The Behaviour of Shoot Apices of Lolium temulentum in vitro as the Basis of an Assay System for Florigenic Extracts

Previous experiments have shown that shoot apices excised from plants of L. temulentum that had been exposed to one long day (LD) could form inflorescence primordia in vitro if gibberellic acid (GA3) was present in the medium whereas apices from plants in short days (SD) could not. We show here that apices from older plants grown under high irradiance can undergo inflorescence differentiation in vitro after one LD even in the absence of GA3, as can apices from plants in SD if GA3 is present in the medium. For apices excised from both SD and one LD plants, the order of effectiveness of gibberellins for inflorescence induction was 2,2-dimethyl GA4 > GA5 > GA3 > GA1, in agreement with their ranking for effectiveness on intact plants. Application of GA3 to leaves before apex excision can substitute for GA3 in the growth medium. The presence of GA3 in the medium is not required until 4-6 days after excision from plants given one LD, and appears to be necessary for differentiation beyond the spikelet primordia stage. Three hypotheses concerning the relation between GAs and the LD stimulus to flowering, as affected by plant age, are considered.