These open-top growth chambers are used to study the effects of the air pollutant ozone on the plants growing inside them. All chambers receive charcoal-filtered air, and some receive a regulated supplement of ozone.

Ozone plays an indispensable role in protecting our planet from the sun's ultraviolet rays. If the ozone blanket in the stratosphere were to disappear over time, Earth eventually would become a gigantic rocky landscape like Mercury or Mars. But here on the surface of our planet, ozone plays a much different role. Formed by the interaction of automobile exhaust and factory emissions with oxygen, ozone is an air pollutant that contributes to various respiratory problems. It also may damage crops, forest trees, and other plant life.

Plant pathologist Eva Pell has studied ozone's effects on vegetation since 1968, examining how the gas causes deterioration at the cellular level in plant life ranging from trees to vegetables. Many graduate students seeking to gain insight into plant stresses and defenses have participated in Pell's studies over the years, and six master's and Ph.D. candidates currently are part of her research team. One of these students, Cosima Wiese, who came to Penn State with a dual bachelor's degree in biology and German from Bucknell University, has been studying the effects of ozone on tobacco plants."Scientists often use tobacco to study vegetative response to air pollution because the plants are easy to grow, and it's not too difficult to develop genetically transformed plants with a desired trait," says Wiese. "We've been looking at a wild type and two genotypes of tobacco that produce smaller quantities of an enzyme called ribulose bisphosphate carboxylase/oxygenase, or rubisco."

Rubisco is abundant in plants and plays an important–though not completely understood–role in photosynthesis, which is how plants use sunlight to transform water and carbon dioxide into carbohydrates essential for their growth."This enzyme makes up 50 to 70 percent of the total soluble protein in the leaves," Wiese explains."Along with helping with photosynthesis, it is important for nitrogen storage in plants and is crucial for other nutritional functions, too."

Rubisco also is affected by ozone."When plants are exposed to elevated levels of ozone, one of the things that happens is they start to senesce more rapidly. That means they age and die before their time,"says Wiese."It's easy to see this in the leaves, which turn yellow and fall off. Levels of rubisco in these plants begin to drop prior to their decline. The two processes–senescence and decreased rubisco–coincide so often that we suspect they may be closely linked. We're wondering, do the decreased quantities of rubisco actually trigger the senescence?"

To help answer this question, Wiese has been trying to learn why rubisco is so sensitive to ozone in the first place."Some of Dr. Pell's previous studies suggest that the biochemical composition of the enzyme may make it susceptible,"says Wiese."But it's also possible that the loss is related only to the quantity of rubisco in the leaf."One way to find out if the latter is the case is to study plants with low quantities of the enzyme. While the wild type in Wiese's study produces normal amounts of rubisco, one of the genetically transformed genotypes produces 45 percent less and the other produces 65 percent less."We exposed all three of these tobacco genotypes to high levels of ozone and studied profiles of their rubisco levels over time,"Wiese says."We were hoping to be able to tease out what was actually going on. If the reason for rubisco's disappearance was biochemical, then we expected that roughly the same percentage of the enzyme would be affected in each of the plants. If its disappearance depended on quantity, then we expected to see some dramatic differences among the plants."

Doctoral student Cosima Weise (left) studies the effects of ozone on tobacco plants with her adviser, Eva Pell, Steimer professor of agricultural sciences.

What the researchers found was more of a surprise."It turns out that the tobacco, or at least the cultivar we were using, reacted very differently to ozone than other plants that have been studied,"Wiese says."For one thing, we never saw the usual symptoms of accelerated senescence. Instead of gradually yellowing, the way leaves normally do under these conditions, the tobacco leaves stayed green except for some patches where the cells instantly died and the area turned brown, or necrotic. We also discovered that the genetically transformed plants with less rubisco were far more sensitive to ozone than the wild species were. On these plants, the areas of leaf necrosis developed so rapidly that it was almost as though you could watch it happen. On the plants with 65 percent less rubisco to begin with, entire leaves turned brown and dropped off. It seems that ozone damage is related to rubisco, but not necessarily the way we thought. Does having a lot of rubisco help protect plants from ozone damage? We're not sure."

Wiese did some additional reading and found that certain characteristics of the cells in tobacco leaves may make the plants especially sensitive to ozone. Anatomical studies on these genetically transformed genotypes show that the plants have large air pockets between their leaf cells. These air pockets allow gases, such as ozone, to flow in and surround cells."Since more of the cell surface is exposed to ozone, more cells are likely to die,"Wiese explains."Over all, the tobacco plants responded so unexpectedly that our original questions certainly weren't answered. But uncovering this response of tobacco to ozone has been an interesting discovery in itself."

Wiese completed her master's studies in 1996 and has begun her Ph.D. work in Pell's lab."I wanted to keep working with Dr. Pell because she's an excellent mentor,"says Wiese, who is deciding whether to pursue a career in industry or academia after getting her doctorate."For my Ph.D. work, I plan to continue studying ozone, but I am switching gears a bit,"she says."I've always been interested in how plants defend themselves when they're attacked by pollutants, fungi, bacteria, or insects. I especially want to learn more about how they sense these attackers in the first place and what their initial responses are–such as manufacturing a defensive chemical compound. This kind of research has practical applications for both the environment and agriculture–but the main appeal for me is that it could answer a lot of fundamental questions about how plants survive."

–Rose Pruyne