Emergence Emergencies

May 31, 2009 07:00 PM

by Bill Wiebold, University of Missouri extension corn and soybean specialist
Seeds are like tiny miracles. We plant them and expect that they will emerge, but often take for granted how much science is happening inside. That's particularly true in years like this when Mother Nature dishes out some challenges.
Although weather conditions are similar to 2008 [at least in Missouri], it appears corn emergence problems are more severe and widespread this year.
Causes of poor corn emergence can be difficult to assess and are highly dependent on soil conditions near the seed. Seed treatments can help get us past less than optimum conditions, but disease and insect pests aren't the only bad things that happen to high quality seeds.
Newly planted corn seeds absorb water, and this increase in water content turns on all the life processes of the seed. Unfortunately, water absorption itself can damage the seed if the water is cold. Cold water damages cell membranes, which surround the entire cell and all of the cell organelles. Damaged cell membranes can cause cell death if the damage is severe. But perhaps more problematic, is that weakened membranes allow cell contents to leak from the cell. Cell leakage of sugars and proteins is possible even if the membrane damage is not severe enough to cause cell death. These sugars and proteins act as a magnet for insects and pathogens.
Cold water imbibition damage is difficult to diagnose because all the damage occurs below ground and the resulting cell leakage stimulates other problems that are more often considered the cause of emergence problems.
The first signs of germination are when the first root and the coleoptile emerge from the seed. The coleoptile is a leaf modified to appear like a tube. It is what we see when we say that the corn is "spiking.” Both of these structures must rupture the seed coat to emerge.
As these ruptures tear openings in the seed, proteins and sugars spill into the surrounding soil. These sugars and proteins are food sources for pathogens, and fungi can multiply rapidly as they feed on the leakage. Tears in the seed coat are entry points that allow pathogens and insects to invade the seed.
Life processes require abundant oxygen. As seeds proceed through germination to emergence, oxygen demand rapidly increases.
The amount of oxygen in the soil depends on the amount of pore space. Soil texture affects total pore space, and soil compaction decreases pore space. Water is stored in the same soil pore spaces as oxygen. In other words, more water in a soil means less oxygen.
Here's the rub: if oxygen supply is limited, actively growing seeds will deplete oxygen even further. That can cause cell and seedling death.
All of these problems probably occur at some level each year, but the effects on corn stands are usually minimal because spring weather conditions typically improve after planting. The worst case scenario is a combination of fluctuating soil temperatures and frequent precipitation.
Seeds remain relatively "safe” if soil conditions prevent germination. Minimum temperature for corn germination is usually given as 50 degrees Fahrenheit. However, slow germination will occur at temperatures in the high 40's. Dry soil doesn't contain enough water to stimulate germination. If the seed coat has not been damaged during handling, disease pathogens and even most insects will not damage seeds after planting into cold and dry soils.
However, once germination begins, it is a race between seedling establishment and the seedling enemies. Fluctuating soil temperatures and water status allow germination to begin, but then slow the process to a point where the germinating seed and the emerging seedling are at a disadvantage.
The bottom line is once seeds have been planted, they are the mercy of Mother Nature. Protection of the seed with effective seed treatments can help, but will not eliminate all the problems leading to poor stands. Delayed planting sometimes works, but will not be much help if soil conditions deteriorate after planting. Reducing activities that compact soil around seeds is your best defense. Compaction reduces root health and delays emergence.

For More Information
You can find another explanation of the science behind corn seedling struggles from University of Illinois extension specialist Emerson Nafzinger at: http://ipm.illinois.edu/bulletin/article.php?id=1121.


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