To resolve open questions about plant plumbing—how plants transport water from roots through stems and how they respond to stresses such as drought—science teams from around the world met September 1–3 at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley for an intensive round of x-ray and other experiments.
The scientists studied live American chestnut saplings with a 3D imaging technique (x-ray microtomography) at Berkeley Lab’s Advanced Light Source (ALS), then explored the same samples using other laboratory tests. About 15 researchers, representing groups from across the U.S. and as far away as Australia, participated in this “Plant Hydraulics Working Group,” and together they will work to analyze the data and interpret the results.
Drought conditions are a source of stress that can create gas bubbles in plants’ water transport systems, which can lead to blockages, much the way embolisms occur in our own blood vessels. To understand how plants repair themselves or are permanently damaged by this process, scientists have studied the internal structure of plants’ stems using x-rays as well as performed other studies in which they force water through cut plant stems to observe how efficiently they transport water in response to varying conditions.
An ALS conference room was converted into a makeshift plant laboratory for the research event. The samples were fed blue-dyed water to easily identify which vessels were transporting water—key to moving sap through healthy plants—and those that appeared blocked by bubbles or other types of obstructions. At a separate station, a conductivity apparatus measured the flow of water through cut stem segments. The technique, which factors in the length and width of each segment, can inform researchers about the maximum flow of liquid through a given stem section if all of the section’s vessels are active in water transport. It can also detect when flow declines due to gas bubbles.
The x-ray microtomography technique, on the other hand, is like a CT scan, but with microscopic resolution. So, while benchtop methods require cutting up the stem of a plant to examine it, x-ray microtomography creates a complete 3D representation of the stem at a microscopic level while the plant is still intact. Over the last several years, ALS Beamline 8.3.2 has hosted some of the most detailed, consistent work in this area. Below is an example of a microtomographic volume rendering from earlier research involving grapevine stems. [Knipfer et al., Plant Physiol. 171, 1024 (2016).]
One question the researchers hoped to resolve at this event is whether x-rays impact measurements in any way, versus sample preparation from more conventional laboratory techniques. The results could show whether x-ray studies can potentially undercount blocked vessels and also whether hydraulics studies using cut stems can introduce additional air bubbles or otherwise alter natural samples and give false readings.
At ALS Beamline 8.3.2, the samples were separated into groups based on how much water they received prior to the experiments. Each tree was wrapped in plastic to preserve its moisture during each x-ray scan, which lasted about 12 minutes as each tree was rotated 180 degrees in the focused x-ray beam.
The collaborative effort to study plants prepared under different watering conditions will provide new data and insight about how to better tend to crops and other plants under stress and in natural conditions, and could also lead to improved understanding and forecasting for drought-related die-offs of trees and other plant species.
Read the full story by Glenn Roberts (Berkeley Lab).