Production of reproductive mango shoots occurs when they are initiated to grow from quiescent buds on resting stems exposed to cool, floral-inductive temperatures in subtropical and high-latitude tropical climates (Davenport and Núñez-Elisea, 1997; Núñez-Elisea et al., 1996). In contrast, reproductive flushes generally occur after extended periods of stem rest in constantly warm temperatures of the low-latitude tropics (Bueno and Valmayor, 1974; Davenport, 2009; Davenport and Núñez-Elisea, 1997; Núñez-Elisea and Davenport, 1995). The age of the last flush of vegetative stems, thus, appears to be the primary factor regulating floral induction in warm climates. Bueno and Valmayor (1974) indicated that leaves must become brittle as evidenced by an audible crackling sound when hand-crushed to indicate leaf age. Núñez-Elisea (1986, 1988) reported that stems must be at least 6 months of age. Empirical observations (Davenport, 2006, 2009) have indicated that terminal stems (terminal intercalary units) must have attained a dark green color and achieved a minimum age of 4 months since the previous limp, red-leaf stage in easily induced cultivars and 5 months for the more recalcitrant cultivars to obtain a reproductive shoot response in the low-latitude tropics.
Frequent vegetative flush events occur in young trees and in mature trees in high fertility conditions with an abundance of water. Such trees continually produce shoots that are induced to be vegetative as a result of short periods of stem rest between flushes (Davenport, 2003). Foliar-applied potassium, ammonium, or calcium nitrate stimulates shoot initiation of mango and is widely used in the low-latitude tropics to stimulate flowering (Núñez-Elisea and Caldeira, 1988). Potassium nitrate (KNO3) is a water-soluble nitrate salt that is commonly used to stimulate flowering in numerous mango cultivars (Barba, 1974). It stimulates more vigorous flowering in mature stems than in younger ones (Bondad and Linsangan, 1979).
Previous anecdotal observations indicated that ‘Haden’ and ‘Tommy Atkins’ mango stems need to be at least 4 to 5 months old to reproductively respond to nitrate compounds. Insufficient stem maturity may, thus, explain the meager flowering response often observed in mango trees after spraying nitrates (Medina-Urrutia and Núñez-Elisea, 1997). To be successful in stimulating flowering, the nitrate salt must be applied after the resting stems of mango have reached sufficient age to overcome any inhibitory influence they may have on the flowering response (Davenport, 2000).
Davenport (2009) provided several lines of evidence to indicate that KNO3 as well as ammonium and calcium nitrate [NH4NO3, Ca(NO3)2] initiate shoot growth but do not determine the inductive fate of bud morphogenesis. The ratio of the leaf-generated florigenic and vegetative promoters at the time of shoot initiation, not nitrate, is thought to be responsible for receptive buds to induce reproductive shoots. Kulkarni (2004) shared the view that the floral stimulus is already present in stems at the time that buds are forced in response to KNO3 and suggested that nitrates may also sensitize buds to the floral stimulus. Others have proposed that KNO3 is floral-inductive in mango (Yeshitela et al., 2005). Alternately, nitrogen sufficiency achieved by potassium nitrate is considered a possible explanation (Kulkarni, 2004).
Another key factor when managing mango flowering is synchronization of vegetative growth in tree canopies, which allows all the stems in a canopy to be in the same physiological stage of maturity when KNO3 applications are made (Davenport, 2000). Synchronized growth is best accomplished by tip-pruning all terminal stems on trees (Davenport, 2003, 2006). Tip-pruning not only produces a specifically timed uniform flush of vegetative growth throughout the canopy, but it removes growth- and flower-inhibiting factors in stems derived from the previous season's flowering and fruiting panicles (Davenport, 2000, 2009). Tip-pruning mature trees quickly results in a synchronous flush of lateral vegetative shoots if water is adequate and leaf nitrogen levels are in a range of 1.1% to 1.4% (Davenport, 2003). If the lateral stems produced by this pruning event subsequently remain in rest for 4 or 5 months (depending on cultivar) in warm temperatures, then flowering will usually occur when shoots are initiated to grow by foliar application of potassium, calcium, or ammonium nitrate (Davenport, 2006).
The application of KNO3 to synchronized mature canopies has been empirically determined to be effective under tropical conditions as a mechanism to induce flowering. Programs have been adopted for the management of flowering in such orchards growing in warm tropical conditions (Davenport, 2003); however, the application of KNO3 and tip-pruning in flowering management strategies have not been thoroughly documented in replicated trials. A better understanding of the role that stem age plays in mango flowering is, therefore, needed. The aim of the current study was to establish the age of the previous vegetative flush necessary to induce flowering in synchronized ‘Keitt’ and ‘Tommy Atkins’ trees under warm, tropical conditions in Colombia.
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