Floral determination and in-vitro floral differentiation in isolated shoot apices of Lolium temulentum L.

Shoot apices of Lolium temulentum excised after exposure of the plants to one long day (LD) undergo floral development in vitro, whereas those excised from plants in short days (SD) remain vegetative. Floral differentiation in vitro is reasonably normal and responds quantitatively to the preceding LD induction when three conditions are met. First, excision must not precede the arrival at the apex of the LD stimulus from the leaves. This appears to begin about 22 h after the start of the LD and is completed after a further 14 h, by which time all isolated apices have become capable of initiating inflorescence differentiation, i.e. florally determined. Second, for apices excised on the day after the LD (Day II) or early on Day III, the presence of gibberellic acid in the medium is required for floral differentiation to occur in most explants. By contrast, neither kinetin (N(6)-furfurylaminopurine) nor indole-3-acetic acid is required or beneficial, while abscisic acid in the medium is inhibitory to both survival and floral differentiation in excised apices. The third requirement is for an adequate supply of sugar, particularly after floral differentiation begins. Sucrose is taken up rapidly to reach high levels in the excised apices, but high sugar concentration in the medium, either alone or with gibberellic acid, does not suffice for floral differentiation to begin, and there is an absolute requirement for receipt of the LD stimulus.