Effects of Enhanced UV-B Radiation on the Drought Semi-deciduous Mediterranean Shrub Phlomis fruticosa under Field Conditions are Season-specific

Abstract

The effects of enhanced UV-B radiation on Phlomis fruticosa L. were recorded during a 1- year field study. Plants received ambient or ambient plus supplemental UV-B radiation (simulating a 15% stratospheric ozone depletion over Patras, 38.3ºN, 29.1ºE) and only natural precipitation, i.e. they were simultaneously exposed to other natural stresses, particularly water stress during summer. Actual, biologically equivalent UV-B doses were 8.55 and 11.21 kJ m^-2 day^-1 during the summer maximum (14 July) and 0.85 and 1.12 kJ m^-2 day^-1 during the winter minimum (27 December) for control and W-B plants respectively, while intermediate values were received for the rest of the year. lho seasonally separated effects could be distinguished. The first was a growth response, observed at late spring, in the absence of any simultaneous stress and at the period most favourable for this shrub, during which it shows maximum photosynthetic performance. The effect was an inhibition of new leaf development and premature falling of old leaves, leading to lower leaf numbers and total leaf areas for the rest of the experimental period. The second effect coincided with the summer drought, during which net photosynthesis, chlorophyll content and photochemical efficiency of photosystem II decreased in the controls, but a trend towards a further decrease in W-B irradiated plants was also evident. Changes in total carotenoids were negligible, leading to higher carotenoid to chlorophyll ratios under enhanced W-B radiation. Photosynthetic parameters recovered to the same levels in control and UV-B irradiated plants after the first heavy rains in autumn. At final harvest, considerable decreases in total dry mass were evident for UV-B irradiated plants, while plant height, UV-B-absorbing compounds, relative water content and leaf specific mass were unaffected during the whole experimental period. UV-B effects may depend not only on co-occumng natural stresses, but on the specific sensitivity of individual developmental stages as well, i.e. they may be season-specific.