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. Dail y 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) produce d varying responses on growth and leaf morphology. Inhibition of growt h and shoot length is attributed to the destruction of endogenous auxi n levels by UV-B. Chl b content decreased more than Chl a. Room temper ature absorption spectra of carotenes and xanthophyll extracts of trea ted chloroplasts showed an increased synthesis of carotene. Xanthophyl ls of UV-B treated seedlings exhibited marked spectral changes which c onfirm that the light harvesting chlorophyll protein assembly is affec ted under short term UV-B treatment. Inhibition by 50% of photosynthet ic 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 carboxyla se 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.