JARS v37n1 - Vegetative Growth of Evergreen Azalea to Application of Gibberellin

Vegetative Growth of Evergreen Azalea to Application of Gibberellin
Pat Halligan, Langley, WA

This report recounts a simple experiment testing the effect of gibberellin on terminal growth buds of evergreen azaleas. For certain applications, such as production of a standard or topiary, constant growth stimulated by application of gibberellin might be used to advantage. First, treatment of one or a few branches could be used to funnel the plant's energy into highly directed growth without pruning or pinching. Second, treatment might save time by forcing constant growth throughout the year. Third, dwarf varieties could be used as standards without grafting onto vigorous varieties. I hope that this report will stimulate a fellow rhododendron enthusiast to carry on, since I do not plan any further studies.
Lengths of new growth were measured periodically through the duration of the experiment and once on 6-13-80, approximately three months after discontinuing treatment. Leaves were counted on test and control branches on 8-29 and 10-27.

Branches treated every other day grew almost continuously throughout the experimental period (Figure 1). During the three months after discontinuing gibberellin, the test branches grew almost as much as during the previous three months. Control branches grew intermittently. During February and March they grew almost as rapidly as the treated branches, but during the other months they grew slowly or not at all.

Experimental results
Fig 1: Growth of branches of azalea in response to
application of gibberellin. Solid shapes are tests.
Open shapes are controls. Squares and triangles are
R. 'Mme. Alfred Sanders'. Circles are R. 'Rose Glow'.
Details of experiment are give in text. Final results
of tests are significantly different from those of the
controls by the t test at P less than 0.01.

Growth of the branches treated weekly was less than that of the controls. Total growth over the period of 8-29-79 to 3-23-80 was 0 inches for the two plants of Mme. Alfred Sanders and 2.5 inches for Rose Glow. I observed that the growth of the control branches was considerably greater than most of the branches on the plants, while those treated weekly were slightly less than average. This probably was due to my selection of only vigorous shoots as controls while selecting average looking ones for tests. This presumably introduced a negative bias against the expected results.
During the period from 8-29 to 10-27 the treated branches grew 3.3 times as long as the controls. Elongation of internodes was 1.2 times the controls, and the number of new leaves was 2.7 times the controls. Thus, increased elongation of internodes accounted for only a small percentage of the increased stem length while increased "real" growth as measured by an increase in new leaves accounted for most of the increased stem length.

The results of this experiment indicate that application of gibbeirellin stimulates growth of azalea branches. The mechanism is suggested by the relative growth rates of test and control branches. In February and March the control branches grew nearly as rapidly as the treated branches. The difference in the final result appears to be due to the fact that the treated branches grew rapidly throughout the experiment while the controls grew rapidly only during one brief seasonal burst. This suggests that the major contribution of applied gibberellin to stem growth lies in the prevention of dormancy or resting of the buds. Increased elongation of internodes appears to have a lesser effect. Azaleas apparently respond positively only to a narrow range of concentrations of applied gibberellin. A concentration of 0.0857% was apparently toxic, while 0.0214% was stimulatory when applied every other day, but ineffective when applied weekly.
The growth of the treated branches three months after cessation of treatment was almost equal to that of the previous three months during treatment. This suggests a residual effect of gibberellin lasting several months.
The plants in this experiment were fertilized very lightly and infrequently as needed to keep the foliage in good color. I would suspect that much more spectacular growth rates could be achieved by fertilizing in addition to applying gibberellin.
The results of this experiment suggest that gibberellin can be used to direct much of the growth of an azalea into a single branch, and to prevent dormancy of terminal bud of that branch. These effects might be useful in the commercial production of a standard or topiary form of an azalea, especially if a more convenient method of application were used. Perhaps gibberellin could be applied with lanolin at less frequent intervals and still be effective.