The objective of this study was to quantify the effect of potassium (K) supply on osmotic adjustment and drought avoidance mechanisms of Eucalyptus seedlings growing under short-term water stress. The effects of K supply on plant growth, nutritional status, leaf gas exchange parameters, leaf water potential (Psi(w)), leaf area (LA), stomatal density (SD), leaf carbon (C) and nitrogen (N) isotopic compositions (delta C-13 and delta N-15 parts per thousand) and leaf C/N ratio under polyethylene glycol (PEG)-induced water deficit were measured. Under both control (non-PEG) and osmotic stress (+PEG) conditions, K supply increased plant growth, boosting dry matter yield with decreased C/N leaf ratio and delta N-15 parts per thousand values. The +PEG significantly reduced LA, plant growth, dry matter yield, Psi(w), number of stomata per plant and leaf gas exchange, relative to non-PEG condition. Potassium supply alleviated osmotic-induced alterations in Eucalyptus seedlings by better regulating leaf development as well as SD, thus improving the rate of leaf gas exchange parameters, mesophyll conductance to CO2 (lower delta C-13 parts per thousand values) and water use efficiency (WUE). Consequently, K-supplied plants under drought better acclimated to osmotic stress than K-deficient plants, which in turn induced lower CO2 assimilation and dry matter yield, as well as higher leaf delta C-13 parts per thousand and delta N-15 parts per thousand values. In conclusion, management practices should seek to optimize K-nutrition to improve WUE, photosynthesis-related parameters and plant growth under water deficit conditions.< Réduire