Authored by: Michaela DiGiovanni Hold a single pine needle in your hand. What do you see? Better yet, what if you pulled out your pocketknife and sliced a thin sliver from the center of that needle? That’s the part that Warnell associate professor Dan Johnson is looking at. That small green speck in your hand holds information on that tree’s health—for example, how it moves water through its systems or how it’s responding to drought or fire. Johnson is one of the world’s experts in understanding these systems, and using the latest imaging technologies he’s been able to dive even deeper into trees’ water transportation systems. As director of the Tree Physiology Lab at Warnell, Johnson’s research focuses on plant hydraulics, seedling germination and drought resistance. By understanding how trees transport water through their systems, he and his team are learning more about how different species can overcome drought-induced stress. Part of this process involves making images of microscopic connections, documenting when they fail and connecting these failures to environmental factors. Because when we understand why trees die, we can better understand how we can help them thrive. The following images were selected from Johnson’s past decade of work. Earlier pictures were taken with magnetic resonance imaging, while more recent images come from a high-powered X-ray called a synchrotron to show extremely detailed cross-sections of needles and other parts of trees. His team also makes corresponding color images with a laser, using a process called confocal microscopy. Different cells reflect in different colors, creating a rainbow of circles that researchers can use to better identify parts of the stem. A closer look PONDEROSA PINE IN A NEW LIGHT This image shows ponderosa pine vascular tissue imaged using fluorescence. The tissue in red is the vein xylem, the tissue responsible for moving water in the plant. Created in 2007, the two-dimensional image was made using fluorescence microscopy. WHERE XYLEM ONCE RULED This image of the vein of a leaf from Golden Chinkquapin (Chrysolepis chrysophylla) was created using cryo-scanning electron microscopy (Cryo-SEM). Cryo-SEM is done at temperatures around -130 degrees Celsius, while normal scanning electron microscopy is done at around 25 degrees Celsius. Cryo-SEM keeps all the water in a frozen state, and empty xylem conduits can be seen in black. These empty xylem conduits show that this xylem has formed cavities due to drought and is no longer functional. EXTREME CLOSE-UP This close-up Cryo-SEM image of a Chrysolepis vein shows individual xylem conduits. This detail captures the helical thickenings on the walls of the xylem conduits. FREEZE FRAME This is a Cryo-SEM of a ponderosa pine needle that has been freeze-fractured. This is fully hydrated, so there are no non-functional xylem conduits. RECONSTRUCTED ROOTS These 3-D reconstructions were created from microCT scans of roots from Texas live oak, left, (Quercus fusiformis) and gum bumelia (Sideroxylon lanuginosum). Conduits in yellow are connected; Texas live oaks have very few conduit-conduit connections, while gum bumelia has many. This has many functional implications for water transport and the spread of disease. Slide/Banner Caption: This composite image shows the cross-section of a ponderosa pine sapling that had been burned two years prior to the images being made. At the top and bottom note the irregular xylem formed when a tree tries to close a wound—in this case, a fire scar.
