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.