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) radiation, due mostly to phenolic compounds. The assessment of the contribution of this function is necessary for an evaluation of the impact of increasing UV‐B radiation. A method is proposed to estimate epidermal transmittance on the basis of chlorophyll fluorescence measurements. Fluorescence of chlorophyll induced by UV‐A (320–400 nm, measuring beam centered at 366 nm, half band width 32 nm) or UV‐B (measuring beam centered 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 shown 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 samples of various species ( Vicia faba, Spinacia oleracea, Rumex scutatus ) from which the epidermis was removed. In epidermis‐free leaves no significant differences were found between adaxial and abaxial leaf sides, suggesting that leaf structure has negligible influence on the F(UV)/F(BG) ratios. On the other hand, fluorescence excitation ratios varied over a vast range when intact leaves from different species and habitats 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 and 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.