Photomorphogenetic Responses of a Long Hypocotyl Mutant of Cucumis sativus L.

Abstract

Summary Photomorphogenetic responses have been studied in a Cucumis sativus mutant ( lh ) which has long hypocotyls in white light (WL) compared to its wild type. Seeds and dark-grown etiolated seedlings of the lh mutant and wild type contain similar levels of spectrophotometrically detectable phytochrome. In continuous WL after 20 d there is a six-fold difference in hypocotyl length as a result of differences in cell elongation since the hypocotyls were estimated to be composed of similar numbers of cells. Although de-etiolation was retarded in the lh mutant in red light (RL), RL/far-red light reversibility of hypocotyl elongation and cotyledon expansion was shown. However, de-etiolated seedlings (seedlings pre-irradiated with 8 h WL) showed no inhibition of hypocotyl growth or promotion of cotyledon expansion in response to RL. Covering of one cotyledon of de-etiolated wildtype seedlings resulted in a marked curvature of the hypocotyl in RL given from above; the lh mutant did not exhibit this simulated phototropism. Little difference in responsiveness of wild type and lh mutant to UV-A and blue light (BL) was observed. It is provisionally concluded that this lh mutant lacks phytochrome function in light-grown plants perhaps due to the absence or lack of function of light stable phytochrome. The usefulness of the lh mutant for understanding the role of phytochrome and the BL-photoreceptor in controlling photomorphogenesis in light-grown plants is discussed.