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Gene Conservation and Exploitation p. 29–34.
Edited by J.P. Gustafson et al., Plenum Press, New York, 1993


THE NARANJILLA (SOLANUM QUITOENSE), THE COCONA (SOLANUM SESSILIFLORUM) AND THEIR HYBRID

Charles B. Heiser
Department of Biology
Indiana University
Bloomington, Indiana 47405


INTRODUCTION

The Solanaceae has been one of the more important families in providing useful plants for humankind. By far the greatest number of these comes from tropical America, white potatoes, tomatoes and chili peppers being the best known. In recent years the fruits of two others, the tree tomato (Cyphomandra betacea) and the pepino (Solanum muricatum) have begun to appear in our markets, both imported from New Zealand where they had been introduced from tropical America. To my thinking two other members of the family, the naranjilla (S. quitoense) and the cocona (S. sessiliflorum), are equal or superior to the last two, but they have yet to reach markets outside of Latin America. These species, both placed in the section Lasiocarpa of Solanum, are the subjects of this paper.

The naranjilla, or lulo as it is often called in Colombia, is a shrubby perennial, nearly 2 m tall, with large purple veined leaves and white corollas. The rotund berries are covered with short, stiff hairs which are fairly readily deciduous so that they are glabrous or nearly so when they reach the market. At maturity the fruits, 4.5 to 5.5 cm in diameter, are orange, which is responsible for the Spanish name meaning "little orange." The flesh or pulp of the fruit, however, is green (Whalen, Costich and Heiser, 1981). The juice is used most commonly for drinks. The fragrance and flavor are unique, but some people have claimed a resemblance to a mixture of pineapple and strawberries, others to that of citrus fruits. The fruit is rich in vitamins A and C (Romero-Castañeda, 1961).

In the sixteenth century Cobo (Patiño, 1962) described plants from the regions of Popayan and Quito that are thought to represent the naranjilla. To this day Colombia and Ecuador, where the plants are generally grown at altitudes of 1000 to 1500 m, remain the principal producers. It is also grown in Venezuela and at one time in Peru. Less than a half century ago it was introduced to Panama and Costa Rica, where it is now well established, and more recently to Guatemala (Heiser, 1985). Attempts to establish it as a crop in Florida were unsuccessful, although the early trials appeared promising (Ledin, 1952). The plant was promoted in a recent publication (National Research Council, 1975) and since that time I have had requests for seeds from people in tropical Asia and Africa.

For a domesticated plant, the naranjilla shows extremely little variability (Heiser, 1972; Whalen et al., 1983). Plants grown in Ecuador are unarmed whereas to the north of that country, they generally have small prickles on the leaves and stems (Schultes and Romero-Castañeda, 1962). In Ecuador five "varieties" are recognized: agria, Baeza, Baeza roja, bola, and dulce, distinguished by very slight differences in the fruits. Because little breeding work has been done with the naranjilla, the plants grown today probably differ little from those found by the Spanish when they arrived in the Americas. Although the naranjilla occasionally escapes from cultivation to establish weedy populations, no truly wild type is known.

The cocona, also known as topiro or tupiru and in Brazil as cubiu, became known to Europeans some half a century later than the naranjilla (Patiño, 1962) and today is widely cultivated in the upper Amazon basin at altitudes up to 700 m or slightly higher. It is a shorter plant than the naranjilla, and it has even larger leaves, generally with little or no purple pigmentation, and greenish-white corollas. The hairs on the berry are softer and more readily deciduous than those of the naranjilla. The mature fruits are orange to maroon and rather variable in shape and size from 4 to 9 cm in diameter. The fruit flesh is nearly white to pale yellow. The aroma and taste of the juice are less pronounced than that of the naranjilla. In addition to being used for juice, the fruits are also used as a vegetable. The fruit is rich in iron and contains vitamins A, C and niacin (Salick, 1989).

A wild type of the cocona (S. sessiliflorum var. georgicum), known from a few collections in Colombia, Ecuador and Peru, differs from the domesticate by being prickly and having much smaller fruits. In Ecuador, a weedy or wild unarmed type, has fruits about 3 cm in diameter, and is known as naranjilla jíbara. Clement (1989) considers the cocona one of the several fruits domesticated in western Amazonia.

My involvement with these species began in 1962. On my early trips to South America I found the naranjilla very abundant and inexpensive in markets in Ecuador but over the years it became less numerous and more expensive. The principal reason for this apparently is the large number of diseases and pests to which the naranjilla is subject. One of the most serious of these is root knot nematodes. Resistance to root knot nematode was discovered in S. hirtum (Heiser, 1971), a species that crosses readily with S. quitoense; and although some effort has been made to incorporate this trait in the naranjilla, resistant varieties have yet to be grown commercially unless it is in Colombia, a country from which I have no recent information.

In 1982 a conference on the naranjilla was held in Quito (Anon., 1984). Before the conference Saúl Camacho took me to the experiment station for the naranjilla at Chaco and there for the first time I saw hybrids of the naranjilla and the cocona. Although more like the cocona in general aspect, the hybrids had a number of intermediate characters. The hybrid was of great interest to me, for I had tried to make it for many years without success. Even more surprising was the account of its origin, for it had been made by a campesino, Raul Viteri. He had heard of plant hybridization being used to improve plants so he had crossed the two species, using naranjilla jíbara as the pollen donor. Months later under the naranjilla he found seedlings whose leaf coloration was intermediate between that of the putative parents. He transplanted some of the seedlings and found that the fruits were not attacked by diseases or insects. The fruits although smaller than naranjillas had a similar color and taste. The seeds were not viable but vegetative propagation was successful (Torre and Camacho, 1981). I made a brief visit to Ecuador in 1985. I didn't visit markets in Quito but in those at Ambato the only fruits I saw were from the hybrid which were called naranjilla or naranjilla chica (small). They were readily recognizable as the hybrid for they were only half the size of true naranjillas and contained very few or no seeds. On this trip I also learned that the experiment station at Chaco had been abandoned by the government because of a recent financial crisis.

On my return to Indiana I renewed my efforts to produce the hybrid and numerous attempts during the next five years were again unsuccessful. No fruit ever developed on the naranjilla as a result of the cross, and although the reciprocal cross yielded several fruits, not a single seed was found.

My next trip to Ecuador was in 1988, and 1 did see a number of what appeared to be normal naranjillas. On two occasions our hostess in Quito purchased some and I was puzzled because I failed to find a single seed in any of the fruits. The answer was provided when Jorge Soria and I made a trip to the new station that had been established for the naranjilla at Palora. Although Palora at an altitude of 950 m is too low for the best growth of the naranjilla, the hybrid thrives there. We learned that a few years earlier some weeds near a field where the hybrids had been growing had been sprayed with 2-4,D. It was later noted that the hybrids downwind of the spray had fruits about twice the size of the other plants. Since that time hybrids in flower have deliberately been sprayed with a very dilute solution of 2-4,D to produce larger fruits, such as those we had seen in Quito. In 1990 it was estimated that two thirds or more of the crop in Ecuador resulted from hybrids sprayed with 2-4,D (J. Soria, pers. commun.).

Again following my return to Indiana I attempted to make the hybrid. using some new accessions that I had acquired from Victor Rodriguez as well as my older ones. In addition to my own desire to obtain the hybrid, there were other reasons for producing it as well. First, the hybrid being grown in Ecuador was very uniform, possibly constituting a single genotype, and hence likely had great genetic vulnerability. Secondly, I thought that it might be possible to produce a larger fruited hybrid by using a large fruited cocona as a parent eliminating the necessity of spraying with 2-4,D. Again a number of fruits was formed when the cocona was used as the female parent. In order to attempt embryo rescue, my colleague Martha Crouch examined a very young fruit in search of embryos. None was found. The other fruits were allowed to reach maturity and in one of them two partially developed seeds were found. These were planted on a nutrient medium (Miller, 1969) supplied by Carlos Miller. Both germinated and were transplanted to soil six weeks later. The parents of this hybrid were a domesticated cocona which I had collected at Yanzatza, Ecuador (8255) and the variety Baeza roja of the naranjilla.

The hybrids (8919) grown in the greenhouse were more vigorous than either parent and morphologically similar to Viteri's hybrid except in two respects. The fruits were much larger, slightly larger than naranjillas, and the fruit flesh was orange instead of green. The increase in size was expected, for the cocona parent had fruits over 7 cm in diameter, but the orange color was not anticipated. Little is known of the genetics of this group of Solanum, so the reason for the different fruit colors remains speculative. One explanation may involve the difference in the parentage of the two hybrids. Viteri used a wild cocona with small fruits that externally are yellow-orange whereas I used a domesticate with maroon fruits; both, however, have a very light colored flesh. The variety of the naranjilla used by Viteri is unknown, but considering the region where he made the hybrid I would judge that it was dulce. The fruit of that variety has a deep green flesh, whereas Baeza roja, the male parent of my hybrid, has a fruit with much lighter green flesh. Another possible explanation in view of the different female parentage of the two hybrids is that a maternal effect may be operating. Viteri's hybrid has a light green fruit flesh approaching that of the naranjilla. Mine is nearer to that of the cocona, but not identical to it by any means. Salick (1989) has postulated a maternal effect to account for the size and shape of fruits in hybrids involving varieties of cocona with different fruit types.

My hybrids gave pollen stainabilities of 28 and 38%. No seed was set on selfing, in sib crosses or in backcrosses made to either parent in both directions. A few seeds, however, were found in some of the fruits in a greenhouse open to pollinators in which several accessions of both of the parental species as well as other species of the section were growing. The low fertility of the hybrids can probably be explained as the result of meiotic irregularities. At diakinesis in one hybrid the following pairing configurations were observed: 12II (3 cells), 11II2I (3). 10II1III1II (8), 10II1IV (13), 10II4I (7), 9II1III,3I (7), 9II1IV2I (6) and 9II6I (1).

The aroma and the flavor of the fruit of the new hybrid are excellent. They are very similar to that of the naranjilla but not identical to it. This opinion is based on my own evaluation and that of three others. I do not yet have reports on it from the people who are growing it in Florida and Ecuador. Possibly the different color of the juice may hinder its acceptance in areas where people expect it to be green; on the other hand, it may make it more acceptable in new areas. Although I hesitate to make predictions, I feel that the juice of both the naranjilla and the hybrid could become popular in the United States and Europe. As for the cocona, it may become more widely grown in the tropics, but I do not believe that either as a juice or as a vegetable, it can compete with the many other juices and vegetables now available in the northern temperate zone.

The enthusiasm for growing the hybrid in Ecuador could mean that attempts to improve the naranjilla by crossing will be abandoned. I hope that this does not prove true. Although there is very little variability in this species, it is now known to hybridize with five other species (Heiser, 1989) so that genes for its improvement may be available. Through backcrossing it may be possible to transfer desirable genes into the domesticated species. Such a procedure can readily be accomplished by scientists in the Third World who may lack the facilities to pursue newer more sophisticated methods for gene transfer.

REFERENCES


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