Regulation of electron transport in C₃ plant chloroplasts in situ and in silico: Short-term effects of atmospheric CO₂ and O

In this work, we have investigated the effects of atmospheric CO₂ and O₂ on induction events in Hibiscus rosa-sinensis leaves. These effects manifest themselves as multiphase kinetics of P₇₀₀ redox transitions and non-monotonous changes in chlorophyll fluorescence. Depletion of CO₂ and O₂ in air causes a decrease in linear electron flux (LEF) and dramatic lowering of P₇₀₀ ⁺ level. This is explained by the impediment to electron efflux from photosystem 1 (PS1) at low acceptor capacity. With the release of the acceptor deficit, the rate of LEF significantly increases. We have found that oxygen promotes the outflow of electrons from PS1, providing the rise of P₇₀₀ ⁺ level. The effect of oxygen as an alternative electron acceptor becomes apparent at low and ambient concentrations of atmospheric CO₂ (≤0.06–0.07%). A decrease in LEF at low CO₂ is accompanied by a significant (about 3-fold) rise of non-photochemical quenching (NPQ) of chlorophyll fluorescence. Such an increase in NPQ can be explained by more significant acidification of the thylakoid lumen. This occurs due to lessening the proton flux through the ATP synthases caused by a decrease in the ATP consumption in the Bassham–Benson–Calvin (BBC) cycle. pH-dependent mechanisms of electron transport control have been described within the frames of our mathematical model. The model describes the reciprocal changes in LEF and NPQ and predicts the redistribution of electron fluxes on the acceptor side of PS1. In particular, the contribution of cyclic electron flow around PS1 (CEF1) and water–water cycle gradually decays during the induction phase. This result is consistent with experimental data indicating that under the steady-state conditions the contribution of CEF1 to photosynthetic electron transport in Hibiscus rosa-sinensis is insignificant (≤10%).