Macadamia integrifolia and hybrids with M. tetraphylla are multiple flushing, subtropical trees which form the basis of the macadamia nut industry in Australia. However little is known about the process of inflorescence or new stem formation in these trees, including floral induction. Axillary buds form both new stems and inflorescences, so this thesis reports investigations into the effect of architectural position in the tree, and of temperature, on axillary bud release from dormancy and floral or vegetative identity.
Location of inflorescence emergence and of new stem production over one year was mapped according to their locations within tree architecture. Bud location down to node level was related to proportion of nodes branching or flowering, and to number of inflorescences or stems per node. Nodes were mapped by position along the growth unit, by growth unit position along the axis, and node position along axes. Interactions of these positions with the size of axis and the size of growth units were also examined.
Position of the node along the growth unit was the strongest factor influencing both proportion of nodes flowering and proportion of nodes branching. The proportion of nodes flowering increased with node proximity to the growth unit base. The proportion of nodes branching was highest by far on node two below the top of the growth unit. Pruning resulted in branching on the node or two immediately below the cut, whether the cut was at the top or base of a growth unit. However pruning did not increase numbers of new stems on the axis.
Pruning did not change number or location of inflorescences on the growth unit pruned, but did increase inflorescences emerging from the growth unit below it, and significantly increased the number of inflorescences emerging from the axis by around 50%.
Growth unit position along the axis also exerted a strong influence on location of inflorescences and new stems. Nodes on growth units closer to the base of an axis were more likely to flower than those on growth units towards the tip. Nodes of growth units closest to the tip were more likely to branch than those at the base.
Axis size influenced the proportion of nodes flowering in five year old trees of variety 741, in which flowering was greatest on shorter axes, but not in twelve year old variety 741 trees or thirteen year old variety 842 trees in which flowering was also studied. The effect of growth unit size on proportions of nodes flowering was different in different varieties and ages.
Axes extending were more likely to branch and less likely to flower than axes that did not extend. Branching as well as extension of an axis reduced the number of inflorescences emerging from that axis. Different applications of mineral nutrients varied the proportions of axes extending and branching in potted trees. Trees receiving moderate levels of fertiliser along with low levels of water extended more and branched less than other treatments, resulting in trees becoming more open in structure over one growth period.
In varieties A4 and A36, temperature appeared to affect flowering in two stages. Warm night temperatures (19°C) followed by cool night temperatures (11.5°C) resulted in substantial inflorescence emergence, but cool night temperatures followed by warm resulted in almost no emergence. Only-warm or only-cool night temperatures each resulted in moderate emergence. Along with the work of previous researchers, this suggests that an early stage of flowering is promoted by warm night temperatures, and a later stage by cool night temperatures. As cool temperatures are known to promote emergence of minute dormant inflorescences, the early stage may be floral induction and subsequent meristem evocation and determination.
Axillary buds were collected during months of warm night temperatures and several months following these. Microscopic examinations of buds showed morphological gradients in development along the growth unit, parallel to gradients in node likelihood of flowering. However no obvious morphological difference between the high branching node just under the growth unit tip and other nodes was detected. A small number of buds from growth units that had just finished elongation appeared to contain round, domed meristems, which are indicative of floral growth. Together with the finding that warm night temperatures promote an early stage of flowering, this prompts the suggestion that floral evocation in macadamias could occur predominantly during the summer flush growth period. One preformed node was found in most buds towards the base of growth units by the time this same flush period was ending. Two preformed nodes were found in most apical buds at this time.
The findings of clear patterns in flowering and branching, and that these change with pruning, as well as the relationship between temperature and control of flowering, may assist orchardists in designing new orchard management programs that increase efficiency and sustainability of macadamia nut production.