Inhibition of the cation-induced reversible changes in excitation energy distribution in thylakoids of BASF 13.338-grown plants

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Measurements of relative quantum yields of Photosystem II and Photosystem I partial reactions and room-temperature chlorophyll a fluorescence of thylakoids in high- and low-salt media showed that the cation-induced changes in the excitation energy distribution were inhibited in the thylakoids isolated from pea plants grown in the presence of sublethal concentration of the pyridazinone herbicide BASF 13.338. Simultaneous measurement of Photosystem II and Photosystem I fluorescence emission kinetics at 77 K showed that the ability of cations to regulate excitation energy spillover from Photosystem II to Photosystem I was inhibited in thylakoids of the BASF-grown plants. Cation regulation of the absorption cross-section of the photosystems was not affected. Electron microscope data revealed that the proportion of stroma membranes relative to grana membranes was markedly less in the thylakoids of the BASF-grown plants. Furthermore, when the thylakoids were resuspended in low-salt medium, no unstacking of the grana was detected. The observed inhibition of cation-induced spillover change was presumably due to loss of ability of these photosynthetic membranes to undergo unstacking in low-salt medium.