Different responses of Arbutus unedo and Vitis vinifera leaves to UV filtration and subsequent exposure to solar radiation

An UV-exclusion experiment was conducted on Mediterranean plant species grown in pots: the evergreen sclerophyllous shrub Arbutus unedo and the deciduous woody crop Vitis vinifera. After 130 days, of exclusion of UVB and both UVA and UVB radiations, during leaf unfolding and development, the plants were exposed to the ambient solar radiation (about 80% of the whole solar spectrum). The different radiative treatments were obtained with tunnels, covered with appropriate filters (cutting UV radiation), where the plants grown. The leaf UV absorbing compounds were analysed, during the growing season, in both species, in plants under the different radiative regimes. The flavonols content in leaf epidermis was, also periodically assessed, in both species, by means of a non-destructive optical index (the FLAV index of the Multiplex sensor). Structural characteristics and functionality of the photosynthetic apparatus of the plants were assessed by means of chlorophyll a fluorescence measurements. The UV radiation exclusion provoked a reduction of UVA-absorbing compounds, mainly flavonols, in the leaves of both species. The results evidenced an increase of these compounds over time in plants previously grown under both UV exclusion treatments, especially in V. vinifera. Chlorophyll a fluorescence analysis showed a delay in the development of the photosynthetic apparatus in V. vinifera leaves previously grown under total UV exclusion (both UVA and UVB) and damage to the oxygen evolving complex. No significant effects of the UV treatments on the leaf photochemical properties were detected in A. unedo. Overall the results highlighted the relevance of UV radiation on the leaf physiology of the species considered and its essential role in the photomorphogenesis of protective substances in the leaves.