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Harvard Forest Symposium Abstract 2006
- Title: Dynamic changes in xylem hydraulic resistance via modulation of xylem sap ionic composition
- Primary Author: Brendan Choat (Harvard University)
- Additional Authors: Jeanne Anderson (University of New Hampshire - Main Campus); Noel Michele Holbrook (Harvard Forest); Maciej Zwieniecki (Arnold Arboretum of Harvard University)
- Abstract:
The potential for daily fluctuations in xylem sap ionic concentration to influence the distribution of resistance through the crown of trees and shrubs of Acer saccharum Marsh. was investigated. We measured diurnal changes in the concentration of xylem sap K+ concentration in upper and lower areas of the crown of mature trees and in the subcanopy sapling. Xylem sap K+ concentration was also monitored in saplings that experienced diurnal variation in light levels. Measurements of K+ concentration were compared with bench top measurements of flow rate in branch sections perfused with increasing concentration of KCl to determine how much changes in xylem K+ could alter hydraulic resistance. Flow rate through detached branch sections increased in a log-linear fashion with concentration of KCl in the perfusing solution. We propose that this ion mediated increase in flow rate was effected by shrinking and swelling of pectin polysaccharides in intervessel pit membranes. Xylem sap K+ concentrations varied from 1.33 to 4.39 mmol in branches of the upper canopy and there was a significant increase (P < 0.05) in the concentration of K+ from night time to day time. The concentration of K+ was between 0.76 and 4.29 mmol in lower canopy branches and 0.25 to 3.09 mmol in subcanopy saplings, however, there was no trend between day time and night time K+ concentration in these canopy regions. Potential flow enhancement under natural circumstances was calculated from the range of xylem sap K+ observed from each canopy region and the flow enhancement measured on detached braches. Based on the maximum and minimum xylem sap concentrations observed over the day, the potential alteration in hydraulic resistance ranged from 7.5% in the upper canopy to 16.8% in subcanopy shrubs. In saplings exposed to diurnal variation in light levels, branches in full sunlight had a mean xylem sap K+ of 1.65 mmol, significantly higher (P < 0.05) than K+ in branches in the in full shade, 0.95 mmol, or in branches harvested prior to dawn, 0.88 mmol. The potential change in resistance between sun and shade for these mean values was 6.8%, while the increase for maximum and minimum values observed was 32%. There were also significant linear relationships between changes in xylem sap K concentration and stomatal conductance, transpiration and photosynthesis raising the possibility of a feedback loop between increased evaporative demand and light at the leaf surface and changes in hydraulic resistance in the xylem. Decreases in hydraulic resistance observed in the range observed in this study represent a significant modification of resistance through the branch net work of the crown. This indicates that A. saccharum has the capacity to actively influence the distribution of resistances through the crown in response to microclimatic variation by modifying xylem sap ionic concentrations.
- Research Category: Physiological Ecology, Population Dynamics, and Species Interactions
