Infection of flooded, young corn plants
This wet late spring has resulted in pools of water in low areas of Midwest US fields. One of the effects can be infection by an organism called Scleropthora macrospora. This is a fungus-like organism belonging to a group of organisms called Oomycetes. Also in this group are pathogens causing Downy Mildew and Pythium diseases of corn and other plants. Common among these are the ability to form thick walled spores to withstand stress environments that can release swimming spores when in water-saturated soil. S. macrospora infects more than 140 grass species in addition to corn.
The source of infection of corn is often grasses near a low spot or edge of a field. Oospores in the flooded living and dead leaves release swimming spores (zoospores) when close to the corn submerged leaf tissue these zoospores release a germ tube that infects the plant. The filaments (hyphae) grow towards the meristems throughout the life of the plant. This can initially be seen as fine stripes in the leaves but the most obvious symptom is proliferation of leafy aberrations of the tassel- the crazy top symptom. Scleropthora macrospora also can grow to the ear bud meristem, causing similar multiple ears from a single node- but no grain.
Related oomycetes occurring in warmer, subtropical and tropical environments can cause similar symptoms. These downy mildew diseases can also cause the proliferation of the tassels and ears. Susceptible genotypes can have severe grain loss from these diseases. Scleropthora macrospora infection is usually limited to a very small area near grass in a low part of the field.
Infection occurs when the plants have less than 6 leaves. Symptoms that show late in the season, but the problem began with excessive rain that occurred only a few weeks after planting. That early moisture that may contribute to large yields can allow forgiving this pathogen for forming these unusual corn structures in a few spots of the field. In addition, it is just part of the interesting biology of corn.
Corn seedlings and low temperatures
Spring temperatures in temperate zones are variable. Movement of maize from its central Mexico origin to much cooler and variable environments required many years of selection for performance under temperature stresses not experienced by its ancestors. Fortunately, the genetic variability within corn and the efforts by people to select for adaptation has resulted in varieties that survive at above freezing temperatures.
Low temperatures do result in some reduction in photosynthesis in seedlings but the biggest response is in other metabolism affecting most growing plant functions. Cell division and cell elongation are greatly reduced as the temperature approaches 50° and probably stops at lower temperatures. Metabolism in these young leaves includes creation of more chloroplasts, affecting the color of the new leaves as they slowly emerge with a yellowish color. Cell elongation is reduced under cooler temperatures causing the new leaves to be shorter than when grown under higher temperatures.
Movement of the sugars produced by photosynthesis from leaf tissue to the new meristem cells and root tissues is a metabolic-driven process. Consequently, sugars can accumulate in the leaf tissue when corn seedlings are too cool. Accumulation of water soluble sugars could have a negative effect on the osmosis of cells, essentially dehydrating them. One adaptation for this condition is production of the red pigment anthocyanin in leaves, resulting in a reduction of photosynthesis. Hybrids differ in occurrence of red leaves when seedlings are exposed to low temperatures. It is temporary and is not thought to have effect on final productivity of the plants.
Genotypes vary in response to low temperatures. This can cause hybrid seed production problems in that it affects the timing of flowering between the two parents. If the male parent development reacted to low early season temperatures more than the female parent, the result can be little pollen at the time the female parent plants are silking. This can result in less seed and higher probability of genetic purity problems. Seed producers develop strategies to negate this potential as it is part of the seed business.
Corn seedling diseases
Midwest corn in spring of 2017 is starting the season favoring seedling disease symptoms. It is difficult to sort out the real cause of seed not emerging or emerging much later than adjacent plants. Seeds are planted in environments that vary every few inches for water holding capacity, organic content and microbes. Furthermore, each individual seed varies slightly in its cellular membrane status. With imbibition causing swelling of the membrane bound cell contents, some seed can have problems getting effective metabolism for early cell growth to push out the root and stem structures.
Cell metabolism includes producing the response to attacks by potential pathogens in the soil. These anti-pathogen chemicals (phytoalexins) are usually produced with a complex system of detecting the microbe and concentrating the phytoalexin into the area of the attack. Weakened seed not only are likely to release more carbohydrates and proteins into soil because of membrane injury, but also be less capable of responding to the microbes invading root and mesocotyl tissue.
Diagnosis of seedling disease becomes complicated also. Pathologists can isolate a fungus such as a Fusarium species or an oomycete like a Pythium species, but the actual cause probably involves some interaction between the microbes, metabolic quality for the ‘diseased’ seedling, and a complex environment not only providing potential pathogens but also affecting the seedlings metabolic rate. Soil organisms are affected by the environments as well. Leakage of carbohydrates directs their growth toward the seedling roots but temperatures favor some over others. Pythium’s swimming spores do well in cool wet environments but can be inhibited by certain seed treatments that have very little effect on fungi such as Fusarium species. Other seed treatments can inhibit the latter group of microbes but are less effective against Pythium. Corn seed genetics and seed quality can be greater factors than either group of chemicals. Cold wet heavy soils for a prolonged time can overcome all methods of defense.
After the stress on the seedlings is reduced, remaining plants that emerge can give normal production especially if they are uniform in growth with adjacent corn plants. The metabolism of these plants will promote the recovery and normal root growth. Those plants that survive but emerge later than adjacent plants will have difficulty competing for light and mineral uptake which will be reflected in grain productivity.
Adventitious corn roots
After the first corn leaves emerge, the hormonal message to the mesocotyl tissue is to stop pushing upwards. Apical meristem, at the tip of the mesocotyl is now below the soil surface where the first leaf is attached. Photosynthesis now drives the metabolism of the young seedling as it switches from dependence upon the seed endosperm for carbohydrates. Cell division in the meristem produces new leaves, each attached to the young stem under the soil surface and attached in distinct clusters of newly dividing cells called nodes. These nodes are then stimulated to produce roots at about the time the 4th leaf appears in the young seedling. Because the roots are being produced from stem tissue, they are called adventitious roots. As the primary root, that had grown initial seed, loses its energy source, adventitious roots become the main roots for the plant.
The first 4-5 nodes of the young stem remain underground, each producing the roots for the plant, even as the first leaves remain attached at the same locations. What appears to be stem in a 4-5 leaf seedling is a compilation of leaf sheaths tightly wrapped together while the actual stem remains beneath the surface. The underground stem portion, formerly attached to the mesocotyl, with adventitious roots becomes known as the crown. Eventually the mesocotyl deteriorates as it is deprived on nutrition and loses resistance to the many soil organisms.
As the stem growing point eventually emerges above the soil surface a few exposed nodes will often form the brace roots to further support the adult plant.
Coleoptile reaches the soil surface
Soon after the primary root breaks through the kernel pericarp, the embryo stem portion grows upwards. Enzymes in the cells assist in production of hormones and cell wall components need for the elastic cell walls to expand. Turgor pressure caused by adequate water infusion assists in cells growth, auxins assist in the geotropism guiding the growth towards the upwards, as the cells on the lower side of the shoot tissue elongate more than the upper side until growth is vertical. Initial elongation occurs in the mesocotyl tissue with the meristem at it tip, surrounded by the coleoptile, a short leaf tissue, functioning as a shield protecting the more delicate meristem tissue.
The meristem (growing point) in the mature seed already possessed the groups of cells that would elongate into the coleoptile, followed by other lateral groups of dividing cells for the first 5 or 6 nodes all near the tip of the meristem. The first node, furthest from the tip of the apical meristem is at the base of the coleoptile.
As the coleoptile reaches the red spectrum of daylight, hormonal molecules transmitted to the mesocotyl cause stoppage of the its cell elongation. Remaining growth upward comes from growth of the coleoptile. The second furthest node from the tip of the meristem now produces the initial first true leaf, usually a short leaf unwrapped from the coleoptile. With a little more time, the other 4-5 nodes around the meristem begin increasing cell division to eventually produce the next 4-5 true leaves in the young seedlings. These nodes from which the leaves derive, remain under the soil surface, compacted within a few inches of tissue. This underground tissue is sometimes referred to as the crown.
These are the processes of vigorous seed in conditions favoring penetrable soil of adequate temperature, oxygen and moisture with no chemical interference. These conditions are not always present for each seed.
Corn seedling cell walls
The first appearance of corn seedling tissue emerging from the imbibed seed is the new root. This young seedling primary root can be considered as three regions: meristem, elongation and mature. The cell elongation area provides the main initial force for pushing through the kernel pericarp. Cell elongation and maturation involves production of many new molecules composing the cell walls as they grow. The pectins, hemicelluloses and celluloses that compose the new cell walls are composed of several sugar-related molecules joined together through specific reactions, assisted by enzymes, heat energy and chemical energy such as from ATP.
Energy and components for the biosynthesis of these cell wall components comes mostly from the endosperm. Starch in the endosperm is broken down with enzymes into sucrose molecules, moved to root cell cytoplasm where other enzymes break down the sucrose into glucose and fructose. With other enzymatic action, the fructose is made into more glucose. Modification of these sugars allows other new carbohydrate based molecules that become linked to form the more complex polysaccharides such as pectin, hemicellulose and cellulose for the new cell walls.
We see what looks like a rather simple process- seed swells, root protrude and a few days later the stem emerges from the seed. What we don’t see is a complex utilization of stored energy, production of complex proteins some of which act as enzymes assisting in linking molecules together and thus giving outer strength to cells. Also, unseen is production of anti-microbial compounds to ward off the many organisms attracted to the very molecules stored and manufactured in the seed. We don’t witness the genetics that programs for these processes. Humans successfully selected for these features from a wild plant species, adapting it to worldwide growing environments. The complexity of corn seed germination still can be challenged in a cold, wet spring such as being experienced in the 2017 central U.S.A. spring, but it is amazing that these processes work even under tough environments
Corn germination physiology
Successful emergence from the soil is dependent upon the physiological events after imbibition of the seed. Imbibition allowed activation of enzymes needed for the following metabolic processes but temperatures influence the speed of those steps. Important initial enzymatic activity is dissolving the starch in the endosperm into water soluble glucose. These molecules are absorbed in the scutellum where sucrose is formed and then moved into the root and shoot cells of the embryo. The carbohydrate is transformed in the mitochondria to form the ATP energy utilized for cell elongation, mitochondria reproduction, more enzyme creation, more starch digestion and a continuation of the cycle as the radical pushes through the pericarp.
Cellular respiration is an obvious key to these processes. The multiple steps ultimately moves electrons from oxygen and glucose molecules ultimately capturing the energy that holds the atoms to the respective molecules to a new molecule called adenosine triphosphate (ATP). Ultimately the process results in creation of CO2 and H2O molecules as well as the ATP. All of this activity is occurring in the mitochondria on membranes that are essentially electron transport systems. Thus, mitochondrial membrane damage during imbibition, with ageing or during stress while developing in the seed field can have a great effect on the germination process.
Cell elongation continues as the hypocotyl surrounding the apical meristem pushes upward (geotropism), still using the energy stored in the endosperm. Once the leaf tissue reaches the surface, chloroplasts turn green and photosynthesis turns on production of new carbohydrates, significance of the endosperm is reduced.
Adequate water, oxygen and heat are needed for corn germination processes to succeed. It is to credit of grower’s agronomic practices and seed producers that usually all happens within expectations.
A good summary of the physiology can be found here:
Recent cold and wet environment shortly after corn planting favored the fungal-like organism known as Pythium. Among the unique characteristics of these species and other Oomycetes are development of special thick-walled spores for dormancy during winter and dry environments and swimming spores released in moisture. The latter are attracted to the carbohydrates and other chemicals released as seeds imbibe and germinate. After attachment to the tissue, Pythium species form filaments (hyphae or mycelia) invading the plant tissue. Primary roots and mesocotyl are the most frequent site of invasion. Lower temperatures and water saturated soil favor the Pythium and disfavor the corn seedling growth. If these conditions are present before the development of secondary roots in the young plant, while dependent upon nutrition from the seed endosperm, the plant will wilt.
It is reasonable to expect that Pythium will be among the causes of poor stands in some fields this spring as early May weather favored the invader and disfavored the corn planted in the last week of April. Individual seed, already weakened by the internal damage after imbibition, having trouble recovering at the low temperatures, probably are most vulnerable to invasion by the organism favored the same environment. Even if the soil temperature increases to above 50°F, allowing cellular respiration to increase in the young tissue, low oxygen levels in saturated soils will add to the difficulty of the seedling to produce new tissue ahead of the invading Pythium mycelia.
Analysis for causes of poor stands after the recent weather will be complex because of the interaction of seed quality differences among the seed, soil water holding capacity, time of planting, and presence of organisms such as Pythium. Seed treatments selected to ward off Pythium species can be overcome by races of the fungus, further confusing the analysis. In addition, several less pathogenic fungi will invade the damaged seed and seedling tissue making the analysis more confusing.
I am not sure if most people outside of agriculture understand the complexities involved in growing a successful corn crop. And those of us involved in some aspect of agriculture have difficulty understanding these complexities as each new bit of information seems to lead to more questions.
Cold, wet germination
Much of the Midwest corn belt is testing seed quality the hard way this year. Daytime temperatures in the mid 40°F for at least two days and about 3 inches of rain in Northern Illinois. Some fields were planted to corn 1-3 days before the cold, wet weather arrived. All the planted seed will imbibe, expanding the membrane-bound organelles within embryo cells, but no further metabolism will be activated at these low temperatures. Those individual seeds that are already weak will be further damaged from the expansion. Once the temperatures increase cellular metabolism will begin the repair process. Vigorous individuals will begin normal enzymatic digestion of stored starches and movement of these to mitochondria in the embryo, and normal cell growth will occur. Not all seed will have the same chance, however, some dying before it can push out the root or shoot from the kernel’s pericarp. Some may make it eventually but will be slower than the vigorous neighbor seed. Some will have torn first leaf when they emerge or may not have the strength to push to the soil surface.
PSR tests seed lots for percent emergence and uniformity of emergence in warm and cold tests. Four hundred seeds are planted into a greenhouse container with a special artificial soil mix, selected for uniform moisture distribution. Warm tests are placed on heated greenhouse benches with minimum of 70°F. Cold tests utilize the same soil mix but are placed in a cold room at 50°F for 7 days before moved to the greenhouse bench.
Most seed samples that we test show the good uniform emergence as in one of the samples in the following photo and occasionally a sample has uneven emergence as in the other tray. The second photo shows poor uniformity in a sample. Not only is the total count of germinated seed low but some already have the 2nd leaf while others are emerging late, some with torn leaves.
These seedlings are grown in a uniform environment, not devoid of microorganisms but without pathogens such as Pythium species. It is consistent with the view that despite being part of the same sample from one container, each seed can have a different seed quality at least at the time of testing.
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.