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Grenzwurzeln? It's just another word for basal roots.

Many people know a German word or two that bring a smile: strudel, gesundheit, or maybe fahrvergnugen. For Rich Zobel, it's "grenzwurzeln."

That's the name - translated roughly as "boundary roots" - that a group of German scientists pinned on a specific type of plant root back in the mid- 1800's.

But through the years, other scientists have discounted the existence of these roots as a separate type, contending they are simply a variation of the better known adventitious roots. But in the 1970's, Zobel not only began believing in grenzwurzeln - he grew them.

"The scientific literature has always indicated there were only three types of plant roots," says Zobel, a plant geneticist with the Agricultural Research Service at Ithaca, New York.

"There were the tap root, the first to emerge in germination; the lateral roots that do the real exploring in the soil; and adventitious roots that initiate from the stem and keep the plant standing up."

Zobel first recognized grenzwurzeln in the early 1970's while working with mutant tomato plants.

"A mutant is caused by one or more genes doing something different," he explains. "It turns out that tomatoes have quite a few genes that don't have lots of copies in the chromosome. So in a tomato plant, a gene that's making the difference is easier to find.

"I thought that if I could find certain crucial genes in tomatoes first,

WJL<I could then look for those same genes in other crops such as corn."

When Zobel crossed a mutant tomato plant that had no lateral roots with one that had no adventitious roots, he should have gotten an offspring with only a tap root, according to the conventional wisdom that said only those three types of roots existed.

Instead, the crossbred offspring had a tap root, plus half a dozen other large roots sprouting from the plant's base. These Zobel dubbed "basal roots," in reference to their point of origin on the plant, but he says they're the same as the Germans' grenzwurzeln.

Two years later, Zobel discovered another believer in grenzwurzeln - a scientist named Lenore Weinhold.

"She had published a paper in 1966 that showed basal roots existed in monocotyledonous plants such as wheat and corn, as well as in dicotyledonous plants like tomatoes and soybeans," says Zobel. "But nobody really paid attention, just as they'd ignored the earlier scientists."

Zobel isn't particularly surprised by the confusion about how many types of roots exist. In fact, he says the entire science of roots is as tangled as an uprooted corn plant, starting with the names of the various roots.

"Just look at the tap root," he says. "The name "tap root" refers to the tendency of the first root to emerge from the germinating seedling and grow directly downward, "tapping" the soil. But it's also called both the radicle and the primary root, because it's the first one and the main root for supporting seedling growth and development.

"Frequently, additional roots develop in the plant embryo and emerge soon after the tap root. These roots are called seminal roots in grasses, but they're also called laterals, adventitious roots, primary laterals, and primary adventitious roots."

Until Zobel's and Weinhold's work, the seminal roots of grasses were thought to be some sort of variation of an adventitious root, rather than a separate type.

Looking beyond and complicating the question of identity, Zobel points out that roots' roles can change with time.

"When a plant is very young and all it has is its tap root, that's the root taking up water and nutrients to nourish the plant," he notes. "But when the basal roots form, the tap root generally stops taking up nutrients. Then, when the laterals arrive, they take over from the basal roots.

"The laterals are the ones that finally do the really important exploring and absorbing," he continues. "The tap root, basal, and adventitious roots simply provide the central plumbing through which nutrients and water move in the plant."

In direct contrast to the common conception of the tap root, that particular root is not especially efficient at taking up either water or nutrients, Zobel says.

"But it doesn't have to be," he adds. "It's really involved in nutrient and water uptake only in the very early stages of the plant's growth, and at that time the plant seedling is so small it doesn't need very much."

Instead, as plants grow, it is often the basal roots that probe ever deeper into the soil in search of more water.

"A real difference between monocots and dicots is the ability of the roots to get larger and increase the volume of water they can carry as a plant gets older," says Zobel.

"In monocot plants, the roots don't do that; they just put out new and larger adventitious roots. But the basal roots in a dicot grow thicker and thicker, to allow more water to be taken out of the soil."

Zobel says all plants have the potential to produce all four root types: tap, lateral, adventitious, and basal.

"However, at any given time you may not find all four on a specific plant," he adds. "Many dicots don't show adventitious roots all the time.

New Roots to the Rescue!

"Occasionally, when they're under extreme stress, such as drought, a dicot will grow adventitious roots. If, for example, you deliberately kill a dicot's basal roots and tap root, you can usually get that plant to grow adventitious roots."

This ability of plants to respond quickly to stress with a new array of roots has been demonstrated in Zobel's experiments at ARS' U.S. Plant, Soil, and Nutrition Laboratory at Ithaca.

"We use a system called aeroponics where you hang plant roots in a fog," he explains. "By changing the density of the fog, you can make the plants |think' they're in a drought and then relieve the |drought' and see what happens. We've done this with corn, tomatoes, and soybeans.

"In our soybean work, we reduced the amount of water in the fog for about 2 days and then increased it. Within 12 to 24 hours, the soybean plants grew new adventitious roots a couple of inches long, to get every bit of water available. This is the first time this has been seen under artificial conditions."

Lateral roots, responsible for acquiring the lion's share of water and nutrients from the soil, are simply branches sprouting from other roots, Zobel notes.

"In soybeans, if the top layer of soil dries out, the laterals are so small that they dry up, too," he says. "But the basal roots don't; they're the emergency back-up system. Then when the soil is wet again, the plant puts out new laterals from the basal roots."

Plant roots are strongly affected by the environment in other ways as well. For example, if carbon dioxide concentrations are higher than 0.5 to 2 percent in the soil's "plow layer," the [CO.sub.2] stimulates the plant's roots to grow outward rather than downward, leaving a vacant region directly beneath the plant, Zobel says.

"The final form of a root system depends as much on the environment in the soil as it does on root types," he notes. "Temperature, gases, soil structure, soil chemistry, and microorganisms in the soil all control the rate and pattern of root development.

"For example, how deep roots will go into the soil can be greatly modified by soil physical conditions, such as compaction or presence of a soil pan - a layer of dense soil below the surface that's too difficult for the roots to penetrate. This can make rooting patterns differ across a single field."

Nutrient Uptake Secrets

Zobel and ARS plant physiologist Leon V. Kochian have been studying the ways in which plant roots take up soil elements such as potassium and nitrate.

Kochian has already shown that when soils contain excessive aluminum, a major obstacle to crop growth, the harmful effect of the aluminum on the plant hinges on whether aluminum comes in contact with the root tip or elsewhere on the plant root. [See "Understanding Aluminum Toxicity in Plants," Agricultural Research, November 1992, p. 23.]

In studies at the Ithaca lab, Zobel, Kochian, and research assistant T.G. Toulemonde used microelectrodes to check tomato plant roots' ability to take up potassium and nitrate.

They found that uptake rates of potassium were similar for 7-day-old tap roots and 7-day-old basal roots at a point 4 centimeters from the tip.

But at the root tip itself, the basal roots took up significantly more potassium than did the tap roots, Zobel says.

"Nitrate uptake was different, though," Zobel notes. "As far as 2 centimeters from the root tip, nitrate uptake by tap roots increased fairly steadily in plants that were 1 and 2 weeks old. But at 3 weeks and older, the tap root's nitrate uptake dropped off dramatically rather than remaining constant with distance as was widely believed."

Basal roots showed a similar pattern of nitrate uptake, but at much lower levels, he adds.

"For the last 150 years, scientists have thought they knew what a root is, how and why it develops, what it does, and how and why it does it," Zobel says. "It's becoming increasingly apparent that for most plant species and growing conditions - including greenhouse conditions and hydroponics - we don't know the answers to these questions after all.

"There are very practical applications for this information. For example, maybe we could develop a soybean plant with an especially strong tap root. In the Southeast, there are compacted areas of the soil where roots can't penetrate. A soybean with a stronger tap root could push on down to where the water may be.

"I suspect that if we keep studying roots, we can someday manipulate them for improved crop growth."
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Author:Hays, Sandy Miller
Publication:Agricultural Research
Date:Aug 1, 1993
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