Maize pollination and fertilisation

Pieter Rademeyer, Chief Agronomist, Pannar Seed
Contact details: 082 776 5583 or pieter.rademeyer@pannar.co.za

Maize plants are monoecious plants which means that both the male inflorescence (tassel) as well as the female inflorescence (cob) occur on the same plant. Because these two inflorescences or flower types are separated on the plant, it is essential for the maize plant to produce sufficient amounts of pollen to enable the plant to produce seed.

Early in the maize plant's life cycle (5-leaf stage), the growing point will change from normal cell division to a reproductive structure. The growth point or embryonic tassel (male) is close to or just below the soil surface at the 5-leaf stage. As the plant grows, the tassel is pushed through whorl of leaf sheaths as the stem elongates and increasingly takes on the shape and appearance of the tassel as the plant grows older. The maize cobs (female) also begin to develop during the 5-leaf stage. During the 5-leaf stage, the plant first pays attention to the number of rows on the maize cob. Any stress during this stage can cause cobs to form fewer rows which will only be visible later in the season.

Successful pollination of the maize plant depends on several factors such as the production of sufficient and viable pollen during the tassel and silking stages. The silks must be able to receive the pollen (pollination) after which fertilisation and the development of the kernel embryo and endosperm (seed) will take place. This means that there must be synchronisation between the silk development and pollen production.

Conditions required to get more kernels on the cob (yield) depend on good general management, fertilisation, high levels of light intensity, minimum weed competition and sufficient photosynthesis (supply of sugars) from the plant. Remember that the first important period in determining the yield potential occurs at the 5-leaf stage and no deficiencies or stresses should prevail at this stage, as it will affect the final harvest.

Tassel (Male Flower)

The emergence of the tassel is regarded as the last vegetative development stage (VT) of the maize plant. Once the last tassel branch is visible, the plant is in the VT stage. A normal tassel consists of a central spike (rachis) with several branches. At high plant populations the tassel will be smaller with a subsequent lower pollen production and at lower plant populations larger tassels will develop with much more pollen produced per plant.

Photo 1: When the whole tassel becomes visible, the maize plant is in the last vegetative stage (VT), the tassel is starting to shed pollen from the central axis or spike (rachis). (Photo: Pieter Rademeyer)

Photo 2: The male flowers in the grass family are called a floret. Each spikelet contains two florets that each produce three pollen packs (anthers). (Photo: Beth Thompson)

The maize plant produces about 200–2 000 flower packs or florets in which the three pollen packs (anthers) are stored. Each pollen pack is composed of a filament and an anther. As the filament lengthens, it will push the pollen packs (anthers) out of the flower pack (floret). Where tillers are present, their tassels normally appear about 7-10 days later than those of the main plant, which extends the pollination period. Some genetic backgrounds can develop a cob where the tassel is supposed to be (an open maize cob without husks) on tillers, which are unable to extend the pollen shedding period. All plants will not necessarily come into seed at the same time, there is always a percentage of plants that will start to shed pollen a day or two sooner or later. This feature gives a few extra days of available pollen to complete pollination.

Pollen and Pollen Production (Male)

The pollen grains are found inside the pollen packs (anthers). It is different from some other plants where the pollen grains are carried on the outside of the anther. When the pollen packs or anthers are exposed, they absorb the evening moisture which then quickly evaporates the next day, causing the anthers to dry out and consequently burst open at the one end, releasing the pollen grains gradually from the opening. Factors such as humidity, temperature as well as wind speed will affect the rate of pollen release. As pollen exits from the pollen packs it is easily spread by wind. One pollen pack can release viable pollen for at least three days. If the wind speed is high, the packet will release the pollen quicker.

Photo 3: The pollen filaments are clearly visible on which the pollen packs (anthers) are attached. There are three pollen packs per flower pack. (Photo: Bob Nielson Purdue University)

Wind is necessary to transport the pollen around the field to ensure that good pollination takes place. Pollen is a very delicate substance, made up of almost pure protein. Due to the large amount of pollen that the maize plant must produce, it requires a lot of photosynthesis (plant energy). This has the disadvantage of putting the maize plant under pressure and makes the plant extremely sensitive to stress during the flowering stage. This is then also the most sensitive period in the maize plant's growth cycle.

Diagram 1: Drought sensitivity at the different growth stages of the maize plant

Pollen remains viable for about 18 to 24 hours after being shed. The median for pollen shedding for a tassel is 5- 7 days. Anything between 2 and 25 million pollen grains per plant is produced. Consider that if a maize cob contained 1 000 kernels, there would therefore be between 2 000 and 25 000 pollen grains available for each kernel. The process of pollen shedding peaks in the morning.

Photo 4: A closeup of the anthers with the pollen in them. The anthers open on the one end as the moisture content decreases when the temperature and relative humidity decreases through the morning. The pollen grains exit from the opening. (Presentation: University of Missouri)

Photo 5: Pollen grains. As the pollen ages the colour intensity increases. The colour can range from slightly white to dark yellow. (Photo: Pieter Rademeyer)

If it rains all day, no pollen shedding will take place. Continuous rain for a few days without ceasing (longer than seven days) will affect the process of pollen shedding, as well as the synchronisation between the silks and pollen shedding, resulting in poor pollination. When water is applied through irrigation, the period that the plant stays wet is too short to prevent pollination from taking place. Heat can affect the production and viability of the pollen. Concomitant heat and drought are usually one of the major causes of poor pollination. Drought plays a much greater role in poor pollination than heat. As pollen leaves the anther, it contains a high percentage of moisture. The moisture is necessary for the survival of the pollen.

Pollen becomes sterile at temperatures above 38°C. It is often argued that pollination did take place notwithstanding the high temperature – it should be borne in mind that pollination occurs mainly in the early morning and that high temperatures are only reached later in the morning. The process of pollen germination on the silk is a quick process.

Cob and Silk (Female)

Cob development begins early in the maize plant's life and is initially microscopically small and grows bigger as the maize plant develops. If the leaves of a maize plant are carefully removed, the small ear shoots in the leaf sheaths will be visible on the stem. As many as 8-10 maize ear shoots are initially formed per plant. There are hybrids that will produce only one cob per plant even under the best conditions, while others tend to bear more than one cob. Single-cobbing hybrids will give preference to the upper cob, as it dominates all the other ear shoots, while multi-cobing hybrids will give preference to more than one cob, starting with the upper or primary cob. This trait is a genetic characteristic and cannot be altered.

The female part of the maize plant consists of an egg cell (ovary) to which the silk is attached. The ovary develops to become the seed. The silk (style) has sticky connective tissue on the end called the stigma and is covered with fine hair (trichomes). The silks begin to elongate at the 12-leaf growth stage. With the hundreds of silks that extend, there must be order within the cob. The rows of flowers are usually straight with small grooves or channels between them on the inside of the husks that helps with the elongation process of the silks to the tip of the cobs. In most cases the process is flawless, but problems can arise where the silks start to curl (silk balling or jamming) within the husks, which results in only the kernels on the base of the cob being pollinated and fertilised.

Maize streak virus as well as differences of more than 10°C between minimum and maximum temperatures can contribute to silk balling. Drought stress can in turn slow down silk development, it can adversely affect the susceptibility of the silk to pollen and can also be detrimental to silk and pollen synchronisation. It is usually characterised by very poor pollination or no pollination at all.

Photo 6: Silk balling or jamming (Photo: Dr Rikus Kloppers)

As soon as the first silks become visible, the maize plant enters the first reproductive or R1 stage. Silks appear over a period of 3 to 5 days. Under cool conditions, the silks will emerge slower. The first silks to emerge from the husks originate from the egg cells at the base of the cob, after which the silks of the other egg cells appear progressively from the bottom of the cob towards the tip of the cob, with the silks at the tip of the maize cob appearing last. Once silks appear, it is immediately receptive to pollen. Silks can remain receptive to pollen for 6-10 days. For the kernel to develop, the silk must first be pollinated (pollen grains fall on the silk, germinate and grow down towards the egg cell on the maize core where it fertilizes the egg cell).

Photo 7: Stigma (hair), style (silk) and ovary (kernel) of the maize (Photo: University of Missouri). The silk is attached to an egg cell (ovary) that develops after fertilisation to become the seed.

Photo 8: The silk (style) has sticky connective tissue on the end called the stigma and is covered with fine hair (trichomes) that help capture pollen grains. (Photo: University of Missouri)

If the plant shows wilting symptoms due to heat or drought, the silk is also wilted. Withered plants and silks will not pollinate and fertilise as well, compared to normal conditions, due to a receptivity problem. Silk consists of more than 90% water. If the plant enters a stress period, it will give preference to the water pressure (turgor) of the leaves and other larger structures and insufficient moisture may be available for silk development. Silk growth is driven by a high water potential, at good water potential conditions silks will emerge sooner and vice versa.

During the first two days of silk development and appearance, the silk growth rate is highest, it then gradually decreases until day 10 at which time virtually no silk growth occurs. The fastest silk growth always occurs in the evenings and early morning hours. The silk obtains its water from the ovary, the ovary absorbs water from the cob, and the cob obtains its water from the stem. The silk is at the last in line in terms of moisture supply. Fortunately, the silk is protected by many husk leaves that protect it from dehydration. Silk will become unreceptive for pollen at about 32-34°C. Low humidity can be problematic, especially when accompanied by warm winds. Should silks become unreceptive today and temperature and humidity improve the tomorrow, normal pollination will occur.

The pollen can germinate anywhere on the silk. In hot environments, the full cob pollinates in three days, while in cooler areas the process can take up to five days to complete. Unpollinated silks will continue to elongate and extend until about 10 days after first emerging. If very long silks are detected it may indicate poor pollination. As silks grow older, their susceptibility to pollen decreases.

Photo 9: Long silks may be an indication that poor pollination took place. (Photo: AK Geldenhuys)

Pollination and Fertilisation

Pollination occurs when the pollen grain lands on the silk and the pollen tube germinates. After the pollen has germinated the pollen tube will grow into the silk - the silk looks like a straw in which the pollen tube can grow. When the pollen grain germinates, the pollen tube can grow along the outside of the silk until it finds an opening to enter and begin elongating down the inside of the silk tube. Openings are usually found near the fine hairs on the silk.

The germinating pollen tube can only grow in a medium with enough moisture which the silk must supply to the pollen grain. It may sometimes happen that after the pollen growth or elongation process has begun, it may cease due to insufficient moisture provided by the silk. The tube is then "clogged”, and that unfertilised kernel will be perceived as infertile on the cob. Unfertilised egg cells can appear on any part of the cob, and kernels in the immediate vicinity of the unfertilised egg cells will compensate and take on a more rounded shape. The process of fertilisation is one of the quickest processes in nature. After the pollen has landed on the silk, it takes 15-25 hours to complete the fertilisation process. A pollen grain contains two sex cells (nuclei or gametes). One male nucleus fertilises one female gamete (embryo or germ) and the other nuclues fertilises two other female gametes to form the endosperm of the kernel - this phenomenon or sequence of events is known as double fertilisation. The phenomenon is especially seen when white maize is planted on a land where there was yellow maize grown the previous year. Due to the many pollen grains produced by the maize plant, some volunteer plants from the previous crop can cause major grain grading problems (the yellow-grain colour is dominant over a white-grain colour). Where yellow pollen fertilises a white grain hybrid, the endosperm of the kernel will have a yellow colour, but not the other way around.

Silks are weaned off the ovary (kernel) within a week of conception. The success of fertilisation can be observed by carefully removing the husk leaves and shaking the bare cob so that silks can detach from the kernel attachment. The silks of unpollinated ovaries will still stick to the cob until a late stage.

Photo 10: Silks remain stuck on the cob if pollination/fertilisation did not occur. (Photo: Pieter Rademeyer)

Photo 11: In some exceptional cases silks can stay attached to mature kernels. (Photo: Pieter Rademeyer)

Under severe stress conditions, it may happen that even though fertilisation took place, the kernel could abort shortly afterwards. The kernels will then shrivel and look translucent with a white cap. Stress conditions one week before to two weeks after the flowering stage usually result in large yield losses. This is attributed to poor pollination and fertilisation. These yield losses cannot be reversed. Any negative aspect that may affect the plant after pollination, e.g. defoliation, disease, cold and heat can negatively affect the development of the kernels as well as the number of kernels that need to be formed. It has been established that for every day's moisture stress during the flowering stage, up to 10% crop loss per day may occur (See diagram 1).

Photo 12: Aborted kernels

The maize plant will always give priority to pollen production over silk production. Under drought stress conditions, silks can emerge too late, without any pollen being available. Much research is being done to improve silk/pollen synchronisation. There are some maize hybrids whose silks will emerge before the tassel and the trait is very valuable should a drought/stress situation occur during the pollination period. In these cases, synchronisation will rarely be a problem. There are also hybrids that give preference to the formation of pollen first and silk development afterwards.

Pollen will survive at normal temperatures above freezing. However, sterility of pollen can occur with freezing and lower temperatures. Insects such as the bollworms that feed on the silks, can cause poor pollination, or kernels may only form on one side of the cob. In some cases, this can cause silk balling within the husk leaf sheaths.

Photo 13: Bollworm feeding damage can result in poor pollination especially on one side of the cob.

Herbicides and especially 2.4D will cause poor pollination if it drifts onto the silk. It is always advisable not to spray chemicals around the silk and tassel stage of the maize plant. Boron plays an important role in all plants to facilitate good pollination. In maize, the typical deficiency symptom is curved cobs that are only pollinated on one side. (not to be confused with bollworm damage).

Summary

The maize plant remains one of the most interesting plants. If everything is done correctly and in time and the season runs smoothly from day one, the maize producer can expect to harvest between 600 and more than 1 000 kernels for every single seed planted. On the other hand, any stress conditions will erode the yield potential. Nature is unpredictable and has the greatest impact on all life.