Site-dependent differences in transmittance and UV-B-absorbing capacity of isolated leaf epidermes and mesophyll in Urginea maritima (L.) Baker

Abstract

The spectral transmittance of isolated 'intact' upper and lower epidermes as well as the extractable UV-B-absorbing capacity of epidermes and mesophyll were studied in the leaves of exposed and deeply shaded, field-grown plants of Urginea maritima (L.) Baker. Epidermal transmittance in the visible part of the spectrum was high (>80%) in all cases. Transmittance in the UV-B (280-320 nm) was comparatively high (c. 14%) in both the upper and lower epidermes of shaded plants, but more than an order of magnitude lower in exposed plants, with the lowest values observed on the upper leaf epidermis. UV-B transmittance was negatively correlated with the methanol extractable UV-B-absorbing capacity of the epidermes, but was independent of epidermal thickness. The UV-B-absorbing capacity of the mesophyll, when expressed on an area basis, was not affected by exposure. However, it was significantly higher in shaded plants, when expressed on a dry mass basis. The results indicate that although the concentrations of the UV-B-absorbing components of the whole leaf or its epidermis fluctuate according to the site-dependent radiation stress, the opposite is evident for the mesophyll. Therefore, high irradiance in U. maritima, apart from inducing an increase in UV-B-absorbing compounds on a whole leaf basis, also caused a change in the distribution of these compounds between epidermis and mesophyll.