Studies on Cation-induced Thylakoid Membrane Stacking, Fluorescence Yield, and Photochemical Efficiency

Abstract

Trypsin digestion of photosynthetic membranes isolated from spinach (Spinacia oleraces L.) leaves eliminates the cation stimulation of chloro-phyil fluorescence. High concentrations of cations protect the fluorescence yield against trypsin digestion, and the cation specificity for this protection closely resembles that required for the stimulation of fluorescence by cations. Trypsin digestion reverses cation-induced thylakoid stackng, and the time course of this effect seems to parallel that of the reversal of cation fluorescence. High concentrations of cations protect thylakoid stacking and cation-stimulated fluorescence alike. The cation stimulation of photo-sytem II photochemistry remains intact after trypsinization has reversed both cation-induced thylakoid stacking and fluorescence yield It is concluded that cation-stimulated fluorescence yield, and not the cation stimulation of photosystem II photochemistry, is associated with thylakoid membrane stacking. In 1966 Izawa and Good (11) first observed the dramatic effect of cations on the chloroplast membrane structure. Incubation of chloroplasts in low salt medium resulted in the unstacking of grana that could be reversed upon readdition of high concentrations of monovalent cations or low concentrations of divalents. These effects have since been confirmed by others (7. 9, 15, 19). A rather similar cation dependency for both the room temperature increase in variable fluorescence and the PSII photochemistry was first noted by Homann (10) and Murata (16, 17) and subsequently by others (12). Thus, divalent cations in the range of 1 to 5 mm and monovalent cations in the range of 50 to 150 mm stimulate membrane stacking, fluorescence yield and PSII photochemistry in a reversible fashion. We have demonstrated (13, 14) that the cation influence on PSII fluorescence and photochemistry can be completely separated from one another after treatment of chloro-plast membranes with the proteolytic enzyme trypsin. Thus we believe that two different cation interaction sites are involved, in spite of the similar cation requirements manifested by the two responses.