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Chloroplasts were isolated from pea and sunflower leaves having various water potentials. Oxygen evolution by the chloroplasts was measured under identical conditions for all treatments with saturating light and with dichloroindophenol as oxidant. Evolution was inhibited when leaf water potentials were below -12 bars in pea and -8 bars in sunflower and the inhibition was proportional to leaf water potential below these limits. Inhibition was more severe in sunflower than in pea chloroplasts. In sunflower, it could be detected after 5 minutes of leaf desiccation, and, up to 1 hour, the effect was independent of the duration of low leaf water potential.In high light, the reduction in activity of sunflower chloroplasts paralleled the reduction in CO(2) fixation by intact sunflower plants having low leaf water potentials. Stomatal apertures and transpiration rates were also reduced under these conditions and were probably limiting. In low light, intact sunflowers required more light per unit of CO(2) fixed when leaf water potentials were low than when they were high. This increased light requirement in the intact system was of a magnitude which could be predicted from the reduced oxygen evolution by the isolated chloroplasts. It was concluded that moderately low leaf water potential affects photosynthesis in at least two ways: first, through an inhibition of oxygen evolution by chloroplasts and, second, by closure of stomata in intact leaves.

Inhibition of Oxygen Evolution in Chloroplasts Isolated from Leaves with Low Water Potentials