Endogenous cytokinin distribution patterns at budburst in Granny Smith andBraeburn apple shoots in relation to bud growth

The possible relationship of branching habit to cytokinin content of apple shoots (Malus x domestica Borkh.) was investigated. One-year old apple shoo ts are acrotonic (distal branching), more strongly so in Granny Smith than in Braeburn. In the first trial, long, I-year old Granny Smith and Braeburn apple shoots were sprayed on 29 August 1995 to break rest with dinitro-o-c resol (DNOC) oil (5%). The cytokinin contents of the xylem sap, the combine d bark and buds, and the wood were determined in distal and proximal shoot halves over the next 6 weeks. Budburst (terminal and lateral buds) was firs t visible (green tip) in both cultivars on 20 September 1995. A greater inc rease in cytokinin content of distal xylem sap, coupled with elevated cytok inin in the distal wood, reflect the overall acrotony of both cultivars. Th e strong acrotony of Granny Smith is reflected in the higher cytokinin conc entration in distal portion 1 week before the proximal portion of the shoot . The differential distribution of cytokinin reflects the pattern of budbur st and may be correlated with growth habit. In a subsequent trial, Granny S mith shoots chilled and forced in the absence of roots showed an increase i n cytokinin content of the bark and buds, and the wood as growth resumed. T his was roughly comparable in magnitude to the increase observed under fiel d conditions. The cytokinin increase in rootless shoots and differential di stribution of cytokinin prior to sprouting, support the hypothesis that sho ot-derived, rather than root-derived, cytokinins act to trigger spring budb urst. (C) 2001 Elsevier Science B.V. All rights reserved.