Seed quality and water use characteristics of a bambara groundnut (Vigna subterranea L.) landrace differing in seed coat colour.

Abstract

Bambara groundnut (Vigna subterranea L.) is an underutilised African legume that fits the same ecological niche as Arachis hypogea (groundnuts). Because of its reported drought tolerance and high water use efficiency there are now renewed efforts to study bambara groundnut with a view to promoting it as an alternative crop in marginal production areas. It is still cultivated using unimproved landraces, and little is known about their seed quality. There is need for information describing aspects of their seed quality in order for farmers to successfully produce the crop. The study evaluated seed quality and seedling water use characteristics of selected seed coat colours of bambara groundnut. Lastly, the study investigated the effect of water stress imposed on maternal plants on subsequent yield and seed quality of bambara groundnut. A single bambara groundnut landrace was characterised into four distinct selections based on seed coat and speckling colour; plain red, plain cream, cream with brown speckles (brown speckled) and cream with black speckles (black speckled). Seed quality (viability and vigour) was evaluated using the standard germination, electrolyte conductivity and imbibition tests as well as water activity, seed coat thickness and mineralogy. Seedling water use characteristics were evaluated under varying water regimes (25%, 50% and 75% field capacity). Measurements included plant growth and physiological (chlorophyll content index and chlorophyll fluorescence) responses up to 21 days after planting; thereafter seedling water use efficiency was determined. Irrigation was withdrawn thereafter in all water treatments to determine physiological and metabolic responses (total soluble sugars, antioxidants and phenols) to terminal stress. A field trial was grown in 2013/14 summer season under irrigated and rainfed conditions. Yield and yield components as well as subsequent seed quality (viability and vigour) of progeny was determined from harvested material. Darker coloured seeds and seeds with similarly coloured speckles showed better viability while the plain cream landrace selection was more vigorous. Seedling water use efficiency in bambara groundnut improved with decreasing water availability. Drought avoidance strategies and acclimation to water stress were also found to be present at the seedling establishment stage. Yield was negatively affected by water stress. Subsequent seed viability and vigour were respectively higher in seeds produced under irrigated and rainfed conditions. The study concluded that although bambara groundnut is a water use efficient crop, water stress may affect yield and subsequent seed quality.