Reprint From CMS Yearbook 1974

Macadamia trees, especially of Macadamia tetraphylla, are noted for their strong tendency to develop upright branches forming sharp-angled crotches with the trunk. Such crotches are weak and are the cause for branches being torn away from the trunk by strong winds and, sometimes, for breaking away under their own sheer weight.

Training and pruning practices, which overcome this fault to some extent, have been developed by Storey, Hamilton, and Fukunaga (7) (3) and by Shigeura (8). Warner and Gitlin (10) stated that mature untrained trees with predominantly upright growth may be topped to stimulate new lateral shoot growth as well as to reduce height. This provides some latitude in selecting lateral branches, shaping the trees, and developing a strong, wide-angled, spreading, scaffold structure. However, this operation may involve considerable expense, and usually results in reduced crop yields for several years. The best policy for developing orchards of well-shaped, strong trees is training them when they are young so that corrective measures will not have to be taken later.

Dominance of upright growth over lateral growth has been overcome in some species of plants having this tendency with the use of cytokinins, auxins, and other plant growth regulators, which suppress growth of the terminal bud and stimulate growth of lateral buds (1, 2, 3, 4, 5, 6, 7, 11). This method provides a wide choice of new shoots for developing a strong, wide-angled, permanent scaffold branch structure.

Seedling trees of the tetraphylla cultivar ‘Stephenson’ were used in the experiment. The central leader of ‘Stephenson’, as of virtually all tetraphylla seedlings, bears the leaves in whorls of four at each node. Three dormant buds lie in a vertical line in each leaf axil. Potentially, under the proper stimulus, all are capable of shooting, and it is possible for twelve branches to develop at a node. The uppermost shoot emerges at a sharp angle, the center shoot at an intermediate angle, and the lower shoot at a wide angle. The Storey-Hamilton-Fukunaga system of training is based on eliminating the two upper shoots and fostering growth of the lowest.

The results of the use of cytokinins and other plant growth regulators reported by the authors cited previously suggested the possibility of using one or more of these materials in the early training of Macadamia trees, so an experiment to investigate the possibility was begun in 1972. The material selected for use was the cytokinin PBA⁴.

In June 1972, seeds of ‘Stephenson’ were planted singly in 4-liter plastic pots. In April 1973, when the resulting seedlings averaged 70 cm in height and 2.9 cm in girth at 15 cm above the soil level, 312 were selected as closely as possible for uniformity in size and vigor.

Aqueous PBA solutions at 250,500 and 1000 ppm and containing 0.1% of X-77⁵ wetting agent were sprayed to run-off at weekly intervals (April 6 to 27). The 26 treatments were 2 controls and 3 concentrations of PBA, at 4 dates in which the apex was either removed or left intact. There were 12 single-seedling replicates in randomized complete blocks in the greenhouse. Terminal shoots of seedlings in which the apex was removed were cut off 5 days before spraying.

Applications of PBA to seedlings with the apex intact induced sprouting of axially buds (Fig. 1, 2). Terminal shoots were not killed, but grew at a reduced rate. Lateral bud growth was initiated on 50% of plants treated with 4 foliar sprays of PBA at 500 and 1000 ppm, and 42% with 250 ppm. The fewer the frequency of applications, the smaller was the percentage of plants with laterals developing. Four sprays of PBA at all concentrations were significantly different from 1 application; other differences are noted in Table 1. There was a significant difference when more than 2 weekly sprays were applied regardless of concentration; rate of application was non-significant.

No lateral buds were stimulated by any concentration with only 1 application. Lateral buds that were stimulated grew at the 3rd or 4th node below the terminal. Each seedling averaged 3 vigorous shoots at the same node. Other laterals stimulated along the trunk were weak and died within a few weeks. The vigorous shoots grew at an angle of 15’ to the trunk. When they approached a length of 25 cm, they began drooping from their own weight. Adding weight gradually up to 65 g to each shoot about 15 cm from the trunk for 2 weeks widened the angles to about 50° .

Although PBA itself cannot be considered as a practical means for widening crotch angles, it does induce more bud sprouting at more nodes than topping the seedling. The results suggest that chemical control of bud sprouting, especially sprouting of all three buds in a leaf axil, may facilitate training by the Storey-Hamilton-Fukunaga method by eliminating the necessity for periodically stubbing back the branches emerging from the two upper buds in order to enhance development of the lowest buds, which produce branches with the widest crotch angles.

1. Batjer, L.P., M.W. Williams, and G.C. Martin 1964. Effects of N-dimethylamino succinamic acid (B-nine) on vegetative and fruit characteristic of apples, pears and sweet cherries. Proc. Amer. Soc. Hort. Sci. 85: 11-16.

2. Bukovac, Martin J. 1968. TIBA promotes flowering and wide branch angles. Amer. Emit Grower 88: 18.

3. Kender, Walter J., and Stephen Carpenter. 1972. Stimulation of lateral bud growth of apples trees by 6-benzylamino purine. J. Amer. Soc. Hort. Sci. 97: 377-380.

4. McCarty, C. D., S. B. Boswell, and R. M. Burns. 1971. Chemically induced sprouting of axially buds in avocados. Calif. Agr. 25(12): 4-5.

5. Poll, L. 1968. The effect of cytokinin N₆ benzyladenine on bud break of fruit trees. Horticultura 22: 312.

6. Sachs, T., and K. V. Thimann. 1964. Release of lateral buds from apical dominance. Nature 201: 939-940.

7. __________________ , and___________________ - 1967. The role of auxins and cytokinins in the release of buds from dominance. Amer. J. Bot. 54: 136-144.

8. Shigeura, G. T. 1956. Training Macadamia nut trees at Keaau orchard. Calif. Macadamia Soc. Yrbk. 2: 37-39.

9. Storey, W. B., R. A. Hamilton, and E. T. Fukunaga. 1958. The relationship of nodal structure to training Macadamia trees. Amer. Soc. Hort. Sci. Proc.61: 317-323.

10.Warner, Robert M., and Harris M. Gitlin, 1971. Effect of hedge pruning on Macadamia nut yields. Hawaii Farm Sci. 20: 809.

11. Williams, M. W., and E. A. Stahly, 1968. Effect of cytokinins on apple shoot development from axially buds. Hort. Sci. 3: 68-69.