Induction changes in photosystems I and II in plant leaves upon modulation of membrane ion transport

The steady-state regime of linear photosynthetic electron transport implies concerted operation of photosystems I and II (PSI and PSII) in plant leaves. Acidification of the thylakoid lumen is known to cause down-regulation of PSII photochemical activity but it is not yet clear how the proton accumulation in the lumen affects the PSI activity and coordinated operation of the two photosystems in intact leaves. Chlorophyll fluorescence and absorbance of oxidized chlorophyll P700 in the near-infrared region ΔA 810–870 (ΔA 810) are convenient noninvasive indicators of the redox state of PSII and PSI components, respectively. Simultaneous measurements of chlorophyll fluorescence and ΔA 810 in pea leaves revealed that some kinetic stages in the induction curves occur synchronously both in dark-adapted and preilluminated leaves. After the treatment of leaves with ionophores promoting or inhibiting the light-induced thylakoid pH gradient (valinomycin, nigericin, monensin), the induction curves of ΔA 810 and chlorophyll fluorescence were consistently modified. The results suggest that characteristic stages of ΔA 810 induction curve, representing the second and the third waves of P700 photooxidation, are closely related to ΔpH generation, although the bases of ΔpH dependence differ for these two stages. The second wave of ΔA 810 depends presumably on stroma alkalinization as a precondition for photoactivation of electron flow from PSI to terminal acceptors. The third wave of ΔA 810 is apparently due to retardation of electron flow between PSII and PSI upon acidification of the lumen.