Landraces of Phaseolus vulgaris (Fabaceae) in Northern Malawi. II. Generation and maintenance of variability

Bean (Phaseolus vulgaris) landraces in northern Malawi were found to be heterogeneous for many qualitative and quantitative traits. An earlier 21- character evaluation of 25 lines in 15 landraces found significant differences in all characters among lines and among landraces. The present study has concentrated on two factors that are likely to be involved in the generation and maintenance of this heterogeneity: seed- handling practices and natural outcrossing. Seed- handling practices during harvest, storage, and marketing at the village, local market, and national levels were found to result in physical and usually nonselective mixing of many seed types. Experiments determined that outcrossing does occur at a low level among beans in Malawi and can result in the generation of many new seed phenotypes. A scenario was developed integrating seed- handling practices and outcrossing that lays a foundation for understanding the generation and maintenance of variability in these bean landraces. The process whereby landrace heterogeneity arises and the ways it is maintained, if understood, should assist plant breeders in the development of improved germplasm for subsistence farmers.