Inhibition of non-photochemical quenching increases functional absorption cross-section of photosystem II as excitation from closed reaction centres is transferred to open centres, facilitating earlier light saturation of photosynthetic electron transport

Induction of non-photochemical quenching (NPQ) of chlorophyll fluorescence in leaves affords photoprotection to the photosynthetic apparatus when, for whatever reason, photon capture in the antennae of photosystems exceeds their capacity to utilise this excitation in photochemistry and ultimately in CO2 assimilation. Here we augment traditional monitoring of NPQ using the fast time resolution, remote and relatively non-intrusive light induced fluorescence transient (LIFT) technique (Kolber et al. 2005; Osmond et al. 2017) that allows direct measurement of functional (sigma'(PSII)) and optical cross-sections (alpha'(PSII)) of PSII in situ, and calculates the half saturation light intensity for ETR (E-k). These parameters are obtained from the saturation and relaxation phases of fluorescence transients elicited by a sequence of 270, high intensity 1 mu s flashlets at controlled time intervals over a period of 30 ms in the Q(A) flash at intervals of a few seconds. We report that although sigma'(PSII) undergoes large transient increases after transfer from dark to strong white light (WL) it declines little in steady-state as NPQ is induced in shadeand sun-grown spinach and Arabidopsis genotypes Col, OEpsbs, pgr5bkg, stn7 and stn7/8. In contrast, sigma'(PSII) increases by similar to 30% when induction of NPQ in spinach is inhibited by dithiothreitol and by inhibition of NPQ in Arabidopsis npq1, npq4 and pgr5. We propose this increase in sigma'(PSII) arises as some excitation from closed PSII reaction centres is transferred to open centres when excitation partitioning to photochemistry (Y-II) and NPQ (Y-NP) declines, and is indicated by an increased excitation dissipation from closed PSII centres (Y-NO, including fluorescence emission). Although Ek increases following dissipation of excitation as heat when NPQ is engaged, it declines when NPQ is inhibited. Evidently photochemistry becomes more easily light saturated when excitation is transferred from closed RCIIs to open centres with larger sigma'(PSII). The NPQ mutant pgr5 is an exception; E-k increases markedly in strong light as electron transport Q(A) -> PQ and PQ -> PSI accelerate and the PQ pool becomes strongly reduced. These novel in situ observations are discussed in the context of contemporary evidence for functional and structural changes in the photosynthetic apparatus during induction of NPQ.