​2019 corn season in USA has started with unusual stress from wet fields in much of the corn growing areas. Not only was planting delayed but effects on seed environments has resulted in uneven emergence in some fields. Although nearly every plant in the single cross hybrid is genetically identical, too much water, or lack of water, seed quality, tillage, and soil compaction and inconsistent planting depth all may contribute to uneven emergence of these seed.  Multiple studies have attempted to evaluate the effect of uneven emergence on final yield.  One study published in 2012 in Journal of Plant Nutrition 35:480-496, 2012 (http://www.tandfonline.com/loi/lpla20) the yields and nitrogen uptake of plants from seeds planted between earlier planted seeds, finding that these individual plants yielded significantly less grain than adjacent plants of the same hybrid.
 
The multiple variables interacting with studies of delayed emergence makes the exact determination of effect emergence on final yield very difficult.  Shading of leaves by adjacent plants reduces photosynthesis.  Delayed silk emergence may miss pollen timing.  Competition for nutrients may be inhibited by earlier and greater root growth of adjacent plants.  Genetics of each hybrid may be affecting the reactions of each hybrid differently.  Although the exact affect in each hybrid-field environment should be expected to differ.
 
In the early 1970’s I was attempting to understand why stalk rot occurred in only some individual plants of a single cross hybrid and not in other adjacent plants.  If the cause was a fungus that was common, why did one plant develop stalk rot but not the genetically identical other plant?  My first hypothesis was that these were late emerging plants.  I marked some of these plants and followed their development though the season. Instead of developing stalk rot these plants had very narrow stalks, flowered later than adjacent plants, had deformed tassels with abnormally few glumes and very small, poorly pollinated ears. Not being sure that these plants were not inbred impurities in that hybrid, I intentionally planted seed between earlier emerged seedling.  This was done at the plant densities of that time with 5 commercial hybrids. The effects on plant develop was the same as observed the previous season, confirming that genetically identical plants are affected by interactions with adjacent plants.

My brief experiments were done with hybrids of the 1970s, commonly bred for much lower densities than is common in the USA today.  It should be expected that each hybrids reaction to delayed emergence will be different as well as each field environment will be different. We can acknowledge that uniform emergence is optimum but prediction of the exact result on final grain yield is complicated.

​The biological entity that we call a corn seed is more complex than it appears.  Each seed of each hybrid may appear to be identical upon first glance, but a closer study reveals external differences in terms of size and shape and, perhaps, external damage.  Each seed within a container may have been produced by pollination by the same male parent onto the silk of the same female parent.  They may have identical genetics.  But position on the ear in the production field may reflect slight differences in environmental exposures ranging from pathogens in the seed production field to handling during seed harvest, drying, shelling and bagging.  Seed treatment application, including the important drying process, may not be equal for each seed.  Potentials for variation continue as the seed is distributed to growers with varying storage conditions. 
 
Internal biology of each seed can be affected in each step.  Even a dry non-germinating environment, the critical cellular membranes are vulnerable to damage that only becomes exposed when imbibition allows cell activity.
 
Moisture is needed for germination but too much water, especially in some soils, can suppress availability of oxygen needed for cellular respiration.  Membrane function is essential to all cellular activity.  RNA produced with enzymatic activity in the cell nucleus is transmitted through the nuclear membrane to the membrane intense ribosome.  Among these enzymes are those that split the starch molecules in the endosperm in to glucose molecules that are moved to the mitochondria.  The membranes in mitochondria become the site in which enzymes utilize oxygen, water and glucose to produce the energy source known as ATP, that provides energy for other cell functions including the elongation and duplication of cells for seed germination.
 
Once planted, the seed engages many field environment variables that potentially could interfere with normal germination and emergence from the soil surface.  Temperature and moisture extremes, absorption of damaging chemicals, pathogens, insects and soil hardness can be factors interfering with normal emergence from the soil.
 
Everyone involved in corn seed attempts to limit the risks of poor field emergence.  Genetics of the hybrid, especially of the seed parent, are selected for reduced vulnerability to seed damage.  Seed production methods are adjusted to limit physical damage to the seed. Growers use tillage and planting methods to provide best soil environments for the seed.  In most cases all these efforts come together with a good uniform emergence in the field.  Uncontrollable weather can be involved when all the efforts have failed.  Surely production of a biological entity like a corn crop is more complicated than production of inanimate things.  

​Excessive rain soon after corn seed germination, especially in low areas of fields with heavy soils, is frequently associated with stunted plants. Much of that is caused by lack of oxygen to the roots. Oxygen is needed to maintain metabolism in root cells not only for production of new root tissue but also of other functions including defending against potential pathogens.
 
A comparison of corn seedling root structures growing in aerated and non-aerated conditions showed that the cells between the outer epidermis layer and the inner vascular tissue tended to collapse in the seminal roots lacking oxygen. These cells tended to be empty of cytoplasm but instead became empty spaces separated by the cell walls.  Lack of cytoplasm was apparently the cause of reduced active uptake of potassium and assumedly other minerals by the seminal roots.
 
This study (Plant Physiol. (1980) 65, 506-511)showed that corn seedlings in oxygen deficient media tended to develop nodal roots sooner than those with adequate root oxygen as an apparent reaction to stress of the seminal roots. Prolonged oxygen stress ultimately resulted in less total root volume.  
 
Symptoms of mineral deficiency in young corn plants in excessive, prolonged water areas of fields is associated with oxygen deficiency in corn roots.  This results in less mineral uptake into roots and transfer of the minerals through the vascular system to the shoots.  Prolonged oxygen deficiency results in reduced total root volume, less minerals available for shoot growth and potentially less water uptake in late season dry environment.  
 

​A cool seedling environment often results in purple leaves in some field.  Isolated corn plants with extensive purple colored leaves and stalks late in the season have some relationship in cause.  Purple coloration in plants is the result of anthocyanin pigments.  Anthocyanin pigments absorb the green spectrum of natural light, reflecting much of the blue and red portions.  Chlorophyll pigments absorb the blue and red wave lengths, reflecting the green.
 
Anthocyanins are water soluble pigments that tend to accumulate in the vacuoles of the epidermal cells.  Reflection of higher energetic light blue and red wavelengths of the light could be reducing the availability of that energy to the chloroplasts in the mesophyll, perhaps offering some protection from overload of sugars in those cells.  Absorption of the higher energy wavelengths of sunlight by anthocyanins has been hypothesized as protecting cellular membranes from photo damage. 
 
Anthocyanin synthesis is associated with the accumulation of sugars in leaf tissue.  Conditions that favor photosynthesis but inhibit distribution of the sugar from the leaf tissue can trigger synthesis of anthocyanins.  Seedlings inhibited by soil conditions reducing root growth and thus movement of sugars to root tissue.  Reduced phosphorus available, perhaps because of reduced roots growth, inhibits transport of sugars from the photosynthetic cells.  Cooler temperatures may inhibit metabolism involved in sugar transport.  Insect or pathogen damage to leaves that block the phloem cells of the leaf vascular cells, can cause accumulation of sugars in leaves.  Purple color is common in plants with poor pollination a few weeks after flowering as sugars have no place to be transported.
 
Corn hybrids vary in anthocyanin synthesis genetics, some with a higher tendency to develop purple colors when experiencing sugar transport stresses.  Early season development of purple leaves in most plants of a field probably involves cool temperatures and will be only a temporary condition.  Even hybrids not tending to show purple could be undergoing a similar stress but not have the genetics for the anthocyanin response.
 
It is good to observe occurrence of purple leaves in corn for analysis of possible causes.  If temporary and generally distributed in a field, it is probably temperature related.  If scattered, then it may lead to treatments reducing the occurrence this season or the next.

​Low temperatures have been a major feature of this spring in northern US Corn Belt. Initial concerns are effects on germination and emergence but after emergence, heat energy affects several aspects of the corn plant. Studies have shown that hybrids vary in tolerance to cold temperatures.  Much of this involves the cellular membranes, including their ability to repair after membrane damage.  This affects loss of electrolytes and their function of nearly all cellular processes from photosynthesis to protein synthesis.  Many of the proteins are used as enzymes active in photosynthesis and cell duplication and elongation.  This generally results in reduced plant height and smaller leaves if plants grown at low temperatures than at higher normal plants.  Root volume is also less when corn plants grow under lower temperature environments.  This may be caused by a reduction of photosynthates reaching the root tissue.
 
Chlorophyll levels are lower when corn leaves are at lower temperatures, perhaps because of membrane damage and reduced syntheses of chlorophyll.  Reduction of photosynthesis is hypothesized to account for reduction of stomata and thus transpiration as well as CO2 movement into the plant.  Cellular respiration in which the glucose from photosynthesis is changed to a metabolically useful chemical energy (ATP) is also reduced, contributing to the reduced cellular elongation and duplication.
 
A study comparing commercial hybrids for tolerance to growth under low temperature environment was published in 2015 in Canadian Journal of Plant Science and can be found at 
https://www.nrcresearchpress.com/doi/full/10.1139/cjps-2015-0286#.XNnaYC_MxR4
 
Temperature is one of the factors that favor different hybrids each year.

​Initial nodal roots, growing from the first nodes of stem in the crown area tend to grow laterally before turning downwards.  Each branch of each lateral nodal root has its own root tip.  Nodal roots are formed near to stem vascular system of the stem, even in the crown, allowing efficient transport of minerals and water into the stem, through the xylem and then further to the expanding leaves.  Photosynthesis products are moved through the phloem into the vascular system of the roots to allow expansion of the nodal roots. 
 
Temperature influences the metabolism rate in the root tissue and thus affects the growth rate of the nodal roots.  Although water movement into the root tissue and movement up the plant is mostly influenced by other physical factors such as outside relative humidity, transport of nutrients such as glucose from leaves to roots in phloem is slower if temperatures are reduced.  Auxins influencing the root tip expansion are produced in leaf tips, moved through phloem and therefore are slower to reach root tips if temperatures drop.
 
Genetics influence the pattern of nodal root growth, some hybrids with more lateral roots than others.  It is common for corn hybrids to develop roots from 6 underground and 3 above ground nodes during the life of the corn plant.  Genetics and environments influence the actual number.  Soil types and depth, organic distribution in soil and water distribution all become factors in determining the most efficient nodal growth pattern appropriate for a field.  A change in any of these factors may favor a different set of genetics the next season.

​First root to emerge from the germinating corn seed is the radical extension of the embryo.  It is mostly unbranched and growing downwards.  Next are the seminal roots emerging from the tissue between the radical and the mesocotyl where the scutellum is connecting the endosperm to the growing embryo. Those roots often are branched.  The radical and seminal roots are essential to collecting water and minerals for a few weeks as the mesocotyl pushes upwards to the soil surface.
 
With the light penetrating a short distance below soil surface, the tip of mesocotyl forms the first node and the coleoptile pushes upwards.  The base of coleoptile forms the first stem node.  Within the coleoptile is the first leaf connecting to the stem at the second node.  By the time the first leaf is fully expanded, and the 2nd and 3rd leaves exposed, all connected to the underground stem, the crown, lateral nodal roots extend.  These nodal roots grow laterally first but eventually gravity effect on the cell elongation causes these roots to point downwards.  Lateral roots may develop from crown nodes 1-5 and eventually above ground nodes as well.
 
Lateral roots become the main source of water and mineral absorption as the mesocotyl is cut off from the energy from photosynthesis in the leaves.  Seminal roots served their functions to support the early emergence of the shoot.  Genetics and environment now become the major influences on the direction and branching of the main root system of the corn plant.
 
An interesting summary of root development of young corn can be found at http://www.kingcorn.org/news/timeless/Roots.html

​Much of the USA corn growing areas are experiencing a wet, cool spring.  It is likely that corn seed already planted in these soils will experience temperature and excessive water stress and that some soils will become compacted, leading to not all seedlings emerging at the same time.  PSR's seed tests also detect some seed samples that have a biological problem in which not all the seed germinate at exactly the same time.
 
It is not clear why some individual seeds within a seed lot are slower to elongate the shoot and root, allowing the seedling to emerge from the soil.  One hypothesis is that the slower seedlings had deterioration of the membrane structure in embryo cells, possibly in the mitochondria. This is enhanced with imbibition swelling the membranes, resulting in tearing the tissue.  Membranes in mitochondria are the sites of cellular respiration in which sugar molecules are enzymatically changed into ATP molecules, the energy source for almost all cellular activity, including cell elongation.  Other cellular membranes are major locations for transport of components needed for cell elongation as well.
 
There are multiple cells in the corn embryo, each with many mitochondria.  Complete death of the seed occurs when too many membranes are destroyed for the seed to grow.  Sufficient amount of functioning cell organelles, however, can lead to repair of damaged membranes, eventually gaining enough momentum for cell elongation in the shoot and root tips.  This can result in eventual emergence from soil surface.  Heat energy and oxygen in soil are important to this process as well.
 
Seedlings emerging later than adjacent seedlings can struggle for the whole season, as they are shaded from light and at competitive disadvantage for minerals in the soil.  This can also result in plants with delayed flowering, possibly because of insufficient moisture to push out the female silk, missing the normal pollination.
 
Detection of these individual seeds with internal membrane damage is not easy and may not be detected with standard germination testing.  When planted in artificial soil mixes, most seed lots with high germination percentages emerge uniformly.  Those with non-emerging seedlings will also tend to have more late emerging seedlings as well.  Rarely one finds a seed sample in which the percent emergence is 95% but includes most at three leaves exposed while some only show the coleoptile. Even in these cases, there will be some with each level of development, showing only the ‘spike’, or one leaf, or two or three.  How to classify germination of these seed?  They are alive and they germinated but how will the delayed emergence of these plants detract from field performance of the seed lot?  
 
Uneven emergence in the field could have several causes.  Analysis of main factors is difficult.  Seed biology is one of those factors.