© 2004 Heron Publishing—Victoria, Canada
A potential role for xylem–phloem interactions in the hydraulic architecture of trees: effects of phloem girdling on xylem
hydraulic conductance
Maciej A. Zwieniecki (1, 2), Peter J. Melcher (1, 3), Taylor S. Feild (1, 4) and N. Michele Holbrook (1)
1. Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA / 2. Corresponding author (mzwienie@oeb.harvard.edu) / 3. Department of Biology, Ithaca College, Ithaca, NY 14853, USA / 4. Department of Botany, University of Toronto, Toronto, ON M5S 3B2, Canada / Received August 20, 2003; accepted January 18, 2004; published online June 1, 2004
Summary
We investigated phloem–xylem interactions in Acer rubrum L. and Acer saccharum Marsh. Our experimental method allowed us to determine xylem conductance of an intact branch by measuring the flow rate of
water supplied at two delivery pressures to the cut end of a small side branch. We found that removal of bark tissue (phloem
girdling) upstream of the point at which deionized water was delivered to the branch resulted in a decrease (24% for A. rubrum and 15% for A. saccharum) in branch xylem hydraulic conductance. Declines in hydraulic conductance with girdling were accompanied by a decrease in
the osmotic concentration of xylem sap. The decrease in xylem sap concentration following phloem girdling suggests that ion
redistribution from the phloem was responsible for the observed decline in hydraulic conductance. When the same measurements
were made on branches perfused with KCl solution (~140 mOsm kg–1), phloem girdling had no effect on xylem hydraulic conductance. These results suggest a functional link between phloem and
xylem hydraulic systems that is mediated by changes in the ionic content of the cell sap.
Keywords:
Acer rubrum, Acer saccharum, hydrogel, xylem sap ion concentration.
