THE ROLE OF GIBBERELLINS IN THE THERMO- AND PHOTOCONTROL OF STEM ELONGATION IN FUCHSIA

Stem elongation in Fuchsia x hybrida was influenced by cultivation at different day and night temperature or in different light qualities. Internode elongation of plants grown at a day (25°C) night (15°C) temperature difference (DIF 10) in white light was almost twofold of that of plants grown at the opposite temperature regime (DIF-10). Orange light resulted in a threefold stimulation of internode elongation as compared to white light DIF-10. Surprisingly, internode elongation in orange light was similar for plants grown at DIF-10 and DIF 10. In white light an increase in irradiance level did not affect shoot length at either DIF treatment. Flower development was accelerated at DIF-10 as compared to DIF 10 in both white and orange light. To examine whether DIF- and light quality-induced changes in shoot elongation were related to changes in gibberellin metabolism or plant sensitivity to gibberellins, the stem elongation responses of paclobutrazol-treated plants to exogenously applied GAs were determined. In the absence of applied gibberellins, paclobutrazol (> 0.32 μmol plant-1) strongly retarded shoot elongation. This inhibition was nullified by the application of GA1, GA4, GA9, GA15, GA19, GA20, GA24 and GA44 of about 10 to 32 nmol plant-1, but not by GA12 which only slightly increased stem elongation at an amount of 100 nmol plant-1. The results are discussed in relation to possible effects of DIF and light quality on endogenous gibberellin levels and gibberellin sensitivity of fuchsia, and their effect on stem elongation.