Concomitant measurements of stem sap flow and leaf turgor pressure in olive trees using the leaf patch clamp pressure probe

Stem sap flow (Q) and leaf turgor pressure (Pc) were measured simultaneously on 4-year-old, 2.4 m tall ‘Arbequina’ olive trees in a hedgerow orchard. Measurements were performed on well-watered control trees as well as on 60RDI and 30RDI trees (RDI = regulated deficit irrigation). The 60RDI trees received 59.2% of the crop water needs (ETc), and the 30RDI trees received 29.4% of ETc. Pc was determined non-invasively using the magnetic leaf patch clamp pressure probe (ZIM probe). The patch pressure Pp measured by the probe is inversely correlated with turgor pressure at Pc > ca. 50 kPa. Pc is coupled with xylem pressure; thus Pp yields information about the development of tension in xylem. In the case of the control trees a positive correlation between Q and Pp was generally found, i.e. Q increased usually with increasing Pp and decreased with decreasing Pp, as expected. However, Q peaking did not always coincided with Pp peaking at noon. Occasionally, Q peaking preceded or followed Pp peaking with a time difference of up to 3 h in both cases. Under some circumstances, the onset of Q after sunrise was greatly delayed, even though a pronounced increase of Pp was observed. A delayed onset of Q after sunrise resulted in hysteresis phenomena, i.e. the linear increase of Q and Pp in the morning hours did not coincide with the corresponding decrease of Q and Pp in the afternoon. The development of severe water stress (Pc < ca. 50 kPa) associated with the increase in the intercellular spaces of the spongy tissue in the leaves resulted in inverted diurnal Pp curves, i.e. minimum Pp values were recorded at noon and maximum Pp values during the night on the 30RDI trees. The effects were reversible as shown by re-watering. By contrast, the magnitude of Q decreased continuously from the turgescent state to the state of severe water stress; maximum Q values were still recorded around noon. The data suggests that short-range tension forces are responsible for water lifting in olive trees and that water uptake from water storage reservoirs must play an important role in the supply of the leaves with water. Furthermore, for setting of irrigation thresholds the finding of shape changes of the Pp curves upon severe water stress seems to be a useful indicator. Such shape changes are detected and monitored more sensitively than changes in the magnitude of sap flow rates or of turgor pressure.