PARENT SESSION
Oral Session #74: Plant Water Relations: Hydraulics and use.
Presiding: B. Engelbrecht
Thursday, August 8. 9:00 AM to 11:30 AM. Greenlee Meeting Room, TCC.
Branch junctions and the flow of water through xylem in Douglas-fir and ponderosa pine stems.
Schulte, Paul*,1, Brooks, J Renee2, 1 University of Nevada, Las Vegas, NV2 Western Ecology Division, Corvallis, OR
ABSTRACT- Branch junctions in woody plants represent a complexity in the flow of water from roots to leaves. One aspect of this complexity concerns the interconnectivity of the xylem tracheids in the stem leading to tracheids in the branches. If flow into a branch changes, perhaps by stomatal reductions, can an adjacent branch take advantage of increased capacity in the stem? This question was addressed in experiments with Y-shaped branch junctions in ponderosa pine and Douglas-fir trees. Junctions were excised from branches of 20-30 year-old trees and connected into a system for measuring flow under an applied pressure. Flow through one branch was stopped while observing for flow changes in the adjacent branch. Hydraulic conductance of the branches and main stem was measured to predict flow based on a branch model as completely interconnected (radial flow between tracheids allows a branch to utilize capacity made available by reduced flow in an adjacent branch) or as independent (xylem below the junction is "committed" to one branch because radial connectivity is low). Stopping flow through one branch lead to an increase in flow through the adjacent branch (10 - 40% increase), however these increases were lower than predicted for a completely interconnected system (30 - 100% increase predicted). These results suggest that branches are partially interconnected within the main stem, but not completely - a distinction of consequence for the interaction between branches in response to changing light or other variables affecting water loss from leaves.
KEY WORDS: xylem, conductance, branch, water