Jerusalem artichoke genotypes grown using 3 water levels. Experiments were conducted on a Yasothon soil series in Northeast Thailand during 2010/11 and 2011/12 and included extended dry periods. Drought reduced tuber dry weight and biomass, and the reductions in tuber dry weight and biomass were greater under severe drought than moderate drought conditions. Over both seasons, CN 52867, HEL 53, HEL 231, HEL 335, JA 76, HEL 65, and JA 102 × JA 89 (8) had consistently high tuber dry weight (1.3 to 4.5 t ha -1 ) and HEL 53, HEL 61, HEL 231, HEL 335, JA 76, JA 15, JA 89, HEL 65, HEL 256, and JA 102 × JA 89 (8) had consistently high biomass (2.0 to 6.8 t ha -1 ). These Jerusalem artichoke genotypes are promising parents in breeding for drought tolerance.