Tomatoes are partial to red, potatoes favor pale blue or white, and turnips don't think orange is too bad.
Growing tomatoes over red rather than conventional black plastic mulch increased harvests of number-one quality fruit by 10 to 15 percent, according to plant physiologist Michael J. Kasperbauer and soil scientist Patrick G. Hunt, both with ARS' Soil and Water Conservation Research Center in Florence, South Carolina.
Pale blue or white mulch increased the harvest from potatoes by as much as 15 percent. It's not the colors of the mulch that enhance the harvests, but the differences the colors make in the light reflected onto the plants.
The colors can also have an impact on taste and protein levels in leaves and might even provide protection from insects in some cases.
Mulch itself is commonly used by farmers and gardeners. Soil coverings such as plastic and straw conserve soil moisture and keep weeds down. American vegetable growers use 200 million pounds of plastic mulch a year.
In the spring, heat-absorbing black plastic mulch is used to warm the soil and give plants a head start. In southern areas, white plastic is used to reflect light and reduce soil temperatures to help late season crop production.
But it isn't the temperature, moisture, or weed control benefits that interest Kasperbauer and Hunt. Mulch is simply a convenient vehicle to let them place colored light reflectors below a field-grown crop without interfering with the incoming light.
"Using black mulch is known to be good for plants, but we've found using the fight color is super good," Hunt says.
Light, including sunlight, is made up of various amounts of different wavelengths. Different combinations of wavelengths show up as different colors, as can be seen when sunlight is broken into the colors of the rainbow. So-called white light is actually a blend of many colors of light. When light reaches a colored surface, some wavelengths are absorbed and others are reflected, altering the color of the light reflected from surface.
"Plants are extremely sensitive to the color of light," Kasperbauer says. "They are particularly sensitive to the blue, red, and far-red portions of the light (color) spectrum."
Far-red, also called near infrared, is just beyond the spectrum visible to the human eye.
Light that has a low far-red-to-red (FR/R) ratio will cause a plant to develop a shorter stem and bigger roots. A higher FR/R ratio causes a plant to direct more new growth into the shoots, resulting in a taller plant with more leaves.
"These responses are understandable when you realize each green leaf reflects an increment of far-red. So a plant with many neighbors will get more reflected far-red and a higher FR/ R ratio," Kasperbauer says. "A plant responds to the light signal that says that it has many neighbors by growing taller than its neighbor. With a low FR/R ratio, a plant senses no competition, develops more branches, and sends more nutrients to the roots."
What color mulch is most effective depends on what a grower wants.
"Obviously, if you're interested in a crop like turnips or potatoes, you want to increase growth below the ground; if you are growing tomatoes, more fruit on the plant is what's desirable," Kasperbauer says.
Kasperbauer began his work in photobiology in 1961 with a team of ARS scientists at the Beltsville Agricultural Research Center that pioneered the research on a plant pigment called phytochrome.
Phytochrome, which was discovered in a project headed by Harry A. Borthwick and Sterling B. Hendricks, is considered to be the universal regulator in plants. Plants sense the quality of surrounding light through chemical reactions in phytochrome.
Phytochrome exists inside plants in two interconvertible forms. One form absorbs only red light, which causes it to undergo a chemical transformation and become the other form. The other form will absorb only far-red, and then it becomes the other, red absorbing only, form. The ratio between the two forms in the plant depends on the ratio of far-red to red light and regulates the use of resources within the plant.
The colors can also have an impact on taste and protein levels in leaves and might even provide protection from insects in some cases.
In one of his experiments in the late 1960's, Kasperbauer measured the amount of visible and far-red light in sun flecks that reached the soil surface in a field of tobacco and in those on a plant-free road surface near the field, but away from any plants. Sun flecks are bright concentrated spots of light reflected from a surface.
He found those close to growing plants had more far-red than sun flecks on the road surface. The amount of far- red was also higher where plants were closer together.
"The denser the plant population, the higher the FR/R ratio was and the more it told plants to grow taller to adapt to the competition," says Kasperbauer.
During the late 1970's in Florence, Hunt was investigating the effect of environmental stress, particularly water stress, on different strains of nitrogen-fixing bacteria in soybeans.
Hunt found an odd response that could not be attributed to changes in water availability.
"Without irrigation, we found a 10-percent, increase in the yield of soybeans that were planted in rows that ran east to west compared to north-south rows," Hunt says.
"However, with irrigation and no water stress, we got the highest yields in the north-south rows," he adds.
When Kasperbauer joined the Florence lab in 1983, he and Hunt reasoned the row direction might be affecting the FR/Rratio reflected on the plant canopy.
When they examined the light in the field, they found that light falling on and reflected from east-west rows consistently had a little less far-red in it.
Less far-red means more growth in the roots and therefore more nodules for nitrogen-fixing bacteria and drought tolerance. More far-red produces more shoot growth and is most useful for soils with high nitrogen levels and plentiful rainfall or irrigation.
"While the difference isn't much, you're getting an increase for just turning a tractor around-maybe only a bushel or two an acre, but at more than $5 a bushel, that's a nice increase," Hunt says.
Subsequent experiments showed the sun-tracking ability of leaves increased the amount of far-red a plant in a north-south row was exposed to in the course of a day. "Plants in north-south rows could track the sun all day. They got a slightly higher FR/R ratio, especially near the end of the day when the leaves faced the sun and reflected far-red back onto the next row of soybeans," Kasperbauer says.
"Since we already knew plants responded to the FR/R ratio in a
controlled environment and to reflected far-red from competing plants,
the next step was to look atplant responses to light from other
surfaces," Kasperbauer says.
They raised questions about how different soils, which come in a range of colors from almost white sand to the black soil of the Midwest, reflect sunlight. They also questioned if the presence of crop residues in a field represented a "soil color" change and caused changes in the fight reflection.
The two researchers began testing the influence of reflected light on plant growth with boxes of different colored soils from different locations. But they also had to control the soil temperature that is influenced by the color of the soil, since it also affects plant growth.
To isolate the color reflectance effect, they used Styrofoam panels, each covered with a layer of a different colored soil. There were obvious differences in plant growth over different soil colors, even when the insulation panels kept the temperatures the same.
During the following summer, the scientists found painted styrofoam panels gave the same plant growth responses as the soil-covered panels, if they reflected the same wavelengths and wavelength ratios.
Enter Colored Plastic
In 1985, D.R. Decoteau, a Clemson University horticulturist who was working on tomato productivity, began working with Hunt and Kasperbauer.
Since plastic mulch is widely used for tomato cultivation but available only in black or white, the scientists had to paint plastic in order to create other colors.
Paint from the local store was used to convert black mulch into the other colors. Each paint had to be measured with a spectrophotometer to record the wavelengths (colors) of the light reflected off of it.
The magnitude of impact that colored mulch had on the first tomato experiment surprised them, Hunt admits. They expected differences in growth, but red mulch -caused a 20-percent increase the first year in number-one fruit-the ones that bring the most money-that was amazing.
Using red mulch treatment didn't always give as much as a 20-percent increase. The second year, the difference was 37,057 pounds of fruit with red mulch compared to 32,921 for those mulched with black plastic, a difference of about 12 percent. "That's a pretty good return for a change of surface color," Hunt points out.
Since that first year, the scientists have tested red, orange, yellow, blue, green, white, aluminum, black, and various combinations of these colors on crops such as peppers, cotton, soybeans, southern peas, turnips, and potatoes as well as tomatoes. Turnips were used because both the greens and roots are used as food.
For all crops, the key is the amount of reflected far-red and FR/R ratio. "It is not as simple as taking the first bucket of red paint or roll of red plastic off the shelf," Kasperbauer says. "Me amount of far-red reflectance needs to be verified with a spectrophotometer. Plants are even more discriminating between colors than the human eye is."
One spring, when the spectrophotometer was out of service, the researchers mixed two batches of blue paint by eye.
"To our eyes, the two batches looked exactly the same, but the results at the end of the season were different," Kasperbauer says.
When they ran spectrophotometric measurements on the two blues, they were very different in the amount of far- red and the FR/R ratio.
"They looked the same to our eyes, but the spectrophotometer didn't see them that way and neither did the plants," Kasperbauer says.
When colored mulch begins to be produced commercially, the production of each color will have to be very precise. There are many mixtures of color that appear red to the human eye, but it takes a particular red to stimulate a tomato plant correctly. "Not just any red is going to give you more and better tomatoes," Hunt says.
Colored mulch will benefit the home gardener as much as the commercial producer, according to Hunt and Kasperbauer.
A patent application has been filed for the colored mulch approach. ARS is seeking a company with which to sign a cooperative agreement to develop color concentrates that reflect the proper wavelength combinations for several crops and eventually produce a commercial product
In the commercial product, different colors might be used to enhance different stages of a plant's growth, Hunt says.
"Maybe we can layer colors of mulch so as one degrades, it reveals another
color underneath because you might want a color to stimulate rapid root growth around seedlings transplanted into a field and later a colored mulch to increase shoot growth," Hunt says.
Patterns of color are also a possibility. One color could intensify one part of the sunlight, while another color enhances another part of the light. "If you flew over fields of mulch like that, it might look plaid or checkered," Hunt says.
Early on, Hunt tried a combination of blue and fluorescent orange checkers. Fluorescent colors reflect light that is very high in red and very low in far-red.
"From 100 yards away, the light reflected from the fluorescent orange made the plants look like they were on fire," Hunt says.
The combination didn't work very well on potatoes, although it had some beneficial effect on turnips.
"Actually, a few people here think I started with orange just because I graduated from Clemson University with its traditional orange tiger paw," Hunt says.
Turnip studies turned up another possible effect of the colored mulch.
"It seems that the color of the mulch had an effect on the taste of the turnips," Hunt says.
They used turnips for the preliminary taste test because both the root and the greens are food crops and a change might occur in either part.
Blue mulch, with less far-red, appears to cause a greater accumulation of the compounds responsible for turnip flavor.
"Both the turnip and the greens in that case had a more bitey, less sweet taste," Hunt says. "Of course , the taste tests have been very informal at this point and much more work would need to be done to track any taste effects.
Another possible benefit of a custom-colored mulch: repelling insect pests. With the help of entomologist Steven Roach, the researchers recently completed a 2-year study that looked atinsects on cotton grown over seven colors of mulch: red, yellow, green, blue, black, white, and aluminum silver, as well as bare ground.
Each week, the cotton plants were vacuumed to collect and count the insects living on the plants.
Preliminary results showed the color of the mulch has some impact on the number of insects found on the cotton plants, although not all the data have as yet been analyzed.
While a few aspects of the commercial use of colored mulch remain to be fine-tuned, "this may be one of the first major field applications to come from the scientific discovery of phytochrome," Kasperbauer says. "Colored mulch is certainly based on solid photobiology."
--By J. Kim Kaplan, ARS.