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 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.

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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 ×

Terminal drought induces preharvest aflatoxin contamination (PAC) in peanut. Drought resistance traits are promising as indirect selection tools for improving resistance to PAC. The objectives of this study were to determine the effects of terminal drought on PAC and to investigate the associations between surrogate traits for drought tolerance and PAC. Field tests under rainout shelters were conducted in the dry seasons 2004/2005 and 2005/2006. Eleven peanut genotypes were evaluated under irrigated and terminal drought conditions. Data were recorded for physiological traits, total biomass, pod yield, Aspergillus flavus colonization and PAC. ICGV 98305, ICGV 98330, ICGV 98348, ICGV 98353 and Tifton-8 had low aflatoxin contamination in both years. Traits related to drought resistance were associated well with those related to PAC under drought conditions. Specific leaf area, relative water content, chlorophyll density and drought stress ratings are the best traits for use as indirect selection tools for lower PAC. Breeding for drought tolerance using these traits as selection criteria may help to accelerate progress in developing resistance to PAC.Key words: chlorophyll density -drought stress ratingsdrought tolerance index -indirect selection tools -relative water content -specific leaf area Late season drought on peanuts (Arachis hypogaea L.) generally results in yield reduction, low seed quality, high incidences of Aspergillus flavus colonization and high aflatoxin contamination. Aspergillus flavus colonization during the preharvest period is most important as it can serve as initial inoculum for further A. flavus colonization and, ultimately, aflatoxin contamination. Because aflatoxin is well recognized as a potent carcinogen, reduction of aflatoxin production is an important objective for peanut breeding programmes around the world.Breeding progress for reduction of preharvest aflatoxin contamination (PAC) in peanut using field-based selection approaches have been slow because of large and uncontrollable genotype · environment (G · E) interactions (Holbrook et al. 1994, Anderson et al. 1995, 1996. More consistent and simple traits with lower G · E interactions are worth exploring. A relationship between drought tolerance and reduced PAC has been demonstrated. Some drought-resistant genotypes (Rucker et al. 1995) were observed to have lower PAC when subjected to late season heat and drought stress (Holbrook et al. 2000a). From a breeding point of view, selection for drought tolerance could be an efficient strategy for reducing PAC.Under drought stress, the loss of the capacity of peanut seeds to produce phytoalexins, an immune response to counteract fungal colonization resulted in higher PAC. The ability to maintain higher moisture contents in pods during drought periods may be an important trait enabling cultivars to resist aflatoxin production (Wotton and Strange 1985, Dorner et al. 1989). In addition, drought resistance traits are promising as indirect selection tools for improving resistance to PAC in pean...