MEASUREMENT OF LEAF EPIDERMAL TRANSMITTANCE OF UV-RADIATION BY CHLOROPHYLL FLUORESCENCE

In higher plants one of the important functions of the leaf epidermis is the effective screening of ultraviolet-B (280-320 nm, UV-B) radiati on, due mostly to phenolic compounds. The assessment of the contributi on of this function is necessary for an evaluation of the impact of in creasing UV-B radiation. A method is proposed to estimate epidermal tr ansmittance on the basis of chlorophyll fluorescence measurements. Flu orescence of chlorophyll induced by UV-A (320-400 nm, measuring beam c entered at 366 nm, half band width 32 nm) or UV-B (measuring beam cent ered at 314 nm, half band width 18 nm) is compared to that induced by a blue-green measuring light (475 nm, half band width 140 nm). It is s hown that the ratios of UV- and blue-green (BG)-induced fluorescence, F(UV-A)/F(BG) and F(UV-B)/F(BG), are relatively constant among leaf sa mples of various species (Vicia faba, Spinacia oleracea, Rumex scutatu s) from which the epidermis was removed. In epidermis-free leaves no s ignificant differences were found between adaxial and abaxial leaf sid es, suggesting that leaf structure has negligible influence on the F(U V)/F(BG) ratios. On the other hand, fluorescence excitation ratios var ied over a vast range when intact leaves from different species and ha bitats were investigated. Ratios were low in sun leaves and relatively high in shade-and greenhouse-grown leaves. By relating these results to those obtained with epidermis-free leaves, epidermal transmittances for UV-B radiation could be estimated, with values ranging between 1 and 45%. The data demonstrate a large adaptability of epidermal UV-A a nd UV-B transmittance in higher plants. The proposed method may prove a versatile and relatively simple tool for investigating epidermal UV transmittance complementing established methods.