Inhibition of Kinetin Induced Expansion Growth in Cucumber Cotyledons by UV-B

Expansion growth of cotyledons in cucumber (Cucumis sativus L.) var. Long green is induced by light or by the application of kinetin in darkness. Exposure of cotyledons to UV-B (310 nm) radiation inhibits expansion growth induced by light or kinetin. The effect of UV-B is more prominent on the kinetin induced expansion with a maximum of 50% inhibition under experimental conditons. In contrast to UV-B, UV-A radiation (370 nm) and red light (660 nm) promote kinetin induced expansion. The expansion growth by red and UV-A light is reversed by far-red light while UV-B induced inhibition is irreversible, indicating the existence of a separate UV-B photoreceptor other than phytochrome.