When tomatoes see red: the horticultural tricks colored mulch can play

Science News, Dec 13, 1997 by Janet Raloff

Plastic mulches have been part of the agricultural scene for some 40 years. They retain moisture in drip-irrigated rows, retard the emergence of weeds, and warm the roots of young crop plants. Early on, farmers or gardeners shopping for these sheets of Impermeable plastic would find their range of hues limited to what Henry Ford offered 1914 Model T buyers: "Any color, so long as it's black."

Eventually, brown and greenish plastic joined the ranks. Only recently has real color begun to explode on the mulch scene. Half-mile-long sheets of red plastic have been available for a few years, and silver is expected to be marketed more widely this spring. Before long, yellow, blue, and a rainbow of other hues may also beckon from the pages of agricultural and gardening supply catalogs.

The shade of mulch a grower chooses is less likely to reflect personal style, however, than the wavelength of the light spectrum needed by the cultivar it will nurture. In fact, much of the growing interest in colored mulch stems from Michael J. Kasperbauer's more than 30-year scrutiny of light sensors in plants.

Kasperbauer, who is a plant physiologist with the Department of Agriculture's Coastal Plains Soil, Water, and Plant Research Laboratory in Florence, S.C., has spent his career probing how plants respond to light. Those investigations have demonstrated that wavelength--or color--plays as critical a role as intensity. In fact, the more experiments his team performs, the more traits they discover that can be altered by a mulch's color.

These studies are just beginning to suggest how the activation of color sensors in leaves orchestrates key operations in plants as diverse as tobacco, cotton, tomatoes, and carrots. Yet the preliminary data that he and others have already gleaned suggest that by outfitting a plant's soil in the appropriate hue, growers can improve not only the quantity and quality of their yields but also a plant's flavor and vulnerability to pests.

Kasperbauer began studying phytochrome in 1961, just 2 years after it was discovered. In plants, this color-sensitive protein gauges the intensity of certain wavelengths of light, particularly those in the red (640 to 670 nanometers) and far-red (roughly 700 to 750 nm) range. Unlike the red wavelengths, those in the far red are not photosynthetically active--that is, they do not fuel plant growth and vigor. Rather, Kasperbauer has found, they serve as environmental cues.

"Green leaves reflect a lot of far-red light," he says. While these wavelengths fall just beyond what the human eye can see, phytochrome reacts to them like a beacon. Kasperbauer's studies have established that when plants sense a high ratio of far-red to red light, they realize that "lots of neighbors are nearby," threatening to siphon off a disproportionate share of the sun's growth-sustaining energy. Plants respond by redirecting more of their energy into above-ground growth, usually at the expense of roots. The resulting taller, thinner plants compete more successfully for sunlight.

In the mid-1980s, Kasperbauer teamed with Patrick G. Hunt, also at the Florence lab, to investigate whether other natural features might provide related spectral signals that influence growth. They started by looking down--to the stubble left on the ground and to the actual color of the soil. Again, materials that reflect a higher-than-usual proportion of far-red light onto the seedlings--such as brick-red soil or straw residues--spurred the plants' above-ground growth more than black soil, black mulch, or white plastic did.

The potential economic significance of these observations didn't emerge until Clemson (S.C.) University horticulturIst Dennis R. Decoteau joined the team, turning its focus to tomatoes.

Decoteau recalls wondering whether it would be possible to fool a plant into thinking it was in crowded conditions by carpeting the soil with red mulch, thereby inducing a growth spurt in seedlings. In the field, plants surrounded by red plastic indeed grew faster than normal.

They also went on to produce earlier, larger fruit. In studies over the past decade, this far-red reflecting material has enhanced yields by 20 to 50 percent a season, depending on the weather.

Reasoning that the opposite tactic should benefit root crops, the scientists grew turnips in soil covered with an orange mulch. In fact, Kasperbauer notes, the plastic's orange appearance was only incidental. What mattered was that this mulch reflected up onto the plants' foliage a greater amount of red light and only a little far-red light.

Come harvest, turnips grown under the orange mulch indeed proved bigger than those mulched with black or red plastic.

The influence of a mulch's color doesn't end with yield. Three years ago, Kasperbauer and plant physiologist Robert E. Wilkinson of the Georgia Agricultural Experiment Station in Griffin turned their attention to plant waxes. A combination of fats, alcohols, and hydrocarbons, waxes are what make the surface of a leaf shiny They not only keep a plant from losing water, they can serve as a barrier to pathogens and chemicals.