Axillary Buds Sprouting in Macadamia Induced by Two Cytokinins and a Growth Inhibitor

Axillary Buds Sprouting in Macadamia

Induced by two Cytokinins and

A Growth Inhibitor*

S.B. Boswell, Jim Nauer, and W.B. Storey**

Reprint From CMS 1981

Various cytokinins have overcome apical dominance (7, 8), and other growth regulators specifically stimulate axillary bud growth in apples and avocado (3, 4, 5, 6, and 10). A previous paper (2) reported that sprays of PBA resulted in lateral bud sprouting on only 50% of treated Macadamia seedlings. Vigorous shoots grew at an angle of 15° to the trunk on those seedlings that responded to PEA. This angle growth was increased to 50° by adding weight to the lateral branch (2). Cytokinins also increased crotch angles of young ‘Delicious’ apple trees (3).

The natural growth habit of many Macadamia trees is upright and nearly cylindrical. Mature trees in orchards tend to shade each other, causing loss of lower foliage and yield. Tree height may be reduced with growth retardant: or they may be mechanically topped and hedged, which is expensive (9).

Macadamia tetraphylla has a whorl of 4 sessile leaves at a node and 3 dormant buds in the axil of each leaf. Potentially, it is possible for 12 branches to develop at a single node. The objective of the present research was to increase axillary bud sprouting of Macadamia seedlings as an aid in changing the undesirable upright growth habit.

‘Johnson’ Macadamia seedlings were transplanted to 8-liter containers in May 1979. Prior to chemical treatment, 105 seedlings were selected for uniformity of height and circumference for each replication. Treatments were started May 22 1980, when plants average 75cm in height with a circumference of 3.1 cm at 15cm above soil level. All solutions of the 3 chemicals used contained 5% dimethylsulfoxide (DMS0) and 1.5% Tween 20. BA and PP528 were dissolved in DMS0 and diluted with 95% ethanol. PBA was used as a 1-% water solution. PEA was applied at 1000 and 2000 ppm, BA at 2000 and 4000 ppm, and PP528 at 50 and 100 ppm. Applications were made as follows, with 3 controls (untreated plants). PBA and BA were applied as a single spray, 2 sprays a week apart, and 3 sprays each a week apart. PP528 was applied as a single spray, 2 sprays 2 weeks apart, and 3 sprays at 2-week intervals. There were 5 single-seedling replicates per treatment in randomized blocks in the greenhouse. Solutions were applied to run-off with a hand sprayer to all axillary buds downward from the second node below the apex.

Applications of PEA, BA and PP528 to Macadamia seedlings induced sprouting of axillary buds (Table 1). Bud growth was initiated on all plants treated with 2000 ppm PEA and 4000 ppm BA at all treatment regimes. Lower concentrations of PBA and BA were as effective when 2 or 3 applications were used. PP528 at 50 and 100 ppm was significantly less effective than PBA and BA at all concentrations when applied 2 or 3 times (Table 1). No axillary buds grew on the untreated seedlings. Axillary buds at the 3rd or 4th node below the terminal grow well as a result of stimulation. Each seedling averaged 3 vigorous shoots. Most other axillary buds were weak and grew very little in response in the growth regulators applied. The vigorous shoots grew at an angle of 15 to the trunk. This angle may be widened to about 50° by adding weight (2).

The use of PBA and BA to stimulate dormant Macadamia axillary buds would appear to be an aid in shaping the tree. Application of the chemicals to buds below the 2nd node showed no detrimental side effects to terminal growth as previously reported when the entire plant was sprayed with BA (2).

Table 1.

Effect of PBA, BA, and FF528 on dormant Macadamia buds.


  Auxillary buds forced (%)

Treatments Concentrations (ppm) 1 spray 2 sprays 3 sprays

Control 0   OA' OA OA
PBA

1000 2000

 

80BC 100C

100C 100C 100C 100C
BA

2000 4000

  80BC 100C 100C 100C 100C 100C
PP528

50 100

  40AB 60BC 60BC 60BC 40AB 60BC

* Reprinted from Hurt Science, Vol. 16(1), February 1981.

**Department of Botany and Plant Sciences, University of California, Riverside, California.

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