Soil water matric potential rather than water content determines drought responses in field-grown lupin (Lupinus angustifolius)

Drought responses in leaves of lupin (Lupinus angustifolius L., cv. Polonez) were investigated in plants grown in lysimeters either in a sand or in a loam soil in the field. Abscisic acid (ABA) content, water potential (psi(1)) and conductance to water vapour (g(H2O)) were determined in leaves of both irrigated plants and in plants exposed to gradual soil drying. A morning-peak of leaf ABA content was found in both fully watered and droughted plants. During soil drying which, on both soils types, only decreased soil water potential of the upper soil layers, mid-day leaf ABA content increased relative to that in fully irrigated plants before any appreciable decreases occurred in psi(1). In the part of the soil profile from which water was taken up (0-60 cm depth), g(H2O) decreased when the relative available soil water content (RASW) on sand was below 12% and RASW on loam, below 30%. At this point the average soil water matric potential (<(psi)over bar>(soil)) on sand was less than -0.13 MPa and the fraction of roots in 'wet' soil was 0.12, while on loam, the fraction of roots in 'wet' soil was 0.44 while <(psi)over bar>(soil) was similar to that on sand. A critical leaf ABA content of 300-400 ng/g FW was associated with the onset of stomatal closure on both soil types. We suggest that the initial stomatal closure is controlled by ABA which originates from the roots where its production is closely related to psi(soil) and the water potential of the root surface and that psi(soil) is a more important parameter than RASW or the fraction of roots in 'wet' soil for affecting leaf gas exchange. Further drying on both soils led to further increases in leaf ABA and declines in psi(1) and g(H2O) In order to gain further insight, experiments should be designed which combine signalling studies with simulation studies, which take account of soil water potential, root contact area and water flux when calculating the water status at the root surface in the soil-plant-atmosphere-continuum.