Abstract

Cytokinins play a central role in fruit set and development, with many plants accumulating high concentrations of endogenous cytokinins in developing fruits, and applied cytokinins being used to induce fruit set or parthenocarpic fruit development. Cytokinin application to flowers or immature fruit of macadamia increases initial fruit set and delays fruit abscission, without affecting final fruit set. This study quantified and determined the timing of endogenous cytokinin accumulation in macadamia flowers and immature fruit, to assist in developing a cytokinin treatment to improve fruit set and kernel quality. Flowers and very young fruit contained little cytokinin but levels rose greatly between 3 and 10 weeks after anthesis. Zeatin and zeatin riboside were the predominant cytokinins at 10 weeks post-anthesis, with zeatin riboside concentrations being 20 - 230 times higher than concentrations detected immediately after anthesis. Cytokinin accumulation coincides with the period of maximum endosperm volume in macadamia which, along with the predominance of zeatin and zeatin riboside, conforms to the general pattern of accumulation in developing fruits of many other plants. The very high cytokinin concentrations at this stage may explain the inability of pre- or post-anthesis cytokinin applications to increase fruit retention of macadamia beyond 10 weeks post-anthesis.

Key words: Benzyladenine, benzylaminopurine, fruit drop, kernel, nut, plant growth regulators, Proteaceae.