Response of Barley Shoot Apices to Application of Gibberellic Acid and Abscisic Acid: Dependence on Tissue Sensitivity

Abstract

Stimulation by gibberellic acid of rachis internode elongation in barley grown in short days was dependent on either the timing of application or on the amount applied in a dose at day 10. There was no immediate rachis internode elongation in response to gibberellic acid treatment until floret initials appeared and then only if sufficient gibberellic acid was present in the shoot apices of the barley plants grown in short days.

Applications of gibberellic acid promoted the growth of the double-ridge meristem (upper ridge only) and this resulted in a group of abnormal spikelets being formed on the lower six nodes of the inflorescence, all of which were characterized by enhanced growth of the rachilla apical meristem.

In a 2 × 2 factorial experiment with gibberellic acid, abscisic acid was found to slightly reduce the growth rate of the vegetative barley apex between days 10 and 14. Abrupt cessation of primordium formation, coincident with the appearance of stamen initials, was observed following continued application of both gibberellic acid and abscisic acid whereas neither hormone alone had this effect. Enhanced growth of the rachilla apical meristem of the basal six spikelets following application of gibberellic acid resulted in the formation of branched inflorescences, and this result was not altered by the addition of abscisic acid to the treatment.