Altitudinal differences in UV absorbance, UV reflectance and related morphological traits of Quercus ilex and Rhododendron ferrugineum in the Mediterranean region

We studied the variations in different physiological parameters associated with UV-B radiation defense: UV-B radiation absorbance, UV-visible spectral reflectance, carotenoids concentration, leaf thickness, SLW (specific leaf weigth) and trichome density in Quercus ilex growing at 200 and 1200 m and Rhododendron ferrugineum growing at 2200 m. We examined the role of these parameters as protection mechanisms in an altitudinal gradient of increasin g UV radiation in northern Catalonia and in sun and shade leaves. The conce ntration of UV-B radiation absorbing pigments was 15% higher in sun leaves of Q. ilex at 1200 m than in those from 200 m altitude. Sun leaves of R. fe rrugineum presented concentrations three times higher than those of Q. ilex . Reflectance ranged between 5% (in the region 300-400 nm) and 12% (in the region 280-300 nm). The variation of reflectance with altitude followed an inverse trend respect to absorbance in the 280-300 nm region, with higher v alues the lower the altitude, but in the 300-400 nm region, reflectance of the lower site was the lowest. In both species and altitudes sun leaves pre sented higher concentrations of UV-B radiation absorbing pigments and UV re flectance than shade leaves. Quercus ilex trees of the higher location pres ented higher NDPI (Normalized Difference Pigment Reflectance Index) values, indicating higher carotenoids/chlorophyll a ratio. Actual measurements of carotenoid/chlorophyll a ratio confirmed this pattern. The photochemical re flectance index (PRI) presented higher values the higher the location indic ating lower photosynthetic radiation-use efficiency. Specific leaf weight ( SLW) and leaf thickness were larger in Q. ilex trees of higher location tha n in those of lower location. In both sites, sun leaves also presented larg er SLW values than shade leaves. Adaxial leaf hair density in sun leaves wa s significantlly higher in the lower location. UV absorption and linked mor phological traits (SLW, leaf thickness measured in Q. ilex) presented the l arger differences among studied plants at different altitudes and seem to b e the dominant UV protecting mechanisms.