Published on 12/06/00

Plants Have Alternative Way to Stay Fresh

Plants, much like animals, need oxygen to stay alive. However, when plants and animals become stressed, part of the oxygen they use can turn into poison and accelerate the aging process, says a University of Georgia scientist.

Al Purvis, a horticulturist with UGA College of Agricultural and Environmental Sciences, is researching the ways plants cope with stress. His studies could enable farmers to grow plants whose fruits ripen and age more slowly. These fruits would be able to stay fresh longer at the grocery store and in the home.

Breathing Life

Much like putting a log on a fire, a plant gets energy from respiration and the burning of food. "This is all part of the living process," Purvis said.

The earth's atmosphere is about 21 percent oxygen, he said. While animals don't function well and even die without this level of oxygen, plants can live at lower oxygen concentrations. During normal respiration, most of the oxygen the plant consumes is safely reduced to water. But the plant doesn't use all the oxygen. These extra oxygen compounds can become Reactive Oxygen Species.

Bad Breath

Because their makeup is unbalanced, ROS are unstable. They will react with anything to become stable once again. While they are roaming around the body trying to equalize, they can actually attack and kill body tissue. Anyone who has stored a piece of fruit too long has seen ROS at work, Purvis said. The unstable compounds attack and destroy the fruit's cells, making it mushy.

In the early 1990s, Purvis discovered that plants had an alternate line of defense. This extra feature, known as the alternative electron transport pathway, gives plants an advantage over stress that animals don't have, he said.

Coping with Stress

Research at the Coastal Plain Experiment Station in Tifton, Ga. has shown that the alternative pathway reduces the production of ROS in stressed and aging plant tissue.

When a plant is stressed, he said, respiration increases. The faster respiration consumes more oxygen than the body can safely reduce. This increases the level of ROS. Just the simple process of aging can stress a plant or an animal.

"Plants and animals are more prone to disease after they have gone through a lot of stress," he said. "ROS are good and bad. The key is controlling the level."

Good, Bad ROS

ROS play a major role in the ripening of fruit. They're necessary for other life functions, too. But too many can be harmful.

A common ROS is hydrogen peroxide. It reacts with and helps fight infection on flesh wounds. Another, hydroxyl radical, is formed when a cancer patient undergoes radiation treatments. "It's the most potent," Purvis said. "It reacts with everything under the sun."

Plants and animals both can defend against these harmful radicals. Antioxidants, such as vitamin C or E, can shield body tissue from the attacking compounds and reduce them to harmless water.

"We have ways to prevent the harm that is done from these radicals. But sometimes the level of production of these radicals overcomes the degradation," Purvis said.

Now that the plants' alternative pathway has been identified, he said, it's possible to produce plants that are better able to cope with stress.

Brad Haire is the former news editor with the University of Georgia College of Agricultural and Environmental Sciences.