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1985, Controlling piant growth via the gibbereiiin biosynthesis system -i. Growth parameter aiterations in appie seediings. 'York Imperial' apple seediings (Malus domestica Borkh.) were continuousiy suppiied via the roots with paeiobotrazoi [(2RS, 3RS)-l-(4-chiorophenyl)-4,4-dimethyi-2-(l,2,4-tr[azol-l-yi)pentan-3-oi)], a triazole GA biosynthesis inhibitor, at 0.68 uJW in a nutrient solution. In comparison to controls, seedhngs treated with paciobutrazoi for 66 days showed a 91% reduction in shoot length, a 66% reduction in leaf area but oniy a 17% reduction in leaf number. This effect could be reversed by GA3 appiied to the foliage at 71.4 ]iM 0, 19 or 35 days after paciobutrazoi was initiaiiy suppiied and ieaf area vaiues for paciobutrazoi-treated seedHngs given both treatments did not differ significantly from controls. Plots of growth data indicate linearity of shoot longitudinai growth of GAj-treated seediings. Leaf area increase was non-linear after GA3 treatment up to approximately 30 days, when the rate dropped. On a per shoot basis, leaf weight closeiy foiiowed leaf area but on a per unit area basis, paciobutrazoi-treated ieaves were heavier than controis; GA, applications temporarily reversed this trend.Additional key words -Malus domestica, paciobutrazoi.G. L. Steffens (reprint requests) and S. Y. Wang, U.S.