Probing Soybean Physiology

On the surface it seems simple: too little water or too high temperatures during the growing season equals disappointing soybean yields. A closer look, however, reveals a host of entwined physiological changes in the soybean seed that result in this deflating outcome. Soybean physiologists spoke with Biodiesel Magazine about how they're trying to understand these changes in an attempt to improve yields.
By Jessica Ebert | January 17, 2008
Life in all its many forms-from the microscopic to the massive-shares several basic requirements for persistence such as enough resources to grow and develop and favorable environmental conditions for this to proceed optimally. The availability of these seemingly prosaic needs is oftentimes at the whims of the weather. Too much rain and essential soil nutrients get washed away from the roots of plants that depend on it. Too little rain and these same soils become dry deathtraps devoid of the liquid that sustains the living.

The growth and development of a soybean plant is governed by nothing different. Given ample nutrients and water and a propitious temperature for growth, soybean seeds will yield a satisfying bounty. Take these away or alter them in any way and what once was expected to be a generous haul now becomes mediocre at best. "Obviously weather affects yield. It has an effect on the diseases and the insects present, it affects traits like size of seed and protein and oil content," says Walter Fehr, a soybean breeder at Iowa State University. "For each of these, the extent to which the environment exerts an effect differs a lot."

Physiology Facts
Growth, development and yield of soybeans are influenced by a variety's genetic potential interacting with environmental elements and farming practices, according to a soybean growth and management guide published by the North Dakota State University Extension Service. Farmers who understand how soybeans grow and develop can optimize seed yield by minimizing environmental stresses and establishing field practices that maximize the potential of their chosen variety.

From seed to full-grown and mature plant, the development of soybeans can be broken down into two main phases: a vegetative phase characterized by the first leaves of the plant emerging from the soil and culminating in a stem that stands about a foot tall with several nodes or branching points ending in a profusion of leaves; and a reproductive phase defined by the blooming flower, maturation of the bean pod and development of seeds. "The soybean plant is very flexible in terms of how many seeds are formed. The plant will basically cannibalize the leaves and the stem just to make sure those seeds keep growing as long as they can," explains Mark Westgate, a soybean physiologist at Iowa State University. "But the first thing that drought stress or high-temperature stress does is decrease the number of seeds that are successful."

The second adjustment that the plant makes is in the size of the seed. Since seed size is proportionate with the amount of oil and protein that can be harvested-i.e. the bigger the seed the bigger the volume of oil and protein carried by the seed-and growers focus on bringing in as much oil per acre as possible, conditions that favor the development of lots of big beans are desirable, Westgate says. It turns out that a lack of water plays a primary environmental role in this outcome. "Under drought conditions, the plant runs out of reserves and stops growing," he explains. More specifically, an inadequate supply of water signals the seed to stop expanding. "Soybean seed is kind of interesting in that it continues to accumulate dry matter and continues to do biochemistry as long as it can expand," he adds. Since most of this expansion deals with getting more water in the seed, when that process stops, the seed stops growing.

This phenomenon was documented in the 2007 growing season. According to a November report compiled by Seth Naeve and James Orf, professors in the Department of Agronomy and Plant Genetics at the University of Minnesota, despite a midseason drought in parts of Iowa, Minnesota, Wisconsin, Michigan and several East Coast states, rain in the latter part of the growing season allowed the remaining seeds to fully expand and enlarge. The southern range of the U.S. soybean production area, however, suffered from a season-long drought resulting in smaller seeds from these states. "Overall, the midseason drought affected soybean yields more than any other weather phenomena," the authors wrote.
Although dry conditions affect seed size, drought doesn't significantly affect the basic biochemistry of oil formation. "We know this because the rate of growth of the seed doesn't slow down. It keeps chugging along until eventually it hits a brick wall-it's too dry and the seed can't expand anymore," Westgate explains.

Temperature, on the other hand, does affect the biochemistry of oil production as well as seed size. Higher temperatures speed the activity of enzymes thereby making development proceed more rapidly. When the vegetative and reproductive stages of growth occur more quickly, the overall development of the plant finishes sooner, resulting in a smaller seed.

In addition, temperature affects the concentration of various fatty acids in the oil mix. Higher temperatures favor the synthesis of palmitic and oleic acids while an increase in the concentration of linolenic and linoleic-unsaturated fatty acids-coincides with lower temperatures, according to Jim Board, an agronomist at Louisiana State University. The difference has to do with the proteins involved in the synthesis of each. "The makeup of all things is controlled by enzymes and these are differentially controlled by temperature," Board explains. "So for the genotypes we have today, the kinds of conditions that favor increasing protein and oil content are the same kind of conditions that favor prolonged seed filling," Westgate says. "Anything that shortens the filling period is going to decrease the total amount of protein and oil without having a big impact on concentration."
In terms of biodiesel production, varieties of soybean with higher oleic acid content have been shown to reduce nitrogen oxide pollutants. Similarly varieties with a lower concentration of fatty acids such as palmitic and stearic have been shown to improve the cold-weather performance of biodiesel.

However, breeding soybean varieties with more oil and protein has proved challenging. This is partly because the energy cost of making oil is high, Board explains. "It takes a great deal more energy to make a gram of oil than to make seed." Westgate's approach to understanding how this happens and how the pathways that lead to oil and protein formation could potentially be manipulated to increase concentrations, involves following the fate of carbon atoms through the metabolic pathways that produce all the various amino acids (the building blocks of protein) and all the various fatty acids (the building blocks of oil). Since plants use sugar to form these building blocks, Westgate's team labels the 12 carbon atoms of sucrose with a nonradioactive isotope, feeds this sugar to soybeans and traces the carbons as they're routed through various biochemical pathways using mass spectrometry. "This is a powerful technique because it allows us to see the activities of certain pathways and to look at interactions between compartments in a cell," Westgate says. "We're trying to link a basic understanding of biochemistry with genetic approaches to understand how the environment makes the seed shift how much carbon goes into these end products."

Soybean School
To spread the word about soybean seed physiology, Westgate is one of the teachers in an annual winter soybean management school held on the ISU campus. The two-day colloquium covers anything related to soybean production. "We cover everything from seed physiology to crop physiology to diseases, different insects, growth and development, soybean rust, soybean fertility, weed management, and then agronomic decisions to maximize yield and grain quality," explains Palle Pedersen, an extension agronomist at ISU and organizer of the event, which will be held at the end of this month. Attendees range from growers to crop consultants, county extension agents, marketing people and seed company agronomists. "The major goal is to get people up to a level where they understand soybean production and they understand the variables that influence yield and grain quality. In addition, we want them to gain an understanding of our research projects and why we're doing them," he says.

Westgate has a similar goal for the seed physiology session he teaches. "I want people to take home a few key messages about how plants face the environment and how they respond to it so they're a little bit more aware of what the consequences of their decisions about management might be."

Jessica Ebert is a Biodiesel Magazine staff writer. Reach her at or (701) 738-4962.
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