Light inactivation of functional photosystem II in leaves of peas grown in moderate light depends on photon exposure

Abstract

Abstract.To determine the dependence of in vivo photosystem (PS) II function on photon exposure and to assign the relative importance of some photoprotective strategies of PSII against excess light, the maximal photochemical efficiency of PSII (Fv/Fm) and the content of functional PSII complexes (measured by repetitive flash yield of oxygen evolution) were determined in leaves of pea (Pisum sativum L.) grown in moderate light. The modulation of PSII functionality in vivo was induced by varying either the duration (from 0 to 3 h) of light treatment (fixed at 1200 or 1800 ~tmol photons 9 m -2 9 s q) or irradiance (from 0 to 3000 p.mol photons 9 m -2 9 s -1) at a fixed duration (1 h) after infiltration of leaves with water (control), lincomycin (an inhibitor of chloroplast-encoded protein synthesis), nigericin (an uncoupler), or dithiothreitol (an inhibitor of the xanthophyll cycle) through the cut petioles of leaves of 22 to 24-day-old plants. We observed a reciprocity of irradiance and duration of illumination for PSII function, demonstrating that inactivation of functional PSII depends on the total number of photons absorbed, not on the rate of photon absorption. The Fv/Fm ratios from photoinhibitory light-treated leaves, with or without inhibitors, declined pseudo-linearly with photon exposure. The number of functional PSII complexes declined multiphasically with increasing photon exposure, in the following decreasing order of inhibitor effect: lincomycin > nigericin > DTT, indicating the central role of D 1 protein turnover. While functional PSII and Fv/Fm ratio showed a linear relationship under high photon exposure conditions, in inhibitor-treated leaves the Fv/Fm ratio failed to reveal the loss of up to 25% of the total functional PSII under low photon expoAbbreviations: D1 protein=psbA gene product; DTT=dithiothre-itol; Fo=chlorophyll fluorescence corresponding to open PSII reaction centres; Fv, Fm=variable and maximum fluorescence after dark incubation, respectively; Fs, Fm'=steady-state and maximum fluorescence during illumination, respectively; P680=reaction-centre chlorophyll and primary electron donor of PSII; PS=photo-system Correspondence to: Dr. Y.-I. Park; FAX: 61 (6) 2465000; Tel.: 61 (6) 2 4650 66; E-mail: park@pican.pi.csiro.au sure. The loss of this 25% of less-stable functional PSII was accompanied by a decrease of excitation-energy trapping capacity at the reaction centre of PSII (revealed by the fluorescence parameter, 1/Fo-1/Fm, where Fo and Fm stand for chlorophyll fluorescence when PSII reaction centres are open and closed, respectively), but not by a loss of excitation energy at the antenna (revealed by the fluorescence parameter, 1/Fm). We conclude that (i) PSII is an intrinsic photon counter under photoinhibitory conditions, (ii) PSII functionality is mainly regulated by D1 protein turnover, and to a lesser extent, by events mediated via the transthylakoid pH gradient, and (iii) peas exhibit PSII heterogeneity in terms of functional stability during photon exposure.

A mechanistic model of photoinhibition

“…The ratio of these two forms was thought to be independent of total antenna size (and therefore of photoacclimative state). The loss of PSII α reaction centres, however, did not appear to affect the quantum yield of PSII (Oquist et al, 1992 ;Park et al, 1995), and thus may partially protect PSII. The energy-dependent quenching provided by these susceptible reaction centres (when damaged) may protect PSII β reaction centres.…”

“…This decrease in α is due to the closure of reaction centres, which leads to a decrease in the value of φ yield (Park et al, 1995). A loss of up to 25 % of active D1 proteins has been found not to decrease φ yield (Park et al, 1995).…”

“…There may be two main forms of PSII in vivo, PSII α and PSII β (Park et al, 1995). Park et al (1995) proposed that 25 % of PSII reaction centres exist as PSII α (possessing a large light harvesting antenna), which are relatively more susceptible to damage than PSII β . The ratio of these two forms was thought to be independent of total antenna size (and therefore of photoacclimative state).…”

“…This decrease in α is due to the closure of reaction centres, which leads to a decrease in the value of φ yield (Park et al, 1995). A loss of up to 25 % of active D1 proteins has been found not to decrease φ yield (Park et al, 1995). The mechanism behind this is still unclear, but may be because of the functional heterogeneity of PSII reaction centres (Park et al, 1995), or to the channelling of photons from damaged reaction centres to functioning centres (Raven & Samuelsson, 1986).…”

“…A loss of up to 25 % of active D1 proteins has been found not to decrease φ yield (Park et al, 1995). The mechanism behind this is still unclear, but may be because of the functional heterogeneity of PSII reaction centres (Park et al, 1995), or to the channelling of photons from damaged reaction centres to functioning centres (Raven & Samuelsson, 1986). Although 25 % of active D1 may be lost without a decrease in φ yield, after this threshold φ yield decreases linearly with further decreases in D1 (Park et al, 1995).…”