Microstreaming shear as a mechanism of cell death in Elodea leaves exposed to ultrasound

The hypothesis that ultrasonically induced membrane damage and cell death in Elodea leaves is caused by shear stress associated with microstreaming flow generated in the vicinity of oscillating gas-filled channels between the cells is investigated. Cell death thresholds as a function of frequency seem to follow a condition of constant shear stress, with minimum thresholds near the resonance frequency of a gas-filled channel. Theoretical estimates of the shear stress generated within the cells at the ultrasonic intensity of the cell death threshold are in order of magnitude agreement with measurements of the shear stress required for lysis of blood cells. Furthermore, the dependence of the cell death thresholds on exposure duration seems to correspond roughly with the dependence of critical shear stress for blood cells as a function of the duration of the stress. Membrane damage induced by microstreaming shear stress therefore appears to be a plausible mechanism of cell death in Elodea, and a valuable unifying concept for consideration of bioeffects of ultrasonic cavitation.