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 fu nction of photon exposure. Measuring functional PSII units by flash-induced oxygen evolution, it was found that the susceptibility of PSII to photoina ctivation was less in sun-exposed leaves than shade leaves of Vitis riparia Michaux, and enhanced by lincomycin, an inhibitor of chloroplast-encoded p rotein synthesis. Vitis vinifera L. cv. Chardonnay, grown in a glasshouse w ith slightly lower irradiance, exhibited an intermediate susceptibility. Si gnificantly, 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 f or closed and open reaction centres, respectively, declined much more slowl y than did the number of functional PSII units in V. riparia. Thus, measure ments of (F-m-F-o)/F-m may give an impression of little photoinactivation o f PSII, even when nearly half of functional PSII units may be lost. By cont rast, the parameter 1/F-o-1/F-m is a more linear indicator of functional PS II units. The results indicate that grapevines may suffer photoinactivation of PSII, at least when leaf discs are floated on water.