Tomatillo, husk-tomato (Physalis philadelphica)

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Botanical name: Physalis philadelphica Lam.
Family: Solanaceae
Common names. English: tomatillo, husktomato, jamberry, ground cherry; Spanish: tomate de cascara, tomate de fresadilla, tomate milpero, tomate verde, tomatillo (Mexico), miltomate (Mexico, Guatemala)

The tomatillo or husk-tomato (Physalis philadelphica) is a solanaceous plant cultivated in Mexico and Guatemala and originating from Mesoamerica. Various archaeological findings show that its use in the diet of the Mexican population dates back to pre-Columbian times. Indeed, vestiges of Physalis sp. used as food have been found in excavations in the valley of Tehuacan (900 BC-AD 1540). In pre-Hispanic times in Mexico, it was preferred far more than the tomato (Lycopersicon sp.). However, this preference has not been maintained, except in the rural environment where, in addition to the persistence of old eating habits, the tomato's greater resistance to rot is still valued. Possibly because of the fruit's colourful appearance and because there are ways of eating it which are independent of the chili (Capsicum sp.), the tomato achieved greater acceptance outside Mesoamerica and Physalis sp. was marginalized, or its cultivation was discontinued, as happened in Spain. It is relevant to note that only in central Mexico is the fruit of Lycopersicon sp. known chiefly as "jitotomate", since in other parts of the country and in Central and South America it is called "tomate".

P. philadelphica was domesticated in Mexico from where it was taken to Europe and other parts of the world; its introduction into Spain has been well documented. Indeed, it is believed that this species originated in central Mexico where, at present, both wild and domesticated populations may be found.

The name "tomato" derives from the Nahuatl "tomatl"; this word is a generic one for globose fruits or berries which have many seeds, watery flesh and which are sometimes enclosed in a membrane.

Of the great number of species of the genus Physalis, very few are used for their fruit. P. peruviana L. has been grown in Peru since pre-Columbian times. The fruit of P. chenopodifolia is picked in the state of Tlaxcala, Mexico. In Europe, P. alkekengi is grown as an ornamental plant because of the colourful calyx of its fruit, and its fruit also is used in central and southern Europe.

The tomatillo has been a constant component of the Mexican and Guatemalan diet up to the present day, chiefly in the form of sauces prepared with its fruit and ground chilies to improve the flavour of meals and stimulate the appetite. The tomatillo is also used in sauces with green chili, mainly to lessen its hot flavour. The fruit of the tomatillo is used cooked, or even raw, to prepare purees or minced meat dishes which are used as a base for chili sauces known generically as salsa verde (green sauce): they can be used to accompany prepared dishes or else be used as ingredients in various stews. An infusion of the husks (calyces) is added to tamale dough to improve its spongy consistency. as well as to that of fritters: it is also used to impart flavour to white rice and to tenderize red meats.

About ten years ago the crop began to be industrialized in Mexico and agro-industries are currently estimated to process 600 tonnes per year. 80 percent of which is exported to the United States as whole tomatillos. without a calyx and canned, while the remainder is used in the preparation of packaged sauces for the domestic market.

P. philadelphica is acquiring importance as an introduced crop in California as a result of the growing popularity of Mexican food in the United States. Furthermore, numerous medicinal properties are attributed to it.

Official statistics show that, in 1984, 15 248 ha were sown in Mexico, with a total production value of 5 797 million pesos and an average per caput consumption of 2.32 kg. Both in Mexico and Guatemala, wild tomato fruit from cultivated fields has a predominant place in the diet, hence in some regions it is an important product among those gathered in rural areas for immediate consumption and for sale.

Botanical description

P. philadelphica is an annual of 15 to 60 cm: it is subglabrous. sometimes with sparse hairs on the stem. The leaf lamina is 9 to 13 x 6 to 10 mm; its apices are acute to slightly acuminate, with irregularly dentate margins and two to six teeth on each side of the main tooth. of 3 to 8 mm. The pedicels are 5 to 10 mm, the calyx has ovate and hirsute lobules measuring 7-13 mm. The corolla is 8 to 32 mm in diameter, yellow and sometimes has faint greenish blue or purple spots. The anthers are blue or greenish blue. The calyx is accrescent, reaching 18 to 53 x 11 to 60 mm in the fruit, and has ten ribs. The fruit is 12 to 60 x 10 to 48 mm in size and sometimes tears the calyx.

Ecology and phytogeography

The P. philadelphica plant grows from southern Baja California to Guatemala, from 10 m in Tres Valles, Veracruz, to 2 600m in the valley of Mexico.

Genetic diversity

There are many local or indigenous varieties of P. philadelphica which producers recognize by fruit colour and size as well as by the plant's growth habit although, within these varieties, there is wide variation, possibly because of their self-incompatibility. The wild forms are very often found growing in cultivated fields in traditional agricultural systems, mainly in combination with maize, beans and gourd. In Mexico, another type of tomato is found which is sold on the markets as wild from cultivated fields. In actual fact, it is a cultivated tomato with a small fruit; the reason for this fraudulence lies in the fact that the price of wild tomatoes growing in cultivated fields is double that of the cultivated tomatoes.

The diameter of the fruit is bigger in the Mexican tomato (1.08 to 4.9 cm) than in the Guatemalan tomato (1.04 to 2.89 cm). However, these measurements correspond mainly to the cultivated tomatoes. In Guatemala, purplish green, yellowish green and purple tomatoes are preferred; in Mexico, on the other hand, the variation in colour is greater, as there are yellow, various shades of green and purple fruits.

The characteristics showing the greatest variation are fruit size. colour and average weight; the number and weight of fruit per plant; the consistency and colour of the flesh; the colour and length of the calyx; flower size; the number and size of the nodes on the first bifurcation of the plant; stem colour; the size and number of teeth per leaf; branching; earliness and pubescence.

FIGURE 11 Tomatillo, husk-tomato (Physalis philadelphica):details of the flower and fruit with an accrescent calyx, and a cross-section of the fruit

The tomatillo or husk-tomato is a vegetable which is used widely and continuously throughout the year; its current situation is as follows: wild fruit found growing in cultivated fields is picked and sold;

· small-fruited varieties, similar to those found growing wild in cultivated fields, are specifically grown for the market;
· there are numerous local indigenous selections with a large fruit;
· the Mexican varieties Rendidora and Rendidora mejorada, produced by INIFAP's plant improvers, are rarely used.

In several regions of Mexico the species P. chenopodifolia Lam. grows wild in cultivated fields: its use as a potential resource has been recorded.

The species of Physalis in Mexico and Guatemala are not in any immediate danger of genetic erosion. However, extensive explorations must be carried out to collect both cultivated material and wild plants found in cultivated fields so as to consolidate the gene banks and contribute material and information towards the genetic improvement programme for this crop.

At present, INIFAP's gene bank in Mexico has approximately 190 collections of Physalis species, obtained from four of the country's states while, in the gene bank at the University of San Carlos, there are 41 accessions from several regions of Guatemala.

Cultivation practices

Cultivation practices are common to the majority of the solanaceous plants. Transplanting of the tomato is widespread, principally in the areas where frosts make it essential. Its advantages include saving on seed, reduced weeding and the possibility of starting the cycle while there is still another crop on the ground as well as shortening the growing cycle. Generally speaking, weeding is done by hand or using mechanical implements. Most growers use chemical fertilizers (nitrogen and phosphorus): the doses range from 120 to 240 kg of nitrogen and from 60 to 150kg of phosphorus per hectare. Given the resources, growers are confident they can control pests and diseases affecting the crop. However, they would need to know more about the doses, appropriateness, products and cost-effectiveness ratios of these control practices.

The tomatillo or husk-tomato is grown mainly on irrigated land. Because of this, sowing dates vary within each producing area, which explains why this tomato is found on the market throughout the year. In some areas it is grown on dry land, both using residual humidity and during the heavy rainstorms. Sowing density ranges from 17 000 to 25 000 plants per hectare. The fruit is harvested when it reaches its normal size, when it has a firm consistency and generally when the apex of the calyx has begun to break. Small-fruited varieties, selected for this purpose, undergo cultivation practices similar to those used for the large tomato.

The greater percentage of dormancy occurs in the seed recently extracted from the fruit. In less than a year it reaches its maximum germination potential, losing it drastically as from the third year under commercial storage conditions.

For marketing purposes, the small fruit must not fill the calycinal envelope. On the other hand, the large tomato must fill it completely and should preferably break it to reveal part of the fruit (this is visually attractive to the purchaser).

The wild tomatillo found in cultivated fields adapts to various environments but it appears mainly on cultivated ground and sometimes care is taken to prevent its removal during weeding and earthing up. It appears most commonly on parts of land where vegetable waste is concentrated and burned after clearance This tendency may be due to enrichment of the soil with the ash, the effect of which is to stimulate high temperatures in the seeds Its apparent resistance to the herbicide 2,4-D amine, which is widely used on maize, may help its survival and even its spread (through the reduction of competition in the treated fields) in some agricultural regions

The only two Mexican improved varieties, Rendidora and Rendidora mejorada, have the following characteristics: a smaller and more uniform habit; few or no hollow fruits; and firmer fruit of a lime-green colour.

Prospects for improvement

The variety Rendidora was formed from the best collections selected in the state of Morelos, where improvement work was carried out. Rendidora mejorada was derived from this variety In Guatemala, in spite of the wide genetic variation recognized, genetic improvement of this crop is still in its early stages. The characteristics most affected by the environment are leaf size and shape, growth habit and the growing cycle of the plant Soil fertility stands out as an environmental factor in expression of the phenotype

Genetic improvement work in Mexico should aim at: plants with large and firm, deep green (not yellow) fruit; high yield, wide adaptation and resistance to viral diseases and powdery mildew (Oidium spp.) Improvement aims in Guatemala should be the same, except regarding fruit colour, since purplish green and yellowish green tomatoes are preferred in that country

P. chenopodifolia is in the initial stage of domestication and shows a favourable response to agricultural practices; accordingly, it must be collected and evaluated so that the potential for better utilization in the future may be established.

Bibliography

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Azurdia, P.C.A., Carrillo, F., Rodriguez, B., Vázquez, F. & Martinez, V. 1990 Caracterización y evaluación preliminar de algunos cultivos nativos de Guatemala. Guatemala, University of San Carlos/ICTA/IBPGR

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