Low soil temperatures induce water deficits in olive (Olea europaea) trees

Olive trees are often subjected to low temperatures during winter. To quantify the effects of low temperatures on the water relations of olive trees, we studied the responses to low soil temperatures on winter days of variable evaporative demand (ETo) in 1-year-old potted olive (Olea europaea L. cv. Picual) trees in 1996 and 1997. Low night (2.5 and 5.2 degrees C) but ambient day soil temperatures (above 10 degrees C) did not affect stomatal conductance (g(s)), leaf (Psi(leaf)) and stem (Psi(stem)) water potentials. Soil temperature levels inducing water stress in olive trees were determined for winter days with ETo typical for southern Spain (ETo = 1.5 +/- 0.3 mm day(-1)). Leaf and stem water potential decreased and root hydraulic resistance (r(root)) increased when trees were exposed to night and day soil temperatures below 10 degrees C. Stomatal conductance was not affected at soil temperatures between 6.4 and 10 degrees C, but decreased at temperatures below 6.4 degrees C. The soil temperature levels affecting the water uptake of olive trees remained relatively constant over the range of ETo of 1-2 mm day(-1) during winter and early spring months. However, the soil temperature influencing g(s) appeared to be more Variable and was affected by ETo. Olive tree recovery from low soil temperature stress depended on stress duration and severity and interacted with ETo. Recovery of Psi started already during the stress period, probably induced by stomatal closure and high r(root), thus allowing tree rehydration overnight. Root hydraulic resistance contributed the major part of whole-tree hydraulic resistance in response to cold stress, accounting for 76 and 89% at 6.4 and 4.6 degrees C, respectively; which indicates that r(root) is the primary control of the water status in olive trees under low temperatures.