Excessive drying of the upper soil profile is conducive for the development of what some affectionately call “rootless corn” or “floppy corn” syndrome. The problem illustrates a classic example of the importance of the timing of stress relative to the stage of plant development. The permanent (aka nodal) roots of a corn plant initially elongate from nodes near the crown area of the young plant (approximately 3/4 inch below the soil surface) and first become visible
between leaf collar stages V1 and V2. The young roots develop in individual sets or “whorls”, beginning with the lowermost node of the stalk and progressing sequentially over time in an acropetal fashion up to Node #6 or so.
The main growing point or meristem of a root is located just behind the tip of the root and must remain alive in order for the root to develop normally.
Roots elongate downward primarily in response to gravity. There is evidence that roots also respond to soil moisture gradients by elongating toward soil regions with more favorable moisture levels. However, the overarching determinant of roots’ downward growth is likely gravity.
If nodal roots begin their initial elongation in bone-dry surface soil but elongate downward to adequate soil moisture at deeper depths before the meristematic root tip desiccates, then the root will survive and proliferate. If the root tip desiccates prior to reaching soil moisture, the entire young nodal root often dies.
This is particularly true if the axillary meristems along the length of the root (that eventually produce the adventitious branch roots) have not yet differentiated or become active. The desiccating effect of bone-dry surface soils on young, newly elongating nodal roots is exacerbated by sunny weather and hot temperatures. Dry soil warms more quickly, and dramatically, than wet soil. On a warm, sunny day with air temperatures in the high 80s to low 90s F, soil temperature at the 3/4 inch or depth can exceed lethal levels for young roots. This is especially true for residue-free, conventionally-tilled fields.
The appearance of desiccated roots is what one would imagine; they are shriveled and discolored. This symptom is unlike that of any other lethal root stress, including salt injury from fertilizer. These symptoms are NOT like any associated with herbicide injury or insect feeding. An entire set or “whorl” of nodal roots can emerge and die within the course of a few days. If this
happens to the first set of roots (Node #1) as they begin to elongate (between leaf stages V1 and V2), the young plant survives on what’s left in the kernel reserves and what the seminal roots offer in terms of moisture and nutrient uptake until the next set of nodal roots develop and become established. If subsequent sets of nodal roots die in the same manner, the plant continues its dependence on the kernel and seminal root support.
In fact, it is amazing to me how the aboveground appearance of a plant whose nodal roots are desiccating and dying can appear fairly normal up until the fateful windy day when the mesocotyl simply can no longer support the plant and it literally flops over to the ground. “Floppy” corn plants are NOT technically root-lodged; they are simply broken over at the mesocotyl below the crown area of the plant. Obviously, the health of the mesocotyl and the seminal roots determine whether an affected plant can “hang on” until a decent “soaker” of a
rain occurs to replenish soil moisture levels.
I began this article by telling you that the “Floppy Corn” Syndrome is a classic example of the importance of the timing of stress versus stage of plant development. Rooting of young plants is most vulnerable to the effects of dry surface soils up until the nodal root system has been fairly well-established (about V5 or V6).
Consequently, “Floppy Corn” is more likely to occur in a field of younger, recently-planted corn than an adjacent field of older, earlier-planted corn whose nodal root development is farther along.
Unfortunately, very little can be done to prevent the situation from becoming worse. Row cultivation may encourage new nodal root development if moist soil is thrown around the base of the plants. However, if the soil is dry enough to be causing the problem in the first place, there’s probably very little moist soil shallow enough to be brought up by row cultivation.
The ultimate solution to the problem is a good soaking rain or at least enough of a rain to sustain new nodal root development long enough to allow the roots to reach deeper and hopefully wetter soil conditions before the upper soil dries again.
Indiana corn getting “floppy” from rapid soil drying
