The impact of UV-radiation on the physiology and biochemistry of Ligustrum vulgare exposed to different visible-light irradiance

We have conducted a UV-exclusion experiment in which plants grew at 35 or 100% sunlight irradiance, in the presence or in the absence of UV-irradiance, with the aim of exploring the effects of visible-light and UV-irradiance on the physiology and biochemistry of Ligustrum vulgare, a world-wide distributed shrub, which inhabits partially shaded areas of the Mediterranean Basin. We measured relevant physiological and biochemical traits, namely: (i) leaf expansion and leaf area expansion rates; (ii) the net CO2 assimilation rate and the PSII photochemistry; (iii) the concentrations of soluble carbohydrates and photosynthetic pigments; (iv) the activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX); (v) the share of assimilated carbon recovered in individual polyphenols; (vi) the leaf oxidative damage. UV-irradiance had a relatively minor impact on most examined traits, as compared with the effect of visible-light irradiance. UV-induced variations in plant growth and net CO2 assimilation rate were minor. Maximal (Fv/Fm) and actual (ΦPSII) efficiencies of PSII photochemistry varied to a greater extent because of visible-light than UV-irradiance, and full-sun leaves had smaller Fv/Fm and ΦPSII than the partially shaded ones. The conversion state of violaxanthin-cycle pigments was either largely increased by visible-light or unaffected by UV-radiation, as also observed for the activities of antioxidant enzymes (with the exception of SOD). In contrast, UV-radiation greatly enhanced the allocation of carbon to polyphenols, particularly flavonoids, irrespective of visible-light irradiance. Lipid peroxidation and protein oxidation were superior in UV-treated leaves growing under partial shading, whereas leaf oxidative damage was unaffected by UV-radiation in full-sun leaves. We explain the differential UV-induced oxidative damage in partially shaded or full-sun leaves, on the basis of visible-light-induced biochemical adjustments, aimed at avoiding the generation and reducing reactive oxygen forms (ROS). These adjustments included an increase in (1) violaxanthin-cycle pigments, particularly antheraxanthin and zeaxanthin, relative to chlorophyll; (2) antioxidant enzyme activities and flavonoid concentration, which may effectively inhibit the generation and quench ROS once formed.