Effects of soil moisture deficits on the water relations of bambara groundnut (Vigna subterranea L. Verdc.)

The components of leaf water potential (ψ_l) and relative water content (RWC) were measured for stands of bambara groundnut (Vigna subterranea) exposed to three soil moisture regimes in controlled-environment glasshouses at the Tropical Crops Research Unit, Sutton Bonington Campus. Treatments ranged from fully irrigated (wet) to no irrigation from 35 days after sowing (DAS) (dry). RWC values varied between 92–96% for the wet treatment, but declined from 93% to 83% in the dry treatment as the season progressed. ψ_l at midday decreased in both the wet and dry treatments, but the seasonal decline was more pronounced in the latter: seasonal minimum values were −1.19 and −2.08 MPa, respectively. Plants in the wet treatment maintained turgor (ψ_p) at about 0.5 MPa throughout the season, whereas values in the dry treatment approached zero towards the end of the season. There was a linear relationship between ψ_p and ψ_l9 with ψ_p approaching zero at a ψ_l of −2.0 MPa. Mean daily leaf conductance was consistently higher in the wet treatment (0.46–0.79 cm^-1) than in the intermediate and dry treatments (0.13–0.48 cm s^-1 Conductances in the intermediate and dry treatments were similar, and the lower evapotranspirational water losses in the latter were attributable to its consistently lower leaf area indices (L): L at final harvest was 3.3, 3.3 and 1.9 for the wet, intermediate and dry treatments. Bambara groundnut was apparently able to maintain turgor through a combination of osmotic adjustment, reductions in leaf area index and effective stomatal regulation of water loss.