Visualization of Xylem Sap Flow and Freezing Behaviors in Trees Using MRI

1H MRI was applied to non-invasively observe tree water responses (xylem sap flow and freezing behaviors) under stress and to study their dynamics, diversity, and mechanisms. Two compact MRI systems using 1.0 and 0.3 T permanent magnets were developed to monitor xylem embolism progression non-destructively during pine wilt disease. The 0.3 T MRI with a U-shaped RF coil visualized three-dimensional embolism development by vertically shifting the imaging position using a hand lift. The 1.0 T MRI monitored xylem embolism progression under drought in conifer and hardwood seedlings and differences in the vulnerability of xylem conduits to embolism among tree rings. Outdoor MRI enabled in situ visualization of faster xylem sap flow in current-year earlywood than in latewood vessels in a mature ring-porous tree using the q-space method. The basic principles and methodology of cryo-MRI (freezing temperature MRI) using superconducting magnets are detailed. Cryo-MRI detects the distribution/concentration of “unfrozen” water in organs, which allows visualization of organ/tissue water response/management at freezing temperatures (freezing behavior or strategy). MRI successfully imaged extraorgan freezing in winter buds and the diversity and underlying mechanisms of species-/tissue-specific freezing behaviors. MRI is a powerful tool for studying tree stress physiology at the tissue/organ level during climate change.