Flowering and fruit set of mamey sapote [
Calocarpum
sapota
(Jacq.) Merr.] cv. MaganÄa in South Florida
Thomas L. Davenport
*, James T. O'Neal
Tropical Research and Education Center, University of Florida, IFAS, Homestead, FL 33031, USA
Accepted 9 April 1999
Abstract
Flowering and fruit set characteristics were examined in the popular commercial cultivar, MaganÄa, 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.#2000 Elsevier Science B.V. All rights reserved.
Keywords: Floral buds; Anthesis; Fruitlets; Sapotaceae; Tropical fruit
* Corresponding author. Tel.: +1-305-246-7019; fax: +1-305-246-7003.
E-mail address:tldav@icon.hmsd.ufl.edu (T.L. Davenport)
1. Introduction
Mamey sapote is a minor crop with increasing acreage in south Florida. Cuban immigrants have brought their appreciation of the mamey with them (Campbell, 1967; Knight, 1988), and interest in the mamey has increased as these new markets have developed and the area planted has expanded to meet the demand. `MaganÄa' has been evaluated at the Tropical Research and Education Center (TREC), Homestead, FL since 1961 when the first budwood from El Salvador was grafted and then planted on the property in 1962. Additional grafted trees were planted at TREC in 1975 (Campbell and Lara, 1982) as part of a cultivar evaluation collection. Although several commercial cultivars have been developed, scientific knowledge, especially of the reproductive physiology of this crop, has remained limited.
Mamey is an erect, slow-growing, open-canopy tree with a central trunk and few large limbs. Leaves are clustered at branch terminals. Small flowers form on branches ranging in location from axillary buds located at the bases of intact leaves to a proximal distance of 1±2 m down the branches. Flowers consist of eight to ten small, imbricate sepals, five petals forming a cream colored tubular corolla, five stamens, and a five-celled ovary with each cell containing an ovule (Popenoe, 1920). Phenological observations have been made on the reproductive development of several important or potentially important cultivars and reported elsewhere (T.L. Davenport and J.T. O'Neal, submitted for publication). This report extends those observations by focusing on specific flowering habits and fruit set characteristics of one of the most popular commercial mamey cultivars, `MaganÄa.'
2. Methods and procedures
Ten 20-year-old mamey sapote, Calocarpum sapota (Jacq.) Merr. [syn.
Pouteria sapota (Jacq.) H.E. Moore and Stearn] cv. MaganÄa orchard trees at TREC, were used in this study. Tree height ranged from 3.5 to 7.5 m. Trees were irrigated by overhead sprinklers twice weekly during the dry season (November to May) and with periodic rain during the rainy season (June to October). They were fertilized every 4 months with ground application of 277 kg 8N±3P±9K dry fertilizer and 5.6 kg Geigy Sequestrene Fe-1381
/ha and with foliar applications of minor elements at a rate of 5.6 kg Fer-a-gro1/380 l/ha.
2.1. Time of anthesis
recorded daily at 8.00 AM and at 5.00 PM for 21 consecutive (except 3) days from 8 March to 31 March. The time of anthesis was established by ascertaining the number of newly opened flowers present since the previous observation. Newly open flowers were marked with a felt-tipped pen when first observed on 10 of the tagged branches. A second mark was applied to these flowers during the next and subsequent observation times. Multiple marks, thus, distinguished the previous days' tagged flowers from newly opened ones. In the remaining 10 branches, newly opened flowers were removed from the branch after noting its presence in each observation period which left only pre-anthesis floral buds on the branch. Any open flowers present in the subsequent observation were, thus, newly opened. No differences in flowering behavior were noted between branches in which flowers were removed and those in which the flowers were marked. The diurnal totals of both sets were, thus, combined in the reported results.
2.2. Duration of floral openings
Few flowers typically undergo anthesis on a single branch. To obtain sufficient numbers of flowers for observation, 25 floral bud-bearing branches distributed among 10 `MaganÄa' trees were tagged (two branches on five trees and three branches on five). The number of newly open flowers was recorded daily each morning during 14 days of the winter (27 January±10 February), 37 days of spring (18 April±24 May), and 35 days of summer (17 July±20 August). Each newly open flower was marked with a felt-tipped pen in one of several colors to designate its date of opening. The anthesis date for each color group was identified as Day 0. The total number of flowers on all branches remaining open on each consecutive day after anthesis during each season was calculated. The percent of original flowers remaining open on each succeeding day after anthesis in each season was calculated and reported. Each flower continued to be monitored daily for up to 30 days as it progressed through post-anthesis (closed flower) and fruitlet (greenish, without petals) stages. No fruitlets advanced to immature fruit stage (brown surface color) within the 30-day observation periods. Daily maximum and minimum temperatures were obtained from a nearby weather recording station. They were each averaged over the periods of observation in the winter, spring, and summer seasons.
2.3. Fruit set position around branches
compass with the branch axis and comparing the fruit peduncle position with the scale, the fruit position on a branch could be assigned to one of four radial quadrants with 08being vertically up. The quadrants were defined as: Upper, 3158
to 458; Right, 458 to 1358; Lower, 1358 to 2258; and Left, 2258 to 3158. The large variation among trees and among individual branches within trees prevented us from performing a replicated design survey to show statistical differences in any variable. So few organs were present on any given observation day that cumulative summary data were collected across the branches and trees in each survey. The daily sums and means of variables reported here are, therefore, only indicators of trends.
3. Results
3.1. Time of anthesis
Floral buds developed variously in clusters of buds or in single buds along the branches. They generally began anthesis when their bud length attained about 1.5 cm (data not shown). Fully opened new flowers were consistently found during the 8:00 AM observations (Fig. 1). During the 21 days of observation, 370 new flowers opened during the night hours. The total number of flowers opening among the tagged branches on any particular night ranged from 2 to 36. Flowers opening during the day, as determined by the 5:00 PM observations, were found on only 2 days with a total of four flowers. All were only partially open at 5:00 PM indicating that they were beginning anthesis late in the day.
3.2. Duration of floral openings
The number of days in which individual flowers remained open following anthesis correlated with ambient seasonal temperatures. The results are expressed as the percentage of flowers remaining open on subsequent days after anthesis during winter, spring and summer (Fig. 2). The warmer the weather, the shorter the time flowers remained open. During the winter survey when the mean ambient maximum and minimum temperatures were 22.7 and 7.28C, respectively, 136 flowers were surveyed. Some flowers remained open up to 6 days but the number of open flowers steadily declined from Day 0 to Day 6. By Day 7, all flowers had either progressed into the next developmental stage or abscised.
The spring survey had fewer floral openings on the branches. Fifty-seven flowers were marked at anthesis and tracked until closure. The duration of floral openings in spring was about half that of the winter months (Fig. 2). The maximum duration was 3 days. The sharpest decline in percentage of open flowers occurred between Day 0 and Day 1 as the proportion of flowers remaining open fell to about 23% of the original. The mean ambient maximum and minimum temperatures were 31.7 and 20.68C, respectively, during this period.
The duration of floral openings of 52 flowers was observed during the summer months when the mean ambient maximum and minimum tempera-tures were 33.3 and 22.18C, respectively. The percentage of flowers remaining open fell to about 1% of the original within 24 h after anthesis. Most of the
tagged flowers were open only a half-day, having closed by early afternoon of Day 0.
The anthesis dates of an additional set of spring flowers, which were not involved in the floral duration study, were also recorded. These extra flowers were those that opened after all available tagging colors were in use on a particular branch. The daily total number of these extra newly open flowers, combined with those of the 57 marked spring flowers used in the floral duration study, provided a daily estimate of floral openings over this time period. Periodic peaks in the total number of daily floral openings were discernible during the period from 18 April to 12 May when the flowering season was complete (Fig. 3). These observations complimented those made during 8±31 March in which similar periodic peaks in the number of new flowers was also noted (Fig. 1).
High abscission rates of flowers and fruitlets were observed during the period following anthesis. Of the 136 observed winter flowers, only seven remained on the branches after 30 days. Five had advanced to the fruitlet stage, and two were still in the post-anthesis flower stage, for a 5.2% overall survival rate after a month. Similar results were found during the spring 30-day time period. Of the 57 spring flowers, three remained on the branches with two in the post-anthesis stage and one in the fruitlet stage, for a 5.2% overall survival rate. Summer retention rates were higher than those of both the winter and spring seasons. Whereas the 52 summer flowers were similar in number to the spring flowers, 27 (38%) remained on the branches by 30 days post-anthesis during the summer observation. Of these, 20 had advanced to the fruitlet stage.
3.3. Radial axis position of fruit
A survey of an undetermined number of randomly selected, laterally inclined branches across 10 trees on 31 January revealed a total of 154 immature fruit 1.5± 10 cm in length. The combined upper quadrants of these branches bore a total of 53 fruit. The lower quadrant bore a total of 27 fruit, and the left and right quadrant totals were equal with 37 fruit on each side. The percentage distribution of total fruit was, thus, 34% inserted in the upper quadrant, 18% in the lower, and 24% each in the left and right quadrants (Fig. 4). A survey of the number of older, more mature fruit (>10 cm in length) conducted on the same date revealed a total of 123 fruit on the branches of 10 trees. The mature fruit distribution pattern between upper and lower quadrants was opposite to that of the immature fruit.
The upper quadrant bore 12 fruit, the lower quadrant bore 59 fruit, and the left and right quadrants were still essentially equal with 25 and 27 fruit, respectively. The proportion of the more mature fruit that were inserted in the upper quadrants of branches was, thus, only 10% whereas the lower quadrant bore 48% of the fruit. The proportions of the more mature fruit that were inserted in the left and right quadrants were 20 and 22%, respectively (Fig. 4).
4. Discussion
As Sturrock (1959) and Davenport and O'Neal (submitted for publication) noted, mamey sapote flowers are produced in profusion along the branch but only a few survive to the fruitlet stage. They develop variously in clusters of buds or in single buds presumably at nodal meristems distributed along the branches. All of the 365 flowers observed during the 20-day observation period opened during the night hours with anthesis beginning in rare cases as early as just prior to sunset (Fig. 1). The significance of night anthesis is not clear since nothing is known about pollinators possibly visiting the flowers or whether an insect vector is required. We were unable to determine the time of anther dehiscence or discern the longevity of stigma receptivity in this study. Since the flowers remained open at least one day, pollen vectors present during either the night or day period could be important for pollination.
While flowers remained for at least one day, the duration of flower opening varied with the season. During the cool winter months, when the average maximum and minimum temperatures were 22.7 and 7.28C, respectively, flowers remained open up to 6 days. The length of time in which flowers remained open dropped to 3 days in the spring (31.7 and 20.68C) and to less than 1 day during the summer, when the mean maximum and minimum temperatures were 33.3 and 22.88C, respectively. During the warm summer months, flowers often closed on the afternoon of the day of anthesis. The pace of the developmental processes, such as bud development, length of floral openings, pollen tube growth rate, and various flowering events in tropical fruit trees is commonly sensitive to subtle differences in ambient temperatures. Mamey sapote appears to follow this trend.
The daily frequency of floral openings during the spring flowering period occurred randomly along the branches but in an overall pattern of bud bursts that peaked at periodic intervals of about 1 week. This cycling was apparent while conducting the time of anthesis observations from 8 March to 31 March (Fig. 1) and the duration of floral opening observations from 18 April to 12 May (Fig. 3). These cycles followed a dampening sine wave pattern with each succeeding flower burst involving fewer flowers over each cycle until all floral buds of the flush had opened (Fig. 3). This dampening pattern in the frequency of flowering was also evident in Fig. 1. Through this distribution, flowering was maintained over several weeks with only a few flowers open at any one time.
The differential shift in fruit-set proportion from upper to lower quadrants during fruit maturation was noticeable and unusual. Our observations included two independent fruit size groups, immature and older, more mature fruit simultaneously on fruit-bearing branches. The lateral quadrants retained about one-quarter of the total fruit in both developmental groups indicating that differential abscission was not a factor in lateral quadrant fruit development. The greater loss of fruit from upper than lower quadrants suggests, however, that internal physiological factors may be involved.
Similar surveys on branch position fruit set in other cultivars were conducted, in part, to determine if the observations in `MaganÄa' were an aberration or a general characteristic of mamey sapote. Similar shifts in upper-to-lower quadrant fruit set were observed in three other cultivars, Tazumal, Copan, and Mayapan. Limited numbers of fruit in the appropriate sizes, however, made those results less clear (data not presented).
5. Conclusion
The information presented here presents never-before-reported insights into the flowering and fruit set behavior of mamey sapote. It provides the basis for further investigation of the reproductive physiology for improved yield of this commercially small, but valuable tropical fruit crop.
Acknowledgements
References
Campbell, C.W., 1967. The mamey sapote in Southern Florida. Fla. State Hortic. Soc. 80, 318±324. Campbell, C.W., Lara, S.P., 1982. Mamey sapote cultivars in Florida. Proc. Fla. State Hortic. Soc.
95, 114±115.
Knight Jr., R.J., 1988. Miscellaneous tropical fruits grown and marketed in Florida. Proc. InterAm. Soc. Trop. Hortic. 32, 34±41.
Popenoe, W., 1920. Manual of Tropical and Subtropical Fruits, Hafner Press, (1974 facsimile of 1920 edition), New York.
