Thomas L. Davenport

Flowering of mango trees in containers as influenced by seasonal temperature and water stress

Abstract Experiments were conducted in Homestead, FL, during July (mean minimum temperatures about 20°C, non-floral-inductive), and October and November (mean minimum temperatures about 15°C, floral-inductive), to determine if water stress induces floral morphogenesis in mango ( Mangifera indica L.). Suspending irrigation of container-grown cultivar ‘Tommy Atkins’ trees for 25 or 36 days in July caused water stress ( Ψ L of −3.55 MPa and −3.78 MPa, respectively), while irrigated trees remained non-stressed (−0.30 MPa). No flowering occurred, and all apical buds produced a vegetative flush which emerged later in water-stressed trees than in controls. In October, suspending irrigation of ‘Tommy Atkins’ trees for 35 days resulted in Ψ L values similar to those obtained during July. However, all apical buds, including those of irrigated trees, initiated flowers. Water stress advanced floral budbreak by nearly 2 weeks in nearly 40% of buds. In November, trees of four mango cultivars were or were not water-stressed in a glasshouse under warm conditions (mean minimum temperatures about 20°C), and were lightly pruned to stimulate growth of dormant axillary buds. Axillary buds produced vegetative growth only. Lightly pruned trees growing outdoors in cool temperatures initiated axillary floral buds. In warm temperatures (mean minimum temperatures about 20°C), water stress delayed shoot extension, but did not induce floral morphogenesis. In cool temperatures (mean minimum temperatures about 15°C), floral buds were initiated regardless of water stress. Thus, floral morphogenesis was induced by chilling temperatures. In contrast to water stress delaying the development of vegetative buds, the growth of floral buds was stimulated by water stress. Low temperatures thus promoted floral induction of mango, whereas water stress promoted growth of florally induced buds.

Reproductive physiology of mango

Mango flowering involves hormonal regulation of shoot initiation and induction events resulting in reproductive shoot formation. A balance or ratio of endogenously regulated phytohormones, thought to be auxin from leaves and cytokinins from roots, appears to govern the initiation cycle independently from inductive influences. Induction of reproductive or vegetative shoots is thought to be governed by the ratio of a temperature-regulated florigenic promoter and an age regulated vegetative promoter at the time of shoot initiation. Management of off-season flowering in mango trees is being accomplished in the tropics by successfully synchronizing shoot initiation through tip pruning and use of nitrate sprays coupled with management of the stem age to induce flowering such that it can be accomplished during any desired week of the year.

Effect of leaf age, duration of cool temperature treatment, and photoperiod on bud dormancy release and floral initiation in mango

Abstract Floral induction in apical buds of container-grown ‘Tommy Atkins’ mango trees occurred after a cool temperature regime of 18 °C day/10 °C night, 12-h photoperiod, was imposed for a minimum period of 3 weeks on trees bearing leaves at least 7 weeks old. Growth of induced buds during exposure to the cool temperature regime appeared to be necessary for floral initiation, since buds resuming growth in warm temperatures (about 28 °C day/22 °C night) immediately after receiving an inductive, cool temperature treatment produced a vegetative flush. Trees bearing younger leaves or chemically forced (thidiazuron or ammonium nitrate application) to resume growth prior to completing 3 weeks of cool temperature treatment also produced vegetative growth instead of inflorescences. Cool temperatures of 18 °C day/10 °C night with either 11-, 12-, 13-, or 24-h photoperiods resulted in floral initiation, whereas only vegetative growth occurred with warm temperatures of 30 °C day/25 °C night using 11- or 13-h photoperiods. Results of this study suggest that floral induction of ‘Tommy Atkins’ mango occurred during bud dormancy in cool temperatures around 15 °C, and that warm temperatures near 30°C prevented floral initiation of induced buds. Cool temperatures rather than a short photoperiod caused floral induction, whereas warm temperatures rather than a long photoperiod inhibited flowering.

Correlation of endogenous gibberellic acid with initiation of mango shoot growth

Stems of mango (Mangifera indica L.) rest in a nongrowing, dormant state for much of the year. Ephemeral flushes of vegetative or reproductive shoot growth are periodically evoked in apical or lateral buds of these resting stems. The initiation of shoot growth is postulated to be primarily regulated by a critical ratio of root-produced cytokinins, which accumulate in buds and by leaf-produced auxin, which decreases in synthesis and transport over time. Exogenously applied gibberellic acid (GA3) delays initiation of bud break but does not determine whether the resulting flush of growth is vegetative or reproductive. We tested the hypothesis that endogenous GA3, which influences release of these resting buds, may decrease in stem tips or leaves with increasing age of mango stems. GA3 and several other GAs in stem tip buds and leaves were identified and quantified in stems of different ages. The major endogenous GAs found in apical buds and leaves of vegetative mango stems were early 13-hydroxylation pathway gibberellins: GA1, epiGA1, GA3, GA19, GA20, and GA29, as identified by gas chromatography-mass spectrometry (GC-MS). A novel but unidentified GA-like compound was also present. The most abundant GAs in apical stem buds were GA3 and GA19. Contrary to the hypothesis, the concentration of GA3 increased within buds with increasing age of the stems. The concentrations of other GAs in buds were variable. The concentration of GA3 did not change significantly with age in leaves, whereas that of most of the other GAs declined. GA1 levels were greatest in leaves of elongating shoots. These results are consistent with the concept that rapid shoot growth is associated with synthesis of GAs leading to GA1. The role of GA3 in delaying bud break in mango is not known, but it is proposed that it may enhance or maintain the synthesis or activity of endogenous auxin. It, thereby, maintains a high auxin/cytokinin ratio similar to responses to GA3 that maintain apical dominance in other plant species.

Identification of Members of the Dimocarpus Longan Flowering Locus T Gene Family with Divergent Functions in Flowering

Dimocarpus longan is a subtropical fruit crop whose year-round production relies on the application of KClO3 to induce flowering; however, the mechanism by which this chemical causes flowering is yet unknown. To further characterize floral signaling in this species, we have isolated three longan FLOWERING LOCUS T (FT)-like genes and studied their activities by heterologous expression in Arabidopsis. Expression of two of these genes (DlFT2 and DlFT3) accelerates flowering, whereas expression of the third gene (DlFT1) causes delayed flowering and produced floral morphology defects. This anti-florigenic protein may be a member of a class of FT-like family involved in flowering time control in biennial and perennial species. Surprisingly, KClO3 treatment also suppressed the expression of both DlFT2 and DlFT3 in a field trial.

Selection of Highly Informative Microsatellite Markers to Identify Pollen Donors in ‘Hass’ Avocado Orchards

Abstract‘Hass’ is the most popular avocado (Persea americana Mill.) cultivar in the world. It has been characterized as a crop requiring cross-pollination. However, the potential extent of self-pollination and the most effective pollen donors (best cross-pollinizing cultivars) have not been determined. In this study, 56 markers were screened against ‘Hass’ and nine commonly used pollinizing cultivars grown in southern California: ‘Bacon,’ ‘Ettinger,’ ‘Fuerte,’ ‘Harvest,’ ‘Lamb Hass,’ ‘Marvel,’ ‘Nobel,’ ‘Sir Prize,’ and ‘Zutano.’ Seventeen microsatellite, i.e., simple sequence repeat (SSR) markers, were found to be very promising for paternity analysis. Four highly informative SSR markers were selected to accurately and unequivocally identify pollen parents of ‘Hass’ fruit from an orchard interplanted with these pollinizing cultivars. From 2003 to 2006, 7,984 ‘Hass’ fruit were analyzed for their paternity. Overall, the pollen parents of 99.55% of the analyzed fruit could be unequivocally identified with a single multiplex polymerase chain reaction (PCR). Only 36 fruits (<0.45%) required a second PCR reaction to reach unequivocal identification of the pollen parents.

Re-evaluation of the roles of honeybees and wind on pollination in avocado.

Summary Avocado (Persea americana Mill.) flowers, with their synchronously dichogamous behaviour, are considered to be pollinated by honeybees, despite the lack of any direct evidence. Results in southern Florida showed that avocado pollen was transferable by wind, and was dispersed over a brief period of time (15 – 60 min) each day. Ten ‘Hass’ avocado orchards in the Santa Clara River Valley, CA, USA, planted far from any known ‘Zutano’ polliniser trees, were selected to investigate the impact of honeybees on pollen transfer. ‘Zutano’ pollen (5 g per insert) was placed at the entry to beehives (approx. eight beehives per orchard) and refreshed four-to-five times during the flowering season. Successful pollinations were determined by parental analysis of harvested ‘Hass’ fruit from trees located at various distances from the beehives, and at three different stages of fruit development, using microsatellite DNA markers. The results showed no significant difference in the proportions of ‘Zutano’-pollinated fruit with respect to distance and/or development stage between orchards provided with beehives containing ‘Zutano’ pollen and those without supplemented pollen. This strongly suggests that honeybees are not the major pollinators of avocado, and that most avocado flowers are self-pollinated by wind.

Flowering and fruit set of mamey sapote (Calocarpum sapota (Jacq.) Merr.) cv. Magana in South Florida

Flowering and fruit set characteristics were examined in the popular commercial cultivar, Magana, in an effort to elucidate the reproductive phenology of mamey sapote, Calocarpum sapota (Jacq.) Merr. [syn. Pouteria sapota (Jacq.) H.E. Moore and Stearn]. Flowers opened during the night with anthesis beginning around sunset. The length of floral opening varied according to season, ranging from 6 days in winter to a single day in summer. Bursts of new flowers generally appeared in cycles of about 7 days in declining numbers of flowers per burst until all the floral buds of a particular floral bud flush had flowered. Floral buds flowered randomly along a branch with only a few flowers open at any one time. Flower position around the branch was a factor in fruit set. Flowers and small fruitlets encircled horizontal branches in great numbers, but immature fruit most often developed from flowers located on the upper branch quadrant. The lower quadrant contained the fewest immature fruit. As fruit matured, however, more upper quadrant fruit abscised until by harvest, most mature fruit were found on the lower quadrant. These observations provide the first reported in-depth insights into the flowering and fruit set behavior of mamey sapote. They furnish the necessary informational framework for future studies.