Photomorphogenic effects of UV-B radiation on hypocotyl elongation in wild type and stable-phytochrome-deficient mutant seedlings of cucumber

Hypocotyl elongation responses to ultraviolet-B (UV-B) radiation were investigated in glasshouse studies of de-etiolated seedlings of a long-hypocotyl mutant ( lh ) of cucumber ( Cucumis sativus L.) deficient in stable phytochrome, its near isogenic wild type (WT), and a commercial cucumber hybrid (cv. Burpless). A single 6- or 8-h exposure to UV-B applied against a background of white light inhibited hypocotyl elongation rate by ca 50% in lh and WT seedlings. This effect was not accompanied by a reduction in cotyledon area expansion or dry matter accumulation. Plants recovered rapidly from inhibition and it was possible to stimulate hypocotyl elongation in plants exposed to UV-B by application of gibberellic acid. In all genotypes inhibition of elongation was mainly a consequence of UV-B perceived by the cotyledons; covering the apex and hypocotyl with a filter that excluded UV-B failed to prevent inhibition. These results indicate that reduced elongation does not result from assimilate limitation or direct damage to the apical meristem or elongating cells, and strongly suggest that it is a true photomorphogenic response to UV-B. The fact that UV-B fluences used were very low in relation to total visible light, and the similarity in the responses of lh and wild-type plants, are consistent with the hypothesis that UV-B acts through a specific photoreceptor. It is argued that, given the weak correlation between UV-B and visible-light levels in most natural conditions, the UV-B receptor may play an important sensory function providing information to the plant that cannot be derived from light signals perceived by phytochrome or blue/UV-A sensors.