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Plants with more complex water transport structures are more resistant to drought conditions, making them more likely to survive and pass this characteristic on to their offspring.

That’s the conclusion Yale researchers have reached after poring over the fossil records of ancient plants that span tens of millions of years.

Earlier this month, a group of University affiliates working with faculty from Bates College, the University of Maine and Haverford College, among other institutions, published a paper on identifying the impact of droughts and drought resistance in determining plant structure over time. 

“This study is an excellent example of how a collaborative, multidisciplinary approach can yield novel insights into evolutionary questions,” Jonathan Wilson, an associate professor of environmental studies at Haverford College and an author on the paper, wrote in an email to the News. “Bringing together modeling, physiology and paleontology and integrating their data and methods shed a great deal of light on this period in plant evolution.”

Lead author Martin Bouda GRD ’17, a researcher at the Institute of Botany at the Czech Academy of Sciences, studies the network properties of plant parts such as root systems and how these systems affect water intake of plants. In this paper, he was responsible for conceptualizing the water transport within the plants and building computational models to analyze these hydraulic processes. Craig Brodersen, a professor of plant physiological ecology at the Yale School of the Environment, was the principal investigator for the paper. 

According to Brodersen, this new research stems from his team’s previous work on drought tolerance in plants such as grapevines. Previously, they have used new imaging techniques to study the vascular systems of various plants and the effects of different vascular organization on plant survival in droughts.

Brodersen explained that during extreme drought conditions, plants may accumulate air bubbles in their vascular systems. 

“These bubbles block the flow of water from the roots to the leaves,” Brodersen wrote in an email to the News. “One air bubble among the many hundreds to thousands of vessels in a plant might not cause too much harm initially, but these bubbles can spread between vessels wherever there is a connection to an adjacent one. Understanding how the vessel network of a plant is connected can then tell us something about how air bubbles spread, which becomes a greater problem during prolonged drought.”

The researchers used this framework as a guiding question to study the evolution of vascular plants over millions of years, according to Brodersen. 

When conducting this research, the team of scientists referenced a collection of images put together over the past century by various paleobotanists. The images depicted xylems, a specialized tissue responsible for transporting water and nutrients, of extinct plants. The fossil record, which spanned approximately 50 million years, gave the researchers an understanding of the arrangement of various vascular systems in plants. 

“In this particular case, some of our previous work on the very fine structure of a vessel network got me thinking of the big picture instead: our living plants almost all have a gap in the middle of the network, whereas in the earliest land plants, the network occupied a solid cylinder of tissue,” Bouda wrote in an email to the News. “I built a little simulation to see if embolism spread differently on the two different cross-sections. The results were encouraging and fit in well with Craig’s ongoing work on seedless vascular plants.”

While Brodersen produced the actual microscope images of the networks within the plants the researchers were studying, Bouda constructed idealized, possible networks to use as comparison cases for the observed networks.

Then, Bouda added, the researchers worked with Wilson to study fossil plants and realized that the fossil plants’ networks were more vulnerable to droughts than those of present-day plants. 

“[T]here’s a clear pattern of xylem network shapes starting out with simple forms and becoming more complex,” Brodersen wrote. “Using the arrangement of the conduits within those vessel networks we were able to map out the possible pathways that air bubbles could spread from conduit to conduit. We then compared networks from extinct plants to living relatives today such as ferns and lycophytes.”

Using a computer model, the researchers were able to simulate drought conditions and study the effects of those conditions on various xylem configurations. The results suggested that the xylems of the earliest plants, which were also the simplest in arrangement, were the most vulnerable to air bubbles during a drought. Plants with a more complicated vascular system outperformed other species during the drought, according to Brodersen. 

This paper is the result of interdisciplinary collaboration of experts in mathematical modeling, plant anatomy and paleobotany. The researchers applied current plant water transport theory and experimental work to their own inquiry. Brodersen explained that the team at Yale, including Bouda, developed the computer model and anatomical observations used to test their hypothesis. He added that Wilson served as the paleobotanist who helped “guide the species selection from the fossil record.”

Brodersen explained that this research stems from his laboratory’s collaboration with the plant breeding community. The findings could potentially inform the development of more drought-resistant varieties of plants. 

According to Bouda, this paper helps elucidate “something fundamental about the structure and function of vascular plants that has been overlooked for a long time.” While the practical applications of the research are not immediately apparent, Bouda hopes that the findings might help diminish the tradeoff between crop yield and drought resistance, which has traditionally been a problem in agriculture. 

“I thoroughly enjoyed exploring the paleobotanical fossil record,” Bouda wrote. “A lot of this was completely new to me, and I daresay both Craig and I got pretty excited discovering the variety of vascular forms that evolution had tried out over 400 million or so years. In the end, we found that we had taken a roundabout way to broadly reconstruct the dataset used by F. O. Bower a hundred years ago to show that vascular complexity depends on plant size, and that’s when the final piece of the puzzle snapped into place.”

Xylem comes from the Greek word xylon, meaning wood.