A Fierce Competitor

October 29, 2009 07:00 PM

Competition gets fierce in your fields. Scientists are only beginning to learn just how weeds vie with crops as they battle for nutrients and sunlight.
Take velvetleaf, for one. In the presence of corn, velvetleaf accelerates genes that turn carbon into sugars, genes used in photosynthesis and genes that stimulate cell division. Genes that regulate lengthening velvetleaf's stem work harder, too, pushing its growth.
"Velvetleaf grows more quickly in the presence of corn than velvetleaf growing without corn,” says Sharon Clay, South Dakota State University weed scientist, whose study on the relationship between corn and velvetleaf used DNA microarray analysis to look at which genes are expressed more under different regimens. She co-authored the study with David Horvath, USDA–Agricultural Research Service scientist, and Danny Llewellyn, subprogram leader of genomics and plant development at the Commonwealth Scientific and Industrial Research Organization in Australia.
Interestingly, while velvetleaf got tougher, corn in the study became less competitive in the presence of velvetleaf.
"In corn, the genes for photosynthesis were downregulated. Those genes were turned off in the presence of velvetleaf,” Clay says.
Is corn a wimp of a plant? Could be. "We took velvetleaf plants growing by themselves and growing with corn. We found when velvetleaf is in the middle of corn, photosynthesis genes' expression in the velvetleaf is greater than when grown without competition. By that measure, corn is a wimp,” she says.
Velvetleaf: Exotic and Tough
Velvetleaf is in the same genetic family as cotton. A broadleaf annual weed, it is native to China and India, says Sharon Clay, South Dakota State University weed scientist. Velvetleaf was introduced to the U.S. more than 200 years ago as a possible fiber crop, but the weed can cause significant yield loss in corn and soybeans.
Clay and her colleagues did DNA analysis of velvetleaf using the cotton genome information, since the two are so closely related. The cotton genome is mapped and microarrays are available, whereas the velvetleaf genome is not. "Using information about gene expression is a really neat tool that was unavailable until recently. We can use this information to examine actual gene expression in the plants and compare differences between stressed and nonstressed plants,” Clay says.
"Corn decreased expression of genes involved with photosynthesis in the presence of velvetleaf. In the study, velvetleaf was taller when grown with corn than velvetleaf growing by itself. Corn was shorter when grown with velvetleaf than when grown by itself. In addition to light and nutrient competition, velvetleaf also might have some allelopathic chemicals that suppress corn growth,” Clay says.
Corn opponents. Though this particular study focused on velvetleaf's competitiveness at only one stage of corn growth, Clay would like to see if other weeds and grasses have similar effects on corn growth. "We're going to look at grass, and we have a USDA grant allowing us to analyze gene expression in corn at several growth stages. We're hoping we can better define the critical weed-free period and how weeds actually affect the crop,” Clay says.
"These data are just beginning to shed light on how different plants function. But it would appear there's sound science behind the phrase ‘growing like a weed,'” she says.
In addition, Clay is looking at how corn plants compete with each other and respond to what is growing around them. "Corn in 15" rows has decreased photosynthetic ability versus corn in 30" rows,” she says.
"Corn in 15" rows actually increased yield 11% per acre. Per plant, though, there was 50% less yield with the narrow rows. We're almost to a point where we think the main factor may be light-quality effects on the corn, which is how the corn plant perceives its neighbors,” Clay says. 

You can e-mail Charles Johnson at cjohnson@farmjournal.com.

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