Three phases of energy-dependent induction of $${\text{P}}_{{700}}^{+}$$ P 700 + and Chl a fluorescence in Tradescantia fluminensis leaves

Abstract

In plants, the short-term regulation (STR, seconds to minute time scale) of photosynthetic apparatus is associated with the energy-dependent control in the chloroplast electron transport, the distribution of light energy between photosystems (PS) II and I, activation/deactivation of the Calvin–Benson cycle (CBC) enzymes, and relocation of chloroplasts within the plant cell. In this work, using a dual-PAM technique for measuring the time-courses of P700 photooxidation and Chl a fluorescence, we have investigated the STR events in Tradescantia fluminensis leaves. The comparison of Chl a fluorescence and \({\text{P}}_{{700}}^{+}\) induction allowed us to investigate the contribution of the trans-thylakoid pH difference (ΔpH) to the STR events. Two parameters were used as the indicators of ΔpH generation: pH-dependent component of non-photochemical quenching of Chl a fluorescence, and pHin-dependent rate of electron transfer from plastoquinol (PQH2) to \({\text{P}}_{{700}}^{+}\) (via the Cyt b6f complex and plastocyanin). In dark-adapted leaves, kinetics of \({\text{P}}_{{700}}^{+}\) induction revealed three phases. Initial phase is characterized by rapid electron flow to \({\text{P}}_{{700}}^{+}\) (τ1/2 ~ 5–10 ms), which is likely related to cyclic electron flow around PSI, while the outflow of electrons from PSI is restricted by slow consumption of NADPH in the CBC. The light-induced generation of ΔpH and activation of the CBC promote photooxidation of P700 and concomitant retardation of \({\text{P}}_{{700}}^{+}\) reduction (τ1/2 ~ 20 ms). Prolonged illumination induces additional slowing down of electron transfer to \({\text{P}}_{{700}}^{+}\) (τ1/2 ≥ 30–35 ms). The latter effect is not accompanied by changes in the Chl a fluorescence parameters which are sensitive to ΔpH generation. We suggest the tentative explanation of the latter results by the reversal of Q-cycle, which causes the deceleration of PQH2 oxidation due to the back pressure of stromal reductants.