Received August 31, 2006
Accepted November 5, 2006
Modification of Leaf Apoplastic pH in Relation to Stomatal Sensitivity to Root-Sourced ABA Signals
Wensuo Jia and William John Davies *
State Key Laboratory of Plant Biochemistry, Department of Horticulture, College of Agronomy and Biotechnology, China Agricultural University, Beijing 100094, China
Department of Biological Sciences, Lancaster Environment Centre, University of Lancaster, Lancaster LA1 4YQ, UK
* Corresponding author; email: w.davies{at}lancaster.ac.uk.
The Confocal microscope was used to determine the pH of the leaf apoplast and the pH of micro-volumes of xylem sap. We quantified variation in leaf apoplast and sap pH in relation to changes in edaphic and atmospheric conditions which impacted on stomatal sensitivity to a root-sourced ABA 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 mid rib 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 alkalinising 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 and Commelina did not change significantly as soil dried while pH of tomato sap increased as water availability in the soil declined. Increasing the availability of nitrate to roots also significantly alkalinised 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 ABA-based mechanism.
