Existing antioxidant levels are more important in acclimation to supplemental UV-B irradiation than inducible ones: Studies with high light pretreated tobacco leaves

Greenhouse grown tobacco plants were exposed to supplemental ultraviolet irradiation (280-400 nm, UV-B centered) for 6 days and changes in their photosynthesis (gas exchange and electron transport) and general and specific antioxidant activities were measured. UV irradiation corresponded to 8.95 kJ m-2 d-1 biologically effective dose and was supplemented to below ambient (200 μmol m-2 s-1 photon flux density) photosynthetic photon flux density (PPFD, 400-700 nm). Two groups of plants, which were different in their leaf antioxidant capacities due to one of them having been acclimated to high irradiance (1000 μmol m-2 s-1 PPFD) before the UV treatment, responded differently. High light pretreated leaves lost approximately 25% of photosynthetic activity during the UV exposure and showed no change either in the amounts of UV-absorbing pigments or antioxidant levels. On the other hand, leaves which were exposed to UV irradiation without the preceding high light acclimation had 60% lower photosynthesis by the end of the treatment, and increased antioxidant activities. Our results emphasize the importance of base antioxidant levels over inducible pools in leaf responses to low doses of UV irradiation and may also contribute to hypotheses on acclimation under field conditions.