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To achieve the previsual detection on nitrogen(N) stress, the microelectrode technique was used to measure the membrane potential, membrane capacitance and light induced responses of leaf membrane potentials under four different levels of Ntreatment, respectively. No obvious difference was found in the membrane potential between Nstress and Nsufficient cucumber leave. It shows that the membrane potentials were not suitable for early diagnosis of Nstress in cucumber plants. The research on the changes of membrane capacitance found that it was sensitive to Ndeficiency. The measurement of membrane capacitance could be used for previsual diagnosis of severe and moderate Ndeficiency 2d before the appearance of visually morphological changes. The research on the changes of magnitude of leaf membrane potential response to light found that it was sensitive to Nexcess. The measurement of the magnitude of leaf membrane potential response to light could be used for previsual diagnosis of Nexcess at least 7d before the appearance of visually morphological changes. It shows that the microelectrode technique is suitable for early diagnosis of Nstress in cucumber plants.