Contrasting seasonal morphological and physio-biochemical responses to UV radiation and reduced rainfall of two mature naturally growing Mediterranean shrubs in the context of climate change

Abstract

Naturally growing Arbutus unedo L. and Phillyrea angustifolia L. mature shrubs with similar leaf sclerophyllous index were studied. The aim was to evaluate whether: 1) ultraviolet radiation (UV) effects on plant development differ depending on the type of UV radiation (UV-A or UV-B), 2) rainfall amount modulates plant responses to UV and 3) seasonal climatic conditions influence the response of plants to the UV and/or rainfall treatments applied. To achieve these goals, A. unedo and P. angustifolia plants were subjected to UV-B exclusion (UVA), UV-B + UV-A exclusion (UV0) or near-ambient UV (UVBA) levels, in combination with two rainfall conditions (natural rainfall or rainfall reduction). Leaf morphological (leaf mass per area, leaf density, thickness and area), physiological (leaf relative water content, gas exchange and chlorophyll fluorescence) and biochemical (leaf C and N content, and leaf concentration of photosynthetic pigments and phenols) parameters were analysed seasonally for a year. Effects of UV and rainfall treatments on most of the parameters analysed in both species did not depend on the season. In any of the two species, morphological parameters responded to the treatments, likely because of the high leaf mass per area of the sampled fully-developed leaves. In A. unedo, neither physiological nor biochemical leaf traits varied significantly among UV conditions, probably due to their intrinsic high leaf concentration of phenols, which could also explain the lack of correlation between leaf traits and leaf carbon content. Conversely, leaf nitrogen content was highly correlated with leaf features when A. unedo plants were grown exposed only to UV-A radiation. Overall, A. unedo would be less responsive to UV, but not to a decrease in water availability, with plants displaying a water-spender strategy under reduced rainfall. In contrast, P. angustifolia responded at physiological level to the combination of both treatments. Indeed, plants subjected to low rainfall and only UV-A showed lower leaf transpiration rates and stomatal conductance than UV0 plants, higher leaf electron transport rates than UVBA plants, and the highest leaf water use efficiency. These results suggest that, under reduced rainfall, plant exposure only to UV-A would trigger the water-saving response typical of P. angustifolia, despite the mild reduction (20%) in soil water content applied. Under natural conditions, this UV-A-induced response of plants to mild water deficit seems to be masked by the presence of UV-B radiation.