California Grapevines Under Drought Stress - X-Ray Diagnosis in Ultra-High Vacuum

Wines from the USA are famous and coveted worldwide. With more than 4,500 square kilometers of vineyards, the United States is the fourth largest wine-growing country in the world after Italy, Spain and France. California alone produces about 84 percent of all U.S. wine. However, concern is increasingly spreading among winegrowers due to the abnormally high summer temperatures caused by climate change. This puts the vines under drought stress.

Winegrowers are therefore forced to harvest the grapes before they are perfectly physiologically ripe, which can lead to unbalanced wines and atypical aging notes. Artificial irrigation provides a remedy in some cases, but it involves high costs and is not exactly environmentally friendly.

For Andrew McElrone, professor in the Department of Viticulture and Enology at the University of California at Davis, research into the water transport physiology of grapevine roots is crucial to the future of California's viticulture industry. The plant biologist's ultimate goal is to develop sustainable water use strategies for grape growers. To do this, he wants to provide them with hard data on how much drought their plants can withstand and which species are ideal for an agricultural future with an uncertain water supply.

High Resolution X-Ray Tomography

In his research, McElrone used Beamline 8.3.2 of the "Advanced Light Source" - ALS for short (see infobox). To gain a better understanding of the grapevine's water transport system, McElrone used the high-resolution X-ray tomography available there. Thanks to the brilliant scans, he and his research partners were able to see for the first time exactly how pressure from drought affects the xylem vessels in a plant's water transport network. This is important because the xylem network transports water and nutrients from the roots to the rest of the plant–. The more a drought stresses the network, the greater the tension in the system and therefore the more susceptible the individual vessels and tubes ni the plant are to rupture, and thus to bacterial invasion.

McElrone discovered during his work on the ALS that some grapevines can actually repair this break by forming droplets on the walls of the xylem tubes. Scientists had speculated about this process, but McElrone and his fellow researchers were the first to actually observe it in action. They found that droplet formation does not occur randomly, but rather follows the orientation of the remaining living cells. McElrone continued the research with a number of grape varieties and realized that that the more drought-resistant grape varieties were more likely to be able to resist breakage or have better repair capabilities.

"Our goal is to find out how far we can push drought in different grape varieties before they break, which we can then apply to water use in the fields," McElrone says. "At the ALS, we were able to actually observe the breakage and repair."

Another important finding from McElrone's research on the ALS beamline was that the vines' bridging cells - which provide better connectivity within the plants' vascular architecture - also determine which species are most resistant to pathogens. Thanks to high-resolution electron microscopy scans from the University's Davis lab combined with ALS scans, McElrone's team observed the presence and orientation of bridge cells.

Additional research should also reveal whether there are intact barriers between the bridge cells. The more susceptible vines actually had more open bridge cells, McElrone said. Pathogens were able to penetrate through the network and further into the plant structure. "The resistant species, on the other hand, isolated the pathogens by forming bridge cells with barriers, stopping the bacteria from spreading," the researcher says. McElrone sums up by saying that, "as far as plant biology goes, the images are decidedly revealing."