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.
Millions of pollen grains in a hybrid field, and yet only a small percentage land on the stigma of the female flower. The sticky hairs on the silk assist in holding the pollen in place. Attached pollen grain rehydrates from the moisture in the silk. A germ tube emerges through the pollen grain pore, invading the silk usually through the silk hair. It continues to be dependent upon silk moisture as the germ tube grows down the center of the silk, probably with the assistance of hormones to guide it. A nucleus at the tip of the tube is active in providing the DNA codes for the growth, while two other nuclei are carried along the tube. Although a few pollen grains may simultaneously be germinating on an individual silk, usually only one succeeds in progressing all the way. The silk begins to collapse as the winning pollen tube grows, shutting off water to later pollen grains- and to potential fungal invaders. The winning pollen tube reaches the ovule in up to 24 hours, depending upon the distance from where it penetrates the silk. Nutrition for pollen tube growth and metabolism is supplied by the silk.
After reaching the ovule, the other two haploid nuclei in the pollen tube are released into the cytoplasm of the ovule. One nucleus migrates towards and joins the egg cell nucleus to form a diploid embryo. The other nucleus migrates to the cell that already has two haploid nuclei to form the triploid nucleus of the endosperm.
The genetics of the next generation is set for this seed. Hopefully, the winning pollen grain was the one intended by the humans involved.
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.