Cytokinins and fruit development in the kiwifruit (Actinidia deliciosa). II. Effects of reduced pollination and CPPU application

The significance of changes in cytokinin content during early fruit growth was examined in the kiwifruit (Actinidia deliciosa var. deliciosa cv. Hayward). Fruit growth was modified by the reduction of seed number or by the application of the synthetic phenylurea cytokinin N‐(2‐chloro‐4‐pyridyl)‐N‐phenylurea (CPPU). The influence of these treatments on cell division was monitored by flow cytometry and changes in the endogenous cytokinins were measured at days 10 and 20 after anthesis, using high‐performance liquid chromatography and radioimmunoassay. Total cytokinin levels appeared not to be limiting growth since the highest total cytokinin concentration was detected in unpollinated fruit, which abscised by day 25 after anthesis. However, compared with control fruit which had the highest concentration of zeatin (Z) 10 days post anthesis, Z levels were low in unpollinated fruit. It is hypothesised that an increase in Z is the critical change in cytokinin metabolism required for the initiation of cell division and fruit growth. The synthetic cytokinin CPPU promoted fruit development, but there was a decrease in the endogenous cytokinin concentration. Zeatin was not detected in CPPU‐treated fruit. Cell division was reduced in unpollinated fruitlets but there was no significant difference (P > 0.05) between the other treatments. Differences in final fruit size appeared to be due to cell expansion.