Long-day induction of flowering in Lolium temulentum involves sequential increases in specific gibberellins at the shoot apex

One challenge for plant biology has been to identify floral stimuli at the shoot apex. Using sensitive and specific gas chromatography-mass spectromet ry techniques, we have followed changes in gibberellins (GAs) at the shoot apex during long day (LD)-regulated induction of flowering in the grass Lol ium temulentum. Two separate roles of GAs in flowering are indicated. First , within 8 h of an inductive LD, i.e. at the time of floral evocation, the GA(5) content of the shoot apex doubled to about 120 ng g(-1) dry weight. T he concentration of applied GA(5) required for floral induction of excised apices (R.W. King, C. Blundell, L.T. Evans [1993] Aust J Plant Physiol 20: 337-348) was similar to that in the shoot apex. Leaf-applied [H-2(4)] GA(5) was transported intact from the leaf to the shoot apex, flowering being pr oportional to the amount of GA(5) imported. Thus, GA(5) could be part of th e LD stimulus for floral evocation of L. temulentum or, alternatively, its increase at the shoot apex could follow import of a primary floral stimulus . Later, during inflorescence differentiation and especially after exposure to additional LD, a second GA action was apparent. The content of GA(1) an d GA(4) in the apex increased greatly, whereas GA(5) decreased by up to 75% . GA(4) applied during inflorescence differentiation strongly promoted flow ering and stem elongation, whereas it was ineffective for earlier floral ev ocation although it caused stem growth at all times of application. Thus, w e conclude that GA(1) and GA(4) are secondary, late-acting LD stimuli for i nflorescence differentiation in L. temulentum.