LIGHT INACTIVATION OF FUNCTIONAL PHOTOSYSTEM-II IN LEAVES OF PEAS GROWN IN MODERATE LIGHT DEPENDS ON PHOTON EXPOSURE

To determine the dependence of in vivo photosystem (PS) II function on photon exposure and to assign the relative importance of some photopr otective strategies of PSII against excess light, the maximal photoche mical efficiency of PSII (Fv/Fm) and the content of functional PSII co mplexes (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 mu mol photons . m(-2) . s(-1)) or irradiance (from 0 to 3000 mu mol photons . m(-2) . s(-1)) at a fixed duration (1 h) after infilt ration of leaves with water (control), lincomycin (an inhibitor of chl oroplast-encoded protein synthesis), nigericin (an uncoupler), or dith iothreitol (an inhibitor of the xanthophyll cycle) through the cut pet ioles of leaves of 22 to 24-day-old plants. We observed a reciprocity of irradiance and duration of illumination for PSII function, demonstr ating 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 inhi bitors, declined pseudo-linearly with photon exposure. The number of f unctional PSII complexes declined multiphasically with increasing phot on exposure, in the following decreasing order of inhibitor effect: li ncomycin > nigericin > DTT, indicating the central role of D1 protein turnover. While functional PSII and Fv/Fm ratio showed a linear relati onship under high photon exposure conditions, in inhibitor-treated lea ves the Fv/Fm ratio failed to reveal the loss of up to 25% of the tota l functional PSII under low photon exposure. The loss of this 25% of l ess-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 chlor ophyll fluorescence when PSII reaction centres are open and closed, re spectively), but not by a loss of excitation energy at the antenna (re vealed 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 gradien t, and (iii) peas exhibit PSII heterogeneity in terms of functional st ability during photon exposure.-