Winter stem xylem pressure in walnut trees: effects of carbohydrates, cooling and freezing

Pressure transducers were attached to twigs of orchard trees and potted tre es of walnut (Juglans regia L.) to measure winter stem xylem pressures. Exp erimental potted trees were partially defoliated in the late summer and ear ly autumn to lower the amount of stored carbohydrates. Potted trees were pl aced in cooling chambers and subjected to various temperature regimes, incl uding freeze-thaw cycles. Xylem pressures were inversely proportional to th e previous 48-h air temperature, but positively correlated with the osmolar ity of the xylem sap. Defoliated trees had significantly lower concentratio ns of stored carbohydrates and significantly lower xylem sap osmolarities t han controls. Plants kept at 1.5 degreesC developed xylem pressures up to 4 0 kPa, just 7% of the theoretical osmotic pressure of the xylem sap. Howeve r, exposure to low, nonfreezing temperatures followed by freeze-thaw cycles resulted in pressures over 210 kPa, which was 39% of the theoretical osmot ic pressure. A simple osmotic model could account for the modest positive w inter pressures at low, nonfreezing temperatures, but not for the synergist ic effects of freeze-thaw cycles.