Tomato tumors: red light means grow.

Tomato tumors: Red light means grow

Two scientists have discovered that visible red light induces tiny tumors on the leaves of tomato and certain other plants growing in an environment shielded from the sun's ultraviolet rays. Conversely, they report, barely visible infrared or "far-red" light suppresses the tumors. The findings may suggest ways for gardeners to illuminate their greenhouses to prevent the tumors -- which slow growth, cause leaves to curl and occasionally kill plants -- and could shed light on how organisms regulate cell division and enlargement, says study coauthor Theodor W. Tibbitts, a horticulturist at the University of Wisconsin-Madison.

Previous studies have suggested that, unlike humans and many other living things, certain genetically predisposed plants need a small amount of ultraviolet light to prevent nonpathogenic tumors, or those that are not induced by another organism. Many members of the solanaceae family, including tomatoes, potatoes and eggplants, and several other plants such as sweet potatoes and pea pods are particularly susceptible to such tumors. Scientists also have suspected that visible wavelengths play a role in inducing the tumors, but Tibbitts and co-worker Robert Morrow say they are the first to determine the specific wavelengths involved and to suggest that a particular plant-pigment molecule, known as phytochrome, mediates tumor growth.

Tibbitts and Morrow punched disks out of the leaves of healthy tomato plants and illuminated the disks with blue, green or red light from filtered lamps. They found that tumors afflicted 60 percent of the surface of red-illuminated disks, compared with 3 percent of the green-illuminated surfaces and none of the blue. The more red light, the more tumors. But far-red light (700 to 800 nanometers) reversed the effect, and the more far-red light the better, Tibbitts and Morrow report in the December PLANT PHYSIOLOGY.

These effects tipped the researchers off to phytochrome, known to switch on and off by changing its form in response to the balance of red and far-red light. The growth of nonpathogenic tumors is probably not linked to photosynthesis, they say, noting that blue and green light help drive photosynthesis and far-red light does not reverse or inhibit it.