The relative significance for stem elongation and flowering in Lolium temulentum of 3?-hydroxylation of gibberellins

Abstract

In previous experiments with many gibberellins (GAs) and GA derivatives applied to Lolium temulentum L., quite different structural requirements were evident for stem elongation on the one hand and for the promotion of flowering on the other. Whereas hydroxylation at carbons 12, 13 and 15 enhanced flowering relative to stem growth, the reverse was the case at carbon 3 (L.T. Evans et al. 1990, Planta 182, 97–106). The significance of hydroxylation at carbon 3 is examined in this paper. The application of inhibitors of 3β-hydroxylation, including C/D-ring-rearranged GAs, reduced stem growth but, in the case of the two acylcyclohexanediones, increased the flowering response when applied on the inductive long day. Later applications of the acylcyclohexanediones, made after floral initiation had occurred, were inhibitory to flowering, suggesting that subsequent inflorescence development requires 3β-hydroxylated GAs. Applications of the 3α-hydroxy epimers of GA1, GA3 and GA4 gave slightly less promotion of flowering in comparison with the 3β-hydroxy GAs, but far less promotion of stem elongation, except in the case of 3-epi-GA4, which was comparable to GA4. The 3α-hydroxy epimer of 2,2-dimethyl GA4 gave less promotion of flowering than its 3β-hydroxy epimer but almost no promotion of stem elongation. The 3α-hydroxy epimers of GA3 and 2,2-dimethyl GA4 did not act as competitive inhibitors of the stem elongation elicited by GA3 and 2,2-dimethyl GA4, respectively. These results extend the differences in GA structure which favour flowering as opposed to stem elongation, and indicate that 3-hydroxylation and its epimeric configuration are of much greater importance to stem elongation than to flower initiation in Lolium.