Management of Flowering in Three Tropical and Subtropical Fruit Tree Species

There exist hundreds of cultivated tropical and subtropical fruit species, most of which are arboreal. Among these are mango (Mangifera indica), which thrives from the equator (zone 11) to the subtropical limits of tropical zone 10b of the northern and southern hemispheres (Cathey, 1990; Jordan, 2001); Citrus sp., which have a growing range that extends for some species from near the equator into subtropical zone 9b, including, for examples California, Texas, and Florida; and lychee (Litchi chinensis), which performs best at the subtropical limits of tropical zone 10b (Fig. 1). Cathey (1990), as adapted by Jordan (2001), defi ned tropical Zone 11 as those locations in which the average annual minimum temperature is above 4.4 °C and which never freeze. This area is generally bounded in the north by the Tropic of Cancer (23°27'N) and the south by the Tropic of Capricorn (23°27'S). It includes the low altitude dry and humid tropics as well as high altitude tropics where growing temperatures are always low but within the constraints defi ned by Zone 11. Plants growing in areas classified as Zone 10b experience average minimum temperatures of 4.4 to 1.7 °C with occasional frosts and rare freezes. Subtropical areas include Zone 10a with average minimum temperatures of 1.6 to –1.1 °C and annual mild freezes and Zone 9b, which defines the limits of subtropical plant production. This zone experiences –1.2 to –3.8 °C average annual minimum temperatures and more frequent or intense freezes. A conceptual flowering model has been described to explain the interaction of intrinsic and extrinsic factors regulating vegetative and reproductive shoot initiation and induction for both mango (Davenport and Nunez-Elisea, 1997; Davenport, 2000) and Citrus sp. (Davenport, 1990; Davenport, 2000). Lychee is another tropical fruit species that appears to follow this general flowering model (Davenport, 2000). Although all three arboreal species display different morphologies, they all seem to respond to the same environmental cues to drive phenological changes involved in growth and development. Of the three, however, only citrus is induced to flower in specific response to water stress (Southwick and Davenport, 1986). Unless stated otherwise, the events or regulation of events described hereafter refer to all three species. Individual stems borne on branches of these trees spend most of the time in rest. Stems here refer to resting vegetative structures composed of the terminal intercalary unit resulting from the previous fl ush of vegetative growth (Davenport, 1990; Davenport and NunezElisea, 1997). Ephemeral flushes of shoot growth from these stems occur periodically and last ≈2 weeks before returning to the resting state. Most often, these flushes are vegetative. Individual stems typically produce only one reproductive flush per year. If synchronized, the flush will occur at once in all stems throughout the canopy. More often than not, however, sections of tree canopy flush in the tropics instead of the whole, and hence may produce flowers in one section on one occasion and in other sections on other occasions until the entire canopy has fl owered. Although the subject of this article is management of flowering in these tropical fruit trees, one must first recognize that initiation of shoot growth is the first event that must occur in order to produce fl owering (Davenport, 1990, 2000; Davenport and Nunez-Elisea, 1997). Management of flowering, thus, also entails management of shoot initiation. The reader is encouraged to refer to the above-cited review articles in order to fully appreciate the reasons for implementing the management practices described here. Shoot initiation, or bud break, is driven by different factors than those regulating induction events that determine whether developing shoots will be vegetative or reproductive. It is thought to involve the interactive ratio in buds of accumulated cytokinins from roots with declining auxin production and transport from leaves of increasing age (Davenport, 2000). Frequent flush events occur in young trees and in mature trees in conditions of high nitrogen levels and an abundance of water. Other factors that stimulate initiation of shoot development include stem pruning, defoliation, foliar nitrogen sprays, and ethylene. One of three types of shoots is typically induced to develop from initiated buds: vegetative (leaves only), generative (determinate panicle or indeterminate inflorescences), or mixed (composed of a leaf and lateral inflorescence inserted at each node or, in general, a mixture of fl owers and leaves) (Fig. 2). The type of shoots that are evoked upon initiation appear to be governed by the interaction of a putative temperature-regulated florigenic promoter (FP) known to be made in leaves of mango and stem tips of citrus and an age-dependent vegetative promoter (VP), which is likely to be a gibberellin made in the same tissues as the that of the florigenic promoter of each species (Davenport, 1990, 2000, 2002; Davenport and Nunez-Elisea, 1997). The ratio of these two components in buds is thought to induce formation of the distinct shoot types; i.e., high FP to VP ratio induce generative shoots, high VP to FP ratios induce formation of vegetative shoots, and intermediate FP to VP ratios may be responsible for formation of mixed shoots. Vegetative flushes typically occur one to three times per year on individual stems, depending upon species, cultivar, size of the tree, and growing conditions. Reproductive flushes generally occur after extended periods of stem rest in the low-latitude tropics often after relief of extended periods of non-lethal water stress or during cool winter months in the higher latitude tropics and the subtropics. Control of fl owering of mango, citrus, and lychee outside of these natural flowering times focuses on management of shoot initiation at a time when the ratio of these two putative promoters are favorable for fl owering. There are reasons unique to each commodity as to why producers of mango, citrus, and lychee desire to manage flowering. Mango is perhaps the most amenable to floral manipulation. Producers are, thus, most interested in reliable out-of-season flowering in order to provide fruit