Issue No. 8, Article 6/May 25, 2012
Root Problems in Corn Plants
While corn planting is essentially complete in Illinois and the crop in many areas is making good growth, there are reports of "floppy" plants in western and northwestern Illinois and into southeastern Iowa. Given this spring's better-than-average planting conditions, this was not a problem I expected to see.
I wrote about similar problems that were found in a limited area in May 2007, following an early start and dry soil conditions (issue 9, May 25, 2007). It's an easy phenomenon to spot. Plants develop using water provided by the seminal root system up to the 3- to 5-leaf stage or so. The seminal root system becomes inadequate to meet the demand as the plant continues to grow. Normally, the nodal root system then takes over as the plant's main root system, providing both water and anchorage for the growing plant. If the nodal root system fails to develop, the plant may fall over, often after suffering from drought stress. This problem has also been called "rootless" corn, due to the absence of nodal roots.
The problem this year appears to be most common in corn that was planted in the last week of April and is now at stage V3 or so. Some of the affected fields were no-till planted, while others were tilled. We more often see problems with nodal root development in no-till fields, especially when soil dries out after planting and the seed furrow opens up. This, along with the sidewall compaction created during planting, can result in a barrier to nodal root penetration into the bulk soil. If soils stay dry, such plants can become floppy.
Another source of difficulty for nodal roots is what has been dubbed the "high-crown syndrome." This is a relatively rare phenomenon in which the base of the stem (the crown) ends up positioned at or very near the soil surface instead of at its normal placement about 3/4-inch deep in the soil. The nodal roots, which arise from the lower stem nodes, then emerge at or above the soil surface. If the soil is dry they can't penetrate it well, and with little anchor besides the mesocotyl that attaches the stem to the seed, such plants often flop over easily before nodal roots can form.
There are several possible causes of high-crown syndrome, but it's difficult to know the cause with confidence after we see the problem. The crown is usually set when light strikes the tip of the coleoptile as it emerges above ground; a signal from the coleoptile tip (which also stops growing when light reaches it) stops the elongation of the mesocotyl. Thus the crown, positioned between the mesocotyl and the coleoptile, is set at its proper depth in the soil. When soils after planting are very warm, coleoptile and mesocotyl growth are rapid, and if the coleoptile tip emerges at dusk, there can be enough growth overnight to have the crown well above its normal position by the time the light strikes the coleoptile to stop growth the next day.
Another cause of high-crown syndrome is subsidence of the soil due to rainfall after planting, when planting is into dry soils fluffed by tillage. If the planting furrow opens as soils dry after planting (this is most common in no-till), coleoptile growth stops and the crown can be set near the seed, essentially placing the seed and seedling above the soil.
Finally, PGR herbicides such as 2,4-D can, if they reach the seed or seedling during this process, result in rapid growth of the mesocotyl, which can push the crown to the soil surface.The seminal root system typically reaches its maximum size by stage V2 or so, though if nodal roots are unable to form it might make additional growth. As the plant increases in size, the limit to the seminal root system's ability to provide water and nutrients is quickly reached, and without nodal roots to take over, such plants are in real trouble. Once plants can no longer stay upright due to a lack of anchoring roots, water uptake and photosynthesis slow and the supply of sugars starts to decrease, limiting the ability of the plant to grow much more or to form roots even if it had the water to allow this to happen. If it stays dry with the plant lying on the soil, it may eventually break off its mesocotyl anchor and die quickly.
In some areas that have remained dry during May, even fields where the crown is at normal depth may have plants that are struggling to establish nodal roots due to dry surface soils. Roots simply cannot grow into soils from which they can't extract water as they grow. The first sign of inadequate root systems is usually curling of leaves in the afternoon. As water shortage progresses, leaf curling takes place earlier each day, and plants may start to lose their green color.
Plants growing in dry soil often show some degree of purpling as well. Having plants turn purple may actually be preferable to having them turn pale green. This is because purpling results from sugar accumulation in the pIant, and sugars can't accumulate without photosynthesis. There are several causes of sugar accumulation, including lack of places (like roots) for sugars to move to and lack of enough phosphorus to help sugars move. Roots that are growing poorly don't need much sugar, and they don't get to the phosphorus in the soil very well, so plants with poor roots for any reason often turn purple. Some hybrids do this faster than others, but a return to normal root growth quickly alleviates the purpling, with no harm done in most cases.
There is little to be done once corn plants fall over due to lack of nodal roots. Reports are that some of this corn is already being replanted. If rain falls before plants fall over, nodal roots can grow quickly, unless root tips have been dried out and damaged by contact with hard, dry soil. Moving soil into the row to keep plants standing until it rains will theoretically help, but it would have to be done before plants start to fall over, and it is not easy. Some may even try watering down the row; wetting the soil in a band 4 inches by 2 inches over 30-inch rows takes something like 1,500 gallons of water per acre, so this probably is not practical in many fields.
One question is whether plants perched on top of the soil will recover to become fully productive even if it rains and allows nodal roots to penetrate into the soil and grow. Such roots tend to grow downward at a steep angle, which might be a small advantage, or at least no disadvantage. But with their delayed start and the fact that some roots initiated above the soil surface often do not penetrate the soil surface very well, such plants may become fully productive only if the season turns out to be relatively free from stress. But 2012 has not been stress-free so far, at least in most areas.
In better-watered areas where the corn crop established well, the return of warm temperatures has meant very rapid growth. Corn planted in central Illinois in mid-March and not damaged by frost has by now accumulated about 900 growing degree days and so has reached V9-V10, the point at which stem elongation quickly accelerates. Such fields will likely show tassels by mid-June. Corn planted in early April has accumulated about 650 GDD and is at V7, while that planted in mid-April is at V5, having accumulated 520 or so GDD.
Water use accelerates as corn reaches V7-V8, but it is still only about an inch per week. Dry surface soils have meant low evaporation rates, so most water is moving out through plants, which means high efficiency of use. Plants in most areas are still growing well by extracting water from the soil, and the drying of surface soils is encouraging deeper root growth. In deeper soils that can provide 8 to 10 inches of water to a crop, there should be enough water to keep the crop growing well into June. At some point, of course, we will need rain to keep the crop growing up to its potential.--Emerson Nafziger