No
Nodal Roots?
Excessive drying of the upper soil profile can be conducive for
the development of what some of us affectionately call the "rootless
corn" or "floppy corn" syndrome. The problem illustrates
a classic example of the importance of the timing of stress and
plant development.
The permanent (nodal) roots of a corn plant develop initially from
near the crown area of the plant (Nielsen, 2007a). They develop
sequentially in sets or "whorls" from individual nodes
of the stalk. The growing point or meristem of a root lies near
the root tip and must remain alive in order for the root to develop
normally.
Contrary
to popular opinion down at the coffee shop, roots do not grow toward moisture
on purpose. They grow downward in response to gravity (i.e., a gravitropic response).
If nodal roots begin development in bone-dry surface soil but elongate into soil
moisture at deeper depths prior to desiccation of the root tip, then the root
will survive and proliferate.
Figure
1. Broken Mesocotyl
Figure
2. Example of "Floppy Corn" Syndrome
If the root tip (and accompanying meristem) desiccates prior to
reaching soil moisture, the entire young nodal root will likely
die. This is particularly true if the axillary meristems along the
length of the root (that eventually produce the branch roots) have
not yet differentiated or become active.
The appearance
of such a desiccated root is what one would imagine; 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. Because several sets of roots may not have formed below ground,
the crown may "appear" to be at or above the surface. That appearance
is an optical illusion except in a few cases (Nielsen, 2004).
Entire sets or "whorls" of nodal roots sometimes die
in this manner and the plant essentially 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 affected with the "rootless" syndrome can remain fairly normal
up until the windy day when the mesocotyl simply can no longer support the plant
and it 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 occurs
to replenish soil moisture levels.
Figure
3. Desiccated, dead nodal roots
Figure
4. "Normal" Nodal root Development
What Can Be Done? 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.
A "Hindsight" Reminder. "Rootless" corn
can also be aggravated by extremely shallow seeding depths that result in nodal
root initiation beginning near the soil surface rather than at the usual 3/4 inch
depth. This is one of several reasons that growers should avoid choosing seeding
depths shallower than about 1 to 1 1/2 inches.
Related References
