Carbon isotope ratio in cotton varies with growth stage and plant organ

Carbon isotope ratio (C-13/C-12, expressed with a differential notation as delta(13)C) has been proposed as an indirect selection criterion for plant water-use efficiency (WUE=total dry matter produced or yield harvested/wate r used). For efficient modification of WUE in breeding programs, it is esse ntial to determine a sampling strategy, which will provide consistent genot ypic ranking for delta(13)C and maximum differentiation between genotypes. The effects of growth stages and plant organs on delta(13)C values and thei r genotypic variations were studied in cotton cultivars grown in the field under two irrigation regimes. Values of delta(13)C in leaf remained stable during peak flowering and boil development stages and significantly increas ed at boil ripening stage, which could result from the effect of late-seaso n water stress on WUE. delta(13)C varied significantly between plant organs , with lower values obtained in assimilating organs, leaf and bur, and high er values in non-assimilating organs, stem and fiber. This could possibly h ave resulted from carbon discrimination during secondary metabolism. A non- crossover interaction was found between growth stage and cultivar, whereas plant-organ effect did not interact with cultivars, indicating that a simil ar ranking of cultivars can be expected with delta(13)C at various growth s tages and plant organs. Late season leaf samples and bur of the last boil r evealed maximal differentiation among genotypes and water regimes, both amo ng other growth stages as well as among other plant organs. These samples h ave also revealed the highest correlations between delta(13)C and WUE estim ates. It is concluded that leaf sampling during boil ripening stage may be most effective for the assessment of delta(13)C as an indicator of WUE. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.