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Incomplete pollination and why it matters

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.

This article first appeared in American Bee Journal, Volume 159 No 10, October 2019, pp. 1125-1128.

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.

Fertilization Basics

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.

The parts of a flower.

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.

Corn 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.

 After a pollen grain germinates, a tube grows through the length of the corn silk. The pollen tube carries the male gametes to the female ovule, which matures into a kernel of corn.
After a pollen grain germinates, a tube grows through the length of the corn silk. The pollen tube carries the male gametes to the female ovule, which matures into a kernel of corn. Photo by Rusty Burlew
If any of the corn ovules are not fertilized, they remain small and undeveloped.
If any of the corn ovules are not fertilized, they remain small and undeveloped. Photo by Rusty Burlew

Berry Crops

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.

The female parts of a raspberry flower, in the center, are surrounded by a ring of male parts. In raspberries, each ovule is in a separate ovary, which grows into a tiny single-seeded fruit called a drupelet. All the drupelets together from a drupe.
The female parts of a raspberry flower, in the center, are surrounded by a ring of male parts. In raspberries, each ovule is in a separate ovary, which grows into a tiny single-seeded fruit called a drupelet. All the drupelets together from a drupe. Photo by Rusty Burlew

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.

There is no shortage of cucurbit pollinators in the ABJ editor’s garden, most notably bumble, carpenter and squash bees in addition to the half-dozen honey bee colonies. Unfortunately, there is also no lack of weeds, which can sometimes obscure cucumber blossoms from the bees (right) and grown cukes from the gardener (center). Photo by Eugene Makovec
There is no shortage of cucurbit pollinators in the ABJ editor’s garden, most notably bumble, carpenter and squash bees in addition to the half-dozen honey bee colonies. Unfortunately, there is also no lack of weeds, which can sometimes obscure cucumber blossoms from the bees (right) and grown cukes from the gardener (center). Photo by Eugene Makovec

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.

(L) In cucurbits, such as this yellow squash, the female flowers have a clearly visible ovary just below the petals. (R) The male squash flower has no ovary and simply falls away after releasing its pollen.
(L) In cucurbits, such as this yellow squash, the female flowers have a clearly visible ovary just below the petals. (R) The male squash flower has no ovary and simply falls away after releasing its pollen. Photo by Rusty Burlew

Pollination Problems

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.

Rusty
Honey Bee Suite

Comments

Alistair
Reply

Hi Rusty

A very timely post as I attended an excellent lecture this week by a Dr Thomas Wood who’d done research into cucumber pollination in Michigan!

Three interesting points:

1. Around 95% of cucumber pollination in Michigan is done by honey bees. In India (where cucumbers originate) about 80% of pollination is done by honey bees (not all Apis mellifera).

2. It takes 8 – 10 visits of honey bees to fully pollinate the female cucumber flower. He also had photos of cucumbers pollinated by only 4 or 5 visits and they did look rather sorry for themselves.

3. Honey bees don’t like cucumber pollen, so they don’t collect it. This helps the plant, because when they get the pollen on their faces while collecting nectar they don’t groom it off into the corbiculae so it is readily available for pollination.

Edward Nice
Reply

Wow Rusty, just wow! That’s a whole textbook of knowledge condensed down to a single article. Very nicely done. I wish my high school biology class had been so clear and concise (or perhaps I wasn’t paying attention 😉).

It is interesting to me how finding something you truly love (like pollinators or beekeeping) provides an anchor and guidepost that opens the door to seeing and understanding the world more broadly. You are one of my ‘definitive’ sources of information regarding pollinators and beekeeping. Keep it coming!

Rusty
Reply

Edward,

Thank you. Your observation about learning works on this end, too. I find an endless array of things to write about that are related to bees and beekeeping even though they are not actually about bees and beekeeping. Even though I have degrees in the life sciences, I learn much as I go along.

Victor
Reply

Thank you for the very interesting plant biology!

Maria Beebe
Reply

Thanks for that! I love vegetable gardening and read a lot about it but I did have an “Aha!” moment here. I always thought the misshapen cuke was due to watering. But it can also be due to pollination. That makes so much sense as sometimes the weeds get a bit out of hand.

Love your blog. Rarely comment but always read it!

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