Custard apple has cryptic axillary buds, hidden from view by the base of the petiole. This has led to confusion about custard apple's flowering habit. Flowering only occurs during early branch development, and can be forced at any time of the growing season simply by removing leaves. Here, we show that flowering is terminal, not extra-axillary, and that the apparent continuation of the main stem beyond the flower is, instead, a sympodial branch. Secondary (including sympodial) branching only occurs during early branch development. Thereafter, axillary bud release is inhibited by the subtending leaf. Here, we show that summer tip-pruning of all branches arrests canopy development until the following spring owing to this inhibition. Although summer tip-pruning prevented new vegetative growth in the canopy, fruit size decreased relative to the control trees by ca. 23%. The reason for this decrease was probably related to increased carbon limitation to growth given that dawn water soluble and total nonstructural carbohydrate concentrations were lower in the tip-pruned trees. Thus, it appears that the reduced competition between fruit development and new vegetative growth in the tip-pruned trees was more than matched by lower photosynthetic capacity in the arrested canopy. Trees grown inside a shade-house were more vigorous than those grown outside. The difference in vigour had little effect on fruit size.
Macadamia (Macadamia integrifolia) is a recurrent flushing tree, with buds alternating between growth and quiescence. Here we report on factors governing bud release. We used staggered tip-pruning (equivalent to branch decapitation) to generate a range of trees with different flush maturities, but uniform maturity within any given canopy. We then tip-pruned the trees again to remove the most recent flush, and monitored the time taken for visible axillary bud emergence. New flushes appeared 9–16 days after tip-pruning. The slowest bud emergence was associated with flushes of intermediate maturity, approximately 7 weeks old. Overall, the variation in bud emergence was most closely related to the growth rate of the most recent flush at the time of pruning, and not related to the levels of stored carbohydrates.
The effects of different levels of carbon limitation on the flower development of custard apple were studied in two defoliation experiments. The duration of flower development was unaffected by moderate carbon limitation, but increased with severe limitation. Both moderate and severe limitation caused a decrease in flower dry weight. Some of the data collected were combined with data from an earlier study to develop a model of the dependency of the duration of flower development on temperature. The model was applied to long-term weather records from Alstonville, northern New South Wales, to illustrate the effects of late 20th century warming on the duration of flower development.
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