The effects of seasons, exposure, enhanced UV-B radiation, and water stress on leaf epicuticular and internal UV-B absorbing capacity of Cistus creticus: a Mediterranean field study

The seasonal variation in leaf epicuticular and internal (cellular)UV-B absorbing compounds was followed in field-grown shrubsof Cistus creticus L. Fully exposed and permanently shaded (undera canopy of evergreen sclerophylls) individuals were used. Nearmonthly measurements were performed for three years. In addition,a 2-year field experiment was established, in which fully exposedplants received ambient or ambient plus supplemental UV-B radiation,simulating a 15% ozone depletion over Patras (38.3 N, 29.1E). A second, six-month field experiment investigated the effectsof additional summer irrigation on UV-B absorbing compounds.It was found that: (a) exposed plants had considerably higherconcentrations of UV-B absorbing compounds (both epicuticularand internal), irrespective of season, (b) exposed and shadedplants exhibited dramatic and yearly consistent seasonal fluctuationsin epicuticular material with a c. 18-fold difference betweenthe summer maxima and winter minima. Internal UV-B absorbingcompounds showed the same seasonal trends, but with far lesspronounced differences, (c) In the 2-year field experiment withenhanced UV-B radiation, an increase in both epicuticular andinternal UV-B absorbing capacity was evident only during thesummer dry period of the first year. Later on, however, andup to the termination of the experiment, the differences wereabolished, (d) Well-watered and water-stressed plants showedthe same levels of both epicuticular and internal UV-B absorbingcompounds. Although the results indicate that concentrations of epicuticularand, to a lesser extent, internal UVB absorbing compounds fluctuateaccording to the seasonally and site-dependent environmentalstresses, no indications for an effect of water availabilitywere obtained. Therefore it may be concluded that the main determinantof the levels of UVB absorbing compounds is the intensity ofsolar radiation. The relative influence of the various spectralregions of solar radiation cannot be assessed from the presentresults. It was predicted, however, that the effects of increasedsolar UV-B radiation, resulting from the expected ozone depletionover the next decades, will be negligible and probably maskedby the much larger seasonal and site-dependent changes. Key words: Epicuticular, exposure, seasons, UVB absorbing compounds, UVB radiation