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Evading the shade.

Can the spectral quality of shaded sunlight affect the quality of your food?

Maybe so, if you're fond of foods cooked in vegetable oil.

Recent findings by Agricultural Research Service scientists suggest that oleic acid levels in soybean seeds - the world's major source of vegetable oil - are related to the blue and far-red portions of sunlight falling on seed pods.

Oleic acid is a monounsaturated fatty acid that may help people ward off heart disease. Some studies have indicated that oleic acid lowers serum cholesterol. Less prone to rancidity, it has better storage and cooking properties.

Blue wavelengths increase the percentage of oleic acid in comparison to other fatty acids in the seeds, according to Steven J. Britz, head of the ARS Climate Stress Laboratory in Beltsville, Maryland. Wavelengths in the far-red part of the spectrum decrease the percentage.

Direct, unshaded sunlight contains all the colors of the rainbow plus invisible ultraviolet and infrared bands. But dense leaf canopies block out much of the blue, Britz explains, while far-red (not quite infrared) wavelengths pass through.

"And that fact goes hand in hand with our observation that seeds from shaded pods tend to have less oleic acid," he says.

Britz and James F. Cavins, head of the ARS Analytical Chemistry Support Unit in Peoria, Illinois, compared the effects of broadspectrum light (containing all visible wavelengths) and blue-deficient light on soybean plants. Their study, which included soybean plants grown in greenhouses and controlled environment chambers, was part of an ongoing research program to examine crop responses to various lighting conditions.

The scientists found that soybean seeds contained 50 percent more oleic acid when pods received direct, broadspectrum light. Removing the blue or adding far-red reduced oleic acid to levels associated with shaded pods. Seed yield, protein, and total oil were the same with either kind of light.

"Evidently," says Britz, "the metabolism of oleic acid into more highly unsaturated fatty acids is retarded by photoreceptors that respond mostly to wavelengths in the blue region of the spectrum."

Now comes the big question: How can soybean growers take advantage of this information?

Soybeans are usually grown close together to maximize yield, Britz points out. So mutual shading - and the loss of blue light as far as the pods are concerned - is probably unavoidable for economic reasons.

But there's more than one way to solve the problem. If we can't get the seed pods out of the shade," Britz says, "then maybe we can help them evade the effects of shade."

Britz proposes the selection and breeding of soybean plants with more efficient blue light photoreceptors as one possibility. The seeds could then make better use of whatever blue light they do receive. Another possibility might be development of pods with different light-absorption characteristics.

"In other words," says Britz, "we need to genetically trick soybean plants into responding as if there were no shade - as if their pods were getting all the blue that direct sunlight normally provides."

That's easier said than done, Britz acknowledges. Breeding specific kinds of photosensitivity into plants could be a trial-and-error process taking years, and there's no guarantee of success.

But the blue light connection could make a big difference, according to plant physiologist Richard F. Wilson, head of the ARS Soybean and Nitrogen Fixation Research Unit in Raleigh, North Carolina. There are more than 14,000 soybean varieties in the ARS soybean germplasm bank, he points out, so faster techniques for genetic screening are always needed. And if more oleic acid is the goal, he adds, then checking for blue light sensitivity in soybean germplasm might indeed be the best way to achieve it.
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Title Annotation:shade may reduce beneficial oils in soybean crops
Author:Miller, Stephen Carl
Publication:Agricultural Research
Date:Nov 1, 1992
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