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 (F-M) and increased minimum chlorophyll fluorescence (F-0) from dark-adapted leaves; however, with increasing UV, F-M quenching was stimulated but increases in F-0 were reduced. The F-V/F-M ratio (where F-V=F-M-F-0) was clearly reduced by visible radiation (VIS): UV-B caused a moderate extra-reduction in F-V/F-M. Exposure of leaves (V. vinifera L. cv. Bacchus) to UV or VIS lamps quenched the F-M to similar extents; further, UV-B doses comparable to the field, quenched F-0. 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 F-M yield for UV- and VIS-inactivated PS II was assumed, and for lower F-0 yields of UV- than for VIS-inactivated PS II. It is deduced, that natural UV can produce inactivated PS II exhibiting relatively high F-V/F-M. The presence of UV- inactivated PS II is difficult to detect by measuring F-V/F-M in leaves. Hence, relative concentrations of intact PS II during outdoor exposure were derived from F-M. These concentrations, but not F-V/F-M, 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.