In this work, an online sampling of plant xylem sap combined with an efficient (CE)-based method was developed and applied to study the kinetics of changes in the sap composition and to assess plant fitness under stress conditions comprehensively. A laboratory-built CE device was developed to provide online sampling and CE analysis of various ionogenic species in the sap during plant stress response. The rapid online sampling and short CE analysis time allow for real-time monitoring of changes in sap constituents in the living plant during the stress response. The developed device was successfully used to analyze chloride, nitrate, and sulfate ions in the plant xylem during the salt stress or stress caused by nitrate deficiency within short time scales.