Light‐grown Plants of the Cucumber Long Hypocotyl Mutant Exhibit Both Long‐term and Rapid Elongation Growth Responses to Irradiation With Supplementary Far‐red Light

Abstraet— Seedlings of wild type (WT) and the lh mutant of Cucumis sativus were grown in white light and given supplementary far‐red radiation either to the whole plant, or via fiber‐optic probes directed at the apical region of the hypocotyl. In both WT and mutant seedlings, reducing the ratio of red to far‐red radiation resulted in significant increases in extension growth. Direct measurement of extension rate by transducer techniques revealed that the mutant seedlings responded to additional far‐red radiation by an increment in extension rate that was equivalent to that observed with the WT seedlings. Assuming that the lh mutant is deficient in phytochrome B (Kendrick and Nagatani, The Plant Journal1: 133–139, 1991), these results indicate that although phytochrome B clearly has an important role in the induction of the shade avoidance syndrome, it is unlikely to be the sole responsible receptor.