HORTSCIENCE 41(7):1541–1546. 2006. Crop Load Affects Vegetative Growth Flushes and Shoot Age Influences Irreversible Commitment to Flowering of ÔHassÕ Avocado Samuel Salazar-Garcı ´a 1 Campo Experimental Santiago Ixcuintla, INIFAP, Apdo. Postal 100, Santiago Ixcuintla, NAY 63300, Me´xico Luis E. Cossio-Vargas Unidad Acade´mica de Agricultura, Universidad Auto´noma de Nayarit, Apdo. Postal 49, Xalisco, NAY 63780, Me´xico Carol J. Lovatt Department of Botany and Plant Sciences, University of California, Riverside, CA 92521-0124 Isidro J.L. Gonza ´lez-Dura ´n and Marı ´a H. Pe ´rez-Barraza Campo Experimental Santiago Ixcuintla, INIFAP, Apdo. Postal 100, Santiago Ixcuintla, NAY 63300, Me´xico Additional index words. Persea americana, phenology, alternate bearing, floral development, girdling, floral determination Abstract. Several studies were undertaken in commercial nonirrigated ÔHassÕ avocado orchards under the subhumid semiwarm subtropical climate of the state of Nayarit, Mexico, with the following objectives: 1) to determine the frequency and intensity of vegetative shoot flushes and their contribution to the production of floral shoots, 2) to quantify the effect of tree fruit load on the occurrence of vegetative shoot flushes during the year and the relationship between vegetative and reproductive shoot number during flowering, and 3) to determine the time when apical buds borne on the major vegetative shoot flushes reached irreversible commitment to flowering (floral determination) through the use of shoot defoliation and girdling. Data trees were selected in two orchards based on their current crop load. Four to five branches per tree were tagged, and the number and intensity of vegetative flushes that developed during 2 years, as well as the type of growth produced by apical buds of shoots of different ages, were recorded at the end of the winter bloom periods for two separate years, 1999 and 2001. In a separate experiment using a different set of trees, winter and summer flush shoots were defoliated (year 1) or defoliated and girdled (year 2) at different stages of bud development from September to January in each case. Four vegetative flushes occurred each year. The winter flush that emerged in Feb. 1998 made the greatest contribution to the 1999 winter bloom—76.5% of the shoots produced floral shoots. Contributions of the summer 1 (late July 1998), summer 2 (early Aug. 1998), and summer 3 (late Aug. 1998) flushes to flowering were intermediate. A total of 30.6%, 36.4%, and 19% of the shoots produced floral shoots respectively. All four vegetative flushes produced a similar number of vegetative shoots during winter bloom. Evaluation of the 2001 winter bloom for trees with high (>95 kg fruit/tree) and low (<70 kg fruit/tree) crops showed no effect of tree fruit load on the production of vegetative or floral shoots by winter or summer vegetative flushes. Irrespective of time of treatment (shoot defoliation and girdling) or shoot age, irreversible commitment to flowering of apical buds occurred by 15 Oct., and this stage was associated with an average of 27.5 chilling days (temperature, #19 °C) for both years. Buds irreversibly committed to flowering were closed and pointed, with partial senescence of bud scales. Anatomically, the buds showed a convex primary axis meristem and four secondary axis floral shoot meristems. The state of Nayarit is the second highest avocado-producing state in Mexico, averag- ing 17,000 ton from almost 2000 ha (CON- APA, 2006). The ÔHassÕ avocado-producing area is located between 21°18# and 21°32# N latitude and from 850 to 1500 m above sea level (asl) in a subhumid warm climate (20 to 29 °C average annual temperatures) with 1300 mm of summer rain. Bloom of the ÔHassÕ avocado in Nayarit occurs once a year and usually takes place during the winter (January to February), concomitantly with a flush of vegetative shoots. Other vegetative shoot growth flushes may occur during the summer and fall. Legal fruit maturity (21.5% mesocarp dry matter) is typically reached by October/November of the same year. To remain competitive, Nayarit ÔHassÕ avocado growers need to increase the selling price of their crop and to maximize the effectiveness of orchard management practi- ces, like fertilization, pruning, or application of plant bioregulators, in solving production problems. The low selling price of the crop is because peak harvest season for the ÔHassÕ avocado in Nayarit and Michoaca ´n (the largest avocado-producing area) is from Oc- tober to December, and thus coincides with that of Nayarit. To minimize this problem and to increase both the selling price of avo- cados and grower income, an ability to shift the time of maturity of the crop to before or after the period of maximum harvest is de- sired. A standard horticultural strategy for shifting the time of fruit maturity is to advance or delay flowering. However, an in-depth study of avocado reproductive phys- iology is a prerequisite to attempts to modify the time of bloom and, thus, change the date of harvest of the ÔHassÕ avocado (Salazar- Garcı ´a, 2000). Knowledge of avocado flowering is crit- ical to the development of orchard manage- ment strategies to increase floral intensity or, alternatively, to promote vegetative growth and decrease flowering. For such strategies to be successful, it is essential to know the time when shoot apical buds reach the irreversible commitment to flowering (ICF), or floral determination. To prevent an inadvertent increase in vegetative shoot growth at the expense of flowering, cultural practices that can prevent flowering, like nitrogen fertiliza- tion, light canopy pruning (hedging, top- ping), or application of some plant bioregulators, must be carried out after ICF. A 2-year study in California showed that apical buds of summer shoots of ÔHassÕ avocado transitioned from vegetative to re- productive growth from 30 Aug. to 15 Oct. (Salazar–Garcı ´a et al., 1998). Latitudinal differences between California and Nayarit may influence tree phenology and may affect the time and stage of development when apical buds become committed to reproduc- tive growth. Depending on environmental conditions, ÔHassÕ avocado trees can produce two or more flushes of vegetative growth every year (Salazar-Garcı ´a, 2000). The magnitude of Received for publication 20 June 2005. Accepted for publication 14 July 2006. Major funding for this work was obtained from the University of California Institute for Mexico and the United States, The Consejo Nacional de Cien- cia y Tecnologı ´a, and the Fondo Mixto de Fomento a la Investigacio ´n Cientı ´fica y Tecnolo ´gica de Nayarit. Additional support was provided by the INIFAP, the USPR Aguacate Hass de Nayarit, and by the Fundacio ´n Produce Nayarit, A.C. We thank Alberto Ante and Bernabe ´ Sa ´nchez for use of their avocado orchards. 1 To whom reprint requests should be addressed; e-mail samuelsalazar@prodigy.net.mx. HORTSCIENCE VOL. 41(7) DECEMBER 2006 1541