The effect of gibberellins on flowering in roses

The gibberellins A(1), A(3), A(5), A(8), A(19), A(20), and A(29) were identified in vegetative shoot tips of Rosa canina by comparing their mass spectra and Kovats retention indices with those of standards. Most wild roses have a shea, flowering season of 2-4 weeks in spring, whereas most modern cultivars flower recurrently. 'Felicite et Perpetue' is a short-season hybrid from a cross between a wild rose and a recurrent-flowering rose, whereas its sport, 'Little White Pet,' flowers recurrently. The concentrations of gibberellins (GAs) were measured in shoot apices of both cultivars. In March (before floral initiation in spring) the concentrations of GA(1) and GA(3) were respectively threefold and twofold higher in 'Felicite et Perpetue' than in 'Little White Pet.' In April (after floral initiation) the concentrations of both gibberellins were substantially greater than in March, and concentrations of GA(1) and GA(3) were, respectively, 17-fold and 12-fold: greater in 'Felicite et Perpetue' than in 'Little White Pet.' It is postulated that, in 'Felicite et Perpetue,' floral initiation occurs when concentrations of GAs are low and is inhibited when concentrations of GAs are high, whereas in 'Little White Pet' concentrations of GAs remain at permissive levels throughout the growing season. Applications of GA(1) and GA(3) to axillary shoots in March inhibited floral development in 'Felicite et Perpetue' but not in 'Little White Pet.' This suggests that the combined concentration of exogenous and endogenous gibberellins might have been raised to inhibitory levels in the former but not in the latter cultivar.