Over the last decade, synchrotron based X-ray micro computed-tomography (microCT) has emerged as a valuable tool to gain new insights into fundamental aspects of plants responses to drought stress. Our results from microCT have contributed to an ongoing and contentious debate regarding methods used to assess plant hydraulic responses to drought stress. Numerous studies have now shown that methods commonly used to assess xylem vulnerability to drought-induced cavitation on excised samples can introduce artifacts and misrepresent the functional status of xylem conduits in vivo for some species. Furthermore, numerous microCT studies have shown that few if any woody plant species undergo diurnal cycles of embolism formation and repair during the growing season, including grapevines. We also recently found that tissue compartments for capillary water storage refill in excised stems but rarely under in-vivo conditions for some species, which questions the paradigm that fiber water storage plays in daily discharge/recharge of stem capacitance in intact trees.
Results/Conclusions
Responses seem to vary between species, and we will first summarize results from a variety of studies to demonstrate how water content of various organs and xylem cell types can change temporally and how the spatial distribution of air-filled tissue may impact patterns of sap flow within the xylem network. Results from microCT will be compared to that from traditional hydraulic and sap flow methods to illustrate potential discrepancies particularly when comparing data from excised stems versus intact plants, and will present results from a spatially explicit model to demonstrate how changes in the water content of various cell types can impact resulting interpretation of sensor output. We will also present results of a recently developed method to track starch content in vivo using microCT and a machine learning algorithm; this novel method can successfully characterize the spatial distribution of starch concentration in living stems at micrometer resolution and track active starch depletion from storage pools under stress.