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JXB Advance Access published online on April 16, 2008
Journal of Experimental Botany, doi:10.1093/jxb/ern064
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© 2008 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)

RESEARCH PAPER

Root hydraulic conductivity measured by pressure clamp is substantially affected by internal unstirred layers

Thorsten Knipfer * and Ernst Steudle{dagger}

Department of Plant Ecology, Bayreuth University, D-95440 Bayreuth, Germany

{dagger} To whom correspondence should be addressed. E-mail: ernst.steudle{at}uni-bayreuth.de

Using the root pressure probe in the pressure clamping (PC) mode, the impact of internal unstirred layers (USLs) was quantified for young corn roots, both in experiments and in computer simulations applying the convection/diffusion model of Knipfer et al. In the experiments, water flows (JVrs) during PC were analysed in great detail, showing that JVrs (and the apparent root hydraulic conductivity) were high during early stages of PC and declined rapidly during the first 80 s of clamping to a steady-state value of 40–30% of the original. The comparison of experimental results with simulations showed that, during PC, internal USLs at the inner surface of the endodermis substantially modify the overall force driving the water. As a consequence, JVr and Lpr were inhibited. Effects of internal USLs were minimized when using the pressure relaxation mode, when internal USLs had not yet developed. Additional stop-clamp experiments and experiments where the endodermis was punctured to reduce the effect of internal USLs verified the existence of internal USLs during PC. Data indicated that the role of pressure propagation along the root xylem for both PC and pressure relaxation modes should be small, as should the effects of filling of the capacities during root pressure probe experiments, which are discussed as an alternative model. The results supported the idea that concentration polarization effects at the endodermis (internal USLs) cause a serious problem whenever relatively large amounts of water (xylem sap) are radially moved across the root, such as during PC or when using the high-pressure flow meter technique.

Key words: Convection/diffusion model, high-pressure flowmeter (HPFM), pressure clamp, pressure propagation, pressure relaxations, root pressure probe, simulations, storage capacity, unstirred layers (USLs), Zea mays L

* Present address: UCD School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland.

Received 9 January 2008; Revised 9 January 2008 Accepted 11 February 2008


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