Inhibition of hypocotyl elongation by ultraviolet‐B radiation in de‐etiolating tomato seedlings. II. Time‐course, comparison with flavonoid responses and adaptive significance

Abstract

UV‐B radiation inhibits hypocotyl elongation in etiolated tomato (Lycopersicon esculentum Mill. cv. Alisa Craig) seedlings acting through a photoreceptor system with peak apparent effectiveness around 300 nm. In order lo further characterize the response and gain insight into its potential ecological significance, the time‐course of inhibition was measured and compared with the time‐course of flavonoid accumulation in the same seedlings. When a background of strong (> 620 μmol m−2 s−1) white light (WL) was supplemented with low irradiance UV‐B (∼ 3 μmol m−2 s−1). substantial (∼ 50%) inhibition of elongation occurred within 3 h of the light treatment. The magnitude of UV‐B‐induced elongation inhibition was similar in wild type (WT) and au‐mutant seedlings, in spite of the large differences between genotypes in rate and temporal pattern of elongation. In comparison to the effect of UV‐B on elongation, induction of flavonoid accumulation in WT and au seedlings undergoing de‐etiolation was a much slower response. Several UV‐absorbing compounds appeared to be specifically induced by light, and some of them accumulated faster under the WL + UV‐B treatment than under WL alone. However, there was little or no delectable effect of WL on flavonoid levels until up to 3 h of treatment, and the specific UV‐B effect was measurable only after 6 h of continuous treatment. Indeed. UV‐B‐screening properties of crude alcoholic extracts were not different between WL and WL + UV‐B treatments until after 9 or 24 h. When the light treatments were applied to seedlings that were just breaking through the soil surface. UV‐B was found to consistently retard seedling emergence. These results suggest that the rapid inhibition of elongation in de‐etiolating seedlings is an evolved response lo UV‐B, which may serve to minimize seedling exposure to sunlight until protective pigmentation responses (triggered by WL and UV‐B) have taken place in the seedlings epidermis.