Published May 2007

Root Development in Young Corn

URL: http://www.kingcorn.org/news/timeless/Roots.html

R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
Email address:
 

uccessful emergence (fast & uniform) does not guarantee successful stand establishment in corn. The next crucial phase is the establishment of a vigorous nodal root system. Success is largely dependent on the initial development of nodal roots from roughly V2 (2 leaves with visible collars) to V6.

Corn is a grass and has a fibrous type root system, as compared to soybeans or alfalfa that have tap root systems.  Stunting or restriction of the nodal root system during their initial development (e.g., from dry soil, wet soil, cold soil, insect damage, herbicide damage, sidewall compaction, tillage compaction) can easily stunt the entire plant’s development. In fact, when you are attempting to diagnose the cause of stunted corn early in the season, the first place to begin searching for the culprit is below ground.

To better understand rooting development and problems associated with root restrictions, it is important to recognize that root development in corn occurs in two phases. The first phase is the development of the seminal or seed root system. The second phase is the development of the nodal or crown root system. 

Corny Trivia: Sometimes you may hear the seminal root system referred to as the primary root system and the nodal root system as the secondary root system. This classification was described by Cannon (1949) and certainly makes chronological sense but always confuses me from the standpoint of importance to the plant.

The Seminal (Seed) Root System

Seminal (seed) roots originate from the scutellar node located within the seed embryo and are composed of the radicle and lateral seminal roots. The radicle emerges first and elongates towards the tip end of a kernel. The lateral seminal roots emerge later from behind the coleoptile and elongate towards the dent end of a kernel.

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Seminal & nodal root systems of late V1 corn seedling.

Closer view of seminal roots and nodal roots from 1st node of crown.

The seminal root system anchors the young seedling and absorbs small amounts of water and nutrients for the first two to three weeks. The rate of growth for these seminal roots slows down dramatically shortly after the coleoptile emerges from the soil surface. Conventional wisdom (or agronomic legend) suggests that though the seminal root system is important during these early stages of development, their contribution toward the season-long support of the plant is minimal if not nil.

A young corn seedling depends primarily on the energy reserves of the kernel until the nodal roots develop. These energy reserves are translocated from the kernel through the connecting mesocotyl “pipeline” to the young stalk and leaf tissues. Therefore, healthy kernel, seed roots, and mesocotyl are vital until the nodal roots are well established.

If damage occurs to seminal roots or the mesocotyl before the later-developing permanent roots become established, stunting or death of the plant will occur. Examples of such damage include salt injury from excessive rates of starter fertilizer, seedling diseases, herbicide injury and insect feeding damage.

Nodal (Or Permanent) Root System

Shortly after seedling emergence, the first set or whorl of the so-called permanent roots begins to elongate from the first of the stalk nodes located near the crown of the young seedling and are distinctly visible by at least leaf stage V2. Individual sets of nodal roots develop sequentially over time from each belowground stalk node plus one or more aboveground nodes. By leaf stage V6, five sets of nodal roots are typically well established and have completely taken over the sustenance of the plant.

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V2 corn seedling.

Seminal and nodal roots of V2 seedling.

Elongation of the stalk tissue begins between leaf stages V4 and V5.  Elongation of the internode above the fifth node usually elevates the sixth node above ground. Subsequent elongation of higher-numbered stalk internodes will result in higher and higher placement of the remaining stalk nodes. Sets of nodal roots that form at above ground stalk nodes are commonly referred to as “brace” roots, but function identically to those nodal roots that form below ground. If surface soil conditions are favorable (moist and not excessively hot), brace roots will successfully penetrate the soil, proliferate, and effectively scavenge the upper soil layers for water and nutrients.

Corny Trivia: Root hairs are lateral extensions of root epidermal cells, grow to a length of several millimeters, and number about 200 per sq. millimeter (Gardner et al., 1985). Their typical life span is only about 2 days at moderate temperatures and less so at higher temperatures (Gardner et al., 1985). Root hairs are visible even on the radicle root of a young seedling. Collectively, the surface area represented by root hairs is very large and can account for a large share of nutrient and moisture uptake by the plant.

A split stalk of an older plant will reveal a “woody” triangle of stalk tissue at the bottom of the corn stalk.  This triangle is typically comprised of four stalk nodes, one on top of the other, whose associated internodes do not elongate. The first internode to elongate is the one above the fourth node, which elongates about 1/4 to 1/2 inch, above which is found the fifth node (usually still below or just at the soil surface). Consequently, five sets or whorls of nodal roots will usually be detectable below ground (one set for each of the below ground stalk nodes).

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First five sets of nodal roots identified on a split stalk of corn.

Length of internode between 4th and 5th nodes of corn.

Effects of Root Damage in Young Corn Plants

Even though the seminal root system contributes little to the season-long maintenance of the corn plant, early damage to the radicle or lateral seminal roots can stunt initial seedling development and delay emergence. Such damage will not necessarily cause immediate death of the seedling as long as the kernel itself and mesocotyl remain healthy, but may result in the seedling leafing out underground. As more and more nodal roots become established over time, damage to the seminal root system will have less and less impact on seedling survival.

Examples of seminal root damage include imbibitional chilling injury, post-germination injury from lethal or sub-lethal cold temperatures, and “salt” injury from excessive rates of starter fertilizer placed too close to the kernel. Symptoms of such root damage include retarded root elongation, brown tissue discoloration, prolific root branching, and outright death of root tissue. If the radicle root is damaged severely during its emergence from the kernel, the entire radicle root may die. Once the radicle has elongated a half-inch or so, damage to the root tip will not necessarily kill the entire root, but rather axillary root meristems may initiate extensive root branching in response to damage to the apical meristem.

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Delayed emerger w/ healthy lateral seminal roots but damaged radicle root. Coleoptile is splitting prematurely and seedling will likely leaf out underground.	Closer view of premature splitting of coleoptile.	Closer view of damaged (dead) radicle root.
Another delayed emerger with healthy lateral seminal roots and damaged, but alive, radicle root.	Closer view of damaged radicle root tip with numerous adventitious roots.

Corn seedlings transition from dependence on kernel reserves to dependence on nutritional support by the nodal roots around the V3 leaf stage. Damage or stress to the first few sets of developing nodal roots during the time period V1 to V5 can severely stunt or delay a corn plant’s development. Damage to the first few sets of nodal roots forces the young seedling to continue its dependence on kernel reserves longer than is optimum. If the kernel reserves are nearly exhausted, continued seedling development is easily stunted and seedling death is not uncommon. Typical stresses that can stunt initial nodal development include fertilizer salt injury, seedling diseases, herbicide injury, insect feeding damage, excessively wet or dry soils, soil compaction (tillage or planter).

Corny Trivia: The primary meristem of a root is located near the root tip. Elongation of cells behind the meristem leads to elongation of the root.

A somewhat uncommon, but dramatic, stunted root symptom is what is referred to as the “floppy corn” or “rootless corn” phenomenon. This problem occurs as a result of the detrimental effects of excessively dry surface soil near the time of initial nodal root elongation in young (V2 to V4) corn plants. Young nodal roots that emerge from the crown area of the plant will die if their root tips (and associated meristematic areas) desiccate prior to successful root establishment in moist soil. The crown of a young corn plant is typically located only 3/4 inch or so below the soil surface and so is particularly vulnerable to dry upper soil conditions.

Related References

Cannon, William Austin. 1949. A Tentative Classification of Root Systems. Ecology 30[4], 542-548.

Gardner, Franklin P., R. Brent Pearce, and Roger L. Mitchell. 1985. Physiology of Crop Plants. Iowa State Univ. Press, Ames, IA.

Nielsen, RL (Bob). 2004. Over-Extended Mesocotyls and Floppy Corn Syndrome. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/articles.04/FloppyCorn-0624.html. (URL verified 5/16/07).

Nielsen, RL (Bob). 2007a. Germination Events in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/GerminationEvents.html. (URL verified 5/16/07).

Nielsen, RL (Bob). 2007b. The Emergence Process in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at  http://www.kingcorn.org/news/timeless/Emergence.html. (URL verified 5/16/07).

Nielsen, RL (Bob). 2007c. Visible Indicators of Germination in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at   http://www.kingcorn.org/news/timeless/GerminationGallery.html. (URL verified 5/16/07