TARO BREEDING in INDIA ABSTRACT Introduction

TARO BREEDING IN INDIA

M.T. SREEKUMARI, K. ABRAHAM AND M. UNNIKRISHNAN CENTRAL TUBER CROPS RESEARCH INSTITUTE,TRIVANDRUM - 695 017, INDIA

ABSTRACT

The genetic improvement of taro (Colocasia esculenta (L.) Schott) is one of the major research programmes of the Central Tuber Crops Research Institute, Trivandrum, which maintains 424 edible accessions. Field evaluation of 369 accessions resulted in the identification and release of two high yielding superior varieties from the CTCRI Headquarters at Trivandrum and a leaf blight tolerant type from its Regional Centre at Bhubaneswar. The varieties developed from germplam selections are triploids indicating the higher yielding potential of triploid taros than diploid taros. By intervarietal hybridization in diploid taros, a large number of hybrids were produced from different combinations. Of the 4280 sergeants evaluated, 802 (18.7%) were rated as above average for important attributes like dwarf plant types, higher corm and cormel yield, non-acridity, long keeping quality and early maturity. From the initially screened segregants, rigorous selection for the above attributes resulted in the identification of seven superior hybrids viz., H-2, H-3, H-4, H-12, H-13, H-120 and H-160. They consistently recorded higher tuber yields and superior quality attributes during the yield trials at the CTCRI. Currently they are being evaluated in farmers’ fields for verification of their merits, prior to variety release. Besides the above high yielders novel types like dwarfs, erect types, profusely flowering lines and CLB tolerant types were identified among the segregants. With the regular flowering of the clones and the possibility of producing full sibs, half sibs and selfs, taro breeding has been pursued at the CTCRI extensively and intensively, for developing hybrid varieties combining the various superior attributes. The latest approach in taro breeding at the CTCRI is to artificially produce triploids by crossing diploids with induced tetraploids to increase productivity. For this, tetraploid taros developed by colchicine treatment are being tested for flowering, fertility, and interploid (diploid x tetraploid) compatibility for the production of triploids.

Introduction

Colocasia esculetna (L.) Schott) is a popular tuber crop consumed as a vegetable in India and South East Asia while it is an important staple food in the Pacific region. It is commonly known as taro, dasheen, cocoyam and occasionally eddoe. It belongs to the family Araceae. Despite its importance as a popular edible tuber crop, very little attention has been devoted to the genetic improvement of taro. Great back of the research work on taro has been chromosome studies (Yen and Wheeler, 1968; Vijaya Bai et al., 1988; Coates et al., 1988; Sreekumari and Mathew, 1991 a, b). Information on the sexual potentialities of the crop has been very fragmentary, and the improvement programme have been largely dependent on the

1 exploitation of the existing genetic variability among the cultivars (Warid, 1970; Kuruvilla and Singh, 1981, Plurre, 1984; Tanimoto and Matsumoto, 1986; Velayudhan and Muraleedharan, 1987; Singh and Singh, 1987; Unnikrishnan et al., 1987; 1988; Lebot and Aradhya, 1991). For a long time it was believed that taro plants do not flower, and therefore fail to produce seeds (Kikuta et al., 1938; Plucknett et al., 1970; Shaw, 1975). However, later reports from different countries indicate that many clones flower and produce viable seeds (Abraham and Ramachandran, 1960; Plucknett et al., 1970; Shaw, 1975, 1984; Jackson et al., 1976, Jos and Vijaya Bai, 1977; Strauss et al., 1980; Ghani, 1984).

Because of its long history as a cultivated crop and of the vegetative mode of propagation, it has been possible to select and preserve distinct types or varieties in taro, which are useful to man. Accumulation of such multiplicity of types must have made it possible for suitable cultivars to be selected in different areas and in various growing conditions of soil, water, altitude, temperature, planting practices, etc. Doku (1980) has pointed out that this large store of variation present in the crop would be immediately available for utilization in the raw state and in all future combinations and recombination if conditions for flowering, hybridization, seed setting and raising of seedlings are discovered.

Research Priorities for Taro Genetic Improvement: -

- Germplasm collection, maintenance, characterization and evaluation - Publishing bulletins and catalogues for easy reference - Evaluation of the collections and identification of suitable cultivars for (1) direct release based on performance in the different field trials conducted at the Institute farm and in on farm trials (2) incorporate in various breeding programmes - Intervarietal hybridization and production of true seeds - Production of selfed seeds - Evaluation of the seedling and subsequent clonal progeny for various desirable attributes - Identification and multiplication of the selected hybrids for the conduct of field trials - Isolation of superior hybrid selections for release - Production of triploids in large scale by crossing induced tetraploids with diploids - Evaluation of the induced triploids and identification of genetically improved types.

The Taro gene bank The taro gene bank at the CTCRI possesses 424 accessions collected from all over India. The cormels are planted in April-May and harvested after six months. Important characters were recorded

2 based on modified IPGRI descriptor and catalogues and bulletins were published. (Unnikrishnan et al., 1987). A wide spectrum of genetic variability was noticed among them for several characters, especially growth habit, pigmentation on different plant parts, crop duration, flowering habit, fertility, open pollinated fruit and seed set, size, shape and yield of corm and cormels, cooking quality, tolerance to leaf blight disease etc. Evaluation trials were undertaken such as row trials, preliminary evaluation trials, advanced yield trials and finally on farm trials prior to the release of superior selections. Two accessions viz. C.149 and C.266 were thus released for general cultivation under the names Sree Reshmi and Sree Pallavi respectively from the CTCRI. A gist of the characters of these two varieties and few of the elite germplasm collections identified are given in the Table1 Table 1. Important characters of the released varieties and other elite germplasm selections Germplasm selections Description 1. C.149 (Sree Reshmi) This is a local collection and is a natural triploid. It grows to a height of 1-1.5 m and has edible petiole, leaves, corms and cormels . Cooking quality is excellent. It matures in 7-8 months and cormel yield ranges from 15-19 t ha-1. Cormels contain 14-16 percent starch and 2-2.5 per cent protein. This was released from the CTCRI during 1987. 2. C-266 (Sree Pallavi) It is another released variety of taro from the CTCRI. It is a triploid collection from Meghalaya. Plant height ranges from 1-1.5 m and the crop duration is 7-8 months with a yield potential of 12-15 t ha-1. Only the cormels are edible. The starch content ranges from 19-23 per cent and protein content 1.8-2.1 per cent. 3. C-9 A triploid accession. It is an early maturing type (5-6 months) with medium height (60-80 cm). The leaves are medium broad and average yield (cormel yield) is 15 tonnes per hectare. Cormels are excellent for culinary purpose but the cormel keeping quality is poor. 4. C-189 It is a high yield triploid with an average yield of 20 t ha-1. The plants are tall types with comparatively long duration. The cormels are many and long fusiform in shape. The most

3 attractive attribute of this selection is the long keeping quality of the cromels (4-5 months) 5. C-303 and C-384 These two diploid accessions are the only ones in the germplasm that flower almost regularly. The flowers are highly fertile resulting in open pollinated seed production The plants are of medium type, cormel yield ranges from 10-14 t ha-1 and cooking quality is good. Both the accessions are usually incorporated in the breeding programme. 6. C-408, C-444/2 and C-481 These accessions are tall, late maturing types. They produce comparatively large main corm (> 500 g) with 8-10 well-developed big cormels. They are identified as triploids, but based on tuber characters they can be treated as ‘intermediate types’ between dasheen and eddoe groups.

Ploidy in relation to tuber yield

Twenty diploids and twenty triploids were evaluated for growth performance and yield. It was observed that triploids are superior to diploids in seven of the nine characters studied. The corm and cormel yield showed very promising and impressive increase in the triploids except in the case of cormel number which was significantly more in diploid plants.. This implies that for selecting high yielding types in taro it is desirable to consider the triploids rather than diploids. The comparative performance and extent of variability noticed within the two ploidy types is given in Table 2.

Table 2. Comparative performance of diploid and triploid taro Sl. Characters Diploids Triploid t value No. Mean CV (%) Mean CV (%) 1 Plant height (cm) 69.7 ± 0.47 33.14 76.3 ± 2.2 31.6 2.168* 2 Tiller number 3.4 ± 0.14 44.12 3.6 ± 0.17 52.8 NS

4 1.090 3 Number of leaves 8.3 ± 0.56 73.49 8.1 ± 2.22 60.49 NS 6.276 4 Shoot girth (cm) 8.8 ± 0.79 98.86 14.6 ± 0.46 34.93 6.149** 5 Leaf length (cm) 20.5 ± 0.84 44.87 30.4 ± 0.58 21.05 9.628** 6 Leaf breadth (cm) 16.0 ± 0.67 45.63 25.4 ± 0.56 24.02 10.823** 7 Corm weight (g) 141.2 ± 8.01 62.11 203.6 ± 10.94 58.59 4.600** 8 Cormel Number 18.8 ± 1.00 65.48 12.6 ± 0.70 60.16 3.050** 9 Cormel weight (g) 206.2 ± 7.69 40.88 447.1 ± 8.03 26.51 6.348** *Significant at 5% level ** Significant at 1% level NS -Not significant

Flowering frequency and floral biology: -

Flowering was seasonal mostly starting by the middle of June (2-3 months after planting) and lasting till the middle of September. Highest frequency of flowering was usually observed during the last week of July. Flowering is rather very low in cultivated forms. However, frequency of flowering in wild taro grown in marshy areas under waterlogged condition was notably high everywhere.. The genetic resources maintained at the Central Tuber Crops Research Institute (CTCRI) showed that natural flowering is neither profuse nor predictable which might be correlated the characteristics of the location, which is as follows: -

Latitude : 8° 40° N Longtitude : 77°.0’ E Minimum night temperature : 19° C Maximum day temperature : 33.4° C Rainfall : 1400-1500 mm Day length : 11 hr 23 min (shortest in Dec.) To 12 hr 39 min (longest in June) Crop season : April-October

In addition to the environmental factors, flowering was also influenced by the ploidy status of the genotype. A few of both diploid and triploid accessions flowered but frequency was more among diploids compared to triploids ranging from 2.5 – 5.0 per cent in the former and from 0.8 to 2.5 per cent in the latter. Flowering data recorded from 120 diploids and 119 triploids during three consecutive seasons is given in Table 3.

Table 3. Frequency of flowering in the two ploidy types of taro during three years in two seasons No. of accessions flowered Year Season* 2n 3n 1990 1 5 (4.2) 2 (1.7) 2 1 (0.8) 0 (0.0) 1991 1 6 (5.0) 3 (2.5) 2 0 (0.0) 0 (0.0) 1992 1 3 (2.5) 1 (0.2) 2 1 (0.8) 0 (0.0)

Season 1 – April planting Season 2 – November planting (Figures in parentheses indicate percentage of flowering)

Flowering ability and floral floral productivity are extremely important for the conventional breeding process. In India, poor flowering is the main factor limiting for planned hybridization. Ploidy variations, incompatibility, female or male sterility, disease, soil conditions, heavy rain, shade, etc. were the other factors limiting for planned hybridization.

Floral biology was studied and pollination techniques standardized. Stigma receptivity was found to last for a considerable length of time. Jos and Vijaya Bai (1977) observed that on the day of opening the inflorescence, the percentage of successful pollination was 85.7.The stigma receptivity was at its peak for some hours after the liberation of pollen also. Limited receptivity was noticed upto 36 hours after the liberation of pollen. and there was no seed set beyond that period. The detailed study had established that trace receptivity could be realized for 44 hours earlier to anthesis and 60 hours later to anthesis (Jos and Vijaya Bai,1987; Sreekumari and ThankammaPillai, 1993).

Intervarietal Hybridization The main targets of taro breeding were: - Genetic variability Ideal plant type Cormel yield Taste quality of cormels Resistance/ tolerance to taro leaf blight Early maturity Longer keeping quality

6 Density tolerance Incidence of flowering and floral productivity.

However, as flowering was irregular and non-synchronous crosses could be made using the available fertile diploids only. For the successful breeding of novel taro varieties with new combinations, a high genetic diversity between the parents is desirable. Here, the genetic diversity of the very few flowering diploids alone could be incorporated in the breeding programme. Accessions derived from one country alone or from crosses between materials from neighboring countries are reported to be not desirable for an effective breeding programme. Generating a very diverse offspring will only be possible if crossing of cultivars from genetically diverse gene pool is carried out (Kreike et al., 2001). By doing so the chance of wild characters will be minimized in the offspring and the improvement of the taro crop can be accomplished in a very short time.

Open pollination and selfing Open pollination occurred in fields planted with different genotypes. Depending on the flowering rate, synchrony etc. high open pollinated fruit and seed formation was obtained from several diploid accessions. Regarding selfing, pollination within the same clone was more successful compared to bagging mature buds two days before anthesis. However, self pollination is usually done for the production of inbred lines meant for genetic studies and due to scarcity of flowering for a planned inbreeding programme, not much progress could be achieved.

Hybrid progeny evaluation

Taro seeds were non-dormant. Seeds were sown inside the glass house. Seed germination initiated within 8 to10 days and the germination percentage in different cross combinations ranged from 60.5 to 82.3. Time to germination and germination percentage did not show any noticeable difference in the cross, self or open pollinated seeds. Seedlings were transplanted to field within 90 to100 days after germination. Altogether 10,898 seedlings were evaluated during the last six years. The sexual progeny exhibited a wide spectrum of variability with regard to almost all characters. 10 to 15.5 per cent of the progeny showed wild characters such as stoloniferous root and highly acrid nature of all plant parts. Such types and also other poor yielding types were discarded at the seedling stage itself and the rest were carried over for clonal evaluation. More than 4000 sexually generated clonal progeny are at present under different

7 stages of evaluation. Recurrent back crossing was proved to be difficult due to the poor flowering ability of the cultivars. From the first clonal generation onwards itself potential new cultivars were selected including dwarf types, blight tolerant types, high yielding good cooking quality types etc. Five hybrids having 18.5 to 22.0 tonnes per hectare yield with good cooking quality are being tested in on farm trial prior to release. It is expected that the best two of them will get released for general cultivation soon as the first hybrid varieties of taro from India.

Flowering in the sexual progeny

Vigorous growth, the continuous occurrence of floral clusters,4 –5 inflorescences per cluster, well developed spadices, a high proportion of fertile female flowers, an abundance of pollen, an intense odour a day before pollen was released, many insects inside or outside the inflorescence, good seed set and well developed fruit head are reported to be the main indices of good and productive flowering in taro (Ivancic et al., 1996). The sexual progeny showed a regeneration of sexuality with all the attributes mentioned above. A similar regeneration of sexuality was observed in another vegetatively propagated tuber crop viz. Greater yam (Dioscorea alata ) by Abraham et al. ,(1985). The progeny thus evinced wide spectrum of genetic variability and the much needed flowering frequency and synchrony of flowering. As all of them were diploids, highly fertile types were abundant for the use as base material for future breeding programme. This sexuality improvement in taro signifies the tremendous scope for the genetic improvement of this species.

Polyploidy breeding in taro The natural triploids in taro were found to be significantly superior in yield compared to diploids. With the objective of producing artificial triploids in large scale so as to enhance the frequency of superior yielders in taro, attempts to produce induced tetraploids for interploid crossing for the production of artificial triploids are underway at the CTCRI.

Induced polyploidy has been widely used as a potential plant breeding method on account of the tendency of colchiploids to manifest gigantism in plant parts (Stabbins 1950, 1971). In seed propagated plants this has only limited application because of the fall in pollen and seed fertility, due to meiotic abnormalities usual with auto-polyploids (Gottschalk, 1978), which is not a problem in vegetatively propagated crops like taro.

Ten desirable accessions based mainly on cormel yield and cooking quality of cormels were selected for the induction of tetraploidy by colchicines treatment. It was

8 successful when 0.2 per cent solution was applied on the emerging shoot tip for 6-8 hours. The occurrence of tetraploids ranged from 0.0 – 31.0 per cent in the different accessions. However all did not establish in the field. Preliminary evaluation for the identification of the higher ploids was easy from the stomatal size and the stomatal chloroplast count which showed noticeable differences compared to the control. However, the occurrence of tetraploids (2n = 56) was confirmed through cytological screening. Evaluation of the tetraploids revealed that some of them were better in performance compared to the control (Sreekumari and Mathew,1991),the result needs further confirmation. Attempts to produce triploids by crossing induced tetraplodis with diploids are underway for which induction of flowering in tetraploids is needed.

Conclusion

The taro breeding in India is limited to the National level research programme. Structuring the genetic diversity is necessary to optimize the use of germplasm by breeders for which molecular level screening is highly warranted. It appeared that floral attributes are unreliable to classify accessions because variation within each variety is so important that the variation between the two becomes doubtful. Shy flowering, non- synchrony and protogyny were recognized as the major breeding barriers for a planned breeding programme. Large scale production and evaluation of sexually regenerated clonal progeny revealed the tremendous scope for the genetic improvement of the crop through hybridization and selection. Indian agriculture has to be accompanied by intense genetic improvement of the crop for which it is essential to have International co- operation among taro breeders and establishing a procedure for germplasm exchange.

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