ABSENCE OF PHYB INHIBITS HYPOCOTYL ELONGATION IN DARK-GROWN LH CUCUMBER SEEDLINGS - AN ACTIVE-ROLE FOR PRB

Cucumber (Cucumis sativus L.) seedlings carrying the long hypocotyl (l h) mutation, which confers a lack of B-type phytochrome (phyB), were s ignificantly shorter than their near-isogenic wild-type counterparts w hen grown in complete darkness, Relative growth rates determined for 5 mm hypocotyl regions were lower in lh seedlings in all growing region s, and the zone of elongation was less extensive in lh hypocotyls, Dig ital imaging microscopy revealed that the pattern of epidermal cell le ngths along the stem axis differed between the Ih mutant and the isoge nic wild-type. These findings (and the fact that experiments were cond ucted under conditions where phytochrome photoconversion to the far-re d-absorbing form does not occur) suggest that the red-absorbing form o f phyB (PrB) is an active positive regulator of development in etiolat ed plants.