Root growth, cytokinin and shoot dormancy in lychee (Litchi chinensis Sonn.)

Potted lychee trees (cv. Tai so) with mature vegetative flushes were grown under three day/night temperature regimes known to induce floral (18/13degreesC), intermediate (23/18degreesC) and vegetative (28/23degreesC) shoot structures. Root and shoot growth was shown to alternate at all temperatures, with root growth occurring during shoot dormancy and in the initial stages of shoot emergence. As temperature decreased, the rate of root growth decreased and the length of the shoot dormancy period increased. Increasing the period of the endogenous shoot/root growth rhythm was sufficient to induce floral initiation. At all temperatures, shoot dormancy was broken after a similar amount of root growth. This occurred at approximately 3, 6 and 8 weeks for 28/23, 23/18 and 18/13degreesC, respectively. The concentration of the cytokinin, zeatin riboside, increased in parallel with an increase in root growth rate, reaching a peak in terminal buds just prior to shoot emergence. Application of zeatin riboside to dormant buds resulted in bud-break, but full emergence did not occur. It is proposed that root growth has a strong influence on the period of shoot dormancy, possibly through the initiation of bud-break by zeatin riboside, and can influence emerging shoot structure. (C) 2004 Elsevier B.V. All rights reserved.