Action of UV and visible radiation on chlorophyll fluorescence from dark-adapted grape leaves (Vitis vinifera L.)

Grapevine plants (Vitis vinifera L. cv. Silvaner) were cultivated under shaded conditions in the absence of UV radiation in a greenhouse, and subsequently placed outdoors under filters transmitting natural radiation, or screening out the UV-B (280 to 315 nm), or screening out the UV-A (315 to 400 nm) and the UV-B spectral range. All conditions decreased maximum chlorophyll fluorescence (FM) and increased minimum chlorophyll fluorescence (F0) from dark-adapted leaves; however, with increasing UV, FM quenching was stimulated but increases in F0 were reduced. The FV/FM ratio (where FV=FM-F0) was clearly reduced by visible radiation (VIS): UV-B caused a moderate extra-reduction in FV/FM. Exposure of leaves (V. vinifera L. cv. Bacchus) to UV or VIS lamps quenched the FM to similar extents; further, UV-B doses comparable to the field, quenched F0. A model was developed to describe how natural radiation intensities affect PS II and thereby change leaf fluorescence. Fitting theory to experiment was successful when the same FM yield for UV- and VIS-inactivated PS II was assumed, and for lower F0 yields of UV- than for VIS-inactivated PS II. It is deduced, that natural UV can produce inactivated PS II exhibiting relatively high FV/FM. The presence of UV-inactivated PS II is difficult to detect by measuring FV/FM in leaves. Hence, relative concentrations of intact PS II during outdoor exposure were derived from FM. These concentrations, but not FV/FM, correlated reasonably well with CO2 gas exchange measurements. Consequently, PS II inhibition by natural UV could be a main factor for UV inhibition of photosynthesis.