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JXB Advance Access originally published online on November 29, 2005
Journal of Experimental Botany 2006 57(1):139-147; doi:10.1093/jxb/erj016
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© The Author [2005]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

RESEARCH PAPER

High apoplastic solute concentrations in leaves alter water relations of the halophytic shrub, Sarcobatus vermiculatus

J. J. James1,*, N. N. Alder1 {dagger}, K. H. Mühling1 {ddagger}, A. E. Läuchli1, K. A. Shackel2, L. A. Donovan3 and J. H. Richards1

1Department of Land, Air, and Water Resources, University of California, Davis, CA 95616-8627, USA
2Plant Sciences, University of California, Davis, CA 95616, USA
3Plant Biology, University of Georgia, Athens, GA 30602, USA

* Present address and to whom correspondence should be sent: USDA-ARS, Eastern Oregon Agricultural Research Center, 67826-A Hwy 205, Burns, OR 97720, USA. E-mail: jeremy.james{at}oregonstate.edu

Predawn plant water potential ({Psi}w) is used to estimate soil moisture available to plants because plants are expected to equilibrate with the root-zone {Psi}w. Although this equilibrium assumption provides the basis for interpreting many physiological and ecological parameters, much work suggests predawn plant {Psi}w is often more negative than root-zone soil {Psi}w. For many halophytes even when soils are well-watered and night-time shoot and root water loss eliminated, predawn disequilibrium (PDD) between leaf and soil {Psi}w can exceed 0.5 MPa. A model halophyte, Sarcobatus vermiculatus, was used to test the predictions that low predawn solute potential ({Psi}s) in the leaf apoplast is a major mechanism driving PDD and that low {Psi}s is due to high Na+ and K+ concentrations in the leaf apoplast. Measurements of leaf cell turgor ({Psi}p) and solute potential ({Psi}s) of plants grown under a range of soil salinities demonstrated that predawn symplast {Psi}w was 1.7 to 2.1 MPa more negative than predawn xylem {Psi}w, indicating a significant negative apoplastic {Psi}s. Measurements on isolated apoplastic fluid indicated that Na+ concentrations in the leaf apoplast ranged from 80 to 230 mM, depending on salinity, while apoplastic K+ remained around 50 mM. The water relations measurements suggest that without a low apoplastic {Psi}s, predawn {Psi}p may reach pressures that could cause cell damage. It is proposed that low predawn apoplastic {Psi}s may be an efficient way to regulate {Psi}p in plants that accumulate high concentrations of osmotica or when plants are subject to fluctuating patterns of soil water availability.

Key words: Apoplast, Great Basin, nanolitre osmometer, predawn water potential, pressure probe, salinity, solute potential, turgor


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