PARENT SESSION
Poster Session #9: Water Relations.
Monday, August 6, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

The effect of phloem girdling on xylem hydraulic conductance.

Zwieniecki, Maciej ¹, Melcher, Peter¹, Feild, Taylor¹, Holbrook, Missy ¹, ¹

ABSTRACT- We investigated phloem/xylem interactions in two tree species (Acer rubrum and Acer saccharum) using an experimental approach that allows for in situ determination of branch hydraulic conductance. This technique determines the hydraulic conductance of the xylem 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 (girdling) upstream of the point at which deionized water was delivered resulted in a decrease (24.1% for A. rubrum and 15.4% for A. saccharum) in stem xylem hydraulic conductance. Declines in hydraulic conductance with girdling were accompanied by a decrease in the concentration of xylem sap. The decrease in xylem sap concentration following phloem girdling (but not observed in non-girdled stems) suggests that ion redistribution from the phloem might be responsible for the observed decline in hydraulic conductance. When the same measurements were made using a KCl solution (~140 mM), we observed no effect of phloem girdling on xylem conductance. These results suggest that a functional linkage might exist between phloem and xylem hydraulic systems, which may influence xylem hydraulic capacity and that this link is mediated by ion concentration.

KEY WORDS: xylem, hydraulic, phloem, girdling