LATE SEASON WATER-STRESS IN COTTON .2. LEAF GAS-EXCHANGE AND ASSIMILATION CAPACITY

Water stress reduces net CO2 assimilation (A) and yield of cotton (Gos sypium hirsutum L.), but our knowledge of the physiology of water stre ss on A and assimilation capacity is incomplete, Experiments were cond ucted in a rain shelter-lysimeter facility in 1990 and 1991 to determi ne if the yields of two short-season cotton cultivars with common ance stry, TAMCOT HQ95 (HQ95) and G&P74+ (GP74), resulted from intrinsic di fferences in A and assimilation capacity. Water stress was imposed by withholding 0, 50 or 75, and 100% of the depleted soil water after flo wering. Results indicated that both stomatal and nonstomatal factors w ere 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. Nonstom atal limitations to A were more important than stomatal Factors when p si(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 h igh c(i) (A(max)) declined with increasing water stress for both culti vars. 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 proce sses for ribulose-1,5-bisphosphate regeneration of the cultivars did n ot differ. Therefore, stomatal and nonstomatal CO2 assimilation proces ses are important in limiting A of water stressed cotton. Intrinsic di fferences in these processes enable some cotton cultivars to better to lerate water stress.