Long-Day Induction of Flowering in Lolium temulentumInvolves Sequential Increases in Specific Gibberellins at the Shoot Apex

Abstract
One challenge for plant biology has been to identify floral stimuli at the shoot apex. Using sensitive and specific gas chromatography-mass spectrometry techniques, we have followed changes in gibberellins (GAs) at the shoot apex during long day (LD)-regulated induction of flowering in the grass Lolium 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 GA5 content of the shoot apex doubled to about 120 ng g−1 dry weight. The concentration of applied GA5 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 [2H4] GA5 was transported intact from the leaf to the shoot apex, flowering being proportional to the amount of GA5 imported. Thus, GA5 could be part of the 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 GA1 and GA4 in the apex increased greatly, whereas GA5 decreased by up to 75%. GA4 applied during inflorescence differentiation strongly promoted flowering and stem elongation, whereas it was ineffective for earlier floral evocation although it caused stem growth at all times of application. Thus, we conclude that GA1 and GA4are secondary, late-acting LD stimuli for inflorescence differentiation in L. temulentum.