From Perspectives on New Crops and
New Uses, Proceedings
of the Fourth National Symposium, Biodiversity and Agricultural
Sustainability
by Charles Heiser and Gregory Anderson
"New" Solanums
1. TOMATO
2. TREE TOMATO
3. PEPINO
4. COCONA
5. NARANJILLA
6. LOOKING BACKWARDS
7. REFERENCES
The nightshade family (Solanaceae) has provided many plants to humankind. The genus Solanum has been particularly significant for it has furnished us with one of our basic food plants, the Irish potato (S. tuberosum), as well as the widely used eggplant (S. melongena).
The genus has recently become even more important with the addition of
two other food plants—both because of recent changes in their names.
The tomato, which has long been known as Lycopersicon esculentum, is now recognized as S. lycopersicum and the tree tomato, previously known as Cyphomandra betacea, has become S. betaceum. Three other Solanums—the pepino (S. muricatum) the cocona (S. sessiliflorum) and the naranjilla (S. quitoense), long valued for their fruits in South America, deserve greater attention.
Tomato
The tomato was originally named Solanum lycopersicum by Linnaeus in 1753. Philip Miller in The Gardeners Dictionary in 1768 used Lycopersicon esculentum
and this became the accepted name until very recently. Various lines of
evidence had suggested that the tomato belonged to the genus Solanum and molecular studies provided the compelling evidence that has led to the readoption of S. lycopersicum (Spooner et al. 1993). The other species of Lycopersicon have also been assigned or re-assigned to Solanum.
More recent studies (Bohs and Olmstead 1997; Olmstead and Palmer 1997)
have provided additional evidence for the inclusion of the tomato in Solanum. The tomatoes are placed in Solanum
in groups very close to the tuberous and non-tuberous potatoes. This
has not led to major changes in tomatoes as yet, but it does make very
clear the relationship of tomatoes and potatoes, and should promote
even greater efforts at interbreeding and genetic transfer among these
two important groups.
Tree Tomato
The tree tomato, originally named Solanum betaceum by Cavanilles in 1799, was transferred by Sendtner in 1845 to the genus Cyphomandra where it remained until recently when Bohs (1995) returned it to Solanum.
As in the tomato, molecular studies prompted the recent change in the
name (Olmstead and Palmer 1992; Spooner et al. 1993; Bohs and Olmstead
1998). More recent molecular studies have furnished additional support
for its inclusion in Solanum (Bohs and Olmstead 1997; Olmstead and Palmer 1997).
The
tree tomato is hardly a new crop, for it has long been popular in the
Andes, where it is native, and in many other parts of the tropics. It
has been grown successfully in New Zealand and has been exported to the
north temperature zone for around two decades. In New Zealand, the
plant was renamed tamarillo, a made-up name. The new name does have
some advantages, for although many people have claimed a resemblance of
the tree tomato to the tomato it is mostly superficial. Both the aroma
and taste are quite different from that of the tomato. The fruit is
eaten raw as well as for the juice, preserves, jellies, and as a
vegetable, either cooked or raw in salads. Some improvement of the
plant has taken place in New Zealand in recent years, and on a visit to
Ecuador in 1983 we were happy to learn that New Zealand had sent
improved forms to South America.
Although the tree tomato now
reaches the United States from New Zealand and South America, it
probably first became familiar to many people here some 30 years ago
through advertisements of a nursery company offering plants for sale. A
few years later we saw another advertisement stating "Does 60 pounds of
tomatoes from one yield sound unreasonable—not if you own the tree
tomato." Other advertisements with extravagant claims have appeared
from time to time, but we hadn't seen any for several years until this
past spring when two companies offered it again claiming yields of 60
pounds. This time however, we saw a release from the Associated Press
in a local newspaper headlined, "Gardeners should beware of catalog
exaggerations" and the tree tomato was one of the plants featured.
Several
wild species of the tree tomato in South America also have edible
fruits. In addition, various parts of the plants, particularly the
leaves, of the tree tomato and several wild species have been used in
folk medicine in Latin America. More information on the ethnobotany of
the tree tomato may be found in Bohs (1989); see also National Research
Council (1989). A wild ancestor of the tree tomato has not yet been
identified unequivocally but recent studies indicate that it most
likely had its origin in the southern Andes, perhaps Bolivia, and that
the cultivated tree tomato was derived from a wild form of the same
species (Bohs 1998).
Pepino
The pepino (cachum in Quechua), Solanum muricatum,
shows some similarities to the tree tomato in that it is also native to
the Andes where it is an important fruit; it has also recently been
grown in New Zealand (Hewett 1993) whence it is exported to the United
States, Europe and Japan. Like the tree tomato, its future as a crop
outside of the Andes is uncertain. There are many differences—the
pepino is eaten raw and it was probably domesticated much earlier than
the tree tomato; in addition, its origin is more complex and the two
plants are very different in appearance. Pepino is the Spanish word for
cucumber, and the plant was so named by the Spanish because of a slight
resemblance to some cucumbers. The typical fruit of the pepino is
usually broadly ovate, white or ivory colored, often with a few purple
stripes, when mature. However, there are many fruit color and shape
varieties among the 'indigenous' cultivars in South America e.g., see
the cover of the 1993 American Journal of Botany 80(6). It is generally
eaten out-of-hand, frequently after peeling, for its sweet pleasant
taste. The plant itself somewhat resembles the Irish potato; in fact,
both species are placed in the subgenus Potatoe. There is great
variation in the pepino not only in its fruits but in vegetative
features as well; the leaves may be simple or compound.
Some
improvement of the plant has taken place in New Zealand, and at present
it is being tried in a number of countries. The history of the plant,
both ancient and modern, has been treated in some detail by Prohens et
al. (1996).
Contrary to the situation in the tree tomato, and
similar to the naranjilla (see following) there is no clearly defined
wild ancestral form of the pepino. This highly variable cultigen is
know only from cultivation. Extensive field studies have failed to
indicate a wild form. However, many studies and approaches have
identified three wild species that are most closely related to the
pepino, and that have been treated as possible progenitors (e.g.,
Anderson 1979; Anderson et al. 1996; Anderson and Jansen 1998; Jansen
et al. 1998): the widespread S. caripense, which ranges from Costa Rica
to Peru, S. tabanoense, which is found in a few localities in Colombia,
Ecuador, and Peru, and S. basendopogon, which has a very limited
distribution in Peru. Recent molecular studies (Anderson et al. 1996;
Anderson and Jansen 1998; Jansen et al. 1998) suggest S. basendopogon
as a less likely element in the origin of the pepino. The chloroplast
DNA evidence is quite strong in support of S. tabanoense as central to
its origin; over 85% of the pepino collections from South America are
linked in a cladistic study with this species (Anderson et al. 1996).
The remaining pepino collections are associated with S. caripense,
implying either a second origin, or possibly post-origin hybridization.
Although multiple origins are known for a few other domesticated
plants, in each of these, only one wild species is involved; none is
known involving more than one species. The molecular results, however,
do not rule out the possibility that the pepino evolved from a single
species, most likely S. tabanoense, followed by hybridization with S.
caripense, and possibly other closely related wild species (perhaps
including S. basendopogon). Hybridization among the wild species, and
with the pepino, is not obviously rampant in the wild but, is
relatively easily performed in plants grown in the greenhouse. Either
the unprecedented multiple origin hypothesis, or post-origin
hybridization, would help explain the great variability in the fruits,
flowers, and leaves of this polymorphic species. Work now in progress
should help explain more about the origin of the pepino, including its
place of origin. Southern Colombia and northern Ecuador, at present,
would seem to be the most likely places.
Cocona
Although not yet grown commercially outside of Latin America, two other South American Solanums
deserve wider cultivation. The cocona or tupiro, as it is known in
Spanish speaking countries, or cubiu in Brazil (S. sessiliflorum) is a
shrubby perennial, generally a meter or more tall with extremely large
leaves (to 9.5 cm long). It bears maroon, orange-red or yellow fruits
up to 10 cm in diameter with yellow flesh. The fruits are covered with
small, soft readily deciduous hairs. The plant is widely cultivated,
usually in very small plots, in the upper Amazon basin from sea-level
to 700 m or higher. The berries have a pleasant acidulous flavor,
somewhat like citrus, and are used as fruits, chiefly for the juice,
and also cooked as a vegetable (Whalen et al. 1981). Parts of the plant
have also been used in medicine. The species is quite variable,
particularly in size, shape, and flavor of the berries, as might be
expected in a species domesticated for its fruits. A wild type (S. sessiliflorum var. georgicum)
with smaller (3–4 cm), globose, orange berries and prickles on the stem
and leaves is the likely ancestor of the domesticate (Heiser 1972). The
nutritional value, cultivation and production of the cocona have been
treated by Salick (1989, 1992) who believes that its wider cultivation
could significantly improve human subsistence in tropical lowlands.
Naranjilla
The naranjilla or lulo (S. quitoense) belongs to the same section of Solanum (Lasiocarpa)
as the cocona. It is a taller plant (ca. 2 m), less branched and with
slightly smaller leaves (Whalen et al. 1981). The leaves, particularly
younger ones, the veins and petioles are often purplish, making the
plant sufficiently attractive to be grown occasionally as an indoor
ornamental in the United States. The globose berries are around 5 cm in
diameter, orange and covered with short stiff hairs which have usually
rubbed off by the time the fruits have reached the markets. The pulp is
green and gives a green juice, the form in which it is usually used.
The taste is unique, but has been described by some as like that of a
mixture of pineapples and strawberries. The plant was found by the
Spanish in Ecuador and Colombia (Patiño 1963) where most of it is still
grown. It was introduced to Panama and Costa Rica in the middle of this
century where it is also grown today. Introductions to other places,
including Florida, have been largely unsuccessful (Heiser 1985).
Recently Bernal et al. (1998a) have postulated that the form similar to
the cultivated plant that grows spontaneously as a weed in Colombia is
the original wild type. We feel, however, that these plants may be
nothing more than escapes from cultivation, no different from escapes
in Costa Rica where the plant was introduced a half century ago.
Moreover, it is difficult to imagine no change occurring in a plant
that has been cultivated for several hundred years. On the other hand,
no other wild ancestor has been reported and it is most unlikely that
S. quitoense descends directly from any other species in section
Lasiocarpa. For a domesticated plant it shows very little variability.
In Colombia and Central America the plants usually have prickles on the
stems and leaves whereas in Ecuador plants are unarmed. Several
cultivars are recognized in Ecuador, based on slight differences in
fruits.
Over 20 years ago Heiser (1969) wrote that "the fruit
[of the naranjilla] yields one of the most delicious beverages known."
Why then hasn't it become more widely appreciated? Many early visitors
to Colombia and Ecuador spoke highly of the juice (Patiño 1963) so it
is difficult to believe that the Spanish didn't carry seeds to other
regions, but it was not successfully introduced elsewhere. The
naranjilla grows best at between 1200 and 2300 m and it is neither a
tropical plant (pollen aborts at high temperatures) nor a temperate one
(it is killed by freezing and fruits need four months to mature). The
juice has been canned but its unique flavor is lost in doing so.
Freeze-drying techniques have shown promise in Ecuador but no attempts
have been made to use them to make juice for export, probably because
no certain markets are known. Fresh fruits, of course, could also be
exported if produced in sufficient quantities. Root knot nematodes and
various insect pests and fungal diseases limit its production in all of
the countries where it is presently grown (National Research Council
1989). There are two promising recent developments toward its
improvement: the use of interspecific hybrids with the cocona and the
introduction of a nematode resistant variety.
In Ecuador a local
farmer, Raul Viteri, crossed S. quitoense with the wild variety of S.
sessiliflorum (Torre and Camacho 1981) and the resulting hybrids were
vigorous and highly productive. The hybrid was propagated vegetatively.
The chief disadvantage of the hybrid was that the fruits were much
smaller than those of the naranjilla. This was overcome by spraying
plants with dilute solutions of 2,4-D after it was learned by accident
that such spraying would cause enlargement of the fruits. From the late
1980s to the present most of the naranjilla in Ecuador was from this
hybrid which came to be know as the "Puyo hybrid." In the markets they
could be distinguished from the pure naranjillas by the lighter green
color of the pulp and the absence of filled seeds.
After
learning of Viteri's hybrid, Heiser who had previously made many
attempts to secure this hybrid, again attempted to do so. No fruit was
ever secured with S. quitoense as the female parent1.
Several fruits without seeds were secured with S. sessiliflorum as the
maternal parent, and finally in 1989, using a large fruited cocona, a
fruit was secured with two partially filled seeds. These were
germinated on nutrient agar and gave rise to vigorous hybrids,
differing primarily from the Puyo hybrid in the much larger berry (7 cm
in diameter) having orange colored flesh (Heiser 1993)2. Dr. Jorge
Soria carried cuttings of the hybrids to Ecuador where they were
planted at the experiment station of the Instituto Nacional de
Investigaciones Agropecuarias at Palora. After multiplication they were
distributed to a few farmers for trial and became known as the "Palora
hybrids."
The most recent reports from Ecuador (J. Soria pers.
commun.) state that the cultivation of the Palora hybrid is increasing
rapidly in Ecuador and has spread into southern Colombia. In addition
to the larger size of the fruit the Palora hybrid has other advantages
over the Puyo hybrid. It survives for three years in cultivation in
contrast to one and a half years for Puyo, produces about twice as much
annually and does not require spraying with 2,4-D. Moreover, the juice
of Palora takes nearly 24 h to start oxidizing, whereas the juice of
Puyo and that of pure naranjilla both begin oxidizing in less than an
hour. Some buyers, however, are paying more for Puyo than Palora
because of the color of the juice (greenish vs. orange respectively)
and the thicker rinds of the latter. The thick rind, however, would be
an advantage in shipping. Perhaps the cultivation of Palora will
increase even more after it is formally released and its advantages
become more widely known. The Palora hybrid may fare well in other
countries in competition with the cocona, a fruit that is not very
important in Ecuador.
One of the most serious pests—if not the most serious—of the naranjilla is root knot nematode, various species of Meloidogyne
(National Research Council 1989). Although the naranjilla is a
perennial and ought to yield for several years, because of these
parasites it is often grown as an annual in many places. Resistance to
nematodes was discovered in plants of S. hirtum grown in the greenhouse at Indiana University (Heiser 1971). Hybrid seed of S. hirtum × quitoense
(for B1 and F2) were send to Soria at CATIE in Costa Rica who found
that many of these plants were resistant to nematodes there.
Subsequently seeds of hybrids were sent to plant breeders in several
countries and a resistant cultivar has now been produced in Colombia
from F2 seeds sent to Dr. Mario Lobo in 1984 (Bernal et al. 1998a,b).
The plant was backcrosssed to S. quitoense
twice, and material from F2 plants of the BC2 was multiplied by tissue
culture. Tests in various parts of the country showed good performance.
In addition the new plant, called "lulo la selva" grows better in full
sun, has fruit of better quality, outyields the traditional lulo and is
also spineless. Moreover, the juice has a longer life because of
delayed oxidation (M. Lobo pers. commun.). The resistant cultivar is
propagated vegetatively. It was formally released in June of 1998.
There are still problems to overcome but if work on the improvement of
the naranjilla continues, the juice eventually may become more widely
available.
Looking Backwards
A
book on the nightshade family (Heiser 1969) appeared 30 years ago. To
us, at least, it is interesting to see some of the changes that have
taken place with various numbers of the family since then. Neither the
tree tomato nor the pepino were available in stores in the United
States at that time. In the book it was written that "several attempts
have been made to make it (the tomatillo, Physalis ixocarpa)
more popular in the United States, but it has never really caught on."
It has now definitely caught on and has become a standard item in
grocery stores and many gardens, not only in the Southwest but in most
of the rest of the country as well. Capsicum peppers, of course, were
widely available in 1969 but mostly the sweet kinds of C. annuum. The pungent sorts were seldom seen in those days and what was available belonged to a single species, C. annuum. Now pungent varieties are common. Furthermore, today more than one species is represented; the extremely pungent C. chinense
is now well accepted by many people. Along with this a huge increase in
the number of pepper sauces has become available. Tomatoes, of course,
were as popular in 1969 as they are today and then, as now, they were
in demand in the winter as well as the summer. Although not mentioned
in the book, we remember that some of us referred to the tomatoes then
available in the winter as "little pink rocks." For several years now
we have been fortunate in having more edible tomatoes in the winter.
Tobacco, of course, received a lengthy chapter in the book where it was
stated that "probably more has been written about tobacco than any
other plant." That is still true. In fact, we can think of no other
plant that has been featured in headlines recently more than tobacco.
In spite of much adverse publicity over the centuries the plant still
shows no signs of disappearing.
References
Anderson, G. 1979. Systematic and evolutionary consideration of species of Solanum, section Basarthrum.
p. 549–562. In J. Hawkes, R. Lester, and A. Shelding (eds.), The
Biology and taxonomy of the Solanaceae. Linnean Society Symposium
Series 7.
Anderson, G. and R. Jansen. 1998. Biosystematic and molecular systematic studies of Solanum section Basarthrum and the origin and relationship of the pepino dulce (S. muricatum). Ann. Missouri Bot. Gard. In Press.
Anderson,
G., R. Jansen, and Y. Kim. 1996. The origin and relationships of the
Pepino (Solanaceae): DNA restriction fragment evidence. Econ. Bot.
50:369–380.
Bernal E.J., M. Londoño B., G. Franco, and M. Lobo A. 1998a. Lulo la selva. Corpoica, Rionegro, Colombia.
Bernal
E.J., M. Lobo A., and M. Londoño B. 1998b. Documento presentación del
material "Lulo la selva." Corpoica, Rionegro, Colombia.
Bohs, L. 1989. Ethnobotany of the genus Cyphomandra (Solanaceae). Econ. Bot. 4:143–163.
Bohs, L. 1995. Transfer of Cyphomandra (Solanaceae) and its species to Solanum. Taxon 44:583–587.
Bohs, L. 1998. Evolutionary relationships of the tree tomato, Solanum betaceum, (Solanaceae) and its wild relatives. Am. J. Bot. 85:116. (Abstr.).
Bohs, L. and R. Olmstead. 1997. Phylogenetic relationships in Solanum (Solanaceae) based on ndhF sequences. Sys. Bot. 22:5–17.
Bohs, L. and R. Olmstead. 1998. Solanum
phylogeny inferred from chloroplast DNA sequence data. In: Proc. Fourth
International Solanaceae Conference. Adelaide, Australia (in press).
Heiser, C. 1969. Nightshades, the paradoxical plants. W.H. Freeman and Co., San Francisco.
Heiser, C. 1971. Notes on some species of Solanum (Leptostemonum) in Latin America. Baileya 18:59–65.
Heiser, C. 1972. The relationship of the naranjilla, Solanum quitoense. Biotropica 4:77–84.
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