Materials and methods

This study was conducted in Puerto Rico at the USDA, Agricultural Research Service Research Farm in Isabela (Coto clay: clayey, kaolinitic isohyperthermic Typic Hapludox) and at the Corozal Agricultural Experiment Station of the University of Puerto Rico (Corozal clay: clayey, mixed, isohyperthermic Aquic Haplohumults). Soil and climatic characteristics are described in Tables 1 and 2. Soil samples from each site were taken ≈2 months before planting by taking 15 borings at a depth of 0–25 cm from each of the 10 projected cultivar rows. Samples were air-dried and passed through a 20-mesh screen. Soil pH in water and 0.01 m calcium chloride (1 soil : 2 water) were measured with a glass electrode. Exchangeable cations (potassium, magnesium, calcium) were extracted with neutral 1 n ammonium acetate and determined by atomic absorption spectroscopy (Sumner and Miller, 2007). Phosphorus was extracted with 1 n ammonium fluoride and 0.5 n hydrochloric acid and determined using the ascorbic acid method (Benton, 2001). Organic carbon was determined by the Walkley–Black method (Nelson and Sommers, 2007). Soil ammonium and nitrate were determined by steam distillation (Mulvaney, 2007).

Table 1.Average preplant soil characteristics at two mamey sapote test sites in Puerto Rico measured to a depth of 25 cm (9.8 inches).

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Table 2.Weather data at two mamey sapote test sites in Puerto Rico (2005–09).

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Six-month-old trees of cultivars Copan, Magaña, Mayapan, Pace, Pantin, and Tazumal grafted onto ‘Pantin’ seedling rootstocks were transplanted to the field on 2 Feb. 2000 (Isabela) and on 25 Apr. 2000 (Corozal) and were arranged in a randomized complete block design with five replications at each location. Before transplanting, the soil was chisel-plowed to a depth of ≈90 cm. Planting holes of ≈1.5-ft deep were dug with an auger connected by a drive shaft to the power-take-off unit of a tractor. On transplanting, each plant received 11 g granular phosphorus provided in the form of triple superphospate. Within a replication, plots for each cultivar contained three trees spaced 20 ft apart and 20 ft between adjacent rows in a triangular array, ≈108 trees per acre. The experiments were surrounded by two guard rows of ‘Tazumal’ seedlings. Irrigation based on tensiometer readings was provided with spinner jets (model DXMAG368X; Maxijet, Dundee, FL) spaced 20 ft apart and providing 13.5 gal/h at 20 psi when the soil water tension at a depth of 30 cm exceeded 50 kPa. Fertilization was provided every 3 months using a 15N–2.2P–16.3K–1.8Mg fertilizer at a rate of 100, 200, and 269 kg·ha⁻¹ in 2002, 2006, and 2009, respectively. Herbicide (glyphosate) for weed control was applied only in strips within the planting row. Weeds between rows were controlled with a tractor mower.

Harvests were initiated in Jan. 2005 at both locations. At this time, grafted trees were ≈6.5-years old and producing fruit in sufficiently large numbers for commercial harvest and sale. At each harvest, number and weight of marketable fruit were recorded and weighed. Fruits were harvested at color break when they started to show a pink rather than green tissue upon light scratching of the fruit skin. Representative fruit from each cultivar is shown in Fig. 1. Fruit totaling 10% of those harvested were used to determine fruit length and diameter. Readings of total soluble solids were also recorded with a temperature-compensated digital refractometer (Pal-I; ATAGO Co., Tokyo, Japan) when the fruit ripened, ≈5 to 7 d after harvest.

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Analysis of variance was carried out using the GLM procedure of SAS (release 9.1 for Windows; SAS Institute, Cary, NC). After significant F test at P ≤ 0.05, mean separation was performed with the least significant difference test.

Results and discussion

Year, location, and cultivars showed significant effects (P ≤ 0.05) on all fruit parameters measured in the study. The only exception was fruit total soluble solids, which did not show a significant location effect. The year × location interaction was significant for total number of fruit, fruit yield, fruit length, and fruit soluble solids but not for fruit diameter and individual fruit weight (Table 3). Except for fruit diameter, the year × cultivar interaction was also significant for most fruit variables.

Table 3.Yield and fruit quality traits of six mamey sapote cultivars planted at two locations in Puerto Rico. Values are means of five replications and 5 years (2005–09).

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At both locations, only cultivars Magaña and Tazumal exhibited an overall increase in the number of fruit and fruit yield as expected as the trees advance in age (Table 4). The rest of the cultivars exhibited erratic production patterns, which were characterized by relatively low production during 1 or 2 successive years following heavy cropping (Table 4). For example, number of fruit and fruit yield in cultivar Copan declined by 63% from 2005 to 2006 at Corozal, increased in 2007 and 2008, and declined again in 2009 by almost 30%. Cultivar Pace showed a drastic decline in fruit production in 2006 at Corozal and in 2008 at Isabela following years of heavy cropping. The possibility of water stress impeding flower initiation or development is ruled out because supplemental irrigation was supplied when necessary. Retention of mature fruit on trees is sometimes practiced [e.g., avocado (Persea americana)] to obtain better market prices. However, this practice can drive the tree into a biennial cropping cycle (Schaffer and Andersen, 1994). In this experiment, fruit were harvested periodically and was not a factor in inducing biennial cropping. Most probably, the high fruit load in some cultivars during 1 or 2 years resulted in depletion of assimilates, which then caused an “off-year” because of light blooming as trees built up carbohydrate reserves (Scholefield et al., 1985). Biennial production is not always characterized by an every-other-year cycle. An “on-year” can be followed by one or more “off-years” and vice versa (Paz-Vega, 1997). The magnitude of this response varied among cultivars and locations as evidenced by the significant year × cultivar and location × cultivar interactions (Table 3). In Corozal, ‘Tazumal’ exhibited an increase in number of fruit and yield throughout the duration of the experiment and leveled off in 2009 (Table 4). A similar response was observed at Isabela except that fruit number and yield declined slightly, but not significantly, in 2007 (Table 4). Similarly, with the exception of 2006 at Corozal and 2005 at Isabela, fruit number and yield of ‘Magaña’ also increased throughout the duration of the experiment, peaking in 2009 at both locations (Table 4).

Table 4.Number of fruit and fruit yield of six mamey sapote cultivars grown at two locations in Puerto Rico. Values are means of five replications.

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At Corozal and Isabela, ‘Tazumal’ had the highest 5-year mean for number of fruit produced (Table 4). However, it is noteworthy that higher number of fruit in this cultivar did not necessarily translate to significantly higher fruit yield. For example, while ‘Tazumal’ produced a significantly higher number of fruit than other cultivars at both locations, fruit yield in this cultivar was not significantly different from that of ‘Magaña’ and ‘Pantin’ in Isabela. (Table 3). An explanation for this response is the fact that, although ‘Tazumal’ produced a greater number of fruit, individual fruit weight in this cultivar was significantly lower than that in the rest of the cultivars (Table 3). At Corozal, cultivars Tazumal and Magaña had significantly higher fruit yield per hectare than the rest of the cultivars, whereas at Isabela, ‘Magaña’, ‘Tazumal’, and ‘Pantin’ had the highest fruit yield (Table 3). At both locations, yield of fruit was significantly lower in ‘Copan’, but it did not differ significantly from ‘Mayapan’ at Isabela and from ‘Pantin’, ‘Mayapan’, and ‘Pace’ at Corozal. Cultivars Pace, Mayapan, and Pantin had significantly higher concentration of soluble solids at Isabela, whereas there were no significant differences among cultivars for this variable at Corozal.

Total number and yield of fruit were significantly different between locations, with more fruit produced in Corozal (25,929 fruit/ha) than at Isabela (17,887 fruit/ha) (Table 3). Although average fruit number and yield were significantly lower at Isabela, this response was mainly associated with drastic declines in cultivars Copan and Mayapan. These cultivars showed an average reduction in fruit number of 65% and in fruit yield of 60% when compared with values in Corozal (Table 3). A possible explanation for why more fruit were produced at Corozal than at Isabela may be that this site is less windy than Isabela. Windy conditions have been reported to be detrimental to some tropical fruit crops (Crane, 2005; Galan-Sauco et al., 1993; Marler et al., 1994), but the effect of wind on physiological processes of mamey sapote is unknown. The possibility that tree nutrition was more favorable for fruit production at Corozal than at Isabela cannot be ruled out even though the source and rate of fertilizer was the same at both locations. Results of nutrient analysis of leaf tissue taken in 2007 showed higher concentration of phosphorus, potassium, and iron at Corozal, whereas nitrogen, calcium, magnesium, and manganese were higher at Isabela (data not shown). Yet, although critical concentrations of nutrients are not reported in the literature for mamey sapote, the leaf nutrient concentration for the above nutrients is within optimal values for many tropical fruit and nut crops (Mills and Jones, 1996). In contrast to ‘Copan’ and ‘Mayapan’, the number of fruit and yield in ‘Pantin’ were similar at both locations, which may indicate that this cultivar is more adaptable to diverse agroenvironmental conditions. Average fruit length and diameter were significantly greater at Isabela than those at Corozal, whereas average soluble solids concentration did not differ significantly between locations (Table 3).

Average individual fruit weight was significantly higher at Isabela than that at Corozal (Table 3). At both sites, individual fruit weight of ‘Magaña’ was significantly higher than the rest of the cultivars, averaging 1047.5 g. Higher individual fruit weight in ‘Magaña’ was the result of significantly higher length and diameter of fruit (Table 3). At both locations, individual fruit weight was significantly lower in ‘Tazumal’, but it did not differ significantly from ‘Copan’ at Corozal. It is noteworthy that fruit of ‘Tazumal’ were not only smaller (Table 3) but also contained between two to four large seeds rendering the edible fraction of the fruit to be significantly less than that in other cultivars. The seed fraction in fruit of this cultivar accounted for 16.5% of the total fruit weight. In ‘Magaña’, this was only 5.9%, whereas it was 8.9%, 9.5%, 11.1%, and 11.2% in ‘Pantin’, ‘Mayapan’, ‘Pace’, and ‘Copan’, respectively (data not shown). The small fruit size and large seed number in fruit of ‘Tazumal’ considerably reduce the marketability potential of this cultivar, which usually requires fruit weighing between 650 and 900 g. Values for individual fruit weight reported in this study for each cultivar fall within the range of those obtained in Florida (Balerdi et al., 2008).

In conclusion, six mamey sapote cultivars were evaluated for the first time at two locations during 5 years of production. These cultivars had significantly higher number of fruit and yield at Corozal (Ultisol) than at Isabela (Oxisol). Cultivar Magaña showed high yield at both locations, but its large-sized fruit may make it difficult to market but may be suitable for the processing industry. ‘Tazumal’ was the highest yielder at both locations, but fruit were the smallest and had several seeds. At both locations, cultivar Pantin had high production of fruit, relatively high yield, and above-average soluble solids concentration values, making this cultivar suitable for planting at various agroenvironments typical of the humid tropics. However, it should be noted that 2 years after completion of this study (11 years after field transplanting), trees in Corozal started to exhibit a high incidence of root rot incited by Phytophthora sp. (≈30% tree mortality) possibly brought about by a combination of unusually high rainfall and poor drainage characteristic of heavy-clay Ultisol soils. This long-term factor must be taken into consideration when establishing new plantings of mamey sapote. Efforts to control this disease through trunk injections of fungicide (fosetyl-aluminum) have been unsuccessful. There has been no mortality of trees in Isabela because of this disease. Efforts are being initiated to screen mamey sapote accessions from the USDA germplasm collection in Mayaguez, PR, to identify materials with phytophthora root rot resistance for use as rootstocks.

Units

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