Late season water stress in cotton .2. Leaf gas exchange and assimilation capacity

Water stress reduces net CO2 assimilation (A) and yield of cotton (Gossypium hirsutum L.), but our knowledge of the physiology of water stress on A and assimilation capacity is incomplete, Experiments were conducted in a rain shelter-lysimeter facility in 1990 and 1991 to determine if the yields of two short-season cotton cultivars with common ancestry, TAMCOT HQ95 (HQ95) and G&P74+ (GP74), resulted from intrinsic differences in A and assimilation capacity. Water stress was imposed by withholding 0, 50 or 75, and 100% of the depleted soil water after flowering. Results indicated that both stomatal and nonstomatal factors were important in controlling A. HQ95 had higher A and g than GP74 over leaf water potentials (psi(L)) ranging from -1.0 to -3.2 MPa. Nonstomatal limitations to A were more important than stomatal Factors when psi(L) was >-1.5 MPa. Stomatal factors limited A when psi(L) was <-1.5 MPa for both cultivars. The initial slope (S-i) and the maximum A at high c(i) (A(max)) declined with increasing water stress for both cultivars. The S-i was greater for HQ95 than GP74 over the range in psi(L) and suggest that HQ95 had higher ribulose-1,5-bisphosphate carboxylase-oxygenase activity than GP74, Increasing water stress reduced A(max) equally in both cultivars. This suggests that electron transport processes for ribulose-1,5-bisphosphate regeneration of the cultivars did not differ. Therefore, stomatal and nonstomatal CO2 assimilation processes are important in limiting A of water stressed cotton. Intrinsic differences in these processes enable some cotton cultivars to better tolerate water stress.