Drought Responses of Two Bambara Groundnut (Vigna subterranea L. Verdc.) Landraces Collected from a Dry and a Humid Area of Africa

Drought adaptation strategies of two bambara groundnut landraces, Uniswa Red and S19-3, collected from contrasting environments in Africa, were compared. Our objectives were to investigate the relative significance of effective stomatal control induced by the abscisic acid (ABA) signalling and osmotic adjustment in regulating plant water relations in general for this legume species. The ABA concentration [ABA] in the leaf increased linearly with declining relative leaf water content, and there were significantly higher [ABA] in Uniswa Red compared with S19-3 at the final harvest in the drought-stressed plants. Estimated by a linear-plateau model, S19-3 initiated the reduction in transpiration at a significantly lower soil water threshold (FTSW = 0.50 +/- 0.024) than Uniswa Red (FTSW = 0.69 +/- 0.023) indicating that the latter was more sensitive in reducing plant water use in response to soil drying. A similar trend was found for stomatal closure during soil drying, although the soil water thresholds at which relative stomatal conductance (g(s)) started to decline were not significantly different between the two landraces. By an early closure of stomata and hence an early reduction in transpiration rate during soil drying, Uniswa Red could be defined as a 'water-saver' such that it maintains leaf water status to a great extent of soil water deficit. This strategy is important for survival during intermittent drought. While S19-3 could be defined as a 'water-spender' with a late closure of stomata, hence a late declining of transpiration rate during soil drying allowed the landrace to maximize its water use despite giving up its leaf water relations. Such drought response together with a fast phenological development of S19-3 indicates that the landrace is capable of escaping from terminal drought while maximizing its water use and productivity when soil water is available.