Agronomic performance and genetic diversity of the root crop yam bean (Pachyrhizus spp.) under West African conditions

Abstract

The yam bean (Pachyrhizus spp.) is a legume root crop usually known as a vegetable crop. Three cultivated species are distinguished: Amazonian yam bean (P. tuberosus), Mexican yam bean (P. erosus) and Andean yam bean (p. ahipa). Within P. tuberosus there are three distinct cultivar groups: the Ashipa and the Jíquima cultivar groups with low dry matter content of the tubers (below 20%) and the Chuin cultivar group with high dry matter content (above 30%) of the tubers. The Chuin cultivar group is used like cassava. The yam bean is nowadays cultivated in Central and South America as well as in South East Asia. Cultivation on large scale for selling is only known in Mexico, Philippines and in Indonesia. The crop is established by seeds, but only the tubers are consumed due to the high rotenone content of the seeds (about 1% of seed weight). Therefore, often the flowers are removed to increase the tuber yield. Until recently very little breeding work had been carried out on thisgenus, and levels of genetic diversity and interspecific relationships within the genus are not well understood. This study was conducted in Benin (West Africa) to investigate the possibility to grow the yam bean in West Africa for its introduction into the farming systems of this region as a new tuber crop rich in protein. Thirty four accessions from the three cultivated species (P. tuberosus, P. erosus and p. ahipa) and ecologically diverse origins were tested in a field trial at two locations in Benin during 2001/2002 and 31 agronomic and 40 morphological traits were recorded. The objectives were: (i) to evaluate the agronomic potential of the accessions and to investigate the effect of reproductive pruning (removing all flowers of the plants) on yield, (ii) to assess the genetic diversity within and between the yam bean species using multivariate statistics of morpho-agronomic traits, (iii) to examine the possibility to assess the genetic diversity with the help of Near Infrared Reflectance Spectroscopy (NIRS).For the evaluation of the agronomic potential, 31 agronomic traits were recorded at both locations. Significant differences were observed among locations, accessions and species for most of the characters. Without reproductive pruning, the mean of tuber fresh matter yield ranged from 12.4 t ha-1 in p. ahipa to 23.4 t ha-1 in P. erosus. Seed yield ranged from 1.5 to 2.9 t ha-1, 3.5 to 4.6 t ha-1 and 2.6 to 2.7 t ha-1 for P. tuberosus, P. erosus and p. ahipa, respectively. Reproductive pruning resulted in an increase of 48, 91 and 61% of the tuber fresh matter yield in P. tuberosus, P. erosus and p. ahipa, respectively. The tuber dry matter yield increased at the same time by 58, 100 and 66% in P. tuberosus, P. erosus and p. ahipa, respectively. The Chuin cultivar group of P. tuberosus showed a high tuber dry matter content (about 30%). Accessions with genes of interest to improve earliness (early maturity) have been identified within p. ahipa. Accessions with high tuber yield as well as high seed yield have been identified in P. erosus. In this species, two accessions were found to have a high tuber yield under both environmental conditions. In all three species high genetic variation was observed for tuber fresh matter and tuber dry matter yields.Multivariate analyses (Principal component analysis and cluster analysis) of 71 morpho-agronomic characters were used to assess the genetic diversity in the yam bean germplasm. Principal component analysis indicated that the variance accumulated by the first two components for morphological and agronomic data was 62.1%. Three large groups of accessions were formed in the principal component analysis as well as in the cluster analysis. The groups consist of p. ahipa, P. erosus and P. tuberosus accessions, respectively. Accession TC118 of P. tuberosus (Ashipa cultivar group) was classified as an isolated group due mainly to its low tuber fresh matter yield, tuber dry matter content and seed yield. No relationship between geographic origin and pattern of diversity was observed.Near Infrared Reflectance Spectroscopy (NIRS) has the potential to analyse the physical and chemical composition of seeds and in this study was applied to compare and analyse the patterns of variation of the three different species of yam bean. Multivariate statistical techniques, including PCA and cluster analysis, were used to compare the spectral data from the seed samples of the accessions of yam bean. Cluster analysis showed often similarity between accessions within the species, but no complete separation between the species was observed.The study showed the potential of Pachyrhizus spp. for its introduction into the farming systems of Benin and of West Africa in general. Accessions with high tuber yield were observed within P. erosus and accessions with high dry matter content of tubers and earliness in maturity were found in P. tuberosus and p. ahipa, respectively. Interspecific hybridizations are possible between these species. Multivariate analyses have shown that there is a great amount of genetic variability between the three cultivated species and also within P. tuberosus because accessions belonging to the Chuin cultivar group differed significantly from the accession of the Ashipa cultivar group.