This study reports experimental evidence for light-mediated changes of stem hydraulic conductance (Kstem) in field-grown laurel plants. Field measurements based on the evaporative flux method revealed that sun-exposed branches had 60% higher Kstem with respect to shade-exposed branches. Xylem sap potassium concentration was ∼3 mM as recorded in shaded branches and up to 12 mM in illuminated ones. Such a difference in [K+] proved to induce significant increase in xylem hydraulic conductance of excised twigs when artificially perfused with various solutions, as a likely consequence of the interaction of cations with the pectic matrix of pit membranes (the so-called ‘ionic effect’). We propose that this mechanism provides plants with a large potential for fine regulation of water flow towards different parts of the canopy exposed to different environmental conditions with the result of optimizing light and water utilization.