...From the pages of Agricultural Research magazine
At the University of
of Horticulture, geneticist
John Bamberg (left) and
physiologist Jiwan Palta
examine the wild potato
species S. microdontum,
which has genes for high
Baked potatoes, mashed potatoes, scalloped potatoes, potato salad,
hash browns, french fries, potato chips . . . .
According to the U.S. Department of Agriculture's Economic Research
Service, potatoes are America's most popular vegetable. The typical
American consumes more than 140 pounds of them every year. That's 50
pounds more than the per-capita consumption of tomatoesthe potato's
closest competitor. Unfortunately, potatoes can suffer from a variety
of ailments that either render them unfit for sale or reduce their market
Says Jiwan Paltaa physiologist with the Department of Horticulture
at the University of Wisconsin-Madison (UW)some of these problems
can be helped by adding calcium. Specifically, increased concentrations
of calcium in potato tissue have been shown to reduce the severity of
tuber defects such as internal brown spot and hollow heart. Increased
levels of tuber calcium have also been correlated with improvements
in tuber yield, grade, and storage quality.
John Bamberga geneticist who manages ARS' U.S. Potato Genebank in Sturgeon Bay, Wisconsinsays that Solanum tuberosum is the only cultivated potato species. But, he adds, about 200 wild, tuber-producing potato species exist. Knowing how beneficial calcium is to potatoesand to the people eating themBamberg and Palta decided to work together to find which of these wild potato species are best at accumulating this important mineral.
Tubers of wild species (right)
are small and otherwise unfit
for the table, but such
plants often have genes for
traits like high tuber calcium
that could make important
contributions to the quality
of commercial cultivars (left).
The Root of the Problem
"Potato tubers are naturally deficient in calcium," says
Palta. "They grow undergroundusually in sandy, irrigated
soiland have about one-fifth the calcium found in the aboveground
stem of the plant."
Palta explains that the plant's main root system draws water and a
water-soluble form of calcium from the soil and sends them where they're
needed mostthe plant stem and leaves. Because the potato tuber
is surrounded by moist soil, it transpires less and accumulates much
Several years ago, Palta and his UW collaborators discovered that tubers
have their own root systems, which supply water and nutrients directly
to tubers. They then demonstrated that tubers could accumulate much
more calcium if they were "spoon-fed" the mineral during bulkingtheir
major growth and nutrient uptake phase. This research has caused a small
revolution in the potato-growing industry.
Potato farmers who once fertilized the soil early in the growing season
with calcium-rich lime and gypsum have modified their habits. They still
fertilize with nitrogen-, phosphorus-, and potassium-rich products early
in the season, but many now add water-soluble calcium, such as calcium
nitrate, Nitro Plus, or N Plus, to their irrigation lines later, when
tuber bulking occurs. This nutrient-rich water is drawn into the tuber
by the stolon roots.
"Applying 100 to 200 pounds of water-soluble calcium per acre
during bulking vastly improves tuber quality," says Palta. "In
general, we've found that the average calcium concentration in tubers
increases 50 to 100 percent, and the incidence of internal defects dramatically
"We have also found that with increased calcium concentration, tubers bruise less during harvest, transport, and storage. And, potato plants are less affected by heat stress when calcium is added to the soil during the stress period," Palta adds.
Screening Potatoes for Mineral Accumulation
Though Palta knew that a potato's calcium level could be increased,
he didn't know whether certain species were better than others at accumulating
the mineral. That's when he started collaborating with Bamberg at the
U.S. Potato Genebank. Together, the scientists screened 21 potato species
for their ability to accumulate tuber calcium when a control level of
calcium was available and when higher levels were supplied.
Bamberg says, "We identified two wild species that are excellent
calcium accumulators: S. gourlayi and S. microdontum."
S. gourlayi ranked first for calcium accumulation in the control
environment, accumulating more than double that of S. tuberosum.
It ranked second in additional accumulation in the treatment environment,
accumulating three times more than S. tuberosum. And while S.
microdontum exhibited only average calcium accumulation in the control
environment, it had the highest calcium increase when grown in the high-calcium
"Both these wild species are in the same taxonomic series as cultivated
potato species, so they can be crossed with S. tuberosum,"
After making their initial findings, Bamberg and Palta began screening
potato plants for extremes of calcium-accumulation capacity. They moved
from screening among species to screening among populations within a
species to screening among genotypes within a population. As a result,
they were able to identify potato germplasm with very high and very
low calcium-accumulation capacity.
Their next step? To begin transferring the genes for super-high tuber calcium accumulation from the wild species to the cultivar breeding pool. Geneticist Bob Hanneman and technician Andy Hamernik, with the ARS Vegetable Crops Research Unit in Madison, will be able to help them do this. Their work includes making raw germplasm useful for breeding new cultivars. Hanneman says, "We serve as a bridge between the genebank and the user community. We put beneficial wild species into forms that breeders can use more readily."
The Genebank's Continuing Mission
Bamberg's and Palta's research is just one example of how ARS scientists
and their university collaborators are looking to enrich different types
of produce with calcium and other essential minerals. It shows how scientists
at the U.S. Potato Genebank fulfill their mission of studying traits
that may be useful for breeding.
Says Bamberg, "The wild, weedy relatives of potato in the genebank
collection are not suited for growing or eating in their natural state.
But, as exemplified by tuber calcium, some wild potato species carry
specific traits of great potential value to the potato industry."
With the help of other specialists, U.S. Potato Genebank scientists
work to make these traits known and available to potato researchers
and breeders worldwide.By Amy
Spillman, formerly with ARS.
This research is part of Plant, Microbial, and Insect Genetic Resources,
Genomics, and Genetic Improvement, an ARS National Program (#301) described
on the World Wide Web at www.nps.ars.usda.gov.
Note to Readers: Dr. Robert E. Hanneman, Jr., died while this article was in press. He worked for ARS for more than 30 years and was head of the U.S. Potato Genebank for 15.
"Calcium-Rich Potatoes: It's in Their Genes" was published in the March 2003 issue of Agricultural Research magazine.