Upon termination of watering of plants of Nerium oleander exposed to high light, photochemical efficiency became reduced as leaf water content decreased. Evidence is presented that this type of photoinhibition reflects to a substantial degree radiationless dissipation of excitation energy, probably mediated by the
carotenoid zeaxanthin. During the imposition of
water stress, the
zeaxanthin content of leaves increased at the expense of
violaxanthin and
beta-carotene as a water deficit developed over a period of several days. The increase in
zeaxanthin content was linearly related to an increase in the rate of radiationless energy dissipation in the antenna
chlorophyll as calculated from the characteristics of
chlorophyll a fluorescence measured with a pulse amplitude modulated fluorometer at room temperature. The increase in the rate of radiationless dissipation was also linearly related to a decrease in PSII photochemical efficiency as indicated by the ratio of variable to maximum fluorescence. Leaves of well-watered shade plants of N. oleander exposed to strong light showed a similar increase in
zeaxanthin content as sun leaves of the same species subjected to drought in strong light. Shade leaves possessed the same capacity as sun leaves to form
zeaxanthin at the expense of both
violaxanthin and
beta-carotene. The resistance of this species to the destructive effects of excess light appears to be related to interconversions between
beta-carotene and the three
carotenoids of the xanthophyll cycle.