Characterisation of Taro (Colocasia Esculenta (L) Scott) germplasm collections in South Africa : towards breeding an orphan crop.

Abstract

Amadumbe (Colocasia esculenta), better known as taro, is a traditional root crop widely cultivated in the coastal areas of South Africa. Taro is showing potential for commercialisation. However, very little is known about the genetic diversity, potential and its introduction and movement in South Africa. This study was undertaken to (1) determine the genetic diversity in the ARC taro germplasm collection using agromorphological characteristics and microsatellite markers, (2) to determine if it is possible to breed with local taro germplasm and (3) to determine the effect of four different environments (Roodeplaat, Umbumbulu, Owen Sithole College of Agriculture and Nelspruit) on ten agro-morphological characteristics of 29 taro landraces Taro germplasm was collected in South Africa in order to build up a representative collection. Germplasm was also imported from Nigeria and Vanuatu. The South African taro germplasm, and selected introduced germplasm, were characterised using agromorphological descriptors and simple sequence repeat (SSR) markers. Limited variation was observed between the South African accessions when agro-morphological descriptors were used. Non-significant variations were observed for eight of the 30 agromorphological characteristics. The 86 accessions were grouped into three clusters each containing 39, 20 and 27 accessions, respectively. The tested SSR primers revealed polymorphisms for the South African germplasm collections. Primer Uq 84 was highly polymorphic. The SSR markers grouped the accessions into five clusters with 33, 6, 5, 41 and 7 accessions in each of the clusters. All the dasheen type taro accessions were clustered together. When grown under uniform conditions, a higher level of genetic diversity in the South African germplasm was observed when molecular (SSR) analysis was performed than with morphological characterisation. No correlation was detected between the different clusters and geographic distribution, since accessions from the same locality did not always cluster together. Conversely, accessions collected at different sites were grouped together. There was also no clear correlation between the clustering pattern based on agro-morphology and SSRs. Thus, in order to obtain a more complete characterisation, both molecular and morphological data should be used. Although the results indicated that there is more diversity present in the local germplasm than expected, the genetic base is still rather narrow, as reported in other African countries. Fourteen distinct taro genotypes were planted as breeding parents and grown in a glasshouse. Flowering were induced with gibberellic acid (GA3). Crosses were performed in various combinations; however, no offspring were obtained. This might be due to the triploid nature of the South African germplasm. It might be useful to pollinate diploid female parents with triploid male parents or use advanced breeding techniques, like embryo rescue or polyploidization, to obtain offspring with the South African triploid germplasm as one parent. The triploid male parents might produce balanced gametes at low percentages, which can fertilize the diploid female parents. Twenty-nine taro accessions were planted at three localities, representing different agroecological zones. These localities were Umbumbulu (South of Durban - KZN), Owen Sithole College of Agricultural (OSCA, Empangeni, KZN) and ARC - Vegetable and Ornamental Plants (Roodeplaat, Pretoria). Different growth and yield related parameters were measured. The data were subjected to analysis of variance (ANOVA) and additive main effects and multiplicative interaction (AMMI) analyses. Significant GxE was observed between locality and specific lines for mean leaf length, leaf width, leaf number, plant height, number of suckers per plant, number of cormels harvested per plant, total weight of the cormels harvested per plant and corm length. No significant interaction between the genotype and the environment was observed for the canopy diameter and corm breadth. From the AMMI model, it is clear that all the interactions are significant for leaf length, leaf width, number of leaves on a single plant, plant height, number of suckers, number of cormels harvested from a single plant and weight of cormels harvested from a single plant. The AMMI model indicated that the main effects were significant but not the interactions for canopy diameter. The AMMI model for the length and width of the corms showed that the effect of environment was highly significant. There is a strong positive correlation between the number of suckers and the number of leaves (0.908), number of cormels (0.809) and canopy diameter (0.863) as well as between the number of leaves and the canopy diameter (0.939) and between leaf width and plant height (0.816). There is not a single genotype that can be identified as “the best” genotype. This is due to the interaction between the environments and the genotypes. Amzam174 and Thandizwe43 seem to be genotypes that are often regarded as being in the top four. For the farmer, the total weight of the cormels harvested from a plant will be the most important. Thandizwe43, Mabhida and Amzam174 seem to be some of the better genotypes for the total weight and number of cormels harvested from a single plant and can be promoted under South African taro producers. The local accessions also perform better than introduced accessions. It is clear that some of the introduced accessions do have the potential to be commercialised in South Africa. The study indicate that there are genetic diversity that can be tapped into for breeding of taro in South Africa. However, hand pollination techniques should be optimized. Superior genotypes within each cluster in the dendrograms as well as Thandizwe43, Mabhida and Amzam174 (identified by the AMMI analysis as high yielding) can be identified and used as parents in a clonal selection and breeding programme. Additionally, more diploid germplasm can be imported to widen the genetic base. The choice of germplasm must be done with caution to obtain germplasm adapted to South African climate and for acceptable for the South African consumers.