


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.
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.
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'.
Cultivars
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.