Calendula officinalis L. bears a short main axis and branches of several orders. The upper axillary branches grow vigorously whereas the lower branches remain arrested at different levels. Foliar application of gibberellic acid (GA3) stimulated the elongation of the main shoot and of the upper few primary axillary branches (PAB’s). The length of the main axis nearly doubled 10 days after application. The endogenous level of GA3 is probably sub-optimal, since external application enhances stem growth. However, treatment caused varying degrees of stimulation of the arrested branches. Whereas all the concentration of GA3 were equally effective in promoting the growth of the first six primary axillary branches after 60 days of application, in plants treated with 500 ppm, the lower primary axillary branches failed to develop. Since the lower axillaries remained inhibited, it is presumed that 500 ppm of GA3 is supra-optimal. At 100 and 250 ppm, however, the growth of the arrested branches at the base was significantly increased.
Thirty days after application, GA3 promoted growth and augmented the number of secondary axillary branches. Curiously at 100 ppm GA3 markedly inhibited the length of the secondary axillary branches of PAB3 to PAB6.
These studies show that a complex mechanism of apical dominance operates inCalendula officinalis.