Repetitive light pulse-induced photoinhibition of photosystem I severely affects CO2 assimilation and photoprotection in wheat leaves

It was previously found that photosystem I (PSI) photoinhibition represents mostly irreversible damage with a slow recovery; however, its physiological significance has not been sufficiently characterized. The aim of the study was to assess the effect of PSI photoinhibition on photosynthesis in vivo. The inactivation of PSI was done by a series of short light saturation pulses applied by fluorimeter in darkness (every 10 s for 15 min), which led to decrease of both PSI (~60 %) and photosystem II (PSII) (~15 %) photochemical activity. No PSI recovery was observed within 2 days, whereas the PSII was fully recovered. Strongly limited PSI electron transport led to an imbalance between PSII and PSI photochemistry, with a high excitation pressure on PSII acceptor side and low oxidation of the PSI donor side. Low and delayed light-induced NPQ and P700⁺ rise in inactivated samples indicated a decrease in formation of transthylakoid proton gradient (ΔpH), which was confirmed also by analysis of electrochromic bandshift (ECSₜ) records. In parallel with photochemical parameters, the CO₂ assimilation was also strongly inhibited, more in low light (~70 %) than in high light (~45 %); the decrease was not caused by stomatal closure. PSI electron transport limited the CO₂ assimilation at low to moderate light intensities, but it seems not to be directly responsible for a low CO₂ assimilation at high light. In this regard, the possible effects of PSI photoinhibition on the redox signaling in chloroplast and its role in downregulation of Calvin cycle activity are discussed.