Stressed But Strong

We think of vegetables as vitamin sources, but thanks to some research done at the University of Wisconsin-Madison, it looks as if vegetables too need to take their vitamins.

Dale M. Norris, the UWM researcher whose work on plants and vitamins was reported in a recent issue of Science News, usually works with insects. He's an entomologist who has concentrated for 40 years on how insect nerve cells react to chemical cues. It turns out that the plasma membranes surrounding the cells of soybean plants contain certain proteins similar to those in the membranes enclosing insect nerve cells. That interesting similarity lured Norris into studying plants.

The soybean cell-membrane proteins are stress sensitive; Norris refers to them as "stress sentinels." They seem to be a first step in the cascade of chemical reactions by which a plant threatened with environmental stress---heat, drought, caterpillar bite or whatever---protects it-self. In reacting to the stressful stimulus, the sentinel proteins apparently cause the production of other chemicals that act as intracellular messengers. The messengers head for the cell nucleus, signaling it that a threat exists. If enough messenger molecules arrive so that the signal is sufficiently strong, the nuclear apparatus switches on certain genes. These genes encode production of the appropriate defensive chemicals, which the cell then produces. In effect, the sentinels send messengers that cause the cell to gear up for chemical warfare.

It's an elegant and effective system, but it has a significant vulnerability. Norris and his colleagues found the stress-sentinel proteins are very sensitive to attach by free radicals, atoms or molecules with one or more unpaired electrons. Free radicals are the muggers of the molecular world; they snatch the electrons they're missing from wherever they can, which makes them dangerous neighbors. The stress sentinels easily lose electrons to free radicals, and then they cannot function. Since free radicals are common, stress-sentinel proteins commonly suffer impaired function, to greater or lesser extent depending on how prevalent the damaging free radicals are and how touchy the sentinels are.

Among the most effective of naturally occurring substances known to combat free radicals are vitamins C and E. In 1987, the Wisconsin researchers began experimenting with the vitamins to see if they would boost plants' stress-protection systems. They drenched potting soil with vitamin-containing solutions. They painted leaves and sprayed foliage with dilute vitamin baths. They painted bands of vitamins around tree trunks. They dosed seeds with vitamins before letting them germinate.

Everything worked. So far, Norris or other researchers have shown vitamins C and E induce resistance to microbes, insects, weed killers, bruising, and drought. Positive effects have been found not only in soybeans but corn, snap beans, broccoli, coleus, and ash and elm trees.

Treating a single leaf can make for healthy stress-sentinel proteins in every leaf. That's apparently not because the vitamin disperses through the plant but because the vitamin's presence affect the plant's whole system. The benefit also lasts fairly well---about 10 days for vitamin C, up to three weeks for vitamin E.

Of course, gardeners won't be able to treat their plants merely by tossing a vitamin pill into the watering can---life seldom works out that simply. The catch is that dosage is crucial. Plants may need vitamins, but they need minuscule amounts in extremely dilute solutions. Sometimes, Norris' group found, amounts in the parts-per-million range seemed to work best. Overdoses can actually harm the plants they were intended to help.

Norris thinks he's solved the dosage problem---in fact, he's applied for a patent on a vitamin tonic for plants. His patent may soon be one of many. Researchers of the huge Hoffmann-LaRoche pharmaceutical company, the largest producer of vitamins in the world, believe their own studies and independent research in Europe corroborate with Norris' findings. They also claim to have found other vitamins that work even better---but they plan to patent that treatment themselves.