Control by Light of Hypocotyl Elongation and Levels of Endogenous Gibberellins in Seedlings of Lactuca sativa L.

Four 13-hydroxygibberellins, gibberellin A1 (GA1), 3-epi-GA1, GA19 and GA20 were identified by full-scan GC/MS in extracts of lettuce seedlings (Lactuca sativa L. cv. Grand Rapids). The results suggest that the early-13-hydroxylation biosynthetic pathway to GA1 functions in the lettuce seedlings. It was also found that GA1 is active per se in the control of hypocotyl elongation in lettuce seedlings. To investigate the relationship between control by light of hypocotyl elongation and levels of endogenous GAs in lettuce, endogenous levels of GAs were quantified by radioimmunoassay in seedlings that had been grown for 5 days in the dark (5D) and in those that had been grown for 4 days in the dark and then under white light for 1 day (4D1L). The endogenous level of GA1 in the upper and lower parts of hypocotyls in 5D seedlings was about four times higher than that in 4D1L seedlings. The response of explants (hypocotyl segments with cotyledons) from dark-grown seedlings to GA1 is known to be similar in the dark and under white light when the explants are treated with inhibitors of the biosynthesis of GA. Therefore, the above information suggests that the high level of GA1 in hypocotyls of dark-grown seedlings is responsible for the rapid elongation of hypocotyl, while irradiation by white light decreases the endogenous level of GA1 in the hypocotyls with a resultant decrease in the rate of hypocotyl elongation.