RESPONSE OF LIGHT‐GROWN WILD‐TYPE and LONG HYPOCOTYL MUTANT CUCUMBER PLANTS TO END‐OF‐DAY FAR‐RED LIGHT

— A long‐hypocotyl mutant ( lh ) of cucumber ( Cucumis sativus L.) has been studied which has previously been shown to lack phytochrome control of growth in de‐etiolated seedlings and thought to be modified with respect to the light‐stable type of phytochrome. We have analyzed the response of lh mutant and isogenic wild‐type (WT) plants to daily treatment with end‐of‐day far‐red light (EODFR). Only the WT responded to this treatment resulting in a large increase in internode length; an increase in petiole length; changes in leaf development (increased area, decreased thickness and reduction in indentation); redistribution of dry matter from leaf blades to stem; increased apical dominance and promotion of tendril formation. There were only small or no significant effects on chlorophyll and total carotenoid content, chlorophyll alb ratio, soluble protein levels and net photosyn‐thetic rates. The lh mutant failed to respond to EODFR treatment, and had the appearance of a shade‐avoiding plant growing in extreme shade. The lh mutant appears to completely lack the phytochrome responses attributable to the type of phytochrome that is active in shade detection. A discussion of the possible roles of the stable and labile types of phytochrome in light grown plants follows.