Although the majority of Missouri’s corn crop has been planted, recent weather conditions have had, and may continue to have, negative effects on the health of the resulting plants. Some of these effects are visible enough to result in replanting. Unfortunately, some effects are hidden below ground and may not become apparent until later in the growing season.
One possible effect from this weather is a weakened or slowly developing root system. Corn plants, like most annual grass plants, produce two root systems. The first root system (primary) is composed of seven roots that all arise from within the seed. These roots anchor the seeding and sustain the seedling for the first couple of weeks after emergence. The main root system (secondary) of the corn plant is composed of numerous roots that originate from stem tissue. These adventitious roots are located at nodes along the stem, both below and above ground.
Adventitious roots begin their development shortly after the seedling has emerged, but it takes several weeks before the roots are capable of sustaining the plant. Until at least several adventitious roots are functioning, corn plants are dependent on the primary root system for water, mineral nutrients, and anchorage in the soil. The only link between these roots and the rest of the plant is a thin piece of stem called the mesocotyl. The mesocotyl is the first stem internode and stretches from the scutellar node (junction between the stem and root tissues) inside the seed to the first stem node. The mesocotyl elongates and pushes the growing point toward the soil surface. Detection of light by the coleoptile usually stops mesocotyl elongation when the first node and the growing point is about three fourths of an inch below the soil surface.
The mesocotyl can be damaged by disease organisms or insects. Damage to the mesocotyl slows or completely stops translocation of water and mineral nutrients to the developing seedling. The mesocotyl also is the connection between the young seedling and the stored reserves of the seed. Timing of damage is critical to understanding the magnitude of its effects. If the mesocotyl is damaged early, then the life of the seedling is at risk. If the mesocotyl is damaged after the adventitious roots have developed the effect is minimal.
The transition between the plant’s reliance on the primary root system to a fully functioning secondary root system is usually smooth with few problems. This year a sequence of weather conditions may happen that may cause some concern. In some fields, cool and wet soils have limited adventitious root growth. If the cool, wet weather is rapidly replaced with hot weather, small root systems may not be able to supply enough water to the leaves to prevent wilting. Some wilting is survivable, but if the stress is severe in intensity or in length of time, plants could be killed.
This first stem node is the site where the first set of adventitious roots arises. As stated earlier, the first node will be about three fourths of an inch below the soil surface. Since the next four or five internodes of the stem undergo little if any elongation, five nodes, all capable of producing adventitious roots, are located just under the soil surface. This is why soil conditions near the soil surface can have a large effect on corn root development. Root development requires pore spaces large enough for root tips to enter, but not too large so that air touches the root tips rather soil particles. Moisture must be present, but if the soil is waterlogged, then critical oxygen is excluded. And, although cool temperature slow root growth, hot temperatures can inhibit growth and even kill the emerging root tips.
Soil conditions that limit adventitious root growth can lead to what is often called rootless corn syndrome. With this syndrome, corn plants may appear normal through V4 or V5. But soon, plants begin to lodge because adventitious roots had not formed properly. Cloddy soils that were tilled too wet, compacted soils, especially within the top inch, and hot dry soils can all lead to rootless corn. Unfortunately, little can be done to counteract the syndrome. Although it may seem an odd wish this year, a timely rain is about the only thing that can stimulate adventitious root growth and anchor the plants.
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REVISED: October 2, 2015