GA3 application alters flowering phenology of 'Hass' avocado

The objectives of the present research were to quantify 1) the contribution that vegetative shoots produced in the summer vs. fall and indeterminate vs. determinate inflorescences make to yield and 2) the effects of GA3 on flowering expression and inflorescence phenology of summer and fall shoots of 'Hass' avocado (Persea americana Mill.) under field conditions. Anthesis started earlier on fall than summer shoots of 10-year-old 'Hass' avocado trees; however, no difference in the date of full bloom was observed. Indeterminate inflorescences that underwent early anthesis set more fruit than those with delayed anthesis, conversely, determinate inflorescences with delayed anthesis set more fruit. Indeterminate inflorescences comprised 90% of total inflorescences and contributed 73% of total fruit yield, but individual determinate inflorescences were at least three times more productive than the indeterminate ones. Summer and fall shoots were sprayed with 0, 50, 100, or 1000 mg(.)L-1 GA3 in November, December or January. GA3 stimulated apical growth of all shoots. If secondary axes of an inflorescence bud were differentiated at the time of GA3 application, the inflorescence developed in advance of inflorescences on branches not treated with GA3. In addition, GA3 caused precocious development of the vegetative shoot of indeterminate inflorescences relative to the flowers in the same inflorescence and relative to the vegetative shoot of indeterminate inflorescences from untreated branches. Stimulation of vegetative growth at the inflorescence apex by GA3 inhibited growth of axillary buds. GA3 at 50 mg(.)L-1 had no effect on the number of determinate or indeterminate inflorescences produced by either summer or fall shoots. Higher concentrations of GA3 increased the number of vegetative shoots and inactive buds produced by both shoot types.