A new validation of the Scholander pressure chamber technique based on stem diameter variations

The Scholander pressure chamber is one of the most widely used techniques f or measuring plant water status. However, the technique has been the subjec t of recent controversies, and its validity awaits new experimental evidenc e. This paper presents a new test based on the analysis of the dependence o n water potential difference (Delta psi) of stem diameter variation (DeltaD ) in walnut (Juglans regia L.). The correlation between Delta psi and Delta D was established (1) on transpiring potted trees, (2) on dehydrating cut b ranches, (3) by perfusing the xylem of branch segments with mannitol and su crose solutions, and (4) by pressurizing segments in a pressure sleeve. The Delta psi was respectively assessed with a pressure chamber (1,2), a freez ing point osmometer (3) and an air pressure transducer (4). A single relati onship was established between Delta psi (ranging from 0 to -2 MPa) and Del taD for all the experiments. This shows that the measured changes of water potential were correlated to similar modifications of water content in the stems, irrespective of the technique used to induce these changes, and ther efore validates the pressure chamber technique and confirms the occurrence of large negative pressures in the xylem of walnut branches.