Effects of Water Stress on the Photosynthetic Efficiency of Plants
The response of Photosystem II (PS II) activity to water deficit, as characterized by measurements of chlorophyll (Chi) a fluorescence, greatly differs among plant species. Only cyanobacteria and lower plants are capable of rapid restoration of the PS II photochemical capacity following their rewetting after strong inhibition due to severe desiccation. In lower plants kept in light and in the air-dried state, which completely arrests PS II photochemical capacity, the minimum fluorescence yield is strongly quenched thus demonstrating a unique mechanism of long-term photoprotection. In addition water stress stimulates the well-known mechanisms of protection against strong light by significantly reducing the light requirement for zeaxanthin synthesis. In higher plants, PS II is highly resistant to mild water stress, as revealed by unchanged Fv/Fm (variable to maximal Chi a fluorescence) ratios. For C3 higher plants, stomatal closure greatly reduces the C02 content in the leaves kept under water stress thus enhancing electron flow with oxygen as the terminal acceptor. Also, the acidification of the intrathylakoid space owing to photorespiratory activity of Rubisco and/or PS I-driven cyclic electron transport in stressed C3 plants results in photoprotection of the photosynthetic apparatus. Both the above mechanisms are not able, however, to fully protect PS II activity against strong light, when water stress becomes more severe. The C3 plants are similar in this respect with CAM plants, in which the potential capacity for C02 production from decarboxylation of organic acids seems to be insufficient for the maintenance of a high trans-thylakoid proton gradient under strong light. Unlike in lower plants, after a significant water deficit, restoration of the initial PS II activity in higher plants requires several days under well-watered conditions. Nevertheless, the restoration of PS II activity proceeds much faster than that of the overall photosynthetic capacity.
KeywordsWater Stress Chlorophyll Fluorescence Crassulacean Acid Metabolism Resurrection Plant Crassulacean Acid Metabolism Plant
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