Environmental changes due to global warming greatly influence ecological systems including those of plants. Diagnostic technologies using non-destructive inspection methods to evaluate environmental stress are few. Acoustic emission (AE) is applied to plants under water stress and the damage is measured. Cavitation phenomena are created by negative pressure in plants because of the lack of water in vessels or tracheids. Since shock waves due to cavitations create AE, the water stress state of plants can be monitored in real time by measuring AE. In this paper, AE events due to cavitation were measured in the drought-treated Ginkgo biloba L., Ficus spp., and Lycopersicon. In all plants examined, AE events increased during drought. In a short-term drought experiment, AE events were stopped by supplying water, but not in a long-term drought experiment. In Ginkgo biloba L. and Ficus spp, a temporary decrease of AE events was observed during drought. Plants adjust their internal water balance through certain self-defense mechanisms.