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.
John B. Bamberg is with
Potato Genebank, Potato Introduction Station, 4312 Hwy. 42, Sturgeon
Bay, WI 54235-9620; phone (920) 743-5406, fax (920) 743-1080.
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.