July 9, 2022
As spring ends and the corn crop takes off, we start to look at the pollination window. Often, an early planting window is targeted to help increase the chances of pollination occurring before any summer heat and drought stress. It can be difficult to distinguish between heat and drought stress during pollination because they often go hand in hand. Generally, if there is adequate soil moisture, stress during pollination is limited even with extreme heat.
Let’s take a deeper look at how this all happens. Corn is a unique grain crop, as it is monoecious – it has flowers that contain only male sexual parts and flowers that contain only female sexual parts on the same plant. The male parts are anthers on the tassel (Figures 1 and 2) and the female parts are the silks on the ear (Figure 3). The start of pollen shed is called “anthesis”. Both flower types must develop together to have successful pollination. This is called the “nick” or “nicking” period. Pollination is the process in which pollen is released and lands on the silks.
Figure 1. Tassels beginning to emerge from whorl with anthers starting to open.
Figure 2. Close up of anthers which hold pollen until released.
Figure 3. Emerged silks are sticky to hold pollen.
Wind is the typical process that helps complete pollination - most pollen grains travel about 50 feet through the air to a silk; however, some can move up to 500 feet. Once the pollen is released it must land on the silks for fertilization to occur. The silks have hairs (trichomes) that are very sticky to help catch pollen grains. The diameter of a pollen grain is about 1/4 the size of a silk, but it is almost three times the size of the trichomes to increase the odds of getting caught on the silks. The silking stage is called the R1 growth stage. Silks emerge first from the butt of the ear and eventually from the tip of the ear. There is a direct correlation to silk elongation and moisture; therefore, if it gets dry, silk elongation can be slowed, and it may be more difficult for the plants to successfully fertilize.
When a pollen grain attaches to a silk, water from the silk is transferred to the pollen grain causing it to germinate and develop a pollen tube. There are breaks on the silks near the trichomes that allow the pollen tubes to enter and elongate inside the silks. It takes about 24 hours for the pollen tubes to elongate. The process requires water, and any reduction in water coming from the silk may result in a collapsed silk that may not allow the pollen tube to finish elongation. The pollen tube enters the ovule where fertilization occurs. After fertilization, the silk separates from the potential kernel. An easy way to check for fertilization success is to carefully remove the husk from an ear during pollination and give the ear a shake. The silks from successfully fertilized embryos fall away while silks remain attached to non-fertilized embryos (Figure 4).
Figure 4. Embryos with missing silks have been fertilized while those with attached silks have not been fertilized.
Pollination processes can be affected by the weather. Pollen shed can decrease at temperatures above 86° F and pollen can be killed at temperatures over 100° F. Pollen grains usually remain viable for about 18 to 24 hours during favorable conditions. One tassel contains about 6,000 pollen bearing anthers and will shed those anthers from a few days to two weeks. This increases the chances of a successful pollination period. Because pollen shed can slow down as daily temperature increases, most pollen is shed in the morning or evening. Additionally, pollen shed typically ceases during a rain event. Although poor pollination and kernel set can happen, corn plants have many checks and balances in place to increase the odds of a successful fertilization period.
For additional information on how stress affects corn pollination, please see Channel Advice, Corn Pollination Affected by Heat and Drought.
Sources:
Wiebold, W.J. 2012. Corn pollination, the good, the bad, and the ugly. Part 3: Boy Meets Girl. Integrated Pest Management. University of Missouri. https://ipm.missouri.edu/cropPest/2012/7/Corn-Pollination-the-Good-the-Bad-and-the-Ugly-Pt-3/
Nielsen, R.L. (Bob). 2020. Tassel emergence & pollen shed. Corny News Network. Purdue University. https://www.agry.purdue.edu/ext/corn/news/timeless/tassels.html
Nielsen, R.L. (Bob) 1996. Drought & heat stress effects on corn pollination. Purdue Pest Management & Crop Production Newsletter. Purdue University. https://www.agry.purdue.edu/ext/corn/news/articles.96/p&c9635.htm
Lindsey, A. Corn pollination. C.O.R.N. Newsletter. Agronomic Crops Network. The Ohio State University. https://agcrops.osu.edu/newsletter/corn-newsletter/2020-22/corn-pollination/
Web sources verified 5/26/22.
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