THE RELATIVE SIGNIFICANCE FOR STEM ELONGATION AND FLOWERING IN LOLIUM-TEMULENTUM OF 3-BETA-HYDROXYLATION OF GIBBERELLINS

In previous experiments with many gibberellins (GAs) and GA derivative s applied to Lolium temulentum L., quite different structural requirem ents were evident for stem elongation on the one hand and for the prom otion 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 3beta-hydroxylation, including C/D-ri ng-rearranged GAs, reduced stem growth but, in the case of the two acy lcyclohexanediones, increased the flowering response when applied on t he inductive long day. Later applications of the acylcyclohexanediones , made after floral initiation had occurred, were inhibitory to flower ing, suggesting that subsequent inflorescence development requires 3be ta-hydroxylated GAs. Applications of the 3alpha-hydroxy epimers of GA1 , GA3 and GA4 gave slightly less promotion of flowering in comparison with the 3beta-hydroxy GAs, but far less promotion of stem elongation, except in the case of 3-epi-GA4, which was comparable to GA4. The 3al pha-hydroxy epimer of 2,2-dimethyl GA4 gave less promotion of flowerin g than its 3beta-hydroxy epimer but almost no promotion of stem elonga tion. The 3alpha-hydroxy epimers of GA3 and 2,2-dimethyl GA4 did not a ct as competitive inhibitors of the stem elongation elicited by GA3 an d 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 initiatio n in Lolium.