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 the wind and, with any luck, lands on the stigmas of another corn plant.
The female flowers are known as silks and the stigmas are at the tip of each one. Once a pollen grain lands on a stigma, it germinates and sends a skinny tube down through the center of the silk. This tube becomes a sliding board of sorts that carries the male gametes to the female ovule at its base.
If fertilization is successful, a fruit (or kernel) forms as the seed matures. If you look carefully at a kernel of corn, especially a fresh one, you can see the little attachment point where the silk met the ovule. When you husk an ear of corn, you can see that one silk is attached to each kernel. To be completely pollinated, every silk needs to be pollinated by a separate grain of pollen. Since an ear of corn contains 700-800 kernels, that’s a lot of pollination.
Wherever a silk was not fertilized, no kernel develops. When you open these ears you think, “Where’s the corn?” Sometimes the unfertilized ovules are near the top end, but sometimes they can be found randomly throughout the ear.
Raspberries are arranged differently than corn, but they suffer from the same problems. Each little nubbin (technically a drupelet) needs to be pollinated separately. If some are “virgin” you will notice the raspberry is not complete or not very big. Sometimes you can see the bare, undeveloped ovules clustered together with no “meat” around them.
Under-pollinated blueberries are generally smaller than fully developed ones. A blueberry is a “true berry,” a single ovary with a hundred or so ovules, which become the seeds after fertilization. When pollination is good, each seed develops and the berry expands to accommodate and protect all the seeds. If few ovules are fertilized, the berry remains small and undeveloped, or sometimes it simply drops off the plant.
Other true berries are built similarly and behave the same way, for example watermelons, tomatoes, cucumbers, squash, and pumpkins. Here, the word berry is a botanical term that describes how the ovaries and resultant fruit are structured. In these examples, you can see that one fruit protects multiple seeds. To get big pumpkins, watermelons, and tomatoes you need to pollinate as many of the seeds as possible, just like in a blueberry.
Strawberries, raspberries, and blackberries — none of which are actual berries — have different structures but still require lots of pollination. Strawberries carry both their ovaries and seeds stuck to the outside of a receptacle, but if they are not adequately pollinated, the fruit will be small and misshapen. Raspberries, blackberries, dewberries and other aggregate fruits require separate instances of pollination and fertilization for each of the tiny drupelets.
Incomplete pollination in Cucurbits
When plants in the gourd family are under-fertilized, you see strange shapes. These plants — which include cucumber, squash, watermelon, pumpkin, cantaloupe and gourds — get fat and juicy wherever the seeds are fertilized, but any part of the fruit containing unfertilized seeds looks shrunken and wrinkly. Sometimes you can see the difference on the inside as well, especially a condition called hollow heart, where there is empty space inside the fruit.
In the old days, fertilized watermelon seeds were fat, dark, and glossy while unfertilized seeds were skinny and pale. Today, however, seedless watermelons are flooding the markets. As you may have noticed, seedless melons are not actually seedless, but contain many soft and mushy white seeds. The plants that produce these melons are infertile triploids, hybridized by crossing a chemically-induced tetraploid melon with a regular diploid one.
From a pollination perspective, seedless melons are problematic because they require much more pollination than regular old seedy ones. To grow melons from an infertile plant, you have to pollinate the seeds with fertile pollen. To do this, farmers must plant both seedless and seedy types close to each other, and let the bees pollinate both. Since the two types are so close together, every pollen load a bee carries is a mix of fertile pollen diluted with infertile pollen. As a result, adequate pollination requires many more bee trips.
According to Seminis-us.com, a regular watermelon requires 7 to 8 bee visits to be completely pollinated, whereas a seedless melon requires 16 to 24 visits — nearly three times as many. Even though the resulting seeds do not mature normally, enough hormone is released from the fertilized ovules to force the plant to set fruit. More fertilized seed results in bigger fruit.
Don’t Dis the Males
On a side note, people frequently complain that their squashes and melons have lots of flowers but no fruit. It helps to remember that most all cucurbits are monoecious, meaning each plant has both male and female flowers. The male flowers — which are far more numerous — open first, often a week or so before the first females. These, being male, do not produce offspring.
You can find the females by looking for the small, undeveloped ovary just beneath the petals. After pollination and fertilization, the petals fall away and the ovary begins to expand. The fruitless male flowers dry up and disappear, having done their part by producing the pollen.
One of the main impediments to complete pollination is a lack of bees or the lack of the right kind of bees. For example, cucurbit pollen is big, heavy, and sticky, so not every bee species is willing to deal with it. In most cases, large bees — such as bumble bees, honey bees, and squash bees — are good pollinators of super-sticky pollen.
Another common problem is weather. If the season is overly dry or unusually wet, the fruit may not set, even with proper pollination. Or, if the weather is unfit for bee flight, the plants may suffer from low bee numbers.
The grower, too, can inhibit pollination by planting the wrong type of pollinizer or planting it too far away from the plants needing pollination. So while it’s up to the pollinators to the deliver the pollen in the quantities needed, it’s up to the farmer or orchardist to plant compatible species in the right ratio and at the proper distance.
Food from Stores
In our modern society where food comes from restaurants or grocery stores, we seldom see misshapen or substandard fruit. But there is plenty. Poorly-pollinated fruit is used for lower-profit products. Weird-looking apples are squeezed into juice, made into pies, or mashed into sauce. Substandard berries become jam or compote. And all those lopsided pears are rolled into lunch boxes in the form of fruit leather.
The high-priced apples on the produce shelves — all impeccably-shaped and symmetrical — result from adequate pollination followed by textbook fertilization in splendid climatic conditions. Picture perfect produce is seldom an accident, but the result of good management decisions and a large helping of luck.
Honey Bee Suite