GIBBERELLIN-A(1) IS REQUIRED FOR STEM ELONGATION IN SPINACH

The effects of the growth retardants 2'-isopropyl-4'-(trimethylammoniu m chloride)-5'-methylphenyl piperidine-1-carboxylate (AMO-1618) and ca lcium 3,5-dioxo-4-propionylcylohexanecarboxylate (BX-112) on stem elon gation were investigated in the rosette plant spinach (Spinacia olerac ea L.) under long-day (LD) conditions. Stem growth induced by a LD tre atment was prevented by both retardants. The inhibition caused by AMO- 1618 was reversed by gibberellin A1 (GA1) and GA20, whereas the effect s of BX-112 were reversed by GA1 only. Six GAs (GA53, GA44, GA19, GA20 , GA1, and GA8) were quantified by gas chromatography-selected ion mon itoring using internal standards. Plants treated with BX-112 had reduc ed levels of GA1 and GA8 and accumulated GA53, GA44, GA19, and GA20. T he relative levels of four additional GAs (3-epi-GA1, GA29, GA60, and GA81) were compared by ion intensities only. Relative to GA81, the lev el of GA29 was decreased by BX-112, whereas the levels of GA60 and 3-e pi-GA1 were increased. Transfer of spinach from short-day conditions t o LD conditions caused an increase in all identified GAs of the early 13-hydroxylation pathway with GA20, GA1, and GA8 showing the largest i ncreases. These findings support the position that, of the GAs belongi ng to the early 13-hydroxylation pathway, GA1 is the primary GA active per se for stem elongation in spinach. The increase in endogenous GA1 in plants in LD conditions is most likely the primary factor for stem elongation.