VARIATION IN WATER-USE EFFICIENCY AND ITS RELATION TO CARBON-ISOTOPE RATIO IN COTTON

Cotton (Gossypium spp.) is often exposed to drought, which adversely a ffects both yield and quality. Improved water-use efficiency (WUE = to tal dry matter produced or yield harvested / water used) is expected t o reduce these adverse effects. Genetic variability in WUE and its ass ociation with photosynthetic rate and carbon isotope ratio (C-13/C-12) in cotton are reported in this paper. WUE of six cotton cultivars-G. hirsutum L., G. barnbadense L., and an interspecific F-1 hybrid (G. hi rsutum X G. barbadense, ISH), was examined under two irrigation regime s in two field trials. The greatest WUE was obtained by two G. hirsutu m cultivars (2.55 g dry matter or 1.12 g seed-cotton L-1 H2O); the ISH obtained similar or somewhat lower values, and two G. barbadense cult ivars and one G. hirsutum cultivar exhibited the lowest values (2.1 g dry matter or 0.8 to 0.85 g seed-cotton L-1 H2O). These results indica te that different cotton cultivars may have evolved different environm ental adaptations that affect their WUE. Photosynthetic rate was corre lated with WUE in only a few cases, emphasizing the limitation of this parameter as a basis for estimating crop WUE. Under both trials, WUE was positively correlated with carbon isotope ratio, indicating the po tential of this technique as a selection criterion for improving cotto n WUE.