Irreversible Commitment to Flowering in Two Mango Cultivars, Poster Board #205

In recent years, the state of Nayarit, Mexico has experienced variations in rainfall distribution and warmer temperatures during the autumn-winter season which have caused erratic flowering of mango. The early-flowering cultivars, such as ‘Ataulfo’, have been less affected than tardy ones such as ‘Tommy Atkins’. To understand this problem and develop management practices to minimize it, it is essential to learn when irreversible commitment to flowering (ICF) occurs in shoots of these two cultivars. This research was conducted during 2006–07, 2008–09 and 2009–10 in three commercial orchards of each cultivar in northern, central and southern Nayarit. The orchards had no irrigation and the climate is classified as warm-subhumid (mean annual temp. > 22 °C; mean temperature of the coldest month > 18 °C; summer rainfall ~ 1,225 mm). The date in which ICF occurred was determined by quantifying the type of growth (vegetative or floral) produced by the apical buds throughout the various vegetative flushes (‘Ataulfo’: spring and summer; ‘Tommy Atkins’: spring and fall) in response to monthly girdling and defoliation treatments. Shoots not receiving treatments were used as control.  ‘Ataulfo’ flowered every year during the 3-year study. Spring shoots emerged from April to May.  Shoots treated between June and August did not flower; flowering occurred in shoots treated from October on (53.2%). Summer shoots emerged in July–August and flowering occurred in treated shoots from November on (32.4%). ‘Tommy Atkins’ did not flower in 2008–09.  During 2006–07 and 2009–10 spring shoots emerged in from April to May and flowering occurred in treated shoots from November on (23% to 56%). Fall shoots emerged in October–November and flowering in treated shoots occurred from February on (20% to 61%).  Control trees of ‘Ataulfo’ showed flowering in 41 to 43% of the shoots whereas in ‘Tommy Atkins’ it was 47% to 52%. Irreversibly committed to flowering apical buds arising from vegetative flushes were always characterized by having a conical shape with a slight separation of the vestigial leaf primordia (bud scales). Microscopic examination of these buds showed elongation of the primary axis and initial development of secondary axes as well as presence of tertiary axes meristems.