Is water stress induced embolism in grapevines a common phenomenon? Hints from MRI and microCT visualizations

Previous research suggested that grapevines are very vulnerable to water deficit induced embolism. Several authors found ~50% loss of conductivity (PLC) when stem water potentials (ΨS) where higher than -1 MPa. Because such ΨS are very common in vineyards, it was believed that drought induced cavitation is a frequent event. Moreover, since stomata regulation takes place at approximately similar ΨS, it was suggested that petiole cavitation could regulate stomatal closure. In the current research we performed in-vivo visualization of petioles and stems during dehydration using Magnetic Resonance Imaging (MRI) and synchrotron-based micro computed tomography (microCT). Additionally, an adjusted hydraulic protocol, that yielded comparable results to those of the visualization techniques, was utilized to compare PLC and stomatal closure in grapevines under water deficit. Our results show that grapevine petioles are more resistant to cavitation than previously presumed. 50% PLC or 50% embolized vessels were measured at ΨS of -1.3 to -1.5 MPa. Very few emboli (less than 10%) were noticed during the majority of stomatal down regulation (from 250 to less than 50 mmol m-2 s-1). Comparison of embolism in the stem and petiole showed that stems are even more resistant to embolism, reaching 50% embolized vessels at -2 MPa. These findings suggest that significant embolism does not commonly occur in grapevines. During water stress, stomatal down regulation will limit transpiration in order to avoid xylem cavitation. Only when that line of defense is breached, embolism in the petiole will hydraulically disconnect the leaves (i.e. evaporative surface) from the stem in order to maintain the stem’s hydraulic integrity.