Gibberellin-mediated suppression of floral initiation in the long-day plant Rhododendron cv. Hatsugiri

In a number of woody perennial species a decrease in gibberellins concentrations in the apical meristems is required for floral initiation to occur. In Rhododendron, applied gibberellins inhibit flowering and gibberellin biosynthesis inhibitors promote flowering. However, unlike previous reports on other Rhododendron cultivars, Rhododendron cv. Hatsugiri is a faculatitive LDP. It was therefore unknown how gibberellins regulate flowering in this cultivar and if non-inductive short daylengths stimulate the productions of endogenous gibberellins to suppress flowering. By inhibiting floral initiation while not stimulating vegetative growth we found applications of GA5 to best match the natural response of Rhododendron cv. Hatsugiri under short-day regimes. GA5-mediated effects on flowering have previously been reported to be due to conversion to GA6, however, GA5 was found to be present in tissue samples at up to 0.57 ng g−1 FW, while GA6 was never found. In addition, foliar applications of [14C] GA5 were not found to have metabolised to GA6. In line with the hypothesis that gibberellins inhibit floral initiation in short days in Rhododendron cv. Hatsugiri, the concentration of GA20, a precursor to many bioactive gibberellins, was higher in leaf tissues from plants in short days, compared to those in permissive long days when analysed using GC–MS.