Irrigation effects on daily and seasonal variations of trunk sap flow and leaf water relations in olive trees

Irrigation effects on whole-plant sap flow and leaf-level water relations were characterised throughout a growingseason in an experimental olive (Olea europaea L.) orchard. Atmospheric evaporative demand and soil moistureconditions for irrigated and non-irrigated olive trees were also monitored. Whole-plant water use in field-grownirrigated and rain fed olive trees was determined using a xylem sap flow method (compensation heat-pulse velocity).Foliage gas exchange and water potentials were determined throughout the experimental period. Physiologicalparameters responded diurnally and seasonally to variations in tree water status, soil moisture conditions andatmospheric evaporative demand. There was a considerable degree of agreement between daily transpiration deducedfrom heat-pulse velocity and that determined by calibration using the Penman-Monteith equation in the field.Summer drought caused decreasing leaf gas exchange and water potentials, and a progressive increase in hydraulicconductance (stronger in non-irrigated than irrigated trees), probably attributable to modifications in hydraulicproperties at the soil-root interface. Negligible hysteresis, attributable to low plant capacitance, was observed in therelationship between leaf water potential and sap flow. A proportional decrease in maximum daily leaf conductancewith increasing vapour pressure deficit was observed, while mean daytime canopy stomatal conductance decreasedwith the season. As a result, plant water use was limited and excessive drought stress prevented. Non-irrigatedolive trees recovered after the summer drought, showing a physiological behaviour similar to that of irrigated trees.In addition to physiological and environmental factors, there are endogenous keys (chemical signals) influencingleaf level parameters. Olive trees are confirmed to be economical and sparing users of soil water, with an efficientxylem sap transport, maintenance of significant gas exchange and transpiration, even during drought stress.