Gibberellins and subapical cell divisions in relation to bud set and bud break in Salix pentandra

In young plants of Salix pentandra, a temperate zone deciduous woody specie s, elongation growth ceases and a terminal bud is formed at day lengths sho rter than a critical length. This is the first step in dormancy development , making survival under harsh winter conditions possible. Early studies str ongly indicate that gibberellin is involved in the photoperiodic control of bud set and bud break. GA(1) action was studied by application under short days to plants where cessation of shoot elongation had occurred, followed by subsequent anatomic investigations of shoot tips. Under short days the f requency of cell division decreased rapidly along with the earlier observed decrease in GA(1) levels. Application of GA(1) to short-day-induced termin al buds rapidly stimulated cell division in apices several days before visi ble shoot elongation in response to this treatment was observed. One day af ter GA(1) application a fourfold increase in cell division frequency in api ces was observed, increasing to a maximum of sevenfold 2 days after applica tion. Long-day treatment leading to induction of bud break after about 4-6 days was followed by slowly increasing frequency of cell divisions. In earl ier studies of this species, short days and gibberellins had no effect on c ell elongation. These data show that increased GA(1) content, by applicatio n or long-day treatment, results in increased frequency of mitosis. This st rongly indicates that GA(1) affects stem elongation in connection with bud set and bud break primarily by affecting cell divisions in subapical tissue s.