Changes Induced by Ultraviolet-B Radiation in Vegetative Growth, Foliar Characteristics and Photosynthetic Activities in Vigna unguiculata

Abstract

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 s^-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 s^-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.