Studies on the Mechanism of Photoinhibition in Higher Plants

Abstract

Cucumber plants (C. sativus L.), grown at low quantum flux density (120-150 microeinsteins/m2 per s), were photoinhibited by a 3-h exposure in air to 10 times the light intensity experienced during growth. Chloroplasts were isolated from photoinhibited and control leaves and the following activities determined: O2 evolution in the presence of ferricyanide, photosystem I activity, noncyclic and cyclic photophosphorylation and light-induced proton uptake. Chlorophyll and chloroplast absorbance spectra and chloroplast fluorescence were measured. Photosystem II electron transport and noncyclic photophosphorylation were inhibited by .apprx. 50%, while cyclic photophosphorylation was less inhibited and photosystem I electron transport and light-induced proton uptake were unaffected. Electron transport to methylviologen could not be fully restored by electron donation to photosystem II. Chloroplast fluorescence induction at room temperature was strongy reduced following photoinhibition. There was no difference in the absorption spectra of the extracted chlorophylls from control and photoinhibited chloroplast but an increase of the absorption in the blue wavelength region was observed in the photoinhibited chloroplasts. High light stress does not result in alteration of the membrane properties, as is the case in low-temperature stress for example, but affects directly the photosynthetic reaction centers, primarily of photosystem II.