© 2001 Heron Publishing—Victoria, Canada
Winter stem xylem pressure in walnut trees: effects of carbohydrates, cooling and freezing
Thierry Améglio (1), Frank W. Ewers (2), Hervé Cochard (1), Michel Martignac (3), Mark Vandame (1), Christian Bodet (1) and Pierre Cruiziat (1)
1. UMR PIAF (INRA—Univ. Blaise Pascal), Site INRA de crouelle, 234 av. du Brezet, F-63039 Clermont-Ferrand Cedex 2, France / 2. Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824-1312, USA / 3. Unité d'Agronomie, Site INRA de crouelle, 234 av. du Brezet, F-63039 Clermont-Ferrand Cedex 2, France / Received June 1, 2000
Summary
Pressure transducers were attached to twigs of orchard trees and potted trees of walnut (Juglans regia L.) to measure winter stem xylem pressures. Experimental potted trees were partially defoliated in the late summer and early
autumn to lower the amount of stored carbohydrates. Potted trees were placed in cooling chambers and subjected to various
temperature regimes, including freeze–thaw cycles. Xylem pressures were inversely proportional to the previous 48-h air temperature,
but positively correlated with the osmolarity of the xylem sap. Defoliated trees had significantly lower concentrations of
stored carbohydrates and significantly lower xylem sap osmolarities than controls. Plants kept at 1.5 °C developed xylem pressures
up to 40 kPa, just 7% of the theoretical osmotic pressure of the xylem sap. However, exposure to low, nonfreezing temperatures
followed by freeze–thaw cycles resulted in pressures over 210 kPa, which was 39% of the theoretical osmotic pressure. A simple
osmotic model could account for the modest positive winter pressures at low, nonfreezing temperatures, but not for the synergistic
effects of freeze–thaw cycles.
Keywords:
freezing, Juglans regia, pressure transducer, sap osmolarity, stem pressure.
