PHYTOCHROME CONTROL OF SHORT-DAY-INDUCED BUD SET IN BLACK COTTONWOOD

In trees and other woody perennial plants, short days (SDs) typically induce growth cessation, the initiation of cold acclimation, the forma tion of a terminal bud and bud dormancy. Phytochrome control of SD-ind uced bud set was investigated in two northern clones of black cottonwo od (Populus trichocarpa Torr & Gray) by using night breaks with red li ght (R) and far-red light (FR). For both clones (BC-1 and BC-2), SD-in duced bud set was prevented when R night breaks as short as 2 min were given in the middle of the night. When night breaks with 2 min of R w ere immediately followed by 2 min of FR, substantial reversibility of bud set was observed for BC-1 but not for BC-2. By comparing the effec ts of the R night breaks on bud set and the length of specific interno des, we determined that the R night breaks influenced internode elonga tion in two opposing ways. First, the addition of an R night break to the SD treatment prevented the cessation of internode elongation that is associated with bud set. Those internodes that would not have been elongated under SDs (and would have been found within the terminal bud ) elongated in the R treatment. Second, the R night breaks decreased i nternode length relative to the long-day (LD) control. In contrast to the clonal differences in reversibility that we observed for bud set, the decrease in internode length (ie the second effect of R) was R/FR reversible in both clones. Based on these results, we conclude that in ternode elongation is influenced by two distinct types of phytochrome- mediated response. The first response is a typical response to photope riod, whereas the second response is a typical ''end-of-day'' response to light quality. Our results demonstrate that SD-induced bud set in black cottonwood is controlled by phytochrome but that clonal differen ces have an important influence on the R/FR reversibility of this resp onse. The availability of an experimental system in which SD-induced b ud sets is R/FR reversible will be valuable for studying the physiolog ical genetics of photoperiodism in trees.