Early signals in field grown wheat in response to shallow soil drying

The aim of the present work was to test under field condition earlier clima te room findings that root-home signals may control stomatal closure and le af extension rate during mild soil drying. Stomatal conductance (g(H2O)) Of flag leaves, leaf and stem extension rates, leaf water relations, leaf ABA content and predawn xylem [ABA] were measured daily in wheat grown in two soil types (sand and loam) in lysimeters in the field during a period of so il drying with high and low evaporative demands. At 3 days after withholdin g irrigation (DAI) on both soil types, when soil water potential (Psi(soil) ) in the upper soil profile of the droughted treatment had dropped to -70 k Pa and with the lower layers still at field capacity, predawn xylem [ABA] i ncreased. At 4 DAI the leaf extension rate decreased and midday leaf ABA co ntent increased. Stem extension rates decreased at 5 DAI in loam and 7 DAI in sand. g(H2O) Started to decrease 6 DAI in loam and 9 DAI in sand. These responses were observed before any detectable decrease in the midday leaf w ater status of the droughted plants had occurred relative to well-watered p lants. The responses were closely related to Psi(soil) and independent of e vaporative demand. We conclude that, in wheat, root-borne signals probably control stomatal conductance and leaf extension rate during mild soil dryin g in the field.