Osmotic adjustment leads to anomalously low estimates of relative water content in wheat and barley
John S. Boyer A C , Richard A. James B , Rana Munns B , Tony (A. G.) Condon B and John B. Passioura BA College of Marine and Earth Studies and College of Agriculture and Natural Resources, University of Delaware, 700 Pilottown Road, Lewes, DE 19958, USA.
B CSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia.
C Corresponding author. Email: boyer@udel.edu
Functional Plant Biology 35(11) 1172-1182 https://doi.org/10.1071/FP08157
Submitted: 27 May 2008 Accepted: 1 August 2008 Published: 28 November 2008
Abstract
Relative water content (RWC) is used extensively to determine the water status of plants relative to their fully turgid condition. However, plants often adjust osmotically to salinity or water deficit, which maintains turgor pressure and obscures the definition of ‘full turgidity’. To explore this problem, turgor was measured by isopiestic psychrometry in mature leaf blades of barley (Hordeum vulgare) and durum wheat (Triticum turgidum ssp. durum) salinised to 150 mm NaCl, or bread wheat (Triticum aestivum) grown in soil dehydrated to varying degrees. Osmotic adjustment maintained turgor in all the plants but despite full maintenance in some of the salinised plants, their leaf RWC decreased substantially. This occurred because excess water was absorbed while the samples were floated on water as part of the RWC measurement. The absorption falsely increased the weight of the ‘fully turgid’ condition, causing RWC to be anomalously low by 10–15%. Cell solution was secreted into intercellular spaces and was seen under a microscope, which is a test encouraged for all RWC measurements. Several alternate methods are suggested for rehydrating tissues while minimising excess water absorption, but no simple definition of ‘full turgidity’ seems possible. In general, direct measurements of osmotic adjustment and turgor are preferred.
Additional keywords: drought, osmotic potential, relative turgidity, salinity, salt stress, water potential.
Acknowledgements
Thanks are extended to Plant Industry at CSIRO for financial support of JSB during this study. Professor Carlos Trejo of Programa de Botanica, Colegio de Postgraduados, Montecillo, Estado de Mexico 56230, Mexico contributed to preliminary measurements leading to this work.
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