Effects of drought on primary photosynthetic processes of cotton leaves.

The effects of drought on Photosystem II (PSII) fluorescence and photosynthetic electron transport activities were analyzed in cotton. Water stress did not modify the amplitude of leaf variable fluorescence at room temperature in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) nor at 77 K. It is therefore concluded that photon collection, their distribution between the two photosystems, and PSII photochemistry are unaffected by the stress. In droughted leaves at room temperature under low exciting light, the transitory maximum (F(p)) and steady state (F(t)) fluorescence levels are increased; under high exciting light, F(p) level and the rise time from the initial level (F(o)) to F(p) are unchanged, whereas F(p) to F(t) decay time is increased. These results infer that the drought slows the rate of plastoquinone reoxidation. This conclusion agrees with a larger proportion of reduced primary PSII electron acceptor Q(A) measured at the steady state under low light. In thylakoids isolated from droughted leaves, PSII mediated electron flow was the same as in thylakoids from control leaves, whereas PSI mediated electron transport was inhibited. It is shown that water stress does not induce sensitization to photoinhibition in cotton.