Photoinactivation of photosystem II in high light-acclimated grapevines

Grapevines are considered well adapted to high irradiance during growth. It is still controversial, however, whether photoinactivation of photosystem II is completely avoided in high light-acclimated grapevines growing in the field. This study examines the functional stability of PSII in leaf discs (floated on water) of field-grown, high light-acclimated grapevines as a function of photon exposure. Measuring functional PSII units by flash-induced oxygen evolution, it was found that the susceptibility of PSII to photoinactivation was less in sun-exposed leaves than shade leaves of Vitis riparia Michaux, and enhanced by lincomycin, an inhibitor of chloroplast-encoded protein synthesis. Vitis vinifera L. cv. Chardonnay, grown in a glasshouse with slightly lower irradiance, exhibited an intermediate susceptibility. Significantly, the dark-relaxed quantum efficiency of PSII, measured as (F_m - F_o)/F_m, where F_m and F_o are the chlorophyll (Chl) fluorescence yields for closed and open reaction centres, respectively, declined much more slowly than did the number of functional PSII units in V. riparia. Thus, measurements of (F_m - F_o)/F_m may give an impression of little photoinactivation of PSII, even when nearly half of functional PSII units may be lost. By contrast, the parameter 1/F_o - 1/F_m is a more linear indicator of functional PSII units. The results indicate that grapevines may suffer photoinactivation of PSII, at least when leaf discs are floated on water.