these requirements (Stiller et al., 2004), and those with the okra leaf trait have also been successful (Stiller et al., Water stress adversely affects both yield and fiber quality of cotton 2004;Thomson, 1994).(Gossypium hirsutum L.) and any improvement in components of water use efficiency (WUE) would be expected to partially reduce Physiological traits associated with water use effithese adverse affects. Six field experiments in Australia and one in ciency or stress tolerance have rarely been used in plant Texas using four Australian and three Texas cultivars determined breeding. This is due to difficulties associated with meagenetic differences in physiological WUE parameters. Four of the exsuring these traits on large numbers of plants, low heritaperiments were grown under dryland conditions and three under irbilities, and complex relationships between these traits rigated conditions. Cultivar differences for net photosynthesis (A ) and yield (Hall et al., 1994). However, the demonstrawere found in only 30% of comparisons, ratio of intercellular CO 2 tion that carbon isotope discrimination (⌬) could proconcentration to ambient CO 2 concentration (C i /C a ) in 20%, and carvide an indirect measure of plant transpiration efficiency bon isotope 13 C discrimination (⌬) in 69%. Cultivars Cascot 014 and (Farquhar et al., 1982; Farquhar and Richards, 1984) Sicot 189 had significantly (P Յ 0.05) higher A than Siokra 1-4 and has renewed interest in breeding for physiological water Siokra L23 and these differences were consistent across experiments. A significant (P Յ 0.05) cultivar ϫ experiment interaction suggests use efficiency. Discrimination against 13 CO 2 in favor of C i /C a would be an environment specific measure enabling confident 12 CO 2 during CO 2 diffusion through the stomata and durdistinction of cultivar differences. Tamcot Sphinx and Cascot 014 had ing photosynthesis in C 3 plants is closely related to the significantly higher ⌬ (P Յ 0.001) than Siokra L23, with the ranking transpiration efficiency integrated over the life of the differing in only one irrigated experiment. Broad sense heritability plant material sampled (Farquhar and Richards, 1984; estimates were 0.65, 0.68, and 0.56 for A, ⌬, and lint yield, respectively. Condon et al., 1992). In recent times, selection for ⌬ Cultivar variation for these physiological traits measured in single in early generation progeny has increased the yield of leaves of cotton, and related indirectly to plant WUE, indicate potenwheat (Triticum aestivum L.) grown under dryland contial for genetic advancement through selection. Further studies to deditions (Rebetzke et al., 2002) and led to the subsequent termine heritability of these physiological traits in segregating popurelease of a new cultivar (Anon., 2002). lations are needed to confirm their usefulness in a cotton-breeding program.
