Effects of solar radiation on photosynthesis, UV-absorbing compounds and enzyme activities of the green alga Dasycladus vermicularis from southern Spain

The effect of different wavebands of solar radiation (photosynthetically ac tive radiation (PAR), ultraviolet A (UV-A) and ultraviolet B (W-B)) produce d by use of cut-off filters on chlorophyll fluorescence of the green alga D asycladus vermicularis was assessed in summer-autumn 1996 at a shallow site in Cabo de Gata-Nijar, southern Spain. Similar experiments were carried ou t under outdoor conditions at Malaga during summer and autumn 1997. In plan ts growing under in situ natural light conditions (2.5 m depth), the yield of variable chlorophyll fluorescence (Delta F/F-m') decreases with increasi ng sunlight. The full solar spectrum (PAR+UV-A+UV-B) has more accentuated, longlasting effects on fluorescence than irradiation deprived of W-B. In ge neral, decreases in Delta F/F-m' do not exceed 30% in the three treatments. Under outdoor conditions, photoinhibition measured as a decrease in optimu m quantum efficiency (F-v/F-m) varies between 40 and 75% with no obvious di fferences between treatments; however, recovery of photosynthesis after sha de exposure is faster in plants treated with PAR+UV-A. Daily changes in nit rate reductase (NR) and carbonic anhydrase (CA) activities are antagonistic during the onset of natural radiation. The concentration of UV-absorbing c ompounds with maximum absorption at 348 and 332 nm is higher than that repo rted for other green algae. These compounds increase in plants exposed to t he full solar spectrum (PAR+UV-A+UV-B) and decrease under PAR alone and PAR +UV-A conditions at noon, which underlines a possible photoprotective mecha nism. Overall, data show that D. vermicularis is able to tolerate high sola r radiation. Two physiological strategies seem to be basically active: dyna mic photoinhibition at noon and an enhanced concentration of UV-screening s ubstances. (C) 1998 Elsevier Science S.A. All rights reserved.