Responses of Olive Trees to Low Soil Temperatures

Responses to low soil temperatures at winter days of high evaporative demand were studied in 20-year-old (fi eld) and 1-year-old potted (controlled conditions) olive ( Olea europaea L. cv. Picual) trees in 1996 and 1997. Low soil temperatures apparently affected tree water status as evidenced by low water potentials and stomatal conductance. Low night (2 and 5°C) but ambient day (above 10°C) temperatures did not affect stomatal conductance (g l ), leaf (ψ l ), and xylem (ψ x ) water potentials of potted olive trees. Tree ψ l and ψ x decreased when exposed to low night and day temperatures (8°C), but g l was not affected. Water potential of those trees recovered very rapidly when the soil temperature was raised above 10°C at midday. When the trees were exposed to soil temperatures below 8°C for 3 days, ψ l , ψ x , and g l immediately decreased. After the first day, g l and ψ x started to recover while g l was maintained at low levels, thus allowing for tree rehydration. Root hydraulic resistance (r root )—a major part of whole plant resistance—increased immediately in response to soil temperatures below 10°C relative to that of control trees. The relationship between ψ x and r root indicated that the root system apparently plays a mayor role in the control of tree water status in response to low soil temperatures. During the winter months, olive tree water uptake seems to be primarily limited by low soil temperatures, even though soil water content is normally adequate due to high seasonal rainfall.