Author: Surakshya Sharma Bsc. Ag 2nd semester, IAAS paklihawa
Corn is unique among grain crops in that it is monoecious. It means it contains both male (tassel) and female (ear) flowers on each plant. However, unlike many other monoecious plants, male and female reproductive structures are separated by several feet in the corn plant. Given the separation of male and female flower (tassel and ear), self-pollination seems difficult. So, corn is cross-pollinated. Only a little percentage (<5%) of kernel are usually fertilized by pollen from the same plant.
Ears are initiated at each node up to about the 12th to 14th leaf node, but typically only the uppermost ear fully develops. Depending upon the varieties and growing condition, a secondary ear may develop at the next lower node. The female florets, containing the ovules that will become kernels upon successful fertilization, are located in paired rows along the surface of the ear. Silks develop and elongate from the surface of each ovary on the ear.
The tassel is the male flower or inflorescence of corn. This male inflorescence consists of many spikelets, which are located along the main spike and lateral branches of the tassel. The spikelets enclose 2 small flowers or florets. Each floret contains the male reproductive structures, referred as the stamen. The pollen grains are held on the anthers, which are located at the end of a thin stem called the filament. An individual plant may produce several million grains of pollen.
Pollination is defined as pollen grains landing on silks. For the process to work, silks must intercept pollen grains and retain those pollen grains. Pollen shed (anthesis) begins shortly after the corn tassel has fully emerged from the whorl.
Spikelets near the main axis of the tassel are the first to open, exposing the anthers that bear pollen grains. Generally, pollen shed and pollen viability are minimally affected by environmental stresses. However, very hot, dry conditions may reduce pollen viability and decrease the length of pollen shed.
Silks possess hairs (trichomes) that help them catch and hold onto pollen grains. 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.
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 the mass of other silks. 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 silk, the pollen grain germinates immediately, producing a pollen tube that grows down the length of the silk, resulting in fertilization of the ovule within 12 to 28 hours. Although many pollen grains may germinate along the surface of the silk, only one grain will generate a pollen tube resulting in fertilization. The water to drive pollen tube elongation comes from the silk.
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. A mass of long, green silks is an indication that pollination has not occurred. This could be the result of silk emergence after most pollen has shed, or delayed pollen shed due to extended rainy, cloudy conditions.
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.