Plant Responses to the Environment — Six Graduate Student Projects

Help for Stressed-Out Plants

"In nature, plants are never faced with just one problem," says graduate student Douglas Bielenberg. "It's always a mix: too cold, too dry, too bright, too wet. What I'm interested in is how plants respond to multiple stresses. Will one stress help a plant to respond to another? Will the stresses work together in a way that makes them even more harmful? How will they interact?"

To find out, Bielenberg is studying the interactive effects of ozone and soil nitrogen levels on leaf aging. "Ozone's an oxidant," he explains. "We hear in the news about how people should take antioxidants because oxidation is related to aging. We also see this in plants. In lab studies, one of the big effects of chronic ozone exposure is accelerated leaf senescence. Nothing appears to be wrong with your plants, but the leaves age and drop off weeks before their time. This could be a real problem for farmers if potato leaves are dying, for instance, while tubers are supposed to be growing. Similarly, if crops like beans lose their leaves before the time when they move nitrogen from their leaves to their seeds, the pods may not fill."

It's hard to point the finger directly at ozone when you're out in the world, though, because ozone is never found apart from other stresses. Bright, hot conditions that can harm plants are conducive to ozone production. Ozone also interacts heavily with insects and pathogens. "Ozone exposure may weaken plants, but you may not see any effects until the plants are attacked," Bielenberg says. "It's difficult to just walk out into the woods and identify ozone damage. It's very complex. Biologists like complexity that's life. But it also makes understanding the world...difficult."

Researchers have studied the interactions between soil fertility and ozone damage since the 1960s. "Initially, they thought they might ameliorate the symptoms by fertilizing plants," Bielenberg says. "But they found that different species responded in various ways to the range of fertility treatments. My research addresses some of the same questions they were asking, but on a physiological level."

Bielenberg's hypothesis is that ozone damage can be minimized by maintaining nitrogen availability in the soil or growth substrate. "In agronomic or natural settings, nitrogen concentration doesn't remain constant throughout the year," he says. "When you apply fertilizers, nitrogen spikes up, then drops away at a certain rate, depending on soil moisture and other factors. You'll also see a peak in nitrogen during snowmelt and spring flush. What effect does declining nitrogen have on a plant? This could be a very important issue for farmers."

To learn more about how nitrogen availability affects ozone damage, Bielenberg grows hybrid poplars outdoors in open-topped growth chambers. The chambers allow him to charcoal- filter incoming air, then add back a specific concentration of ozone. The trees also are grown in a sand culture system so he can control nutrient levels. Using this system, he can compare the effects of constant nitrogen and declining nitrogen during ozone exposure. He looks at an array of physiological measures, including growth, photosynthesis, protein content, and rubisco content (a key enzyme in photosynthesis and important internal source of nitrogen) under different combinations of ozone and nitrogen. He also measures the rate at which leaves drop under the various treatments. Finally, using a stable nitrogen isotope, he will monitor the pathway nitrogen takes as it moves through the plant and determine whether the movement changes in response to ozone concentration or nitrogen availability in the soil.

"If we can understand the underlying physiological basis for the interaction between ozone and soil nitrogen, we can go back to older studies and try to explain what they were seeing," Bielenberg says. "Some really great work has been done in the past. I hope to go back and settle some of the questions that people have guessed at for a long time."