Review

Improvement, conservation and diversity of the taro (Colocasia esculenta (L.) Schott.) in Cuba

Yadelys Figueroa-Aguila^{1  *}  , Marilys D. Milián-Jiménez¹  , Yuniel Rodríguez-García¹ 

¹Instituto de Investigaciones de Viandas Tropicales (INIVIT). Apdo 6, Santo Domingo, CP 53000, Villa Clara, Cuba

ABSTRACT

Taro (C. esculenta) is a fundamental part of the diet of children and the elderly because of its nutritional properties and riches. The objective of this work is to increase the knowledge regarding the improvement and genetic diversity of the taro existing in Cuba. Besides, in Cuba there are currently few clones of taro in the different productive scenarios, due to the characteristics of the multiplication pathways of this species mainly, that prevent the existence of wide sources of variability, both natural and induced. On the one hand, spontaneous mutations are rare and on the other hand, the emission of inflorescences is scarce and not very productive, with pollinating agents that are not efficient and dependent on environmental conditions. The cultivation of taro is widely used in the feeding of different animal species worldwide, including Cuba, which is an important route in the substitution of imports and contributes to food sustainability at the country level.

Key words: selection; hybridization; taro

INTRODUCTION

Plant genetic resources are the basic element for crop improvement through selection and conventional genetic improvement. Its use contributes to the stability and recovery of agroecosystems, provides a fundamental raw material for the genetic improvement of crops and serves as a support for food security. They are currently the basis of evolution since they allow crops to adapt to an infinite number of means and respond to new adverse factors ¹.

In Cuba, each year about 160 000 of tubers and roots are planted, distributed in all regions of the country with a production of 970,000 t annually, of which 16 % is the malanga ( Xantosoma sagitifoium and Colocasia esculenta ), In 2015, C. esculenta 6 954.7 ha ^2,3 were planted .

The Tropical Food Research Institute (INIVIT), since its founding in 1967, was dedicated to prospecting, maintaining, preserving and evaluating the taro germplasm ( C. esculenta ) until constituting the Cuban germplasm collection of this genus, which account in 201 7 with 102 accessions. In these clones, characterization and evaluation studies have been developed ⁴, which need to be expanded to increase the use of conserved germplasm.

The use of improved clones and introduced by INIVIT has allowed to increase agricultural production and its technology ⁵, without additional costs. However, there are currently few clones of taro (C. esculenta) in the different production scenarios. Mainly due to the characteristics of the multiplication pathways of this species that prevent the existence of wide sources of variability, both natural and induced. On the one hand, spontaneous mutations are rare and on the other, the emission of inflorescences is scarce and unproductive, with pollinating agents that are not efficient and dependent on environmental conditions.

In other regions of the world such as Indonesia, Papua New Guinea, Slovenia, among others. The emission of inflorescence in clones that are to be used for breeding is not limiting for hybridization improvement of this crop. Therefore, the number of accessions that emit inflorescences, the availability of the pollen they produce and the viability of the problem do not represent a problem. In Cuba, this issue is of vital importance, since the accessions with more performance emit inflorescence very rarely or never and when they do, they almost never produce pollen and it is rarely viable.

Origin and distribution

The origin of the taro is still under discussion; however, all the authors agree that it is native to the Indo-Malayan region, and it dispersed to East and Southeast Asia, Pacific Islands and Eastern Madagascar and Africa, from where it was introduced to the Caribbean and the Americas ⁶.

The greatest variability for the taro (C. esculenta) is reported in Cuba because Spaniards who arrived from the Canary Islands settled in that area; however, new evidence points to the eastern region as an important source of variability, due to the finding of stoloniferous wild types ⁷.

Taro (Colocasia esculenta (L.) Schott) is one of the most important crops in the Pacific island countries, not only for its contribution to nutrition and income; but also for its important cultural role, since it is part of the customs and traditions of these countries, and it is the fifth most consumed worldwide ².

Most cultivars that are found throughout the Pacific were not brought by the first settlers of the Indo-Malayan region, but arose before arrival ⁸, which were used as native cultivars in the region of Melanesia ⁹. That is why the clones that arrived until Polynesia during the migrations have had a progressive decrease in number and diversity ¹⁰.

The taro (Colocasia esculenta (L.) Schott) belongs to the family of edible araceae, which includes the genera: Colocasia, Xanthosoma, Alocasia, Cyrtosperma and Amorphohalllus; it is a monocotyledonous plant and the species that is planted with Colocasia esculenta comprises two botanical varieties: (i) Colocasia esculenta (L.) Schott var. esculenta, and (ii). Colocasia esculenta (L.) Schott var. Antiquorum. The C. esculenta var. esculenta: it has a wide cylindrical central corm and a few small corms and C. esculenta var antiquorum is known as the type of taro that has a small central globular bulb with several corms . So the taro Colocasia esculenta Schott var. esculenta agronomically is known as the taro type and var. Antiquorum type eddoe. It differs from Xanthosoma spp. in which its petiole is inserted in the lower third of the limbus. The base of the stem or corm constitutes the edible part. These two genera have comparable characters in their morphology and ecology; they are rhizomatous plants with corms eventually rich in calcium oxalate ⁵.

Taxonomic classification

• Kingdom: Plantae

• Division: Magnoliophyta

• Class: Liliopsida

• Order: Alismatales

• Family: Araceae

• Subfamily: Aroideae

• Tribe: Colocasieae

• Genus: Colocasia

• Species: Colocasia esculenta (L.) Schott

Importance of the culture

Taro (Colocasia esculenta (L.) Schott) plays a major role in food, as starch and as leafy vegetables. Worldwide, it is the fifth most consumed among rhizomes and tubers ², and more than 25 % occur in Oceania and Southeast Asia. The importance of cultivation goes beyond its contribution to nutrition and income; in many Pacific island countries, C. esculenta plays an important cultural role, as it is part of the customs and traditions of these countries.

In Cuba, the main rhizome is preferred from the taro (C. esculenta) and the secondary rhizomes are used for sowing; both are edible with special taste and high digestibility ¹¹, this crop is a fundamental part of the diet of children and the elderly because of its nutritional properties and richness.

Phytogenical resources

Plant genetic resources are the basis of the evolution of crops, as natural resources that have allowed to adapt to an infinite number of means and applications and that will allow them to respond to new adverse factors that arise.

Diversity is used to indicate the sum of known and unknown potential genetic information and variability to indicate a portion of the captured or available diversity ¹².

Genetic improvement work for these species is limited in certain areas by the limited number of existing genotypes, problem that with germplasm banks is solved. Some cultivars constitute local ecotypes of great value, thanks to their natural rusticity, which represents an advantage over others, typical of different areas.

Germplasm banks constitute the best-oriented effort to gather and maintain the genetic diversity of crops and to counteract the constant modifications of agriculture, the disruption of ecosystems and the regression of natural vegetation ¹³.

Conservation methods

Field collections play a crucial role in the conservation of materials in natural environments for prolonged periods, in addition to allowing their characterization and evaluation, at least during the first phase, as well as regular propagation and field control of them. . The status of these collections varies considerably with the size, level of reproduction, origin of the germplasm, their national or institutional character ¹³. Species of vegetatively propagated plants, with a long biological cycle and/or with short-lived (recalcitrant) seeds keep in field germplasm banks, although it is convenient to use a combination of storage techniques instead of relying on one ¹³.

Although plants in field germplasm banks are easy to characterize and evaluate, they expose also to losses from attack by pests and diseases, or to adverse conditions such as drought, floods, fires, salinity, pests and wind, among other. That is why complementary alternative methods such as in vitro conservation are perfected and work is being done to improve appropriate technologies for non-orthodox seed species and for vegetatively propagated plants. The foregoing evidence that ex situ conservation capacity must be increased under profitable conditions ¹⁴.

In Cuba, the germplasm of roots, rhizomes, tubers, bananas and bananas is conserved ex situ using different methods, according to environmental conditions and the means and knowledge available. Among the most used techniques are, in addition to the aforementioned gene banks conserved in the field, seed banks, in vitro and in situ banks and cryopreservation ¹³.

The best known ways to conserve genetic diversity are field collections, especially for vegetatively propagated crops. The conservation of these species has its peculiarities and it is necessary to take them into account and study them if it is intended to obtain the best results in their multiplication and conservation. The limitations presented by these collections refer mainly to maintenance costs.

Research at the Institute of Tropical Tubers and Roots Crops (INIVIT) conservation is done in field collections of taro (Xanthosoma spp. and Colocasia esculenta (L.) Schott) and of other species of roots, rhizomes and tubers. These crops, whose variability has been studied from different points of view, need other studies on different aspects related to the conservation and maintenance of accessions in a more efficient way and without genetic erosion ¹⁵.

Botanical- morphological characteristics

Taro is a succulent herbaceous plant that reaches a height of 1-3 meters, without an aerial stem. The central stem is ellipsoidal, known as corm or rhizome. From the rhizome or central corm, lateral corms covered with fibrous scales develop. The color of the pulp is usually white, but there are also colored clones until violet ¹⁰. According to the clone, the shape varies from cylindrical to almost spherical and the type of branching from simple to very branched. The leaves are usually in a peltized form. They occur in the apical meristem of the corm and appear overwhelmed by the base forming a short pseudo-total. The new leaves are rolled between the petioles of the already formed and the older sides wilt and dry. The petiole is cylindrical at the base and grooved at the top, shows a coloration that varies according to the clone; the presence of yellow or pink longitudinal lines and spots or reddish violet spots, especially towards the base, is distinctive in some ¹⁶.

Two or more inflorescences emerge from the apical meristem of the corm, between the petioles of the leaves. They are formed from an enveloping leaf called a spade that surrounds the spadix. They are characteristic structures of the araceae. From the axis of the latter, sessile flowers are inserted and in the lower part, there are pistillate flowers that can be functional or sterile which do not develop, dry and detach. Taro has an erratic seed production, but cases of normal seed formation are known at numerous sites in its geographical distribution ¹⁷.

The spadix is formed by an axis in which many sessile flowers are inserted. In its lower part, completely closed by the basal cavity of the sword, it has pistillate, functional or sterile flowers. The latter do not develop and dry, the pistillate flowers form the spadix section and in it the fertile flowers intermingle with the sterile ones. The female flowers (pistillate) are green, with ovaries and stigmas well developed. The male part consists of sessile flowers. The sword consists of two parts: the lower part that is usually green or red and forms the floral chamber where the female flowers are located and the upper part where the male flowers are found, here the yellow color predominates, but at Sometimes it can be red, purple or spotted ¹⁸.

Some cultivars rarely (or never) produce inflorescences. In many cases, flowering can be successively induced by spraying the plants with gibberellic acid (GA₃ ). The treatment is to be carried out 3-5 weeks after the planting (depending on the conditions climatic and vigor of growth). The first visible indication of flowering is the appearance of the flag leaf (a modified leaf and membranes). Once the flag leaf is exposed, the first inflorescences appear within one to three weeks ¹⁸ and so phases occur until complete development.

The beginning of flowering is usually associated with the issuance of one strong odor, mainly from the spathe, its main goal is to attract pollinating insects. In an inflorescence when spathe spathe is open allows the entry of small insects, to enter the chamber and female flowers distribute pollen on the stigma ¹⁸.

The pollination by the wind can be significant only in some genotypes of open flowering (with the male portion the spadix fully exposed). In a population of these characteristics and with synchronized flowering, rain can cause self-fertilization by washing the pollen grains from the male part of the spadix to the pistillated region. Self-fertilization is possible because the system of self-incompatibility of incompatible genotypes becomes less efficient at the end of flowering, because there is an overlap between stigma receptivity and pollen release ¹⁸.

Thermogenic activity is significant in the formation of inflorescences in taro (C. esculenta), this can be measured over a period of two successive nights, the first night when the inflorescence emit and smell (the female phase ) and a second night, when the end of the male phase arrived. The female phase heat generated in fertile male part and in the appendix sterile and male phase, heat is generated only in fertile male part ¹⁹. The thermogenic activity is synchronized with the natural protogyny of this species and with the pollination of insects in the early hours of the morning.

Genetic improvement of la taro (C. esculenta)

Many cultivated araceae have spread through vegetative media for a long time, which is why they have lost their ability to reproduce sexually. The plants of C. esculenta rarely produce seeds, ie female flowers mature before the male flowers and because many cultivars of C. esculenta are triploid (3n=42). However, certain cultivars of C. esculenta have a stable production of natural seed in Asia and the Pacific ^20,21.

The improvements of the taro (C. esculenta) have lagged behind other crops, such as cassava (Manihot esculenta Crantz), yams (Discorea spp.), Potatoes (Solanum tuberosum L.) and sweet potatoes (Ipomoea batatas (L.) Lam.). Wilson described several methods to obtain improved varieties of C. esculenta and thus achieve their distribution, collection, evaluation and selection of germplasm ²².

A large number of countries have problems with the inflorescence and with the obtaining and germination of the seed. An example of this is shown in Nigeria where several researchers presented serious problems to develop new varieties of taro (C. esculenta) by conventional improvement, the one that has been hampered by the erratic flowering and the lack of botanical seed ²³.

Hybridization of the taro (C. esculenta) is composed of two stages: emasculation and pollination, which determine the success of a genetic improvement program, for which an essential requirement is the perfect formation of the inflorescence, since it therefore, brings the good seed production and high germination rate. Flowering was previously reported as very rare among cultivars of C. esculenta²⁴; however, with the relatively recent development achieved in artificial methods of inducing flowers, pollination methods with hand, the protocol germination ²⁰, obtaining seeds and application of methods of germination, improvement of C. esculenta can be highly successful ²⁵.

In many cultivars of C. esculenta, flowering induction with various concentrations of gibberellic acid has been used ²⁶. At the National Root Research Institute in Nigeria (IITA), the application of gibberellic acid (GA₃ ) in concentrations of 1 g/L - 1.5 gL^-1 is indicated to induce flowering and to achieve taro botanical seed (Colocasia esculenta and Xanthosoma sagittifolium), although flowering can be profuse and prolonged ²⁷.

The production of seeds, their germination and the development of the seedlings to be achieved, can represent a new era in the conventional improvement of the taro (C. esculenta). Consequently, it is expected that, in the near future, better varieties of taro (C. esculenta) will be obtained with higher yield, disease resistance and good culinary quality ²⁸.

Pollen viability

Pollen viability studies are used in plant breeding and the higher the pollen viability, the greater the likelihood of obtaining different combinations of alleles, and increasing genetic variability ²⁹. The viability of pollen can be checked by direct methods, which demonstrate the germination of pollen, and by indirect methods based on cytological variables ³⁰.

It is important to verify the viability of pollen due to the rectilinear bond for fertilization efficiency ³¹. Pollen may become unfeasible during microgametogenesis, where errors in meiotic behavior result in gametes with imbalance. There are also pollen grains with a folded cytoplasm ³. In addition, it is reported that the viability of pollen can vary considerably between individuals of the same species and between samples of the same individual ³³.

There are different methods to assess the viability of pollen, among the fastest and most accurate are staining with vital dyes and germination in artificial media. Staining tests have advantages as indicators of pollen viability, since they are faster and easier than pollen germination ³².

If observed under an optical microscope, a deep red staining and a clean cytoplasm is indicative of a viable or fertile pollen, while a non-colored or pink cytoplasm indicates a non-viable or sterile pollen. Compared to viable grains, sterile grains are deformed, with the granular and / or retracted cytoplasm ³⁴.

Also other researchers ³⁴ determined pollen viability considered where pollen grains rounded and colored red as a viable and constrained undyed, nonviable.

The viability of pollen is determined by various internal and external or environmental factors. Among the specific internal factors, the duration of microsporogenesis, interspecific genetic variability, pollen metabolism, among others ³⁵ stand out. Among the external or environmental factors is temperature, the degree of humidity, among others ³⁶.

Reproductive phenology of the plant

Studies on reproductive phytophenology have revealed that the optimal time to flourish and fructify is determined by biotic factors ³⁷ and abiotic factors ³⁸ or by a combination or interaction of both kinds of factors, related to the type and timing of the dispersion and seed germination ³⁹.

The differences in flowering phenology between plant species show a mechanism for maintaining the high number of species in tropical communities ⁴⁰. Among the attributes of plants associated with different phenological patterns are the way of life ⁴¹ and the vertical distribution in strata.

Reproductive phenology shows seasonal variations throughout the year, which also varies according to the type and species of plants. The different proportions of each way of life in different habitats influence the variations observed in the phenological patterns ⁴¹.

Selection of progenitors

The taro improvement programs (C. esculenta) are on crossings from local biparental cultivars and aim at improving yield and quality. It was observed relatively narrow, but high within progeny phenotypic variation; thus the breeders managed to select hybrids with quality, to cross them with the best clones conserved in the genetic base by hybridization ⁴¹.

The method used was based on horizontal introgression or lasting resistance through numerous cycles of recurrent selection. The main drawback of these programs has been a difficulty in eliminating undesirable traits, such as: irregular forms of the corm, high number of stolons, and high levels of acrimony ⁴². The breeders have tried to look for resistance to TLB (Taro leaf blight), within a wide range of clones and with the help of the Araceae Regional Network to facilitate the collection and exchange of elite cultivars, with varying degrees of TLB resistance ⁴¹.

Taro improvers (C. esculenta) seek to improve the architecture of the plant (the optimal number of offspring, the absence of stolons, optimal number of leaves, vertical petioles), higher yield and good quality corm, high content of dry matter, shape of the corms, the low level of irritating substances (i.e. calcium oxalate crystals). However, a precise evaluation of general combinatorial ability (GCA) and specific combinatorial capacity (SCA) and of genetic distances between local varieties is lacking; breeders have selected parents with high phenotypic value. In addition, molecular studies based on AFLP, SSR markers, so new possibilities have been opened for the study of the relationships between hybrid performance and genetic distances of parents ⁴².

The different types of information related to the parents can be used to predict the hybrid's performance: its phenotypic value, the results of the progeny tests or its genetic relationships ⁴⁰.

FINAL CONSIDERATIONS

• In the INIVIT, intensive work do on the genetic improvement of the Taro Colocasia esculenta and very encouraging results have been obtained in the hybridization improvement of this crop in Cuba ⁴². The use of improved clones and introduced by INIVIT, has allowed to increase agricultural production and its technology, without additional costs; however, there are currently few clones of taro (C. esculenta) in the different production scenarios.

• In other regions of the world where the taro (C. esculenta) is cultivated, the number of accessions that emit inflorescences, the availability of the pollen they produce and the viability of it are no longer a problem. In Cuba, this issue is of vital importance, since the accessions with more performance emit inflorescence very rarely or never and when they do, they almost never produce pollen and it is rarely viable.

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Received: January 23, 2018; Accepted: March 26, 2019

^*Author for correspondence. [e-mail]