Air embolisms exsolving in the transpiration water – the effect of constrictions in the xylem pipes
Martin J. Canny A E , Jed P. Sparks B , Cheng X. Huang C and Michael L. Roderick DA Ecosystem Dynamics Group, Research School of Biological Sciences, Australian National University, Canberra, ACT 0200, Australia.
B Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853, USA.
C Electron Microscopy Unit, Research School of Biological Sciences, Australian National University, Canberra, ACT 0200, Australia.
D Environmental Biology Group, Research School of Biological Sciences, Australian National University, Canberra, ACT 0200, Australia.
E Corresponding author. Email: martin.canny@anu.edu.au
Functional Plant Biology 34(2) 95-111 https://doi.org/10.1071/FP06210
Submitted: 30 August 2006 Accepted: 9 January 2007 Published: 12 February 2007
Abstract
When water flows through a constriction, air can come out of solution (i.e. it can exsolve). This phenomenon is manifested in the transpiration stream of plants. Observations of gas in functioning xylem prompted a hypothesis predicting the daily balance between air and water in wood: a sudden fall in water content at sunrise, followed by an increase in water content during the day. An extended record by time domain reflectometry of volumetric water content (VWC) every 2 h throughout a summer shows the detailed pattern of change of VWC during 25 individual days, giving good agreement with the hypothesis. This hypothesis has wide-ranging consequences for experiments using cut plant parts. Perfusing aqueous solutions through excised xylem also can exsolve air from the water, causing declines in flow. The location of such air was investigated in cryo-fixed perfused vine stems by cryo-scanning electron microscopy. Bubbles formed at residual walls of perforation plates in small vessels, and filled many large vessels. The input surface is revealed as a major source of exsolved air. Precautions to reduce this effect are outlined and discussed.
Additional keywords: cryoSEM, de-aerated water, declining xylem flow, Kelso interface, pressure gradient, TDR measurements, perforation plates, wood water content.
Acknowledgements
We thank Marilyn Ball, Margaret McCully and Paul Kriedemann for comments on early drafts of the manuscript, and Stewart Turner for helpful discussions and for comments on a later draft.
Berger W
(1931) Das Wasserleitungssystem von krautigen Pflanzen, Zwergsträuchen und Lianen in quantitativer Beachtung. Beihefte zum botanischen Centralblatt 438, 363–390.
and is acceptable, but usually implies a chemical reaction that evolves a product. The word ‘exsolution’ is used by geologists to describe the separation of distinct crystal phases within an initially homogeneous solution in alloys and some minerals (Oxford 1993) and, ‘hence more widely, the separating out of any phase from a parent composite solution’. The verb is ‘exsolve’. These terms will be used in this paper.
