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USDA-ARS
plant physiologist
Michael Grusak examines roots
of hydroponically grown
green bean plants.
Photo by Jack Dykinga. |
Drink
your beans!
Calcium, that bone-building
nutrient so important to growing kids, can't always come from cows. For example, dairy
products won't do as a calcium source for children who can't tolerate lactose—milk
sugar.
Green beans are also a good source of calcium, and some beans are better than others.
 Plant physiologists Michael Grusak and Kirk Pomper, who are with the
Children's Nutrition Research Center at Houston, Texas, evaluated six bean varieties for
calcium content.
A variety named Hystyle had about twice the calcium as the Labrador
variety, and these researchers figured out why:
Hystyle
is better at conserving water.
Water
dilutes the calcium moving through a bean plant, reducing the amount reaching the pods
that people eat. The Houston center is a joint
venture of Baylor College of Medicine and USDA's
Agricultural
Research Service. Many ARS scientists study how nutrients feed plants, so yields can
be raised. At Houston, Grusak focuses on how crops can feed people better.
Calcium is especially important for children when their
bones are growing. Other research at the Houston center suggests bone weakness later in
life may be related to how much calcium children get during growth phases in childhood and
adolescence.
The speed at which water gets in and out of a plant depends largely on how quickly it is
transpired from leaves and other surfaces. The opening and closing of tiny pores called
stomates regulate transpiration. The action is both genetically and environmentally
controlled.
"We found the whole-plant transpiration rate to be twice as high in Labrador as in Hystyle, even though the amount of water transpired from
the pod itself is the same for both," says Grusak. The researchers found that the
higher transpiration rate in Labrador plants resulted in lower concentrations of calcium
moving in the plant's xylem stream.
Xylem transports
liquid and nutrients throughout the plant, somewhat as arteries in a person carry
oxygen-rich blood.
Green beans like Hystyle could also be good news for farmers and
environmentalists. These beans require less water, so they reduce irrigation costs.
Taking less water from area lakes, streams, or aquifers helps the environment in two ways.
It conserves water in drier areas, and it reduces the amount of farm chemical runoff going
back into the environment
By Jill Lee, formerly with ARS.
This research is part of
Plant Biological and Molecular Processes, an ARS National Program (#302) described on the
World Wide Web at
http://nps.ars.usda.gov/programs/cppvs.htm.
Michael A. Grusak (mgrusak@bcm.tmc.edu)
is with the USDA-ARS
Children's Nutrition Research
Center, 1100 Bates St., Houston, TX 77030; phone (713) 798-7044, fax (713) 798-7078.
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