CHANGES INDUCED BY ULTRAVIOLET-B RADIATION IN VEGETATIVE GROWTH, FOLIAR CHARACTERISTICS AND PHOTOSYNTHETIC ACTIVITIES IN VIGNA-UNGUICULATA

The effect of short-term ultraviolet-B (UV-B) radiation on growth and foliar characteristics was studied in Vigna unguiculata(L.) Walp. Daily UV-B exposure was varied from 0.9 to 4.5 kJ m-2 d-1 (15-75 min at 1 J m-2 s-1). Low doses of UV-B irradiation (1.8 kJ M - 2 d - 1) produced varying responses on growth and leaf morphology. Inhibition of growth and shoot length is attributed to the destruction of endogenous auxin levels by UV-B. Chl b content decreased more than Chl a. Room temperature absorption spectra of carotenes and xanthophyll extracts of treated chloroplasts showed an increased synthesis of carotene. Xanthophylls of UV-B treated seedlings exhibited marked spectral changes which confirm that the light harvesting chlorophyll protein assembly is affected under short term UV-B treatment. Inhibition by 50% of photosynthetic CO2 uptake was observed in seedlings after 3 days of treatment at 1.8 kJ m-2 d-1, which was accounted for by changes in primary carboxylase as well as at the primary photochemical reactions. Low temperature fluorescence emission spectra of chloroplasts reveal that strong UV-B radiation, besides acting directly at the photosystem level, induces a change in the stoichiometry of PS I to PS II.