LATERAL ROOTS IN MAIZE
Seedlings are now, in US corn belt, growing roots from the first stem nodes. Near the root tip new primordia are initiated, from which lateral roots can grow. The number of lateral roots is affected by genetics and environment. There is a tendency for fewer if soil is exceptionally dry, resulting in more extension downwards of the secondary roots. More lateral roots have some advantage for reducing root lodging but also comes at some energy cost in that because each of the new root tips draws more energy from the leaves. On the other hand, the deeper growth of the main roots is most likely to reach deeper water reserves. More information available at www.plantphysiol.org/cgi/doi/10.1104/pp.15.00187.
I personally witnessed this phenomenon in 1980. A hybrid (A) was frequently noted as having high yields in many environments but in the very dry year of 1980 it was outstandingly higher yielding including much better than its widely used competitor hybrid (B). Demand for hybrid A skyrocketed. 1981 was a near opposite season for water. Hybrid B did very well whereas Hybrid A had decent ears but root lodged badly only shortly after pollination. The drought resistance obtained by having deeper roots but fewer lateral roots was good under dry conditions but Hybrid B, with its tendency to have more lateral roots was better when the topsoil was exceptionally wet for most of the summer, especially as minimal till placed much organic matter in those top layers.
Like most aspects of corn, we look for a balance of structures appropriate for that season’s condition and no hybrid seems to be at its best every year.
ROOTS AND HORMONES IN MAIZE
Primary roots, developing from the corn seed are temporary. Direction of the growth downward is affected by auxins originating in the growing embryo. By the time the first leaves emerge to light, however, much of the auxin production comes from the leaf tips. These plant hormones, along with a few others, are transported quickly through the phloem sieve cells along with carbohydrates and protein, to the root tips, uninhibited by cell membranes. Root tip cells, with less developed cell walls but with membranes do require active transport, and energy, to move the auxins into these newly developing cells.
Root tips cells are meristematic, dividing and producing new cells at a near constant rate in corn up until pollination. New cells on the outside layer (epidermis) of the root are stimulated by the auxins to produce lateral extensions called root hairs. These cell extensions large vacuoles allow active osmosis effectively increasing the absorption of water and minerals. Root hairs only remain active for a 2-14 days, but the continuously dividing root tip cells produce new ones in the newest epidermal cells. Movement of auxins to the epidermal cells does involve other hormones and proteins with several genetics with several environmental influences. For example, studies have shown that decreasing soil moisture results in production of more root hairs in corn. Given the wide genetic variability of corn, surely varieties differ in this response.
Anaerobic soil environments
Water-logged soil will have spaces for oxygen molecules to support uptake of this important element needed for cellular respiration. The few bacteria that can survive these environments without free oxygen gas have another method of obtaining oxygen from compounds as nitrate (NO3). These bacteria take away an oxygen atom to reduce it to NO2, and then continue to reduce to NO and finally the gas N2.
This process not only reduces the nitrate available for eventual absorption in the root tissue as the N2 gas is not absorbed in corn roots but also will escape into the surrounding air. Not only does the wet pond in the field reduce root growth but also can lead to eventual nitrate reduction needed for plant growth after the flooding subsides.
Pythium, a genus of organisms that appear like a fungus but is classified in a separate group called Oomycetes. This group of organisms feature production of an overwintering spore (oospore) that germinates in water with production of swimming spores called zoospores. They swim towards root tissue. After attaching, it produces filaments penetrating the root. The host supplies nutrition to the pathogen, allowing it to eventually produce more oospores. Pythium species can kill the corn seedling.
The multiple dynamics of rain amounts, soil types, drainage, corn growth stage and multiple organisms in the soil influence the affect of early season flooding in corn fields.
Oxygen for roots
Respiration in corn roots, like respiration in the leaves is the process in which glucose is broken down into a chemically useable form of energy (ATP), CO2 and H20. This process requires oxygen (O2). Obtaining oxygen for leaves is available from the atmosphere as it passes through leaf stomata, but how does it get into roots?
Root hairs, those fine extensions of the root epidermis, have thin walls that allow absorption of minerals, and passes of gases such as oxygen existing in small air pockets in the soil. These root hairs also allow the passage of CO2 from respiration to move outside the root tissue. Absorption of oxygen allows the respiration within the cells, in mitochondria, to release energy for other root activity including cell division and active transport of nutrients to other parts of the plant. Experiments with other plant species grown in water culture have shown that roots grew larger and with more root hairs when the water was aerated versus non-aerated. More root tissue with more root hairs increases mineral absorption, better transport of water and minerals to above ground parts and less vulnerability to lodging as the plant grows.
Soil compaction and excessive water that reduces oxygen available to roots can have a detrimental effect on corn plants for the whole season if the flooding is prolonged.
Anthracnose on seedlings
Cold weather of temperate zone winters can be harsh on fungi in the previous crop debris left on the soil surface after harvest. Low temperatures kill most spores (conidia) capable of spreading and infecting new crop corn plants. Although spring moisture can encourage production of new spores from infections in the old leaves, inconsistent temperatures and relative humidity plus sun exposure of the young seedlings can cause result in many potential fungal pathogens to fail infection of the young plants.
Colletotrichum graminicola (cause of anthracnose) produces spores on surface of infected leaves in mucilaginous matrix that offers protection of the spores on the infected debris from temperature fluctuations and dehydration. This allows survival of spores for quick distribution to seedling leaves. Spores germinate and hyphae quickly form appressoria, allowing penetration in the first few seedling leaves. Corn varieties vary in resistance to further spread of the fungus to the growing point or roots. Killing of seedlings can occur in a few varieties but not in most.
Most studies have shown that there is not a strong correlation among susceptibility to the anthracnose seedling disease, anthracnose on mature leaves and anthracnose stalk rot. This fungus’ ability to overwinter in minimally tilled, continuous corn fields with anthracnose in the previous season are most vulnerable to this seedling disease.
An interesting study of this phenomenon can be found at:
Corn Journal 4/9/2019
Energy for growth
Young corn seedlings are provided with energy for growth and development by new photosynthesis in the young leaves. Light, of course is the important, source of that energy as a series of enzymes in the chloroplasts transform the light energy into stored chemical energy of glucose. Glucose is moved to mitochondria in cells, where the energy is captured into the chemically useable energy of ATP. This chemical energy powers the growing points as it divides, resulting in more cells including more chloroplasts and mitochondria. The result is a growing corn plant.
The other, perhaps less obvious energy source is heat. The speed of movement of the process in all aspects of the plant cells is affected by heat energy. This includes the rate of photosynthesis, the rate slowing as temperatures approach freezing and increases until temperatures above 104°F, at which enzyme integrity falls apart. Low temperatures also slow down the movement of the glucose within and to other cells. The low temperature effect on glucose movement appears to be greater than the effect on photosynthesis, resulting an accumulation of sugars in the leaf tissue.
Plants, including corn, tend to react to over accumulation of sugars in leaves, by production of pigments, a form of carotene called xanthophylls. These are often red pigments in corn. They absorb the light energy, protecting the molecules within chloroplasts from damage from accumulation of too much glucose.
Hybrids will vary in intensity of red pigments in plants that are exposed to cold spring weather but, they will recover with warm weather as glucose resumes movement to the growth areas.
Biology of corn seedling diseases
Corn seedlings face soil environments with multiple potential pathogens and saprophytes. If stressed by cold wet weather that favor pathogens such as Pythium species, stimulated to grow towards seedlings leaking metabolites. If warmer, Fusarium species likewise are attracted to the living plant tissue. Resistance mechanism in the seedlings includes structures of the plant tissues and production of anti-fungal metabolites produced in the root and hypocotyl cells. The latter is influenced by heat energy, cell vitality and seedling ‘vigor’.
If an individual plant survives these early potential problems to emerge with 2-3 leaves, as nodal roots take over and the hypocotyl and earlier roots decline, the plant will usually not show the early wilt symptoms. Fusarium may have successfully penetrated earlier and make its way to the apical meristem without causing direct damage. It has been shown that even if the Fusarium hyphae were in the seed before planting, it does little visible damage until perhaps making its way to the new kernels by harvest.
When seedlings wilt in the field, it is difficult to assign a single cause. It is usually seen scattered among undamaged seedlings. Was it biological vigor or quality of that individual seed, microenvironment of that individual seed or scattered presence of a pathogen. Samples of dead seedlings will frequently show presence of Fusarium species but was it a cause of the seedling death or simply a quick invader of weakened plant tissue? This is not an easy problem to correctly analyze.
The next stage
The first critical state of establishing a corn crop is with germination. This first stage allows the embryo radical to emerge and push downwards, establishing the primary root tissue, and the shoot tissue pushing upwards with the hypocotyl pushing the shoot meristem towards the light. As the hypocotyl grows, roots grow at its base and become temporary roots called seminal roots. Energy for these growths are dependent upon stored carbohydrates in the endosperm and conversion into usable energy in cells via mitochondria. Heat energy from the environment assists with these physiological moves. After the hypocotyl pushes the meristem to the light, the first leaves emerge and begin photosynthesis and resulting new carbohydrates for more growth. This extra energy allows the formation of secondary roots to begin at the nodes at the base of the shoot meristem, establishing the primary long-term nodal roots. These nodes remain under the soil surface. As more leaves form, and the lower higher nodes form above the soil surface these roots gain the dual function of absorbing and transporting water and minerals and supporting the stem. We often refer to these as brace roots.
Bob Nelson at Purdue has an informative summary of these root events (https://www.agry.purdue.edu/ext/corn/news/timeless/Roots.html)
If seed and environment cooperate, this next stage gets the corn crop off to a great start.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.