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

, : Inhibition of hypocotyl elongation by ultraviolet-B radiation in de-etiolating tomato seedlings: II. Time-course, comparison with flavonoid responses and adaptive significance. Physiologia Plantarum 93(4): 593-601

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 to 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 ( gt 620 pmol m-2 s-1) white light (WL) was supplemented with low irradiance UV-B ( apprx 3 mu-mol m-2 s-1), substantial ( apprx 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 detectable 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 to 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.


DOI: 10.1111/j.1399-3054.1995.tb05105.x

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