Regulation of the imbalance in light excitation between Photosystem II and Photosystem I by cations and by the energized state of the thylakoid membrane

Abstract

The imbalance in photoactivity between the two photosystems in broken chloroplasts during steady-state electron transport was investigated using modulated chlorophyll a fluorimetry and oxygen evolution. No imbalance in favor of PS II (imbalance term equals zero) was found at low cation concentration (e.g., 10 mM NaCl) where the membranes are unstacked, while some imbalance in favor of PS II (imbalance term about 0.1–0.2) could be observed at ‘high’ cation medium (e.g., 100 mM for univalent, 5 mM for divalent and 100 μM for trivalent cations) where the membranes are stacked. At the high cation concentration the imbalance was particularly noticeable at a pH range 6–7.5 under conditions where the membranes were non-energized, e.g., in the presence of a range of uncouplers and ionophores (an imbalance term of between 0.44 and 1.1). In the absence of uncouplers or ionophores the imbalance term was initially high, decreasing to a low steady-state value during the light-induced energization of the membranes. The increase in the imbalance measured with the addition of gramicidin D was wavelength-dependent, implying changes in the allocation of excitation energy to the photosystems rather than any other mechanism. This effect was reversed at higher pH: At a pH higher than about 8, the imbalance in absence of uncouplers or ionophores was stronger than in their presence. The relation between the state of imbalance and the cross-membrane proton gradient (ΔpH) was not straightforward or simple, as follows: (i) Imbalance was induced by uncouplers and ionophores also at sufficiently low light intensities which produce only very small ΔpH. (ii) Valinomycin (+KCl) had the same effect as uncouplers like gramicidin D, nigericin, NH4Cl and others, although it presumably abolishes membrane potential but not ΔpH. (iii) The effect of gramicidin D and NH4Cl was close to saturation at concentrations which affect ΔpH still minutely. (iv) The same level of large imbalance could be achieved even without uncouplers or ionophores when a much higher cation concentration (approx. 10-fold) than that considered normal to achieve membrane stacking was used. It is therefore concluded that: (a) High cationic levels change the allocation of excitation energy, probably via the screening of negative charges on the thylakoid surface. (b) The effect of uncouplers and ionophores in the presence of high cationic levels most probably reflects, at least in part, the effect on external surface charges, their exposure by membrane energization and their screening by cations.