INFLUENCE OF AN INHIBITOR OF GIBBERELLIN BIOSYNTHESIS, PACLOBUTRAZOL, ON APPLE SEED GIBBERELLIN CONTENT

Tissue-culture-propagated own-rooted cv. Spartan apple trees (Malus domestica Borkh.) planted in 1979 were treated in 1983 and 1985 via a soil-line trunk drench with the plant growth retardant paclobutrazol [(2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2, 4-triazol-1-yl) pentan-3-ol]. Seeds of immature fruits from untreated and treated trees were sampled in 1989 ca 75 days after full bloom. After seeds were freeze-dried, gibberellins (GAs) were extracted, purified and fractionated via C-18 reversed-phase high-performance liquid chromatography (HPLC). Gibberellins A1, A3,A4,A7,A8,A9,A15,A17,A19,A20,A24,A34,A35,A44,A51,A53,A54,A61,A62, A63 and A68 were identified by using C-18 HPLC, gas chromatography-selected ion monitoring and Kovats retention indices. Eight of the GAs identified were also quantified by using deuterated internal standards. The paclobutrazol applications caused a 55% reduction of vegetative shoot elongation in 1989, but both treated and untreated trees had developed a biennial bearing pattern by that time (heavy bloom or "on year" in 1989). Levels of early 13-hydroxylation pathway GAs, viz. GA53, GA19, GA20, GA1 and also GA3, were not altered by treatment. However, GA4, GA7 and GA9 were increased 13.4, 6.5 and 3.8 times, respectively, in seeds of fruit from treated compared to untreated trees.