Researchers in CU Boulder’s Department of Ecology and Evolutionary Biology are taking a closer look at the drought-recovery efforts of these beloved blooms

Drive down even the driest road in Colorado at the height of summer, and you’re bound to see rows of bright sunflowers lining the shoulder. And even if at the end of a long hot day those long-stemmed blooms look a little wilted, there’s a good chance that by the next morning they’re standing tall again.

That observable bounceback has led researchers to believe that sunflowers are capable of a process called “refilling,” in which the plant’s water transport capability, which can fail during intense drought stress, returns to normal after it has been rewatered. Refilling flushes drought-induced air bubbles, called embolisms, out of the plant’s water transport tubes to deliver much-needed moisture throughout the plant. 

 

Study authors (left to right) Jared Stewart, Stephanie Polutchko and Brendan Allen found that, contrary to popular belief, sunflowers were not capable of “refilling” themselves after experiencing drought and subsequent rewatering. (Photo: Stephanie Polutchko)

But according to a recent study by researchers in the University of Colorado Boulder Department of Ecology and Evolutionary Biology, refilling is not the cause of sunflowers’ apparent bounceback.

“We didn't see a single convincing instance where even one of the tubes was repaired,” says Research Associate Jared Stewart. Stewart served as lead author on a recent article in the Journal of Experimental Botany documenting his team’s use of a micro-CT scanner to peek inside sunflowers at various stages of dehydration and rehydration. 

The finding stands in stark contrast to the results of a study the same team performed on a weedy grass, which was published last year in the journal Proceedings from the National Academy of Sciences USA. In that instance, the grasses’ refilling capabilities were remarkable. 

“There wasn't any real gray area with the two findings,” says Stewart. “This grass refilled 100% completely in every single plant, every single time. In the sunflowers, we saw zero refilling.”

So where does that leave the field of plant hydraulics? “In last year's paper we showed that this refilling process does happen,” says Stewart. “But these results suggest it’s probably not nearly as prevalent as people thought.”

Why sunflowers? 

The study grew out of a conversation between Stewart and his then-postdoctoral advisor Sean Gleason, a plant physiologist with the U.S. Department of Agriculture’s Agricultural Research Service, along with CU Boulder colleagues Stephanie Polutchko, an assistant teaching professor of ecology and evolutionary biology, and graduate student Brendan Allen. The four linked up with researchers at Colorado State University and mapped out a way they could leverage CSU’s micro-CT scanner to observe the drought processes in the plants. 

Sunflower was an ideal candidate, Stewart says, because literature on plant hydraulics almost unanimously supported the idea that sunflowers were capable of refilling (in part because of the daily visual evidence of the flower wilting and then perking back up). In fact, for a long time most experts in plant hydraulics believed that refilling was a fairly universal phenomenon. The problem was that there was very little concrete evidence to back up that belief.

“Previously published evidence for this repair process has been scrutinized more and more intensely over the past decade,” says Stewart. The team thus opted for a simple experimental setup to ensure there was no confusion about the results. 

A mouse-sized scanner to the rescue

An essential part of that simple design was using the micro-CT scanner that belongs to CSU’s College of Veterinary Medicine and Biomedical Sciences. The small scanner is designed to scan small, live animals and was engineered to emit as little radiation as possible—so study subjects can be scanned repeatedly without exposure to high levels of radiation. 

A sunflower plant experiencing drought and one week after being rewatered (top), and micro-CT images of its stem (bottom). The white arrows indicate where the stem was scanned, and the yellow areas in the micro-CT images show air bubbles, or embolisms, in yellow. (Image courtesy of study authors)

The setup was perfect for Stewart’s team, who scanned the 11 specimens of the “Mammoth” variety of sunflowers at three different times: while under drought stress, on the morning after re-watering and after one week of recovery under well-watered conditions. The plants were able to withstand the scanning with no visible damage. 

That was not true in past studies on the topic, including research that dates back more than 40 years. “Conventional ways of studying this have involved basically cutting out portions of the plant,” says Stewart. “Then you do the same procedure on a different plant that’s been rewatered.” 

That process of cutting the plant has been suspected to be problematic for more than 100 years, Stewart says, since it introduces air into the incision, making it difficult to measure the embolisms that existed in the tube prior to cutting. But that method has remained one of the most commonly used in the literature and may be a cause for confusion about the topic.

Now, Stewart believes they’ve come up with a gold standard method for studying this phenomenon. “Personally, I wouldn't trust any data generated with any other published methods at this moment,” he says.

A new line of questioning

Now that the team has validated their methodology with the two different studies, they see it as a first step in designing future experiments. 

“There’s a ton of different directions we’d love to go with it,” says Stewart, adding that he hopes their results might be leveraged for agricultural efforts to improve food production and food security, especially as the planet potentially faces an increase in droughts. 

“I think if we got to pick another crop to study and had unlimited time and money, it'd probably be corn or rice,” says Polutchko. “We know that a grass can do it, but it's one that’s an agricultural nuisance. So, can we have a grass that we like to eat that can also do this?”

Ideally, now that the method has been laid out, other researchers with access to similar micro-CT scanners can also conduct their own research, especially on crops with agricultural significance.   

As for the sunflower’s drooping and reperking routine, Stewart, Polutchko and team are still stumped. “Maybe the plant doesn’t refill those embolisms, but it has a different strategy to work around that stress. Those are great questions, and we want to know the answers.”

“In true scientist fashion, you answer one question and 10 new ones pop up!” says Polutchko.

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