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Probing for stress-tolerant plants.

Fiber optic probes are not only alding doctors in diagnosing people; they' re also helping agronomists diagnose stress in plants.

Decapitation from grazing or harvesting, over-exposure to nitrate from manure or commercial fertilizer, soil temperatures too low or too high, or drought--all are enough to make soybeans, alfalfa, clover, and other legumes less efficient nitrogen fixers. But, just like people, some plants may endure more adversity than others with little or no loss in productivity.

R. Ford Denison, an ARS plant physiologist, has built an apparatus that can quickly identify the toughest plants.

If he wants to check their response to grazing, he cuts the plant a half inch above ground. If he's checking the effects of drought or other stresses, he doesn't need to take so drastic a step.

In either case, he brushes just enough dirt away to expose one of the root nodules containing nitrogen-fixing bacteria. Next he places a probe so that two attached optical fibers touch the nodule, one on each side.

High-intensity red and infrared light pass through one fiber, illuminating the nodule. The ratio of red to infrared light returning through the second fiber to a photodetector is used to calculate the concentration of oxygen in the nodule.

Denison says the nodules have to maintain an optimal amount, because too little or too much oxygen would be harmful.

He explains that the apparatus he designed has confumed in the field what has been reported recently from his and other laboratories, including that of Thomas R. Sinclair, an ARS plant physiologist in Gainesville, Florida: Reduction in nitrogen fixation is caused by oxygen starvation of the bacteria inside the legume nodule. "Stress causes the nodule to admit less oxygen,"he says, "and the bacteria's ability to supply nitrogen from the air to the plants slows down." An adequate oxygen supply in the nodule of a detopped plant is a sign that that particular plant tolerates stress well. Denison presented comparison data for alfalfa and birdsfoot trefoil in the March 1992 issue of Plant Physiology.

Says Dennison, "in addition to improving our understanding of plant responses to stress, we see the instrument as a way plant breeders can do something about the oxygen deprivation/stress connection. They can use it in selecting legumes in the greenhouse or field that are superior nitrogen-fixers in the face of stress.

There have been other attempts to monitor the oxygen content of legume nodules, but all were practical only in a laboratory. "This is the first portable instrument for field testing," notes Denison.

He and a Canadian collaborator, David Layzell, have a patent pending on the instrument. A U.S. company, Morgan Scientific, inc., is developing a commercial version.

Denison says he can test each nodule in less than 5 minutes, not counting the time required to find it.By Don Corms, ARS.

R. Ford Denison is at the USDAARS Appalachian Soil and Water Conservation Research Laboratory, P.O. Box 867, Becklev, WV 258020867. Phone (304) 252-6426..
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Title Annotation:research by R. Ford Denison
Author:Comis, Don
Publication:Agricultural Research
Date:Jun 1, 1992
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