Taxonomy and morphology
Papaya, botanical name Carica papaya, belongs to the Caricaceae, a family mainly inhabiting South and Central America; it is the only species of economic importance in the family. It is a small, normally unbranched quick-growing soft-wooded tree - 'almost an herb' says Chandler (1958) - with latex vessels in all parts. The British call it 'papaw' or 'pawpaw', in Brazil it is known as 'mamao' and in Spanish it is called 'papaya' or 'Iechosa' (Venezuela), but in Cuba (where papaya is a term of abuse) the name is 'fruta bomba'.
The stem is hollow between the nodes, except in young plants; it mainly consists of wood parenchyma and bears large triangular scars. The peltate leaves are arranged in a 2/5 spiral; they have long hollow petioles and large, deeply-lobed blades. The plant is dioecious, but hermaphrodite (bisexual) flowers and trees also occur. The female flowers, 3-5 em long, sit alone or in small groups in the leafaxils; the ovary is 2-3 cm long and has five fan-shaped stigmas on top. The male flowers, with ten stamens each, are found on long hanging panicles. Bisexual flowers have either five or ten stamens and some of these tend to become 'carpelloid' (fruit-like), in which case the fruits have a 'catface' appearance and are unmarketable. Different types of hermaphrodite flowers may occur on the same tree, depending on the season, or on the age of the tree.
Male trees are also variable: sometimes a fruit is found at the end of a long panicle. A complete change of sex may take place when an old male tree is cut back: sprouts bearing female flowers (and later fruits) may appear. There is a difference between pure males and sex-reverting males (Teaotia and Singh, 1967). How pollination takes place is not known with certainty; wind is probably the main agent, as the pollen is light and abundant, but thrips and moths may assist (Purse glove , 1968).
Taxonomy and morphology
Fig. 10.1 Stem, leaves and flowers of a hermaphrodite papaya
The fruit is a large, fleshy, hollow berry . Fruits formed from female flowers are oblong to nearly spherical, but if formed from bisexual flowers they are pear-shaped, cylindrical or grooved. Marketable fruits weigh from 0.5 to 2 kg and are 10-20 em long. The thin green skin turns yellow at the bottom when maturity
Male tree with hanging inflorescences sets in. The flesh is yellow to orange, in some cvs reddish, and has a pleasant flavour. Around the cavity lie a thousand or more black seeds, but seedless fruits occur too. Twenty air-dried seeds weigh about one gram.
The root system is said to be extensive and dense (Malan, 1953) or shallow (Kasasian, 1971). Actually, one may expect a deep and well developed root system on good soil, whereas the roots will stay near the surface on a wet or compact soil.
Uses and composition
Papaya is a popular breakfast food in the tropics and is recently being sold regularly on markets of temperate countries. It is also used for fruit salads and desserts. The fruit contains about 85 per cent water, 10-13 per cent sugar, 0.6 per cent protein, much vitamin A and fair amounts of vitamins B1, B2 and C; it contains practically no starch. It is considered to have a mild laxative action and the seeds are used medicinally against worms.
Processed papaya fruit has a neutral taste that is greatly improved by the addition of passion fruit juice to make soft drinks, jams and various preserves. Unripe fruit can be fermented into 'sauerkraut' or cooked as a substitute for apple sauce. From the latex of scratched unripe fruits, papain is prepared; this is used as a tenderizer for meat and for medical and industrial purposes (Foyet, 1972).
Origin, distribution and production
Papaya is native to tropical America and has never been found wild. It probably originated in Central America, thousands of years ago. From there it spread to South America and the West Indian islands; it was taken to the Philippines by the Spaniards and later reached other regions of South-East Asia and Africa. It is now present in every tropical and subtropical country.
There is a sizeable export to the continental United States from Hawaii and Mexico. The import into Europe is growing but has not yet attained a large volume. The Netherlands, for instance, imported only 30 tonnes in 1977 and 73 tonnes in 1982, mainly from Brazil.
Until recently, Tanzania was the main producer of papain and the chief importer is the United States.
Growth and development
Under optimum conditions the growth and development of papaya proceed at a fast rate. Seeds germinate in two weeks and from then on two leaves emerge each week. Flowers and fruits are produced in the leafaxils after a juvenile period. How long this period lasts, depends on the cultivar; for instance cv. Betty begins to flower at the node 24, 'Solo' at number 49 and their hybrid at node 32. Were fruits to set in all axils, one could expect a yield of 100 fruits a year (Storey, 1969). At an average weight of 0.5 kg and a density of 2,000, this would result in a production of 100 tonnes/ha/year. However, half this amount is already a respectable figure.
The tree remains in prime condition for two or three years, but its economic life comes to an end when the fruits are hanging so high that they cannot be picked profitably. A small farmer may cut back such a tree, after which some branches will sprout and bear fruit for some time yet; however, this practice is not recommended for orchards. It takes four to six months, depending on climate, for fruit to mature. Sex in papaya is determined by three genetic factors: Mj is dominant for maleness, M2 is dominant for hermaphroditism and m is recessive, for femaleness. Each ovule or pollen grain, being haploid, can carry only one of these factors. The diploid zygotes carrying two sex factors with capital letters cannot live; thus, the combinations MjMl, M2M2 and M1M2 are eliminated. Of the remaining ones Mjm are male trees, M2m hermaphrodites and mm females. If we cross a female with a male we get mm x Mjm with the result mm + Mjm: half the seeds will produce female trees, half male trees. Similarly the other crosses give predictable issues . With open, i.e. uncontrolled, pollination a cultivar will lose its identity in a few generations. The sex of a young papaya seedling cannot be predicted; one has to wait until flowers appear. All methods to separate male from female plants at an early stage have failed so far.
Storey stipulates an annual rainfall of 1,000-1,500 mm and Anon. (1982) puts this at an evenly distributed 1,500-2,000 mm (Venezuela). Terra (1949) found best growth of papaya on Java with more than 100 mm rain for every month. The potential evapotranspiration has been determined at 1.3 times the class-A pan evaporation (Anon. 1981a). Irrigation must be provided in climates with a long dry season.
Ecology and physiology
Papaya is grown in the tropics up to an elevation of about 1500 m and in the frostless subtropics, from approximately 32° North to 32° South. The minimum temperature for survival is -1°C, lower temperatures kill the tree. Chandler (1958) puts the maximum at 44°C and Anon. (1982) states that optimal temperatures are between 25 °C and 38°C. No mention is made in literature of the minimum temperature for growth; we assume it to be 15 0C. Lassoudiere (1968-9) states that 22 °C-26 °C is probably optimal for growth, whereas 35°C by day and 26°C at night gave the fastest germination; daylength had no effect. Best quality fruit, which is determined largely by sugar content, develops under full sunlight in the final four to five days to full ripeness on the tree (Storey, 1972). As fruit is harvested almost every week, the tree always needs full sunlight.
In South Africa papaws of best quality are usually grown in areas low in air humidity (Malan, 1953). On the other hand, it has been claimed (Anon. 1982) that relative air humidity should be at least 60 per cent. It seems likely that the sugar content of the fruit, one of the best marks of quality, will be higher at low humidity. It is probably correct to say that good papayas can be grown under diverging conditions of air humidity, as long as irrigation facilities are available.
A delicate tree like papaya must be protected against strong wind; therefore, windbreaks must be established a year or more before the orchard is planted.
Like banana, papaya needs good soil: a well-drained, permeable, well-aerated, fertile loamy soil, preferably rich in organic matter, with neutral reaction (pH 6-7) should be chosen, if possible on flat land. Trees in water-logged soil will die by drowning in three or four days (Storey, 1972). As Popenoe (1920) stated: 'papaya is one of the most insistent plants in the matter of drainage'.
It is, perhaps, incorrect to speak of cultivars in a crop that is almost entirely propagated by seed. However, in some papaya strains the type is retained by inbreeding and we may regard them as cultivars. Storey (1969) quotes as examples: 'Solo' of Hawaii, possibly 'Hortus Gold' of South Africa, 'Improved Petersen' of Australia and 'Betty' of Florida. 'Solo' was then in its 25th generation of inbreeding since its introduction from Barbados in 1910.
Several improved 'Solo' lines were selected after prolonged selfing, e.g. 'Solo 5' (1948), 'Solo 8' (1953) and 'Solo 10'. All are hermaphrodites (bisexuals) with a pear-shaped fruit that weighs about 400 g; 'Solo 8' has a high sugar content. They were crossed with 'Betty', a dioecious dwarf cv. from Florida, to induce early bearing. After backcrossing and selfing during many generations 'Sunrise' resulted, a red-fleshed type with the desirable 'Solo' flavour (Hamilton and Ito, 1968).
Later work produced 'Waimanalo', an early flowering, vigorous grower with short internodes and short-necked fruits with firm flesh, excellent texture, high sugar content and a low cavity percentage (Nakasone et at. 1972). 'Waimanalo' is well accepted locally for its
fruit quality, but the fruit is considered too large for export - the normal weight in Hawaii being 450 g.
Two new cvs, 'Higgins' and 'Wilder', were introduced in 1974 after tests on different soil types and rainfall regimes of the four main islands of Hawaii. 'Higgins' performed well in the high rainfall area and 'Wilder' was good in all other areas (Nakasone, 1974).
'Hortus Gold' is dioecious, but only seed from hand-pollinated fruits is used for propagation. It has a golden colour, remains firm and weighs 1.5-2 kg (Malan, 1953). 'Coorg Honey', 'Co. l' and 'Co. 2' are the best cvs of India. In Indonesia 'Semangka' has big red-fleshed fruits and 'Filipino' (see Fig. 10.3) has practically unlobed leaves. Cv. 'Thailand' is popular in East Java (see Fig. 10.5). The bisexual 'Guinea Gold' and the dioecious 'Sunnybank' and 'Hybrid 5' are grown successfully in Queensland, Australia (Agnew, 1968).
Bharath (1969) in Trinidad mentions: 'Santa Cruz Giant', a hermaphrodite with fruit weighing over 5 kg; 'Cedros' which is dioecious and resistant to anthracnose with fruit weighing 3 kg; 'Singapore Pink', a hermaphrodite, very sensitive to anthracnose, that bears fruits weighing 2 kg. 'Pusa 1-15' is an outstanding Indian cv. with high yield, good quality and 13 per cent sugar (Ram, 1981). Anon. (1982) reports good performance of cvs Maradol roja, Cubana and Paraguanera in Venezuela. Local selections in Surinam were crossed with Hawaiian lines; results of this research are summarized in Table 10.3. A potential yield of 100 tonnes/ha was calculated for cvs Santo 3 and 4, but actual yield came to 60 tonnesjha (Soerodimedjo, 1978).
The improvement of cvs aims at: high yield, good flesh texture, high sugar content, intermediate fruit size, small fruit cavity, uniformity and resistance to pests and diseases (Yee, 1970); to this low-bearing habit could be added.