Cytokinins in citrus. II. Fluctuations during growth in juvenile and adult plants

Juvenile and adult plants of ‘Pickstone Valencia’ orange (Citrus sinensis (L.) Osbeck) were compared for differences in shoot and root growth and for endogenous cytokinins in a synchronized growth flush. Dry-mass accumulation in both shoots and roots of juvenile-budded plants was significantly greater than in adult plants. Cytokinins extracted from leaves and roots of plants removed from the dormancy-induction environment, and subsequently during active growth, were mostly of the non-polar type. Buds of juvenile scions appeared to accumulate higher levels of polar cytokinins during dormancy than adult scion buds. In both plant categories, the level of polar cytokinins decreased once growth was stimulated by higher temperatures. However, cytokinin levels in buds were markedly higher than in new shoot, leaf or root material. Polar cytokinins were present in particularly low concentrations in the developing new shoots, and non-polar cytokinins only increased after the extent of shoot enlargement reached a maximum at about 50 days after warm-temperature incubation.The possible involvement of cytokinins in growth differences between adult and juvenile plants seems to be at the stage of bud activation, prior to breaking of dormancy. At this stage, polar cytokinins reached a much higher level of activity in juvenile buds, and this may have resulted in the greater scion growth of juvenile plants.