Variation in carbon isotope discrimination and other traits related to drought tolerance in upland cotton cultivars under dryland conditions

The search for traits related to drought resistance is a main step in the selection of cotton with improved performance under limited water supply. The effect of cultivar and drought conditions on the physiological traits, such as carbon isotope discrimination (Delta), photosynthesis (A), transpiration (E), stomatal conductance (g(s)), leaf osmotic potential (psi(o)) and leaf-water content (LWC), were studied in Andalucia, Spain. The morphological traits, such as leaf area and specific leaf weight (SLW), were also evaluated. In the initial study performed with three cotton cultivars, positive associations between Delta and A, E and g(s) were observed under increasing water stress. The A/g(s) ratio was negatively associated with a, and a strong negative correlation was observed between SLW and Delta. In a further experiment, using a wider group of cultivars, an apparent genotypic variation in Delta was observed in plants after withholding irrigation. Genotypic variation was found for gas exchange (A, E, g(s)) and leaf variables (LWC, SLW, psi(o)). No relationship was found between a and gas exchange or leaf-related traits. Considering both samplings as situations of different water availability, and plotting the data together, a low but positive correlation was observed between Delta and g(s) or LWC, and negative between Delta and A/g(s). A remarkable correlation between LWC and gas exchange traits was found, whereas SLW was negatively correlated with A, A/E, and LWC. In dryland trials (1996 and 1997), the genotypic variation in LWC was positively associated with a. Under these conditions, an association between genotypic variation in Delta and A or A/E was observed. A positive correlation between yield and Delta was detected in 1996. Carbon isotope discrimination might be a useful tool for selecting drought-tolerant cotton genotypes but more studies are required to define more precisely the sampling conditions and the influence of factors affecting Delta and its relationship to crop yield. (C) 1999 Elsevier Science B.V. All rights reserved.