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 Matthiola incana (L.) R. Br. are promoted by exogenous gibberellins (GAs), including GA₄, and also by acylcyclohexanedione inhibitors of GA biosynthesis, such as prohexadione-calcium (PCa) and trinexapac-ethyl (TNE). Here, because it was unclear how GA biosynthetic inhibitors could promote stem elongation and flowering, their effect on GA biosynthesis has been examined by quantifying endogenous GA levels; also, the sensitivity of stem elongation and flowering to various GAs in combination with the inhibitors was examined. Stem elongation and flowering were most effectively promoted by GA₄ when combined with PCa and, next in order, by 2,2-dimethyl-GA₄, PCa, GA₄ + TNE, TNE, GA₉ + PCa and by GA₄. There was little or no promotion by GA₁, GA₃, GA₉, GA₁₃, GA₂₀ and 3-epi-2,2-dimethyl-GA₄. Both the promotive effects of the acylcyclohexanediones on stem elongation and flowering, particularly when applied with GA₄, and the fact that TNE caused a build-up of endogenous GA₄ imply that one effect of TNE at the lower dose involved an inhibition of 2β-hydroxylation of GA₄ rather than an inhibition of 20-oxidation and 3β-hydroxylation of GAs which were precursors of GA₄. Overall, these results indicate that: (1) GAs with 3β-OH and without 13-OH groups (e.g. GA₄) are the most important for stem elongation and flowering in M. incana; (2) growth promotion rather than inhibition can result if an acylcyclohexanedione acts predominantly to slow 2β-hydroxylation and so slows inactivation of active gibberellins, including GA₄. It follows that a low dose of an acylcyclohexanedione can be a 'growth enhancer' for any applied GA that is liable to inactivation by 2β-hydroxylation.