5-30-2012emerson weighs in on rootless corn

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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 goodgrowth, there are reports of "floppy" plants in western and northwestern Illinois and into southeasternIowa. Given this spring's better-than-average planting conditions, this was not a problem I expected tosee.

I wrote about similar problems that were found in a limited area in May 2007, following an early startand dry soil conditions(issue 9, May 25, 2007). It's an easy phenomenon to spot. Plants develop usingwater provided by the seminal root system up to the 3- to 5-leaf stage or so. The seminal root systembecomes inadequate to meet the demand as the plant continues to grow. Normally, the nodal rootsystem then takes over as the plant's main root system, providing both water and anchorage for thegrowing plant. If the nodal root system fails to develop, the plant may fall over, often after sufferingfrom 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 Apriland 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 driesout after planting and the seed furrow opens up. This, along with the sidewall compaction createdduring 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." Thisis a relatively rare phenomenon in which the base of the stem (the crown) ends up positioned at orvery 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 isdry they can't penetrate it well, and with little anchor besides the mesocotyl that attaches the stem tothe 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 withconfidence after we see the problem. The crown is usually set when light strikes the tip of thecoleoptile as it emerges above ground; a signal from the coleoptile tip (which also stops growingwhen light reaches it) stops the elongation of the mesocotyl. Thus the crown, positioned between themesocotyl and the coleoptile, is set at its proper depth in the soil. When soils after planting are verywarm, coleoptile and mesocotyl growth are rapid, and if the coleoptile tip emerges at dusk, there canbe 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, whenplanting is into dry soils fluffed by tillage. If the planting furrow opens as soils dry after planting (thisis most common in no-till), coleoptile growth stops and the crown can be set near the seed, essentiallyplacing 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 rootsystem typically reaches its maximum size by stage V2 or so, though if nodal roots are unable to form

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it might make additional growth. As the plant increases in size, the limit to the seminal root system'sability to provide water and nutrients is quickly reached, and without nodal roots to take over, suchplants 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 abilityof the plant to grow much more or to form roots even if it had the water to allow this to happen. If itstays dry with the plant lying on the soil, it may eventually break off its mesocotyl anchor and diequickly.

In some areas that have remained dry during May, even fields where the crown is at normal depthmay have plants that are struggling to establish nodal roots due to dry surface soils. Roots simplycannot grow into soils from which they can't extract water as they grow. The first sign of inadequateroot systems is usually curling of leaves in the afternoon. As water shortage progresses, leaf curlingtakes 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 mayactually be preferable to having them turn pale green. This is because purpling results from sugaraccumulation in the pIant, and sugars can't accumulate without photosynthesis. There are severalcauses of sugar accumulation, including lack of places (like roots) for sugars to move to and lack ofenough phosphorus to help sugars move. Roots that are growing poorly don't need much sugar, andthey don't get to the phosphorus in the soil very well, so plants with poor roots for any reason oftenturn purple. Some hybrids do this faster than others, but a return to normal root growth quicklyalleviates 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 ofthis 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 therow to keep plants standing until it rains will theoretically help, but it would have to be done beforeplants start to fall over, and it is not easy. Some may even try watering down the row; wetting the soilin 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 productiveeven if it rains and allows nodal roots to penetrate into the soil and grow. Such roots tend to growdownward at a steep angle, which might be a small advantage, or at least no disadvantage. But withtheir delayed start and the fact that some roots initiated above the soil surface often do not penetratethe soil surface very well, such plants may become fully productive only if the season turns out to berelatively 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 hasmeant very rapid growth. Corn planted in central Illinois in mid-March and not damaged by frost hasby now accumulated about 900 growing degree days and so has reached V9-V10, the point at whichstem 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 surfacesoils have meant low evaporation rates, so most water is moving out through plants, which meanshigh 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 to10 inches of water to a crop, there should be enough water to keep the crop growing well into June. Atsome point, of course, we will need rain to keep the crop growing up to its potential.--EmersonNafziger


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5-30-2012emerson weighs in on rootless corn

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