Shade Avoidance

Wyoming Researchers Seek to Determine Whether Factors Stretching Plants Toward Sunshine Can Affect Production
By Andrew Kniss & Tom Schambow*

* Andrew Kniss is associate professor and weed scientist with the University of Wyoming.  Tom Schambow, whose UW master’s thesis work was based on the shade avoidance in sugar-beets project, is now with the Park County, Wyo., office of the USDA Farm Service Agency.  Another M.S. degree candidate, Albert Adjesiwor, is currently working with Dr. Kniss on this research.
     Their research has been supported by a competitive grant from the Wyoming Agricultural Experiment Sta-tion, as well as with funding from the Western Sugar Cooperative.
     This article is based on one initially appearing in a recent issue of Reflections magazine, a publication of the University of Wyoming College of Agriculture and Natural Resources.

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Plants need light. That’s hardly an eye-popping statement; it’s one of the first biology lessons children learn in school.  Plants convert sunlight into forms of energy the plant can use to grow.  Nearly everything humans eat is derived in some way from photosynthesis, be it the tomato picked from a gar-den or the rib eye steak that once grazed on grass.

Sunlight seems like a plentiful enough resource; but in fact, there’s not always enough sunlight to go around. Sure, there’s plenty of sunlight avail-able for the tallest plant.  But since those taller plants use most of the light, shorter plants may not be able to survive — or at least grow to their potential — in the shade.

Plants have been competing with each other for sunlight for millions of years — and, over time, have evolved mechanisms to ensure they get enough of this essential resource.   Plants growing in shade often tend to grow taller than they would in full sunlight. Growing tall allows the plant to com-pete more effectively with surrounding plants.  The tallest plant in dense vegetation will certainly get enough light to survive and reproduce.  But the energy and resources those plants expend in growing tall can come at a cost: they might have a thinner main stem, fewer leaves, a less-vigorous root system, or the amount of seed might be reduced.

Plants can begin this response of growing taller even before experiencing shade.  They can “see” nearby plants by sensing changes in reflected light quality (specifically, in the red and far red spectrum).  When surrounded by other vegetation, many plants initiate what scientists have dubbed “shade avoidance” responses.  Understanding these shade avoidance responses is important for crop production.  

Shade avoidance responses may be triggered in a crop plant that is surrounded by weeds or other vegetation (such as cover crops).  Canadian re-search has shown that corn and soy-bean yield potential can be reduced if these shade avoidance responses are triggered early in the crop-growing sea-son.  Even after weeds are removed by tillage or herbicide use, the crop plants may not be able to recover their earlier yield potential once shade avoidance responses have been initiated.

Shade Avoidance Response from Season-Long Presence of Weedy Environment, Compared to No Weeds, University of Wyoming Laramie Research & Extension Center, 2014.

Shade Avoidance In Sugarbeet

Weed control is critical for all crops, as weeds obviously will compete with the crop for essential resources.  But getting rid of weeds is particularly important in low-statured crops.  Sugar-beets grown for sugar do not produce a tall main stem; but rather, a rosette of leaves that may only reach up to two feet in height.  Consequently, sugarbeet is a relatively poor competitor for light. Our research has shown a 1% yield loss in the sugarbeet crop for each 1% of full sunlight intercepted by weeds.  But this is the effect of direct shading of the crop by weeds, which is different from shade avoidance.

Growth may be affected by shade avoidance responses even before the weeds grow taller than the sugarbeet crop.  Unlike many agronomic crops, sugarbeets do not produce a main stem in the first year of growth; so they have little capacity to grow taller.  Sugar-beets are also different from most agronomic crops in that they are a biennial species.  So instead of harvesting seeds (like for corn, beans, wheat or sun-flower), the large storage root is harvested after the first year of growth.  

For that reason, it was unclear at the beginning of a University of Wyoming research project how shade avoidance responses might affect the growth of a sugarbeet.

UW graduate students Louise Lorent, Tom Schambow and Albert Adjesiwor have, during the past several years, conducted a number of studies to see how sugarbeets respond to reflected light.  In doing so, we’ve em-ployed many different colors of plastic mulch, as well as some different iterations in which we’ve used actual weeds surrounding the beets.  For the “weedy” studies, the sugarbeet plants and weeds were grown in separate containers to ensure the roots did not interact and the weeds were not robbing water or nutrients from the sugarbeets.  We only allowed the weeds to interact above ground, reflecting light toward the beet plants.  We also kept the weeds clipped to prevent any direct shading of the crop.

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