Introduction

Description of the papaya tree


Erect, fast-growing, usually unbranched tree or shrub, 7 - 8 m tall, with copious latex, trunk about 20 cm in diameter, soft, leaves clustered near top of plant, alternate, long-petiolate, blade suborbicular, to 80 cm long, palmately 7-11-lobed; lobes glabrous, toothed, flat; plants dioecious in nature, some monoectous cultivars; flowers aromatic, male in drooping axillary panicles to 80 cm long, with a 5-toothed green calyx and 5-toothed cream to yellow corolla; stamens 10; female flowers solitary or cymose in axils or below leaves, with 5 yellow nearly free petals to 5 cm long; ovary with 5 stigmas; fruit a large yellow to greenish-orange berry, oblong to nearly globose or pyriform, about 7.5 cm long and bitter in wild types, up to 45 cm long, with flesh 2.5 - 5 cm thick, sweet, juicy and of orange color in cultivars; seeds numerous in central cavity, rounded, blackish, about 0.6 cm in diameter, each enclosed in a gelatinous membrane (aril). About 8,000 seed/lb (ca 17,500 seed/kg). Fl. and fr. nearly continuous all year.

Distribution


Native to Central America, papaya has been carried throughout the tropics, where it is extensively cultivated, and as far north and south as 32° latitude. Almost weedy in some areas of tropics.

Ecology


Ranging from Warm Temperate Dry to Moist through Tropical Very Dry to Wet Forest Life Zones, papaya is reported to tolerate annual precipitation of 6.4 to 42.9 dm (mean of 42 cases = 19.2), annual temperature of 16.2℃ to 26.6℃ (mean of 42 cases = 24.5), and pH of 4.3 to 8.0 (mean of 33 cases = 6.1). Papaya is a tropical plant, killed by frost; does not tolerate shade, waterlogging, or strong winds, and may require irrigation in dry regions. It recuperates very slowly from serious root or leaf injury (Malo and Campbell, s.d.). Grows best below 1,500 m in well-drained rich soil of pH 6 - 6.5.

Harvesting


Trees from seed sown in early spring should fruit the following winter and continue to bear some fruit nearly every month in the year thereafter. Bloom to maturity is 5 - 8 months. Average life of a papaya grown under Florida conditions is 2 or 3 years, but trees may live in the wild 25 years or more. Yield declines after the first few years. All inferior and wild male trees in a region should be destroyed so that their pollen cannot fertilize blossoms of trees from which seed is to be selected. Seed for new plantings should be saved from perfect-flowered plants whenever possible. For fresh fruit, they are harvested just as yellowing commences. Fruits should be twisted gently, the harvester wearing cotton gloves so as not to bruise fruit surface. On trees where fruits are not crowded, fruits are cut off with a knife. Fruit should be placed directly in a picking tray that is well padded on sides and bottom with shredded paper or other soft material. The blossom end is more resistant to bruising and so fruit should be packed with this end down. Since papayas are injured by chilling, they should be kept at about 7℃ with relative humidity of 85% - 90%, and may be kept for 7 to 21 days under these conditions. They should be ripened at 21℃ - 26.5℃ as needed for marketing. Papain is harvested, like opium, by tapping the unripe fruits. Latex drips into a suitable container and is sun dried or oven dried at 55℃ to 60℃. The Same fruits are tapped in different cuts at weekly intervals. Tapped fruits are ultimately edible, so that both fruit and latex are harvested.

Germplasm


Reported from the Middle American Center of Diversity, papaya, or cultivars (cvs), thereof, is reported to tolerate drought, high pH, insects, laterites, mycobacteria, slope, and virus (Duke, 1978). Papaya trees differ from each other in that some are either male (staminate) or female (pistillate), whereas others are perfect, having both male and female flowers on same plant. When the sexes are separate, fruits set on the female plant only when pollen is carried from male plants. One male tree is needed for each 25 or so female plants, but one cannot determine the male trees until they flower, about 12 months after germination. In perfect-flowered cvs, seed is more likely to come true than in pistillate plants. The papaya industry in Hawaii is based on the cv 'Solo', which probably originated in Barbados (Malo and, Campbell, s.d.) Fruits are about 15 cm long 5 and 10 cm wide, weighs from 400-450 gm and is pear-shaped. Some hermaphroditic cvs are 'Fairchild (no. 745)', 'Graham', 'Kissimmee', 'Betty', and 'Bluestem'. (2n = 18, 36)

Cultivation


Seeds retain their viability for 2-3 years when kept air-dry in airtight containers. Seeds may be sown in coldframes or boxes during January or in the open in March. Early planting is much to be desired to make a vigorous plant before the beginning of following winter. Seeds germinate in 2-3 weeks. When 2 or 3 true leaves have formed, seedlings should be transplanted, spacing them 5-7.5 cm apart in seedbed. When plants are 7.5-10 m tall, they can be set in their permanent places in the field. Usual planting distance is about 3-4 m apart each way, giving about 1750 trees to the hectare. In selecting plants for field planting, the more vigorous growing plants are usually the males and may be safely discarded except for a few. By planting 2 or 3 plants in a hill, there is a chance for further selection and elimination of excessive males when first flowers appear, about one male plant to each 25 or so females is sufficient. Transplants must be watered and shaded. Mulch gives much better results than clean culture, keeping down weeds, preserving moisture, shading the soil from hot summer sun, and preventing the burning out of humus and nitrates in the top soil layer. Heavy applications of stable manure or commercial fertilizers can often be used with profit. Attempts at grafting and rooting shoots have not been successful on a commercial scale.

Yields and Economics


Trees produce about 50% of their papain yield in the first year, 30% in second year, and 20% in third year. Yields range 70-130 kg/ha. If this represents 5% of the latex, the latex yields are close to one MT/ha/yr. Morton recounts yields of 20-25 kg dried papain per hectare in the first year; 90-100 in year 2, 60-90 in year 3, 30-40 in year 4, and 20 or less in year 5. She equates 1 kg crude papain with 5 kg fresh latex. A tree may bear 30-150 fruits per year, each weighing up to 9 kg. Up to 34,000 kg marketable fruit per ha is a credible yield, but yields of 100 MT fruit are reported (Duke, 1978). Murthy and Natarajan (1982) report that on fertile soils 'Honey Dew' will yield ca 100 MT/ha fruit (with 1750 bearing trees/ha) the first year, 85 the second, and 75 the third. Previously Sri Lanka and presently Uganda and Tanzania are the principal producers of papain. United States is the principal importer of papain, using it mainly for meat tenderizers, beer treatment, chewing gum, and textile and tanning industry. United States imports about 340 tons of crude papain, valued at $2 million. In the tropics the papaya is used for many purposes, and wherever it is grown, is a stable food. Nearly all of Florida crop is used in homes or sold in local markets. Hawaii is the main producer for mainland United States. In 1968, produced 11,775 tons from 830 acres. Many manufactured products are made, especially papaya juice and concentrate.