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Title:   Author  item
                                            KNIPFER, THORSTEN - University Of Californiaitem
                                            BRODERSEN, CRAIG - Yale Universityitem
                                            ZEDAN, AMR - University Of Californiaitem
                                            Kluepfel, Danielitem
                                            McElrone, Andrew
  Submitted to: Tree Physiology
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 4/27/2015
                                    Publication  Date: 6/9/2015
                                    Citation: Knipfer, T., Brodersen, C., Zedan, A., Kluepfel, D.A., Mcelrone, A.J. 2015. Patterns of drought-induced embolism formation and spread in living walnut saplings visualized using x-ray microtomography. Tree Physiology. doi: 10.1093/treephys/tpv040.Interpretive Summary: Drought stress can cause water transport tissue (xylem) in plants to become blocked with air bubbles (embolism). Walnuts are susceptible to drought-induced embolism, but details of this phenomenon in living plants are unknown.  We used micro-computed tomography (a type of CT scan) to look inside walnut xylem subjected to drought and recovery after rewatering. We found that the walnut saplings exhibit little to no embolism under low-moderate water stress. Limited vulnerability was related to minimal interconnectedness of the xylem network (i.e. tubes are not interconnected which limits air spread once formed). This information can be used to modify rapid trait evaluation using more accessible hydraulic methods in order to improve drought resistance in walnut rootstocks.
                                Technical Abstract: During drought, xylem conduits are susceptible to hydraulic dysfunction caused by cavitation and gas embolism. Embolism formation and spread within xylem is dependent on conduit structure and network connectivity, but detailed spatial analysis has been limited due to a lack of non-destructive methods to visualize these processes in living plants. We used synchrotron x-ray computed tomography (microCT) to visualize these processes in vivo for walnut (Juglans microcarpa) saplings subjected to soil dry down, and also evaluated embolism repair capability after rewatering. Initial cavitation was not detected until stem water potentials ('stem) reached -2.2 MPa, and on average 50% of vessels remained functional at 'stem of -4.0 MPa. Visual assessment of vulnerability to cavitation with microCT was quantitatively similar to measurements obtained with the centrifuge method, except that the latter measured some embolism (10-20%) at tensions below 2 MPa. Initial cavitation often appeared in isolated vessels that were not connected to other air-filled conduits or cells. Once significant levels of embolism started to form in a given plant, several vessels in close proximity would cavitate concurrently even though they appeared unconnected and separated tangentially by rays from a transverse perspective. 3D analysis revealed that vessel drift (i.e. twisting of vessel) created some connections between cavitated vessels at some further distance along the stem axis. Automated network analysis on dried segments confirmed that most vessels (~80%) had no direct connections to other vessels in the segment. Spatially, vessels cavitated randomly throughout plant samples; the number of vessels cavitated was equal in each of the xylem regions, while the vessels that cavitated were slightly but significantly larger than the overall mean vessel diameter of the entire stem section. We found no evidence for vessel refilling even two weeks after rewatering despite a rapid recovery of plant water status and obvious rehydration of tissue outside the xylem. Hence, low embolism susceptibility in J. microcarpa is linked to a high frequency of small diameter vessels and low network connectivity, which may aid sapling survival during establishment.


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