EFFECT OF EXPOSURE TO UV RADIATION ON GROWTH, PHOTOSYNTHESIS AND ANTIOXIDANT DEFENSE SYSTEM IN TOBACCO (Nicotiana rustica L. Cv. Basmas) PLANTS TREATED WITH EXOGENOUS POLYAMINES

Tobacco (Nicotiana rustica L. cv. Basmas) plants were exposed to a daily dose of 10 kJ m-1d-1 of UV radiation and treated with 0.5 mM exogenous polyamines (PAs) applied to the nutrient solution for two weeks. Dry weight of leaves increased under UVAB treatment in the absence of PAs and presence of spermidine but not putrescine. In the presence of putrescine, UVA treatment caused a significant increase in Chl a content and the Chl a/b ratio. Photochemical parameters were affected neither by light treatment nor by exogenous PAs with the exception of non-photochemical quenching in UVA treated plants. In contrast to UVA, UVAB resulted in an increased net assimilation rate in the absence of PAs and presence of putrescine. Application of PAs in control plants increased net assimilation rate following elevated stomatal conductance. Both UVA and UVAB caused slight or significant reduction of the activity of antioxidant enzymes and exogenous PAs increased their activity only in control plants. In the leaves, exogenous PAs in combination with UVA resulted in a significant reduction of H2O2 and increased proline concentration. Our results implied that higher photosynthesic rate which was accompanied by improved antioxidant defense capacity was responsible for UVAB-induced growth stimulation. H2O2 content and activity of its scavenging enzymes correlated poorly with membrane damage and plant response to treatments, suggesting involvement of other components of the antioxidant defense system. Our results showed a significant role of proline in the response of plants to exogenous PAs and in protection against UV radiation treatments.