In vivo observation of cavitation and embolism repair using magnetic resonance imaging

Magnetic resonance imaging (MRI) was used to noninvasively monitor the stat us of individual xylem vessels in the stem of an intact, transpiring grape (Vitis vinifera) plant over a period of approximately 40 h. Proton density- weighted MRI was used to visualize the distribution of mobile water in the stem and individual xylem vessels were scored as either water or gas filled (i.e. embolized). The number of water-filled vessels decreased during the first 24 h of the experiment, indicating that approximately 10 vessels had cavitated during this time. Leaf water potentials decreased from -1.25 to - 2.1 MPa during the same period. Watering increased leaf water potentials to -0.25 MPa and prevented any further cavitation. Refilling of xylem vessels occurred as soon as the lights were switched off, with the majority of ves sels becoming refilled with water during the first 2 to 3 h in darkness. Th ese measurements demonstrate that MRI can be used to monitor the functional status of individual xylem vessels, providing the first method to study th e process of cavitation and embolism repair in intact plants.