Genotypic variation for drought tolerance in cotton

Increasing scarcity of irrigational water is a major threat to sustainable production of cotton (Gossypium hirsutum L.). It could be resolved by developing drought-tolerant cultivars. Osmotic adjustment and cellular membrane stability are well-documented traits that help to sustain yield under drought in cereals. However, their utility in cotton is not well established. Here, we studied genotypic variability and relationships among osmotic adjustment, cell membrane stability and productivity traits under field-induced water stress at the flowering stage. We evaluated a set of cotton germplasm comprising 32 cotton genotypes under contrasting water regimes for measurements of productivity including seedcotton yield, number of bolls per plant and boll weight, and physiological attributes such as osmotic adjustment and cell membrane stability in two field trials. The mean reduction in seedcotton yield due to water deficit was 20 and 43% in 2003 and 2004, respectively. Genotypes differed considerably for relative yield losses due to water stress ranging from 20 to 74%. Significant association between number of bolls and seedcotton yield under a water-limited regime suggests boll retention as the principal determinant of yield in a water-deficit-stress environment. Cell membrane stability varied significantly among the cotton genotypes; however, its association with productivity measurements was not significant in the water-limited regime. The significant positive correlation found between cell membrane stability and osmotic adjustment implicates the role of osmolytes in the protection of various cellular functions, including those associated with cellular membranes. Moderate but significant differences for osmotic adjustment were found among the genotypes in both years. Osmotic adjustment was positively associated with seedcotton yield under the water-limited regime and inversely correlated with the drought susceptibility index. These results demonstrated the contribution of osmotic adjustment in sustaining yield under water-deficit stress in cotton. Thus, like cereals, osmotic adjustment may be useful as a selection criterion in breeding programs with the objective of improving drought tolerance and yield in cotton under water-limited environments; however, the role of cell membrane stability as a drought-tolerant trait requires further investigation.