The Content and In Vivo Metabolism of Gibberellin in Apple Vegetative Tissues

Despite the demonstrated importance of gibberellins (GAs) as regulators of fruit tree stature, information on their in vivo metabolism in apple vegetative tissues is still lacking. To determine whether the GA content and metabolism differs between dwarf and standard phenotypes and the influence of rootstocks, [C-14]GA(12), a common precursor of all GAs in higher plants, was applied to vigorously growing apple (Mains xdomestica) shoots collected from the scion cultivar Redcort on MM.106, a growth-promoting rootstock, and dwarf and standard seedlings on their own roots from progeny 806 (a cross between a breeding selection with reduced stature and an advanced breeding selection with a standard tree form). Twenty-one metabolites were identified by high-performance liquid chromatography (HPLC) and used as tracers for the purification of endogenous GAs. The existence of endogenous and [H-2]-labeled GA(12), GA(15), GA(53), GA(44), GA(19), GA(20), and GA(3) was demonstrated by gas chromatography-mass spectrometry (GC-MS); GA(20) was the major GA present, with slightly less GA(19) and GA(44), and with GA(3) present at approximately one-third the level of GA(20). Despite specific searching, neither GA(4), GA(7), GA(1), nor GA(29) was found, showing that [C-14]GA(12) is metabolized mainly through the 13-hydroxylation pathway and that GA(3) is a bioactive GA in apple vegetative tissues. The invigorating rootstock led to a slow GA metabolic rate in 'Redcort'. For self-rooted plants, the same GAs were identified in dwarf and standard seedlings from progeny 806, although standard plants metabolized at twice the speed of dwarf plants. Young branches of dwarf 806 plants treated with GA(3) were one-third longer with more nodes but similar in internode length. We conclude that the dwarf phenotype in progeny 806 is not caused by a lack of certain GAs in the GA biosynthesis pathway downstream of GA(12).