Use of Gibberellic Acid to Manipulate Flowering in the 'hass' Avocado: a Preliminary Report

Inflorescence development of the 'Hass' avocado (Persea americana Mill.) was investigated at the macro and microscopic level. Near the end of shoot extension, two secondary axis meristems were present in the axils of inflorescence bracts. The primary axis meristem changed shape from conical to flattened to conical followed by the initiation of additional bracts and associated secondary axis inflorescence meristems rather than additional leaf primordia. No anatomical changes were associated with commitment to flowering. The transition phase from the vegetative to reproductive condition occurred in less than one month from the end of July through August for summer flush shoots. A period of dormancy was not prerequisite for the shift to inflorescence development. Commitment to flowering resulted in formation of additional bracts with secondary axis inflorescence meristems from August to October, anthesis was seven months later. Eleven stages of external bud and subsequent inflorescence development were correlated with organogenesis for use in predicting specific stages of inflorescence development in the field. Yield had little effect on the rate of inflorescence development, which correlated with the cumulative number of nights with temperatures < 15 °C, but the "on" crop reduced inflorescence number with a concomitant increase in the number of vegetative shoots. Gibberellic acid (GA3) sprays at 25 mg-liter in September reduced flowering intensity in "on" and "off" crop years. November sprays reduced inflorescence number in the "off" year with a concomitant increase in vegetative shoots; whereas the 2-fold increase in inflorescences in the "on" year was not significant. GA3 caused precocious development of the vegetative shoot apex of indeterminate inflorescences. Control trees yielded 18 kgtree (an "off" crop year) whereas GA3 applied in November, January, or March increased yield to 34.8, 27.3, and 33.9 kg-tree, with more green late-harvested fruit (May). Kg-tree of individual fruit weighing 135-177 g and 213-269 g increased 3-fold or 2-fold with GA3 applied in November or March, respectively. GA3 should prove useful in evening out alternate bearing in avocado.