Effect of Paclobutrazol on Accumulation of Carbohydrates in Apple Wood

Apple trees ( Malus domestica Borkh. ‘Spartan’ grafted on MM 106 rootstock) planted in 1976 in an orchard at Beltsville, Md., were treated with paclobutrazol a gibberellin biosynthesis inhibitor, in Spring 1982 and again in 1983. In 1982, paclobutrazol [50 wettable powder (WP)] was applied as a foliage spray on 4, 14, and 25 May 1982, at 333 mgliter −1 . On 27 Apr. 1983, trunks of these trees were painted with 75 g·liter −1 of paclobutrazol. Paclobutrazol did not inhibit shoot growth in 1983 but it retarded the shoot growth significantly during 1984. The carbohydrate content of paclobutrazol-treated wood was generally higher at all sampling dates from the winter dormant period through growth resumption in the spring. Negative correlation coefficients were found between starch and soluble carbohydrates in the wood during the winter sampling dates, whereas positive correlation coefficients were evident during the spring growth resumption period. The increase in carbohydrates induced by the treatment was similar in both years, when growth was not inhibited (1983) or inhibited (1984), indicated that paclobutrazol has an effect on carbohydrate metabolism as well as growth. Chemical name used: β-[(4-chlorophenyl)methyl]-α-(l,l-dimethylethyl)-1 H -1,2,4-triazole-1 -ethanol (paclobutrazol).