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

Carbon isotope ratio (¹³C/¹²C, expressed with a differential notation as δ¹³C) has been proposed as an indirect selection criterion for plant water-use efficiency (WUE = total dry matter produced or yield harvested/water used). For efficient modification of WUE in breeding programs; it is essential to determine a sampling strategy, which will provide consistent genotypic ranking for δ¹³C and maximum differentiation between genotypes. The effects of growth stages and plant organs on δ¹³C values and their genotypic variations were studied in cotton cultivars grown in the field under two irrigation regimes. Values of δ¹³C in leaf remained stable during peak flowering and boll development stages and significantly increased at boll ripening stage, which could result from the effect of late- season water stress on WUE. δ¹³C varied significantly between plant organs, with lower values obtained in assimilating organs, leaf and bur, and higher values in non-assimilating organs, stem and fiber. This could possibly have 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 similar ranking of cultivars can be expected with δ¹³C at various growth stages and plant organs. Late season leaf samples and bur of the last boll revealed maximal differentiation among genotypes and water regimes, both among other growth stages as well as among other plant organs. These samples have also revealed the highest correlations between δ¹³C and WUE estimates. It is concluded that leaf sampling during boll ripening stage may be most effective for the assessment of δ¹³C as an indicator of WUE.