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A microcomputer-based system of continuously measures water stress in crop in a non-destructive measurement method was developed. Produced by cohesion between water molecules or adhesion to the conduit wall, the crop xylem cavitation was accompanied by the vessel walls vibration which is the indicator of the water stress. The physics of ultrasonic acoustic emissions (AEs) were used to measure the crop xylem cavitation events which were the transmission of vibration. The acoustic emissions could be measured both in the audio frequencies and in ultrasonic frequencies between 0.1 and 1 MHz. To optimize the cavitation measurement precision, the PCI-2 data acquisition equipment (DAQ) and R15 sensors, the Physical Acoustics Corp. (PAC), were adopted. The data recorded by the DAQ transferred the microcomputer to be processed and analysis. Some conclusions can be draw: cavitation induce AEs tend to shift to higher frequency when crop wood dehydration phenomena produces; the strongest frequencies of AEs in the range of 100-300KHz; to some extend, the frequencies and counts changed gradually with the environment of the temperature, illumination, humidity, concentration of carbon dioxide, transpiration; an interruption of water supply or over-irrigation can easily result in changing of the AEs Frequency and counts. The system could be useful for agronomists testing irrigation regulation and in search of non-destructive of recording the water stress under the greenhouse conditions.