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First published online November 10, 2006; 10.1104/pp.106.089110 Plant Physiology 143:68-77 (2007) © 2007 American Society of Plant Biologists
Modification of Leaf Apoplastic pH in Relation to Stomatal Sensitivity to Root-Sourced Abscisic Acid Signals1State Key Laboratory of Plant Biochemistry, Department of Horticulture, College of Agronomy and Biotechnology, China Agricultural University, Beijing 100094, China; and Department of Biological Sciences, Lancaster Environment Centre, University of Lancaster, Lancaster LA1 4YQ, United Kingdom
The confocal microscope was used to determine the pH of the leaf apoplast and the pH of microvolumes of xylem sap. We quantified variation in leaf apoplast and sap pH in relation to changes in edaphic and atmospheric conditions that impacted on stomatal sensitivity to a root-sourced abscisic acid signal. Several plant species showed significant changes in the pH of both xylem sap and the apoplast of the shoot in response to environmental perturbation. Xylem sap leaving the root was generally more acidic than sap in the midrib and the apoplast of the leaf. Increasing the transpiration rate of both intact plants and detached plant parts resulted in more acidic leaf apoplast pHs. Experiments with inhibitors suggested that protons are removed from xylem sap as it moves up the plant, thereby alkalinizing the sap. The more rapid the transpiration rate and the shorter the time that the sap resided in the xylem/apoplastic pathway, the smaller the impact of proton removal on sap pH. Sap pH of sunflower (Helianthus annuus) and Commelina communis did not change significantly as soil dried, while pH of tomato (Lycopersicon esculentum) sap increased as water availability in the soil declined. Increasing the availability of nitrate to roots also significantly alkalinized the xylem sap of tomato plants. This nitrogen treatment had the effect of enhancing the sensitivity of the stomatal response to soil drying. These responses were interpreted as an effect of nitrate addition on sap pH and closure of stomata via an abscisic acid-based mechanism.
1 This work was supported by the National Basic Research Program of China (grant no. 2003CB114300), by the National Natural Science Foundation of China (grant no. 30470160), by the Department of Environment, Food and Rural Affairs, UK, and by the Royal Society of London. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: William J. Davies (w.davies{at}lancaster.ac.uk). www.plantphysiol.org/cgi/doi/10.1104/pp.106.089110 * Corresponding author; e-mail w.davies{at}lancaster.ac.uk; fax 441524510217. Received August 31, 2006; accepted November 5, 2006; published November 10, 2006. This article has been cited by other articles:
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