Photoperiod Regulation of Vegetative Growth and Meristem Behavior Toward Flower Initiation of an Indeterminate Soybean

Abstract

Soybeans were subjected to three photoperiod treatments: (1) short days-(SD) 9-h solar radiation; (2) long days-(LD) 9-h solar radiation with a 3-h night interruption; and (3) 10 short days plus long days (SD + LD). Short inductive photoperiods of SD and SD + LD promoted a rapid transition of axillary and terminal shoot meristems to the reproductive condition. Terminal shoot meristems were transformed to floral racemes, and vegetative structure differentiation stopped, coincident with the beginning of branch elongation, indicating a release from apical dominance. Long photoperiods of SD + LD and LD promoted elongation of internodes and leaf expansion. Unlike SD plants, SD + LD plants developed all of the differentiated but not elongated apical internodes, and terminal racemes became visible. In the noninduced LD plants, internode elongation was promoted, and vegetative growth of terminal shoot meristems was prolonged. The latter effect may be caused by (1) a delay in flower induction and (2) independent behavior of terminal meristems with respect to the reproductive condition of the axillary meristems. Photoperiod may regulate endogenous hormonal levels, and correlative controls of the vegetative growth are postulated.