Why plants end up perched above the soil like is not clear. The crown (base of the stem) is usually set when light strikes the tip of the coleoptile as it emerges above ground. At this point, the coleoptile and the mesocotyl stop growing and the crown depth is set as the first leaf breaks through the coleoptile.
One possible disruption of this process can be rapid growth in warm soils, when the tips of coleoptiles that emerge in the evening do not stop growing until the next morning, at which point the crown is already near the soil surface. But because soil temperatures were only in the 50s in late April this year, that is not a likely explanation.
Another cause of high crown placement is soil subsidence due to rainfall after planting into dry soils fluffed by tillage. If the planting furrow opens as soils dry after planting (this is most common in no-till), the crown will be set near the seed, 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, cause rapid growth of the mesocotyl and push the crown to the soil surface.
"Once plants can no longer stay upright due to a lack of anchoring roots, water uptake and photosynthesis slow down and the supply of sugars starts to decrease, limiting the ability of the plant to grow or to form roots," Nafziger said. "If the plant is lying on soil that stays dry, it may break off its mesocotyl anchor and die."
In some areas that stayed dry in May, even fields where the crown is at normal depth have plants that are struggling to establish nodal roots due to dry surface soils. Roots cannot grow into soils from which they cannot extract water.
"The usual first sign of inadequate root systems is curling of leaves in the afternoon," explained Nafziger. "As the 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 be preferable to having them turn pale green. "This is because purpling results from sugar accumulation in the pIant, and sugars cannot accumulate without photosynthesis," said Nafziger.
Sugars accumulate when there is no place (such as roots) where they can move, or when there is not enough phosphorus to help them move. Roots that are growing poorly do not need much sugar and have difficulty reaching the phosphorus in the soil, so plants with poor roots often turn purple. Some hybrids do this faster than others, but a return to normal root growth quickly alleviates the purpling, usually with no harm done.