Abstract
The water potential (Ψ) at which stomata completed closure (Ψ8Lmin) was determined for pearl millet (Pennisetum americanum [L.] Leeke) at two growth stages by monitoring changes in leaf conductance (gL) and Ψ following shoot detachment. Leaf water status was evaluated concurrently using a pressure-volume (P-V) technique. In a pot experiment with young vegetative plants, Ψ8Lmin closely approximated to the estimated Ψ at zero turgor (Ψu) both for control and for drought-conditioned plants which had osmotically adjusted. However, for penultimate leaves of field-grown flowering plants, Ψ8Lmin was found to be 0.61 (irrigated plants) and 0.87 (droughted plants) MPa below Ψu. In drought-stressed field-grown plants, osmotic adjustment (characterized by a decrease in solute (osmotic) potential (Ψs ) at both full hydration and zero turgor) was insufficient to maintain a positive bulk leaf turgor potential (Ψp) once Ψ had declined to below about -1.5 MPa. It is suggested that localized adjustment by the stomatal complex in response to environmental differences, leaf ageing and/or ontogenetic change, is responsible for the uncoupling of stomatal from bulk leaf water status.