The role of gibberellin biosynthesis in the control of growth and flowering in Matthiola incana

Recently, it was found that stem elongation and flowering of stock Matthiol a incana (L.) R. Br. are promoted by exogenous gibberellins (GAs), includin g GA(4), and also by acylgclohexanedione inhibitors of GA biosynthesis, suc h as prohexadione-calcium (PCa) and trinexapac-ethyl (TNE). Here, because i t was unclear how GA biosynthetic inhibitors could promote stem elongation and flowering, their effect on GA biosynthesis has been examined by quantif ying endogenous GA levels; also, the sensitivity of stem elongation and flo wering to various GAs in combination with the inhibitors was examined. Stem elongation and flowering were most effectively promoted by GA(4) when comb ined with PCa and, next in order, by 2,2-dimethyl-GA(4), PCa, GA(4) + TNE, TNE, GA(9) +PCa and by GA(4). There was little or no promotion by GA(1), GA (3), GA(9), GA(13), GA(20) and 3-epi-2,2-dimethyl-GA(4). Both the promotive effects of the acylcyclohexanediones on stem elongation and flowering, par ticularly when applied with GA(4), and the fact that TNE caused a build-up of endogenous GA(4) imply that one effect of TNE at the lower dose involved an inhibition of 2 beta-hydroxylation of GA(4) rather than an inhibition o f 20-oxidation and 3 beta-hydroxylation of GAs which were precursors of GA( 4). Overall, these results indicate that: (1) GAs with 3 beta-OH and withou t 13-OH groups (e.g. GA(4)) are the most important for stem elongation and flowering in M. incana; (2) growth promotion rather than inhibition can res ult if an acylcyclohexanedione acts predominantly to slow 2 beta-hydroxylat ion and so slows inactivation of active gibbberellins, including GA(4). It follows that a low dose of an acylcyclohexanedione can be a 'growth enhance r' for any applied GA that is liable to inactivation by 2 beta-hydroxylatio n.