Chilling Injury and Flooding Effects on Corn Stand Establishment
by Imad Saab, with contributions from Scott Hart
"Planting time is one of the most exciting times of the year for growers. But it can also be one of the most stressful for them, and for young corn seedlings germinating in cold and wet ground. The unpredictability of the weather and the swings in temperatures and precipitation present a host of challenges for young plants. In this issue of Product/Technology Update, we offer some references to help diagnose chilling injury in corn. Accurate diagnosis can help growers learn what to expect, or what to avoid in the future."
DIAGNOSING STAND ESTABLISHMENT PROBLEMS
Careful examination of damaged seedlings can provide clues into the likely causes of stand establishment problems following early planting or abnormally cold weather conditions. Table 1 lists the main symptoms and likely causes of early season damage. Table 2 shows diagnostic images of chilling and flooding damage to corn seedlings during germination and emergence.
Research scientists continue to work to improve early season corn performance through conventional and molecular breeding, as well as through rigorous testing of research and commercial hybrids. By identifying molecular markers and pathways associated with superior cold germination, researchers are advancing the science and understanding of the genetic basis of stress emergence. This knowledge should eventually lead to even stronger early-season performance.
COLD STRESS IMPACTS SEEDLING EMERGENCE
Understanding and managing early planting risks is a crucial step to achieving a productive stand. Corn is very sensitive to cold stress in the early season. The damage to stand establishment is greatest if the crop is planted into cold soil (typically below 50 F) or if planting is followed by snow, cold rain or an extended cold spell. Of all management practices that affect stand establishment and in turn, yield, the planting date decision often has the greatest impact.
The optimal temperature for corn emergence is in the range of 80° to 90° F. Emergence is greatly reduced at lower temperatures and is effectively halted around 50° to 55° F or below. The degree of stress, and potential damage from stress, is determined to a large extent by soil and water temperatures during imbibition and seedling emergence.
For successful emergence to occur, all parts of the shoot (roots, mesocotyl, coleoptile and leaf within) must work in a coordinated way to push the coleoptile above the soil surface and allow the first leaf to unfurl. Damage to any one of these structures will likely result in loss of the seedling and its yield potential.
THE CRITICAL FIRST HOURS
When the dry seed imbibes cold water (typically 50° F or below), imbibitional chilling injury may result. The degree of damage ranges from seed death to abnormalities such as corkscrews or fused coleoptiles (Figure 1 and Figure 2).
The potential for cold-water damage generally decreases as the seedlings emerge. It also decreases if the initial imbibition takes place at temperatures above 50° F. This may help explain observations where early-planted corn, which was followed by favorable weather, emerged better than corn planted later and followed by a cold spell or snow cover.
FLOODING EFFECTS ON EMERGENCE
Flooding can have an equally devastating effect on seedling emergence and survival as cold soils. Most corn hybrids can only survive for 24 to 48 hours under water, with smaller seedlings suffering the most damage.
Flooding damages corn biochemically. By impairing mitochondria, it causes release of free radicals which damage cell membranes. Flooding also causes oxygen starvation and shifts the plant’s metabolic processes to anaerobic fermentation. Resulting acidosis (low pH) can kill the cells. At the minimum, flooding reduces the plant’s metabolic rate, making seedlings more sensitive to disease, insects and herbicides. In fact, many disease-causing fungi such as Pythium thrive in standing water. Seedlings that are weakened by flooding or cold damage usually succumb to disease if the pathogen is present in the soil.
Flooding damage does not only occur in obvious ponded areas of a field. If fields are completely saturated to the soil surface and remain that way due to continual rain or limited drainage, seeds and non-emerged seedlings are under water. Flooding damage may occur in these areas just as in ponded areas.