Endogenous cytokinin distribution patterns at budburst in Granny Smith and Braeburn 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 shoots 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-cresol (DNOC) oil (5%). The cytokinin contents of the xylem sap, the combined 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 first visible (green tip) in both cultivars on 20 September 1995. A greater increase in cytokinin content of distal xylem sap, coupled with elevated cytokinin in the distal wood, reflect the overall acrotony of both cultivars. The strong acrotony of Granny Smith is reflected in the higher cytokinin concentration in distal portion 1 week before the proximal portion of the shoot. The differential distribution of cytokinin reflects the pattern of budburst and may be correlated with growth habit. In a subsequent trial, Granny Smith shoots chilled and forced in the absence of roots showed an increase in cytokinin content of the bark and buds, and the wood as growth resumed. This was roughly comparable in magnitude to the increase observed under field conditions. The cytokinin increase in rootless shoots and differential distribution of cytokinin prior to sprouting, support the hypothesis that shoot-derived, rather than root-derived, cytokinins act to trigger spring budburst. (C) 2001 Elsevier Science B.V. All rights reserved.