Environmental Stress During Pollination
Successful fertilization of mature ovules requires viable pollen to land on receptive silks. Insect pests, such as adult corn rootworms, may clip silks as they feed, resulting in poor pollination with subsequent poor kernel set. Management and diagnostics for adult rootworms are presented in a Crop Focus article (Rice, 2015.)
There are two basic parts to the pollination process. First, viable pollen must land on receptive silks, and second, the silks must support the formation of pollen tubes to allow male gametes to fuse with female gametes inside the ovule. A large portion of mature pollen is usually released from corn anthers in mid-morning, depending upon environmental conditions. A minimum of 100 grains of pollen per square centimeter per day is needed to successfully pollinate a corn field. Pollen may lose viability within a few minutes if air temperatures are high (approximately 104 °F or 40 °C) and water deficit stress is present. Pollen grains contain about 80% water when first shed. These pollen grains die when the water content decreases to about 40%.
A lot of corn is successfully pollinated under higher temperature conditions. If soil moisture is adequate and the corn plant can transpire water rapidly enough to supply necessary water to the pollen, the pollen remains viable long enough to properly shed and complete the fertilization process. However, if the water supply is inadequate, pollen will die prematurely and not complete the fertilization process.
The second part of successful fertilization of ovules is the formation of the pollen tube and deposition of male gametes inside the ovule. This process relies heavily on the female portion of the plant because the silks supply all of the necessary nutrients and water for growth of pollen tubes. Based upon all of the pictures we have seen to date, viable pollen grains adhere to silk trichomes -- not directly to the silks -- to start the fertilization process.
Figure 8. Pollen attached to silk trichomes. Courtesy of Dr. Don Aylor, University of Connecticut.
Trichomes are hair-like projections that extend from the main stem of the silk, much like root hairs extend from a plant root. Within a few minutes after landing on the trichomes, the pollen grains start to initiate pollen tubes. These pollen tubes seem to always grow near the silk vascular bundle. This may occur because these vascular tissues contain a readily available source of water and nutrients essential for growth
Figure 9. Pollen tubes growing along silk vascular tissue. Courtesy of Dr. Antonio Perdomo, Pioneer.
Depending upon water availability and environmental conditions, it may take just a few hours to approximately one day for pollen tubes to grow all of the way to the ovules. When the corn plant is under greater drought stress, pollen tube growth is slower and the potential for successful fertilization decreases.
Environmental stress during pollination can have substantial effects on grain yield. For a specific hybrid, approximately 85% of grain yield is correlated with the number of kernels produced per acre with the remaining 15% being the weight of individual kernels at harvest (see Figure 10.)
*silks exposed to pollen daily
Figure 10. Relating kernel count to grain yield.
The amount of water available for silk growth substantially influences when silks emerge, their rate of growth, their length of receptivity, and their ability to supply water and nutrients to support pollen tube growth and fusion of gametes. Silk growth during pollination and grain production, as well as problems associated with improper or inadequate silk growth, are presented in another Crop Insights article (Strachan, 2016). From a diagnostic perspective, corn plants that are growing under stress during pollination produce ears with portions of the cobs being barren (examples shown in Figure 12). Portions of the cob are barren because mature ovules were not properly fertilized. These unfertilized ovules begin to disintegrate and disappear before the ear reaches physiological maturity.