Scientists have known for a long time that plants grow differently when they are spaced closer together than farther apart. This response is partly due to phytochromes, a group of protein photoreceptors within cells that sense different wavelengths of light and affect the way a plant grows based on its light environment. Closely spaced plants can result in more chlorophyll per unit ground area than those spaced farther apart.
Chlorophyll preferentially absorbs light in the red region of the visible light spectrum, so the light microenvironment near plants tends to be depleted in red wavelengths and enriched in far-red wavelengths, thereby decreasing the red to far-red light ratio (R:FR). In turn, the pool of red-light–absorbing phytochromes decreases relative to the far-red-light–absorbing pool, creating a signal transduction pathway that leads to a change in the way a plant grows.
For this reason, plants that are spaced close together (a low R:FR environment) tend to grow taller faster and have longer, thinner leaves.
Consequently, plant spacing can affect the light environment and plant growth, even before plants start to shade each other and compete for light. These changes constitute a mechanism—an anticipatory growth response—whereby a plant can detect the presence of chlorophyll (and therefore neighboring plants) around it and compete for the light resource by growing taller than its neighbors prior to being shaded.
Even though this response has been studied in many plant species, it is not well understood in corn under field conditions, particularly in terms of the effects on corn grain yield. To address this knowledge gap, researchers at the University of Wisconsin–Madison conducted field experiments in 2005 and 2006 to determine the effects of early-season (prior to canopy closure) R:FR light environments on corn morphology and grain yield. The results were published in the January–February issue of Crop Science. Low, control, and high R:FR treatments were established by varying corn plant density (107,600, 53,800, and 3,000 plants ha–1, respectively).
At canopy closure, R:FR in the low, control, and high R:FR treatments was 0.23, 0.49, and 0.99, respectively. At this time, each treatment density was thinned to 3,000 plants ha–1 to avoid confounding effects of R:FR treatments and shading among plants after canopy closure. Plants were grown to maturity under conditions where soil moisture and nutrient availability were similar among R:FR treatments. Across years, grain yield per plant in the high R:FR treatment was 1.5 and 2.0 times greater than in control and low R:FR treatments, respectively.
Greater yield in the high R:FR treatment was largely due to greater tiller mass and grain yield. The results indicate that the initiation of tillers at higher plant densities and lower R:FR values may cease at or slightly before canopy closure; that is, this response in corn may be induced when R:FR falls below a threshold level, as has been suggested in some other crops. For example, the R:FR value of 0.49 at canopy closure in the control R:FR treatment may have been sufficient to cease initiation of most tillers, which would account for similar tiller mass between the low and control R:FR treatments at physiological maturity.
Furthermore, because tillers and cobs are close to ground level at the time of canopy closure, it is likely that a signal transduction pathway via one or more intermediate signals (such as a plant hormone) transmits the R:FR information from leaves in the canopy to plant tissues closer to the ground.
Overall, these results indicate that early-season R:FR was an important factor affecting corn productivity under field conditions. Greater understanding of the R:FR-dependent molecular pathway that controls the development of tillers and cobs may provide direction for increasing productivity of elite corn varieties.
Adapted from Markham, M.Y., and D.E. Stoltenberg. 2010. Corn morphology, mass, and grain yield as affected by early-season red:far-red light environments.
Crop Sci. 50:273–280.
Photo: Corn plants exposed to high red:farred light ration throughout the growing season typically had several tillers some with grain-bearing ears. Photo by the authors.