Balsam poplars are a medium-sized deciduous tree commonly planted as a wind shelter or for alley cropping, to create lanes for growing agricultural crops. Because of their economic value for these applications – and considering warmer, drier conditions brought on by climate change -- plant breeders are interested in coming up with new hybrid types that are even more resistant to drought.
Researchers from the University of British Columbia recently used the Canadian Light Source at the University of Saskatchewan to look inside two types of balsam poplar saplings, to learn more about how their water transport system is affected by lack of moisture in soil.
Under normal conditions, water is transported upwards from the roots of a tree to its leaves through a continuous column of water. However, in drought conditions, pockets of air form in that column – like an embolism that develops in the blood vessel of a human -- blocking the transport of water and nutrients to the leaves.
“What other researchers have found is that these drought-induced embolisms are one of the main causes for tree mortality that we are observing globally under dry conditions,” says Thorsten Knipfer, an assistant professor of plant physiology in UBC’s Faculty of Land and Food Systems, and lead researcher on this new study.
Xylem is the tissue in plants responsible for transporting water from the roots to the stem and leaves. Using X-ray computed tomography, the UBC team discovered that balsam poplars use their xylem fibers to store water then subsequently release it into the xylem’s vessels, the pipe-like cells that house its hydraulic column; this mechanism reduces the risk of embolism forming there.
“So internally stored water is utilized that makes the plant less susceptible to air embolisms, which was cool to see,” says Knipfer. “This is the first time that somebody obtained visual evidence of this phenomenon in an intact plant (with X-ray CT).” Before now, he says, researchers thought the primary function of xylem fibers was to provide mechanical support to the tree.
The BMIT beamline enabled Knipfer and colleagues to peer inside an intact plant. In previous studies, researchers typically examined a section of plant material that has been cut from the main body of the plant, exposing the hydraulic column to air and causing embolisms to form.
Their findings, says Knipfer, could help plant breeders develop new hybrid varieties of poplars that blend the drought-resistant capabilities of the poplars they studied with the desirable fast-growing capabilities of another genotype.
“We know from the literature that there are other types of woody species where water is not stored in fibers, which is somewhat surprising because fibers make up the largest volume of the entire xylem in the stem,” says Knipfer. “You’d think that this volume would have to be used by the tree in some way -- that it’s not just this empty compartment. Showing in a balsam poplar that it serves as a compartment for water storage is a very significant finding.”
Image: Adam Jones, Ph.D./Global Photo Archive/Wikimedia Commons
Chu, Cheyenne, Mina Momayyezi, Jarvis A. Stobbs, Raju Y. Soolanayakanahally, Andrew J. McElrone, and Thorsten Knipfer. "Drought‐induced fiber water release and xylem embolism susceptibility of intact balsam poplar saplings." Physiologia Plantarum 175, no. 5 (2023): e14040. https://doi.org/10.1111/ppl.14040
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