The Story of Incomplete Pollination
If you ask a woman whether she is pregnant, she generally answers “yes” or “no.” Very seldom do you hear other versions, such as “more or less,” “partially,” or “somewhat.” She either is or she isn’t — go or no-go — and the reality is easy to comprehend.
In plants, however, the situation is very different. Plants can be partially pollinated, meaning some of the ovules in the ovary became fertilized and some did not. Although most of us don’t ponder this overly much, growers certainly do. An incompletely pollinated crop can mean the difference between profit or loss. Even if you are not selling seeds, pollination is a very big deal.
You’ve probably seen an apple that was round and robust on one side but flat on the other. Or maybe you’ve eaten a raspberry that was missing some of its juicy little nubbins and you wondered why. Cucumbers are sometimes fat on one end and pencil thin on the other, and an ear of sweetcorn may be missing hundreds of kernels. All of these fruits share the same ailment: incomplete pollination.
Whether a grower sells a crop by weight or volume — or even by appearance — he loses money when pollination is insufficient to grow the fruit to full size. A partially pollinated cucumber, for instance, weighs less and is considerably smaller and less attractive than a fully-developed one. And even if the resulting seeds are healthy and viable, incomplete pollination yields fewer of them.
Reproduction in Flowering Plants
In plants, two major steps are required to produce seed. The first, pollination, occurs when grains of pollen containing male genetic material land on the stigma of a flower of the same species. This is where pollinators do their thing: While traveling from flower to flower, they move the pollen to the right place.
The second step, fertilization, occurs when male gametes finally unite with female gametes. This union occurs at the base of a female flower, inside the ovary which contains one or more ovules.
Principles of pollination and fertilization seem pretty basic on the surface, but plants are so variable that the subject becomes sketchy when you look below the surface. For example, some plants are monoecious, meaning that both male and female flowers occur on one plant, and some are dioecious, meaning that male and female flowers are on different plants. Then you have so-called perfect flowers that have both male and female parts in each individual flower on every plant. Go figure.
And that’s just the beginning. Some flowers are self-fertile, completely capable of fertilizing themselves, while others absolutely require cross pollination with another individual. And of course there is plenty of middle ground — those plants that can fertilize themselves but set more fruit with the aid of cross pollination.
Regardless of the different flower types, the basic mechanism of fertilization is the same. Once pollen from the proper source lands on the stigma of a flower, a string of events begins to unfold.
First, the pollen grain must germinate. Remember that the female part of a flower has three sections, the stigma, style, and ovary. After germination, a pollen tube grows from the pollen grain, through the complete length of the style, and into the ovary. The male genetic material travels the length of this tube until it unites with an ovule inside the ovary.
An ovary is just a holding cell for eggs. Plants may have an ovary with one compartment, called a carpel, or it may have multiple carpels. Each carpel, in turn, may have one or multiple ovules. The important thing to remember is that once ovules are fertilized, they become seeds, and the ovary itself grows into a fruit that protects the seeds within.
For example, an apple-ready-to-eat is a mature ovary containing five carpels (those fibrous parts of the core) and each carpel has two or three seeds. If all the ovules in an apple flower are fertilized, the resulting apple will be round and attractive. However, when not all the ovules are fertilized, the apple may end up lopsided or small.
The difference in size and shape is a result of chemical signals. A fertilized ovule releases hormones that cause the fruit to grow. If the ovule is not fertilized, there is no seed to protect, so the plant doesn’t waste energy making a fleshy covering for it.
These fleshy coverings not only protect the seed from premature drying, UV radiation, and pathogens, but they attract animals that move the seed from place to place. Many animals eat the fruit-covered seeds and excrete them elsewhere, effectively dispersing the plant.
Both Parts are Vital
As you can see, both pollination and fertilization are important for adequate fruit set. Without sufficient pollination, the male gametes will never arrive at the right place. But even with good pollination, a crop can fail if conditions don’t allow fertilization to take place.
It is easy to blame a lack of pollinators for everything that goes wrong in the field, but it’s not that simple. Excess heat or cold, too much rain or not enough, poor soil fertility, insufficient sunlight, damaging winds, insects, and disease can all prevent a good crop regardless of the amount of pollination.
One of the easiest places to see the effects of incomplete pollination is in corn. Corn, like all grasses, is wind pollinated, so it is not dependent on animal pollinators. Even so, pollination can be haphazard. Incomplete pollination is often seen on the perimeter of a cornfield, especially on the windward side. The prevailing winds usually blow in the same direction, so plants downwind do much better than those on the windward edge.
If you are not familiar with a corn stalk, here are some basics. Corn is monecious so each stalk has both male and female flowers. The yellow tassel at the top of the stalk is covered with male flowers. At maturity, the tiny flowers open and spill forth lots and lots of pollen. This is carried on….