The effects of seasons, exposure, enhanced UV-B radiation, and water stress on leaf epicuticular and internal UV-B absorbing capacity ofCistus creticus: a Mediterranean field study
The seasonal variation in leaf epicuticular and internal (cellular) UV-B absorbing compounds was followed in field-grown shrubs of Cistus creticus L. Fully exposed and permanently shaded (under a canopy of evergreen sclerophylls) individuals were used. Near monthly measurements were performed for three years. In addition, a 2-year field experiment was established, in which fully exposed plants received ambient or ambient plus supplemental UV-B radiation, simulating a 15% ozone depletion over Patras (38.3° N, 29.1° E). A second, six-month field experiment investigated the effects of additional summer irrigation on UV-B absorbing compounds. It was found that: (a) exposed plants had considerably higher concentrations of UV-B absorbing compounds (both epicuticular and internal), irrespective of season, (b) exposed and shaded plants exhibited dramatic and yearly consistent seasonal fluctuations in epicuticular material with a c. 18-fold difference between the summer maxima and winter minima. Internal UV-B absorbing compounds showed the same seasonal trends, but with far less pronounced differences, (c) In the 2-year field experiment with enhanced UV-B radiation, an increase in both epicuticular and internal UV-B absorbing capacity was evident only during the summer dry period of the first year. Later on, however, and up to the termination of the experiment, the differences were abolished, (d) Well-watered and water-stressed plants showed the same levels of both epicuticular and internal UV-B absorbing compounds. Although the results indicate that concentrations of epicuticular and, to a lesser extent, internal UVB absorbing compounds fluctuate according to the seasonally and site-dependent environmental stresses, no indications for an effect of water availability were obtained. Therefore it may be concluded that the main determinant of the levels of UVB absorbing compounds is the intensity of solar radiation. The relative influence of the various spectral regions of solar radiation cannot be assessed from the present results. It was predicted, however, that the effects of increased solar UV-B radiation, resulting from the expected ozone depletion over the next decades, will be negligible and probably masked by the much larger seasonal and site-dependent changes.