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Harvard Forest Symposium Abstract 2007

• Title: The path of xylem flow differs depending on whether water is under pressure or tension
• Primary Author: Heidi Renninger (Boston University)
• Abstract:

  Many techniques used to measure hydraulic conductivity and vulnerability to embolism in woody species use positive pressure to push water through samples in order to measure flow rates and in turn calculate the conductivity of the sample. However, the cohesion-tension theory predicts that water in trees and other woody species is under tension and therefore is pulled through the conducting tissues. This study sought to determine whether water travels in the same path through the conducting tissues depending on whether it is pulled through the stem by the negative pressure of transpiration or pushed through the stem by positive pressure. We hypothesized that water under tension is inherently more cohesive and therefore will be less dispersive in its path than water under positive pressure. Likewise, because the anatomy of the conducting tissues in woody species is designed to support water under tension, the intervessel pit membranes may be stretched by positive pressure again leading to greater dispersion in the flow path. The study was performed at Tiputini Biodiversity Station on individual fronds from Iriartea deltoidea palms. After dusk, a hole was poked into a frond and a cut-off needle of a syringe (0.5 cm long) was placed into this insertion point. The needle was attached to an IV bag filled with gentian violet dye. The following morning the tubing of the IV bag was unclamped and the dye was allowed to be drawn into the conducting tissues along with the transpiration stream. That evening, the frond was cut from the tree and all of the leaves were removed. The frond was immediately brought into the lab and another syringe needle was introduced into this same insertion point. This needle was attached to an IV bag containing safranin (red) dye and this dye was introduced into the frond using positive pressure. The frond was then cut at various places along its length downstream of the insertion point to determine the location of the violet and red dye. The red and violet dyes were found in different vascular bundles within the frond with the dye that was introduced with positive pressure (red) being found in more vascular bundles than the dye under tension (violet). It appears that water under tension moves more directly to the leaves that are exerting the transpirational pull, while water pushed through the frond by positive pressure moves more diffusely. The next step in this study will be to perform this experiment on angiosperm and coniferous species at Harvard forest to determine if these same patterns are observed.

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• Figures:
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