Corn Growth and Development


Leafing Out Underground
When corn germinates but fails to emerge and leafs out underground, you are usually looking at a failure of the coleoptile. There are several common causes for leafing out underground.

The first is simply excess light striking the coleoptile before it has emerged completely from the soil. Dry or cloddy soils allow light to penetrate deeper than firm moist soils do, and this can cause the coleoptile to sense light even though it has not reached the surface. When this happens the coleoptile tip will split normally, allowing the leaves to extend underground. This is a very common problem in no-till and minimum till plantings when the planter fails to close the planting slot completely. This partially open slot allows light to strike the tip of the coleoptile, causing it to open prematurely. In many cases herbicide injury is assumed to be the culprit, but as we will discuss later, leafing underground from herbicide has distinctly different symptoms.

Crusting is another common cause of leafing underground. Crusts can restrict the movement of the coleoptile, but the five-leaf miniature seedling inside the coleoptile can continue to grow and expand. This causes the side of the coleoptile to rupture, allowing the leaves to exit while underground. When the coleoptile ruptures on the side, you can generally rule out premature light interception as the problem. Cold conditions can greatly aggravate a crusting problem, causing even more leafing underground.

Some herbicides can cause corn to leaf out underground. These herbicides can weaken the coleoptile wall, causing it to rupture very similar to what you see from cold weather and crusting, but you will see twisting or “cork screwing” of the coleoptile and the mesocotyl. Both cold and crusting can aggravate this situation by delaying emergence. The longer the shoot is under the soil surface, the more herbicide the coleoptile will absorb, weakening it.

Pre-emergence Problems
Whenever conditions delay emergence of a germinated seedling, physiological changes begin to occur within the seedling. Compounds which are toxic to the seedling can build to levels that will cause the death of the seedling. The most common culprit in this situation is cold. Either cold soils or cold weather can delay emergence of the seedling and cause it to die. The best solution is to delay planting until soils are warm enough for rapid emergence.

Post-emergence Problems
Once the seedling is above the ground, the mesocotyl must remain intact to provide water and nutrition to the above-ground portion of the plant. Any time you find yourself looking at sickly plants from emergence to the V-5* stage of development, the first place to look for trouble is the mesocotyl. The mesocotyl should be firm, white and round. If it is anything but, you have found the location of your problem. Seedling blights will commonly attack the mesocotyl causing a gray, brown, watery, or shrunken appearance. This reduces the flow of nutrients and water to the leaves, causing stunting, yellowing and sometimes death. Remember, the mesocotyl is the plant’s only life-support line during these early stages. Anything that damages it will reduce the plant’s viability. No known herbicides attack specifically at the mesocotyl, so look for insects or diseases as the source of the problem.

The last critical step required to send the young corn plant correctly on its way is development of the nodal or permanent root system. These crown roots form from the early nodes of the plant and will provide the bulk of the water and nutrients for the plant after stage V-5. Although these roots do not play a major role in anchoring the plant early, they begin to develop strength shortly after emergence.

Rootless or floppy corn syndrome is the most common problem we see in nodal root development. Nodal roots will form only when moisture is present. If the soil is dry, the nodal roots will stop or fail to develop at all. This is common in no-till, especially sod. If the planting slot is not closed properly, the area around the crown dries out and nodal roots fail to form. It is critical to maintain moist soil at the one-half to three-fourth inch depth until the crown roots are developed. Adequate moisture at planting depth will allow the plant to continue to grow, but if the soil above planting depth is too dry to form nodal roots, the plant will eventually fall over under its own weight. This is not related to herbicide injury or insecticide failure, but is caused simply by a lack of moisture at crown depth.

These problems, among others, can hinder your efforts to establish that championship stand you are looking for. We'll soon post a page on Mid-season Corn Development, where we will address common conditions to look for as the plant develops.

* The Iowa State University Leaf Collar Staging Method is used in this series of articles to identify the stages of corn plant development.