As I described in Parts 1 and 2 of this series, corn is unique among grain crops in that it is monoecious. This means that it possesses flowers that have only male sexual parts and flowers that have only female sexual parts on the same plant. Also unique among grain crops, these flower types are separated on a plant by a distance of several feet.
Successful kernel formation is dependent upon both flower types developing in sync with each other (nick), pollen grains landing on exposed silks (pollination), the joining of a male gamete with a female gamete to form the kernel embryo (fertilization), and the joining of another male gamete with two other female nuclei to form the endosperm (second fertilization). This double fertilization is truly a miracle and happens millions of times in our cornfields. Unfortunately, the weather events that most of Missouri has experienced this summer can interfere with each of these events.
In Parts 1 and 2, I described the development of the male and female flowers. Under normal conditions the growth and maturing of these two flower types prepare them so that pollen shed occurs within a day or two after silk emergence from husks. This timing is given a colloquial term – nick. Pollen shed and silk receptivity to that pollen last for at least six days, so some leeway is built into corn plant development to help ensure that pollen is shed when silks are available to catch the pollen. But, several factors, including our unprecedented weather events, can interfere with the timing and lead to “poor nick”.
Water pressure (turgor) drives silk elongation. Silks are highly susceptible to water loss and loss of turgor pressure. They are highly elongated so their surface area to volume ratio is extremely large. They appear to lack much ability to regulate the amount of dissolved molecules in their cells. Dissolved molecules, like sugars, is one way that plant cells can cause water to flow from outside the cell to inside the cell. And, they must elongate a long distance from their source of water.
Water enters silks from the ovary to which they are attached. The ovary receives water from the cob, and the cob receives water from the stem through the shank. Silks must compete with nearly all other plant parts for water. They are at the end of a long chain of these plant parts. So, they are at a competitive disadvantage when water becomes scarce. Fortunately, the place where the silk attaches to the ovary and nearly the entire length of the silk is covered by multiple layers of husks. This helps protect them from harsh environments. But, when water availability is limited silks are often affected first and to a greater amount than other plant parts, such as leaves.
When soil moisture content is low or heat causes increased water loss from plants, silk turgor pressure decreases and silk growth slows. In fact, there are clear diurnal growth patterns for silk. Most rapid elongation of 1.5 to 2.0 inches per day occurs at night or in the morning when water status in the plant is at its greatest. During the afternoon, when plant water status decreases, silk elongation also decreases – sometimes to zero. Drought will extenuate this pattern, and if drought stress lasts through the night, silk growth may be nearly zero for the entire day. Elongation rate changes with days after initial silk growth. Under adequate water levels, the fastest growth rate is on the first two days and growth slows with each succeeding day until by day 10 when growth is quite slow.
Drought affects silk growth far greater than pollen development. Under most commonly experienced droughts, pollen development and pollen shed will continue on schedule. Slow silk growth means that fewer silks will have emerged from the husks while pollen is being shed. Silks that emerge after pollen shed is completed will not capture pollen, an essential step to producing a kernel. With extreme drought, it is possible that nearly all of the pollen grains are released from the tassels by the time most of the silks have emerged. This results in a few kernels being produced on an ear rather than the normal 600. Most of the yield loss from poor nick is permanent because corn has little capacity to compensate for small seed number by increasing seed size – even if adequate precipitation returns.
This year’s drought was more intense than most and accompanied by high temperatures. Water evaporation rates from soil and plant leaves were much greater than usual because of high temperatures, low humidity, and fast wind speeds. Plant water status dropped much lower than we’ve experienced in recent droughts. Water pressure also drives stem cell elongation. With low water pressure, stem cells remained small and plants remained short. In 2012, it is not uncommon to see corn plants 2 to 3 feet shorter than normal. In fact, in some fields tassels did not emerge or only partially emerged from the leaf whorl. Pollen grains released from tassels still covered by leaf sheaths have no chance to fall onto silks. Often when drought causes a poor nick it is because of the effects of drought on female flowers. But, in 2012 there were some instances of poor nick caused by drought effects on the male flowers.
Pollination is defined as pollen grains landing on silks. For the process to work, silks must intercept pollen grains and retain those pollen grains. Corn pollen is carried by wind, but the individual grains are relatively heavy. Because of their weight, most pollen grains are carried less than 50 feet from the plant producing them. But, a few pollen grains may travel 500 or more feet, especially if winds speeds are fast.
Pollen grains are released into the air and fall toward the ground. Since silks cannot move; they must be located in the path of these falling pollen grains. Other plant parts often block the movement of the pollen. LAI is the ratio of leaf area to land area. A productive corn field may have an LAI of 6. That means that for every acre of land area there are 6 acres of leaf area. Unfortunately, silk surface area is small in comparison to leaf area. The silk area index (SiLI) is only 0.003. Each individual silk is small, and although an ear may produce 1000 silks, the opportunity for a pollen grain to land on leaf material is far greater than landing on silk (Figure 1). Although a tassel produces at least 2000 pollen grains for each silk, most of those pollen grains will land somewhere other than on a silk.
Silks possess hairs (trichomes) that help them catch and hold onto pollen grains (Figure 2). Hairs on the silks are stickier than hairs located on other plant parts, such as leaves. So, pollen grains may bounce or roll off of leaves, but if they land on silks they are usually retained. Both pollen grains and silks are small and their small sizes reduce the probability that they will meet each other. The diameters of pollen grains are about one-fourth the size of silks, but nearly three times the size of the hairs on the silks.
One other characteristic of ears can lead to poor pollination of some kernels. As described in Part 2, silks from flowers near the tip of the ear start to elongate later and elongate slower than other silks. As the nearly 1000 silks emerge from husks, silks from the tip ovaries are often late to emerge or buried within mass of other silks (Figure 3). So, even with adequate water, silks from tip kernels may not be pollinated.
Once pollination has occurred and at least one pollen grain has stuck onto a silk, several important steps are necessary to complete fertilization. The pollen tube “germinates” and the pollen tube emerges from the pollen grain. This requires water and that water comes from the silk. So, water flows from the silk and into the pollen grain.
The pollen tube must find an entry point into the silk. Pollen tubes can grow along the outside surface of silks, but the longer they remain on the outside, the greater their vulnerability to dehydration and death. Silks have a waxy cuticle just like other plant parts, and pollen tubes may not be able to go through the cuticle. Fortunately, there are breaks in the cuticle especially on the hairs. Pollen tubes usually find places to enter the silks and begin elongating down the inside of the silks.
There are pathways within silks that help direct the elongating pollen tubes. It is possible for more than one pollen tube to enter a silk, but only one pollen tube will grow down the entire length of the silk and enter the ovule for fertilization. The water to drive pollen tube elongation comes from the silk. It takes about 24 hours for the pollen tube to grow the entire silk length and if the silk loses water during that time the pollen tube may not complete its journey. One common result of reduced water status is a collapse of the silk near the ovary. If this happens, pollen tube growth is prevented. Anything that prevents the pollen tube from entering the ovule will prevent fertilization and a kernel will not form (Figures 4, 5, 6).
Three nuclei (plural of nucleus) move from the pollen grain and into the pollen tube. One of the three nuclei directs pollen tube growth and will be not involved in fertilization. The other two nuclei travel down the pollen tube and enter into the ovule once the pollen tube completes its journey. One male nucleus combines with the female gamete to form the embryo within the kernel. The other male gamete joins with two female nuclei to form the endosperm of the kernel. These two events are called double fertilization and are required for the kernel to form and begin growth. A day or two after fertilization occurs, the silk separates from the kernel and changes color as it dries.
Corn is an amazingly productive plant. One kernel planted returns 600 kernels at harvest. Its unique reproductive characteristics have allowed for an efficient and profitable breeding industry to develop. But, a chain of events must occur in the correct sequence and be properly timed for farmers to reap the benefits of corn’s high productivity. The summer of 2012 has taught us, again, how important Mother Nature is in determining corn yield and the economic viability of Missouri’s farmers. Lack of precipitation affected silk elongation and the ability of pollen tubes to grow through the entire length of the silk. High air temperatures may have had some direct effects on pollen viability, but the primary effect of temperature was increased water evaporation that accentuated the problems related to scarce water availability.
Aylor, DE, NP Scultes, and EJ Shields. 2003. An aerobiological framework for assessing cross-pollination in maize. Agricultural and Forrest Meteorology 119:111-129.
Nielsen, RL (Bob). 2000. Effects of Stress During Grain Filling in Corn Corny News Network, Purdue Univ. Online at http://www.agry.purdue.edu/ext/corn/news/timeless/grainfillstress.html.
Westagte, ME and JS Boyer. 1986. Reproduction at low silk and pollen water potentials in maize. Crop Sci. 26:951-956.
REVISED: October 1, 2015