Effect of High Cation Concentrations on Photosystem II Activities

Abstract

The effects of wide concentration ranges of NaCI, KC, and MgCl2 on femcyanide reduction and the fluorescence induction curve of isolated spinach (Spinacia oleraces) chloroplasts were investigated. Concentrations of the monovalent salts above 100 mM and MgC12 above 25 mm produced a decrease in the rate of ferricyanide reduction by thylakoids uncoupled with 2.5 mm NH4Cl which cannot be attributed to changes in the primary photochemical capacity of photosystem IL Salt-induced decreases in the effective concentration of the secondary electron acceptor of photosystem II, plastoquinone, reduce the capacity for secondary photochemistry of photosystem II and this could contribute to the reduction in ferricyanide reduction by uncoupled thylakoids at high salinities. The rate of ferricyanide reduction by coupled thylakoids is little affected by salinity changes, indicating that the rate-limiting phosphorylation mechanism in electron flow from water to ferricyanide in coupled thylakoids is salt-tolerant, whereas the rate-limiting reaction in uncoupled ferricyanide reduction is considerably affected by salinity changes. Salt-induced changes in the fluorescence induction curve are interpreted in terms of changes in the rate constants for excitation decay by radiationless transitions, exciton transfer from photosystem II chlorophylis to other associated chlorophyll species, and photochemistry.The ionic environment of the thylakoid membrane plays an important role in regulating many of the physicochemical reactions involved in the primary photosynthetic processes. Changes in both the conformational and energetic states of the thylakoid membrane can be induced by changes in the external cation concentration. Divalent cations can affect the degree of stacking of the thylakoids; stacking increases as the external cation concentration increases (10). Such changes in membrane stacking have been related to changes in energy transfer between PSII and PSI (16), although recently it has been shown that changes in stacking -can occur unrelated to changes in fluorescence yield and energy transfer (23). The majority of studies on cation regulation of the primary photosynthetic processes have been made with monovalent cation concentrations in the range 0 to 150 mm, and 0 to 20 mm for divalent cations. Both 100 mm monovalent and 5 mm divalent cations produce an increase in PSII activity at the expense of PSI due to a decrease in energy spillover from PSII to PSI (2). Changes in energy distribution within the thylakoid are thought to be related to cation-induced changes in membrane ultrastructure, which produce a spatial separation of PSI and PSII Chl matrices, thus decreasing the possibility of exciton migration from PSII to PSI (8,18). Cations can also affect the rate of electron flow through PSI, PSII, and both PSII and PSI in series (2) effects can often be attributed to changes in the distribution of energy within the photosynthetic apparatus (2). Coupling of phosphorylation to electron transport can also be cation-controlled (1...

Correlated influence of cation concentration and excitation intensity on PS II activity-I. Influence of high salt concentration on spinach chloroplast activity

“…Mills and Barber [22] have called it the 'irreversible quenching' and Baker [1] noted that it is also present in uncoupled chloroplasts; he stated that effects of high cation concentrations are not principally attributable to phosphorylation, since coupled and uncoupled chloroplasts exhibit similar activities, as we have also found [ Figure 4]. Numerous authors, such as Krause [16], have shown an Mg +÷ efflux from thylakoid related to light-dependent proton pumping.…”

“…However, even in saturating light conditions, DCIPr is significantly increased by low concentrations of Mg +÷, although to a much lesser extent, as already noted by Bose and Arntzen [3]. Baker [1] observed an increase in DCIPr at up to 25 mM of Mg +÷ in saturating light.…”

“…This is true at saturating light intensities (as observed by Baker [1] for ferricyanide reduction) as in limiting light. So in a very high salt environment (600mM Na), DCIPr is not significantly modified by uncoupling, whereas at low concentrations DCIPr is noticeably enhanced.…”

“…In fact, other, and in some cases opposite, effects of cations can be demonstrated with even higher concentrations (up to 200mM of Mg ÷+ or 600mM ofNa ÷ [1] ).…”

Salt Tolerance in Crop Plants Monitored by Chlorophyll Fluorescence In Vivo

“…This could result from a partial block in electron transfer after PSII, but stimulation of PSII activity could be a contributing cause. In isolated chloroplasts, the photoreduction of ferricyanide is stimulated progressively with increasing concentrations of NaCl up to 30 to 50 mm (2,24), an effect possibly related to a cation-dependent alteration in membrane ultrastructure that changes the distribution of absorbed light energy in favor of PSII at the expense of PSI (3). Since PSII is the rate-limiting photosystem at nonsaturating light intensities, it is interesting to speculate whether the enhanced growth of sugar beet in the presence of salt is related to an increase in the production of photosynthate.…”