Late Season Water Stress in Cotton: II. Leaf Gas Exchange and Assimilation Capacity

Abstract

Water stress reduces net CO2 assimilation (A) and yield of cotton (Gossypium hirsututn L.), but our knowledge of the physiolog3, 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&P'/4 + (GP74), resuited 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 bad higher A and g than GP74 over leaf water potentials (~/L) ranging from-1.0 to-3.2 MPa. Nonstomatai limitations to A were more important than stomatal factors when WL was >-1.5 MPa. Stomatal factors limited A when ~L was <-1.5 MPa for both cultivars. The initial slope (Si) and the maximum A at high ci (Am~) declined with increasing water stress for both cnitivars. The S~ was greater for HQ95 than GP74 over the range in ~L and suggest that HQ95 had higher ribulose-l,5-bisphosphate carboxylase-oxygenase activity than GP74. Increasing water stress reduced Amx equally in both cultivars. This suggests that electron transport processes for ribulose-l,5bisphosphate regeneration of the cultivars did not differ. Therefore, stomatal and nonstomatai CO2 assimilation processes are important in limiting A of water stressed cotton. Intrinsic differences in these processes enable some cotton cnitivars to better tolerate water stress. D ROUGHT is an important limitation to cotton production. Reduction in photosynthetic activity and increases in leaf senescence are symptomatic of water stress and adversely affect cotton yield (Krieg, 1981; Constable and Rawson, 1980; Marani et al., 1985). Other effects of water stress include reduced cell growth and enlargement, leaf expansion, assimilate translocation, and transpiration (Hsiao, 1973). Although the consequences of water stress are well known, our ability to manipulate genetically these processes and improve drought tolerance is limited. However, genetic differences exist in the rate of A for cotton. Karami et al. (1980) found that an okra leaf cotton genotype had significantly higher A under water stress than its isoline with "normal" leaf morphology. They attributed their results to differences in source-sink relationships, nonstomatal limitations, and leaf senescence between the genotypes. Pettigrew et al. (1993) attributed the higher A of cultivars with "okra" shaped leaves to a higher specific leaf weight (g/m 2 leaf area) and higher leaf chlorophyll concentrations compared to "normal" leaf cultivars. They hypothesized that the genotypic differences in A were due to a higher concentration of the photosynthetic apparatus per leaf, caused by in

Plant Water-Stress Response Mechanisms

“…Water stress is also found to inhibit chlorophyll synthesis and subsequently decrease chlorophyll content of leaves. In severe stress, photosynthesis may be more controlled by the chloroplast's capacity to fix CO 2 than by the increased diffusive resistance (Faver et al 1996;Herppich and Peckmann 1997).…”

Respostas de plantas de Eleusine indica sob diferentes condições hídricas a herbicidas inibidores da ACCase

“…O déficit hídrico afeta a bioquímica, a fisiologia, a morfologia e os processos de desenvolvimento das plantas, reduzindo a fotossíntese por meio de redução na área foliar disponível para interceptar a radiação solar; redução da difusão do CO 2 para dentro da folha devido ao fechamento dos estômatos; redução da habilidade dos cloroplastos para fixar o CO 2 que neles penetra; e enrolamento e senescência das folhas (Levitt, 1980;Jones, 1985;Faver et al, 1996). Esses fatores podem influenciar a eficiência dos herbicidas.…”

Cotton genotypic variation in the photosynthetic response to irradiance

“…While genotypic differences have been detected in cotton leaf photosynthesis (Pettigrew et al 1993, Pettigrew and Meredith 1994, Faver et al 1996 and in the photosynthetic response to changing CO 2 concentrations (Pettigrew and Turley 1998), little evidence exists documenting differences among cotton genotypes in the photosynthetic response to changing PPFD. Therefore, the objective of this research was to determine if differences in the photosynthetic response to different PPFD existed among a diverse group of cotton genotypes.…”