Robert Pore's Ag Blog

Understanding nitrogen metabolism is of critical importance to crop management, as nitrogen availability is one of the major factors limiting crop growth and yield.

All of the nitrogen in a plant, whether derived initially from nitrate, nitrogen fixation, or ammonium ions, is converted to ammonia, which is rapidly incorporated into organic compounds through a number of metabolic pathways beginning with the activity of the enzyme glutamine synthetase (GS), which catalyzes formation of the amino acid glutamine from ammonia and glutamic acid. An individual nitrogen atom may pass many times through the GS reaction, following uptake from the soil, assimilation, remobilization, and delivery to growing roots and leaves, and ultimately, deposition in seed as storage proteins. Thus GS is likely to be a major check-point controlling plant growth and crop productivity.

In research reported in The Plant Cell, scientists Antoine Martin and Bertrand Hirel from the National Institute of Agronomic Research (INRA) in Versailles, France, together with colleagues from institutions in the U.K., Spain, and Japan, present new information on the roles of two forms (isoenzymes) of cytosolic glutamine synthetase (GS) in maize, which underscores the importance of this enzyme and nitrogen metabolism in cereal crop productivity.

Improving nitrogen use efficiency of crop plants, i.e. reducing the amount of costly nitrogen fertilizer inputs that farmers need to apply to crops while at the same time maintaining and even improving yields, is an important goal in crop research.

As noted by Dr. Hirel, “a more complete understanding of the roles of GS enzymes in nitrogen metabolism and grain yield in maize and other crop plants (including rice, wheat and barley) may lead to improvements in fertilizer usage and crop yield, thus mitigating the detrimental effects of the overuse of fertilizers on the environment“.

The roles of these two GS isoenzymes, products of the Gln1-3 and Gln1-4 genes, were investigated by examining the impact of knock-out mutations on kernel yield. GS gene expression was impaired in the mutants, resulting in reduced levels of GS1 protein and activity.

The gln1-4 phenotype displayed reduced kernel size whereas gln1-3 had reduced kernel number, and both phenotypes were evident in the gln1-3 gln1-4 double mutant. Shoot biomass production at maturity was not affected in either the single mutants or double mutants, suggesting that both gene products play a specific role in grain production. Levels of asparagine increased in the leaves of the mutants during grain filling, most likely as a mechanism for circumventing toxic ammonium buildup resulting from abnormally low GS1 activity. Phloem sap analysis revealed that, unlike glutamine, asparagine is not efficiently transported to developing maize kernels, which could account for the reduced kernel production in the mutants. Constitutive overexpression of Gln1-3 in maize leaves resulted in a 30% increase in kernel number relative to wild type, providing further evidence that GS1 plays a major role in kernel yield.

Some of the major cereals, such as maize, sorghum, and sugar cane, exhibit C4 photosynthesis, which enhances the efficiency of photosynthesis at high temperature (most C4 plants originated in tropical climates). In standard C3 photosynthesis (present in rice, wheat, and most temperate crop plants), CO2 entering the leaf is converted to a 3-carbon compound via the C3 pathway, utilizing energy derived from the light reactions of photosynthesis.

In plants that have C4 photosynthesis, the C3 pathway enzymes are localized in specialized “bundle sheath” cells which surround the vascular tissue in the interior of the leaf. CO2 entering mesophyll cells at the leaf surface initially is converted to a 4-carbon compound, which is shuttled into the bundle sheath cells and then decarboxylated to release CO¬2. CO2 released into bundle sheath cells then enters the standard C3 pathway. This CO2-concentrating mechanism allows plants in a hot and dry climate to take up CO2 at night and store it, and release it again inside bundle sheath cells during the day, thus solving the problem of how to maintain a high concentration of CO2 inside the leaf during the daylight hours, when stomata often must be kept closed to prevent water loss.

Using cytoimmunochemistry and in situ hybridization, Martin et al. found that GS1-3 is present in maize mesophyll cells whereas GS1-4 is specifically localized in the bundle sheath cells. Thus the two GS1 isoenzymes play non-redundant roles with respect to their tissue-specific localization, and the activity of both is required for optimal grain yield. This work illustrates the close coordination between nitrogen and carbon metabolism in photosynthetic tissues, and reveals that nitrogen metabolism plays a critical role in optimizing grain yields.

In a NASA study, scientists have concluded that when Earth's climate warms, there is a reduction in the ocean's primary food supply. This poses a potential threat to fisheries and ecosystems.

By comparing nearly a decade of global ocean satellite data with several records of Earth's changing climate, scientists found that whenever climate temperatures warmed, marine plant life in the form of microscopic phytoplankton declined. Whenever climate temperatures cooled, marine plant life became more vigorous or productive. The findings will appear in the journal Nature on Dec. 7.

The results provide a preview of what could happen to ocean biology in the future if Earth's climate warms as the result of increasing levels of greenhouse gases in the atmosphere.

"The evidence is pretty clear that the Earth's climate is changing dramatically, and in this NASA research we see a specific consequence of that change," said oceanographer and study co-author Gene Carl Feldman of NASA's Goddard Space Flight Center, Greenbelt. Md. "It is only by understanding how climate and life on Earth are linked that we can realistically hope to predict how the Earth will be able to support life in the future."

Phytoplankton are microscopic plants living in the upper sunlit layer of the ocean. They are responsible for approximately the same amount of photosynthesis each year as all land plants combined. Changes in phytoplankton growth and photosynthesis influence fishery yields, marine bird populations and the amount of carbon dioxide the oceans remove from the atmosphere.

"Rising levels of carbon dioxide in the atmosphere play a big part in global warming," said lead author Michael Behrenfeld of Oregon State University, Corvallis. "This study shows that as the climate warms, phytoplankton growth rates go down and along with them the amount of carbon dioxide these ocean plants consume. That allows carbon dioxide to accumulate more rapidly in the atmosphere, which would produce more warming."

The findings are from a NASA-funded analysis of data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) instrument on the OrbView-2 spacecraft, launched in 1997. SeaWiFS is jointly operated by GeoEYE, Dulles, Va. and NASA.

The uninterrupted nine-year record shows in great detail the ups and downs of marine biological activity or productivity from month to month and year to year. Captured at the start of this data record was a major, rapid rebound in ocean biological activity after a major El Niño event. El Niño and La Niña are major warming or cooling events, respectively, that occur approximately every 3-7 years in the eastern Pacific Ocean and are known to change weather patterns around the world.

Scientists made their discovery by comparing the SeaWiFS record of the rise and fall of global ocean plant life to different measures of recent global climate change. The climate records included several factors that directly effect ocean conditions, such as changes in sea surface temperature and surface winds. The results support computer model predictions of what could happen to the world's oceans as the result of prolonged future climate warming.

"When we compared changes in phytoplankton activity with simultaneous changes in climate conditions, the agreement between the two records was simply astonishing," Behrenfeld said.

Ocean plant growth increased from 1997 to 1999 as the climate cooled during one of the strongest El Niño to La Niña transitions on record. Since 1999, the climate has been in a period of warming that has seen the health of ocean plants diminish.

The new study also explains why a change in climate produces this effect on ocean plant life. When the climate warms, the temperature of the upper ocean also increases, making it "lighter" than the denser cold water beneath it. This results in a layering or "stratification" of ocean waters that creates an effective barrier between the surface layer and the nutrients below, cutting off phytoplankton's food supply. The scientists confirmed this effect by comparing records of ocean surface water density with the SeaWiFS biological data.

Federal assistance to farmers and ranchers suffering from the ongoing drought fell short Tuesday as the Senate voted down a bipartisan emergency aid amendment to the Agricultural Appropriations Bill.

Both Sens. Ben Nelson and Chuck Hagel of Nebraska sponsored the amendment and voted for the $4.95 billion emergency assistance bill.

The amendment, whose prime sponsor was Sen. Kent Conrad, D-N.D., fell three votes short of the 60 votes it needed to pass. The vote was 57-37.

With six senators failing to vote for the amendment, Conrad may introduce it later this week for a second try.

“I’m discouraged that Congress keeps dancing around this critical aid,” said Nelson. “It’s unacceptable for Washington to keep dragging its feet when our farmers need action now.”

Nelson is concerned because a report released by the U.S. Department of Agriculture last week says this year’s net farm income will be 20 percent lower than last year’s. Also, Nebraska farmers and ranchers are estimated to have lost more than $340 million in production this year alone due to the ongoing drought.

Nelson also said that Nebraska farmers have had to spend an extra $51 million in energy costs as their irrigation pumps ran more than usual this year because of drought conditions.

“The failure to provide this needed relief threatens many small, rural businesses and communities and it threatens our nation’s food and fuel security efforts,” said Nelson. “We can’t stop a natural disaster like drought, but we can and should help when a disaster hits.”

Hagel said disaster assistance funding is long overdue.

“Nebraska’s agricultural producers have endured one of the worst sustained droughts in U.S. history,” Hagel said. “When it is finally passed into law, this assistance will provide important relief for Nebraska’s producers who have suffered severe losses to crops and livestock due to the ongoing conditions.”

If another attempt later this week comes up short, Hagel said, “Disaster assistance must be the top priority when Congress reconvenes in January.”

The disaster assistance amendment would have done the following:
‰Cover crop losses in both the 2005 and 2006 production years for producers who lost at least 35 percent of a specific crop.
‰ Provide funds to cattle producers in disaster counties for increased feed expenses.
‰ Provide funds for the Emergency Conservation Program.
‰ Provide funds for additional personnel for the Farm Service Agency county offices to implement the disaster assistance program.

Nelson said that the Drought Monitor at the University of Nebraska continues to show that much of Nebraska is suffering from severe to extreme drought.

“Yet, even the secretary of agriculture continues to question the need for disaster relief and this Congress stubbornly refuses to provide relief to those farmers and ranchers that have been harmed by this particular natural disaster,” he said.

According to Tom Buis, president of the National Farmers Union, while a majority of senators supported Conrad’s amendment, it was defeated on a procedural point of order supported by a small group of senators.

Buis will be in Grand Island Saturday at the Nebraska Farmers Union state convention.
“Farmers and ranchers all across the country have suffered devastating losses and this assistance would have provided a helping hand to see them through these difficult times,” he said.

As more and more ethanol plants come online, demand for corn has driven up corn prices.

And those higher corn prices have led corn producers to not rely on the government’s loan deficiency payment (LDP) program to supplement their incomes.

According to Corinne Alexander, Purdue University agricultural economist, producers will need to learn to live without the LDP for as long as the biofuels market inflates corn and soybean prices.

Alexander said that means farmers should sharpen their marketing skills in order to maximize their profits in volatile grain markets.

“The biofuels boom makes marketing much more important for farmers,” she said.
With biofuels production driving up the demand for corn and soybeans, crop prices are too high to trigger LDPs, Alexander said.

She said the LDP is a longstanding support payment that pays producers the difference per bushel between local market prices and local loan rates on program crops when prices dip below loan rates.

“What we’ve seen over the last five weeks or so is that corn and soybean prices have increased dramatically,” Alexander said.

Cash corn price Tuesday at the Aurora Co-op elevator in Grand Island was $3.40 per buhel.

According to the Nebraska USDA Agricultural Statistics Service, corn prices statewide have risen dramatically since September’s $2.08 per bushel average. In September of 2005, statewide corn prices averaged $1.72 per bushel.

“Going into this year, we were saying that there may be a chance for an LDP this fall,” Alexander said. “Well, we are way above LDP levels.”

Alexander said going forward with price levels well above the loan rates, farmers aren’t going to have that downward price protection offered by the government.

“They are going to face the full spectrum of price variability,” Alexander said.
In the past, most farmers factored the LDP into their grain marketing decisions, Alexander said.

“There also tended to be large storage returns, so producers would use storage hedges or forward contract on the cash market so that pricing early was often a winning strategy,” she said. “Now we’re seeing prices increase substantially for future deliveries, so waiting to price seems to be a better strategy.”

Even farmers who miss the price peaks or want to wait longer for still higher prices can use other marketing techniques, Alexander said.

“Should there be another energy price spike that increases demand for corn again, they can use a sell-and-defend strategy, where you price your grain with a forward contract and then buy an out-of-the-money call,” she said.