Water becomes an essential influence on corn pollination. Silk elongation is a process of cell growth dependent upon osmotic pressure to expand the cells as they develop. Drought-stressed plants tend not to extend silks far enough to be exposed to pollen or if they do make it that far it may be delayed beyond the time that viable pollen is available. Although silk emergence is dependent upon moisture available for cell elongation, male flower development tends to continue at the normal timing. If the drought is accompanied with high temperatures, not only is the release of the pollen enhanced but also pollen viability is reduced. Water in the plant is dependent upon roots characters such as structure and depth. Plant density, variety genetics, soil type and fertilizers all influence the emergence of the silks under all circumstances.
On the other hand, slightly excessive water, perhaps along with cooler than normal weather, can cause some varieties to push silks out from the husk covers before pollen is released. This exposes the silks to unintended pollen from other fields and to pathogen such as those that cause common corn smut, Diplodia, Aspergillus and Fusarium (to be discussed in future blogs)
Pollen adheres to the silk hairs (trichomes) and quickly hydrates as it absorbs water from the silk cells. This hydration is needed for the initial growth of the pollen tube as well as activation of the enzymes allowing the penetration of the silk. Moist silk and the invasion of the pollen tube allows provision of the nutrition and hormones that guide the growth of the pollen tube towards the ovule. As that tube progresses toward the ovule that silk quickly dries behind it. This helps prevent further invasion of the silk by potential pathogens.
Female flowers have extended the stigmata (silks) through the husk exposing them to the air. Male flowers, beginning first from the oldest florets, have extended anthers emerging and filled with pollen grains. With a drop of relative humidity, the oldest anthers will open at the lower tip, releasing the pollen grains. The grains are sufficiently dry to be viable and yet float with the slightest of a breeze. Some claim corn pollen can travel ½ mile in 15 minutes with sufficient wind but considering all the variables, i.e. genetics, relative humidity and amount of wind, it becomes difficult to generalize. Seed producers, attempting to produce pure hybrids are well aware of the influence of pollen distribution.
New corn pollen has a light yellow color but as it ages and desiccates in dry air it becomes dark yellow. Pollen will germinate when moistened by growing a germ tube. Pollen landing on the silk hairs (trichomes) produce enzymes that allow penetration of the germ tube into the silk. Nutrition in the pollen grain is sufficient to grow about ¾ inch (2 cm). Nutrition from the silk is needed to allow continued growth down the several inches of silk channel to the ovule. Although several pollen grain may initially penetrate the silk only one usually is allowed to reach the ovule, as the silk channel basically collapses as the germ tube progresses. Pollen grain penetration of a silk occurs within 5 minutes but germ tube growth to the ovule may require 40-60 minutes.
Once the germ tube reaches the micropyle of the ovule, the ovule causes the germ tube to burst, releasing the sperm. One sperm cell migrates to the egg cell with its monoploid nucleus fusing with the monoploid egg cell nucleus to form a diploid zygote. The other sperm nucleus enters the central cell, fusing with its two monoploid nuclei forming a triploid endosperm.
This rather complex process of pollination leaves plenty of opportunities for things to go wrong which will be discussed in upcoming blogs.
Teosinte, the ancestor of corn, has lateral meristems at each node, producing about 10-20 female flowers in two rows attached to a narrow rachis and each flower with a long stigma protruding beyond the modified leaves that surround the flowers. In most Teosinte species the male flowers develop from the apical meristem, much like a corn tassel. As people selected for mutants that had more grain like corn, they also maintained the leaves (husk) that helped protect the ovules and, later, the developing grain from invasion by insects and pathogens. It was also convenient as a wrap for cooking corn (tamales anyone?)
The ear of corn is composed of parent plant tissue and DNA surrounding a group of flowers attached to a central stem (rachis) we know as the cob. Each flower has a single stigmata that we know as silk that extends beyond the outer leaves (husk). The silk tissue, husk, cob and the outer layer of cells of the ovules are parent plant material and therefore controlled by genetics of the hybrid plant. Benefiting from the vast genetic diversity, breeders over the millennia selected for variants with different husk characters that met their specific needs. Heavy insect pressure environments favored those with longer and perhaps thicker husk leaves. Short season environments requiring quicker field drying the mature grain favored those with thinner and shorter husks.
Silk growth also had to accommodate the husk length of husks in order to get exposure to pollen. Silk growth is largely a cell elongation process. Like all cell growth, water pressure is needed to extend the silks, thus it is dependent upon the plant environment. Genetics also is a significant factor that requires breeders to select for good silk extension even with drought pressure. Timing of the silk emergence from the husks is also important because of the limited time in which viable pollen is available. Although the first silks to enlarge are the oldest at the bottom of the ear, those with a shorter distance, perhaps an inch from the bottom reach there first.
One or more of the lateral meristems, which are located at each base of each leaf but attached to the stem node, is stimulated by hormones to produce female flower parts. In corn each node of the modified lateral meristem includes two ovules, one of which degenerates. The ovule diploid cell undergoes meiosis, initially producing 4 monoploid nuclei but three degenerate, leaving a megaspore cell with one monoploid (haploid) cell. This single set of 10 chromosomes on hybrid plants represents a random mix of chromosomes from each of the hybrid plant’s parents. Thus, just as with pollen, there is a minimum of 1028 different sets of genetics among the ovules on a single plant.
The nucleus of the megaspore cell undergoes three successive mitotic divisions resulting in 8 nuclei and a total of 7 cells. Most important of these is the egg cell with a single monoploid nucleus and a large central cell with 2 monoploid nuclei. The central cell is destined to become the endosperm after pollination. Two of the other cells (called synergid cells) adjacent to the egg cell apparently produce attractants to guide the pollen tube to the egg cell. A small opening, called a micropyle, at the tip of the embryo sac, is conveniently located where the silk is attached to the ovule. This composes the embryo sac of the female.
This development process continues in the 500-1000 ovules a few weeks before pollination. Environment and genetics of the hybrid plant influences the actual number of ovules that develop.
Corn apical meristem switches to producing male and female flowering parts, but quickly changes to male development only. Each glume in the tassel is an individual floret containing three anthers. Within these immature anthers are hundreds of microspore mother cells in which meiosis occurs. As a result, each of these cells with 2 sets of the 10 chromosomes (diploid) before meiosis now contain 4 microspores, each with only 1 set of the 10 chromosomes (monoploid). Whereas the diploid stage in hybrid corn, included 1 set from the parent male parent and 1 from the female, after meiosis, each microspore includes a random mix of two parents. There are a minimum of 1024 different combinations of the two parental genetics among the microspores. The 4 microspores separate over a 4-day period and begin to become separate pollen grain with thicker walls. Nutrients are absorbed from the liquid contents of the anther during the microspore and pollen grain stages over about 10 days, at least in one study. During this period, the anther dehydrates as it is filled with pollen grain. By the end of this period, the pollen grain has many starch granules, two haploid nuclei, a thick outer wall and a thin inner one. Total time from beginning of microspore production to mature pollen is 14-17 days. Each pollen grain remains viable for only about two days after maturity and less when under high temperatures.
A pore at the end of the anther opens to release the pollen. This process involves dehydration and is affected by drops in surrounding relative humidity. There is no release during rain and pollen release is common in mornings as relative humidity drops with rising daytime temperatures.
Each floret of the tassel has slightly different time of development as the apical meristem expanded. Consequently, each finishes the process of pollen development at different times, frequently over 10 days. A typical hybrid tassel has about 6000 anthers, although hybrids and environments vary. It is common for a single tassel to produce millions of pollen grains.
Corn ancestor Teosinte, originally growing in southern Mexico, was stimulated to flower only when exposed to short days (long nights). This was an apparent advantage because it matched the wet season of that location. People selected and moved those early corn-teosinte mutants out of that environment increasingly further from the equator, changing the flowering to be less dependent upon long nights but more related to temperature. Once again we benefit from the genetic diversity among the corn genetics and the practical selection by corn breeders over 8000 years.
Photoperiodism in plants is evident as we see many species that bloom at the same time every year. A protein called florigen is produced in leaves and moved through the phloem to the meristems that were producing stem and leaves and stimulates changes to cause it to produce flowers. It is the regulation of the gene, that is causing the gene to be active and therefore produce the RNA, and, ultimately the protein, that is more complex. There are at least 4 genes involved in the photoperiod response by corn. Adapting corn to the temperate zone summers done long before anyone acknowledged presence of genetics was done by farmers over centuries. A recent reference on genetics involved in corn flowering can be found at Genetics. 2010 Mar; 184(3): 799–812. Now we know that the photoperiod aspect is controlled by genes, but also that heat is a factor in the plant’s switch to producing reproductive structures at apex and at least on nodal bud. Tropical corns do eventually flower in the US Midwest but in some cases only close to the fall frost date. Our company does some breeding projects with tropical material. Those planted in April in our greenhouse will reach the ceiling before forming tassels after 4-5 months but those planted in December, with our short winter days, will flower by in 2-3 months and only reach a height of 5-6 feet. Tropical hybrids grown in Brazil have plant heights and flowering times very similar to US corn belt hybrids growing in Midwest summers.
One study that I did many years ago compared the heat units to time of apical meristem showing a tassel to the maturity rating for many commercial hybrids. Timing of that differentiation, occurring in June correlated very closely with our final maturity ratings for those hybrids. This supported the hypothesis that it is the heat units beginning immediately after planting that is most significant in determining the maturity of a corn crop. Heat after switching the growing points from producing stem and leaf tissue to tassel and ear tissue has an influence but the earlier season affect is greater. Maturity in most corn belt corn is controlled by several genes affecting response to accumulating heat soon after planting. Tropical corns are also influenced by heat but other genes affecting response to number of hours of continuous darkness have a greater affect on time to flowering.
Resistance to most potential corn diseases involves at least 3-4 genes beyond structural resistance to pathogen invasion. These genes are activated after the invading organism is detected. Often, it seems, one of the genes has a more major affect then the others. At least, what has been observed that some inbreds, being homozygous, appear to be extremely susceptible to a disease whereas most inbreds are not and that this trait is recessive. That means it is largely modified when the other parent of a hybrid is more resistant. In these cases, corn breeders will witness only a few inbreds among the hundreds in a corn nursery that show the disease. I cited in the last post, the case of Race 1 of Bipolaris zeicola recently showing up on seed production fields in one inbred. I saw that same race in breeding nursery 25 years ago in one of many closely- related, new inbreds. Stewart’s bacterial blight has a similar history of extreme susceptibility in a few popular inbreds such as N28 and A632 during the 70’s, and Goss’s wilt in inbred A632 during the 70’s. In each case, these diseases were less damaging in hybrids because the dominant version of the gene, often from the other hybrid parent, had modified the susceptibility.
The immense genetic diversity available to corn breeders and the large number of genes in corn, includes the possibility of inadvertently selecting recessive versions of genes that interfere with normal resistance to less common potential pathogens. If the new inbreds are not immediately exposed to these organisms the vulnerability is not known. Wider exposure in seed production fields often are the first opportunity to find occurrence of new diseases, although common practice of fungicide spraying reduces fungal infection. New diseases will show up in hybrid fields and it is important that observers be alert and report back to the seed company. Genetic diversity among corn parent seed is great and seasonal changes are available but some lead time to make the changes is needed. Corn has genetic diversity and so do it’s many potential pathogens.
One study of a single corn inbred (B73) indicated that it had 30,000 genes. We benefit from a species with a huge genetic potential and a pollination system that encourages new mixes of genetics. This has allowed the species to be used for food in a wide range of environments. Some of those genes are turned on in response to the many microbes searching for the products of photosynthesis for their nutrition. Microbes have genetics too! Some, like rust and smut fungi, survive by attacking living corn cells, drawing carbohydrates to the cell, and then moving on via spores before the host cells respond to the fungus. Many fungal pathogens of corn simply kill a limited area of the leaf tissue, feed on the dead tissue, produce spores and infect new areas. Corn varieties differ in how quickly and strongly they respond to the invasions.
One fungus that I find interesting is Bipolaris zeicola. It was formally known as Helminthosporium carbonum. There are genetic variants of this species that apparently feeds only on dead leaf tissue, often caused by insect damage or simply physical injury. These variants apparently lack the genetics for either penetrating the live corn leaf tissue or overcoming the resistance system of most corn varieties. At least one variant of this fungus produces a toxin that kills corn plant cells but most corn varieties have dominant gene that effectively blocks this toxin. However, very occasionally, a mutation of that dominant corn gene does occur while developing new inbreds. If this mutation, now a recessive gene, becomes homozygous during the inbreeding process, the inbred is vulnerable to the toxin. The result is practically no defense to this variant (race 1) of B. zeicola. The pathogen kills small leaf area of leaf, produces spores and spreads to new leaf tissue and eventually causes the whole corn plant to die early. Because susceptibility is recessive, and the dominant toxin-resistant gene is present in most corn inbreds, this creates a problem for seed producers but not for hybrid growers. Good that corn has genetic diversity.
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.