pollination – The Garden Professors™

Some like it hot… but most do not: How high temperatures delay pollination and ripening

Ah, summer – vacations (pre-COVID), swimming pools (pre-COVID), ice cream, vegetable gardens, and, in many places, really high temperatures. These things all go hand-in-hand (or at least they did before the pandemic). Many gardeners feel that the heat of mid-summer goes hand in hand with garden production; those high temps driving production on those fruiting plants like tomatoes and peppers. But…..could they be wrong?

We’ve had lots of extra hot days this summer in Nebraska, so it stands to reason that we should have really great production on those garden favorites like tomatoes, right? Then tell me why our extension office has received numerous questions this year about why tomatoes aren’t setting on or ripening. Heck, we even had a Facebook post about tomatoes not ripening in the heat go viral (well, for our standards – 300,000 views/2,000 shares). Could it be a disease? Nope – it’s the heat. High daytime temperatures can have a big effect, but the effects are compounded when nighttime temperatures are high as well.

Tomatoes not ripening? You're not alone. Temperatures above 85 degrees will slow down the ripening process. Temperatures above 95 can stop the process all together. #NebExt #NeWX
Posted by Nebraska Extension in Douglas-Sarpy Counties on Wednesday, July 22, 2020

It turns out that high heat does two things in many of those fruiting vegetables (and of course fruits) that we grow. First, it inhibits pollen production, which in turns reduces fruit set. Second, heat inhibits gene expression for proteins that aid in ripening/maturation of the fruit. Heat stress also reduces photosynthesis (Sharkey, 2005) in many different plants, which would slow down plant processes (such as fruit development and ripening) as it reduces the availability of sugars to fuel these processes. So high heat can not only reduce the number of fruits developing on the plant, but also slow down the ripening process for fruits that have already set. And if you think that these effects only happen at super extreme temps, most of the research studying temperature effects of this nature use a common “high ambient temperature” of 32°C/26°C for daytime/nighttime temperatures. For us U.S. Fahrenheit-ers, that’s 89.6°F/78.8°F, which isn’t really all that hot for most of us.

Many studies show that application of this “high ambient temperature” to crops such as tomatoes, beans, and corn during the pre-fertilization phases of reproduction (ie – flower/pollen development) can negatively effect fruit set. The introduction of Porch and Jahn (2001) gives a pretty good overview of literature detailing the effect in beans (Phaseolus vulgaris). I’ll sum it up here: heat stress while the pollen is forming (called sporogenesis) led to pollen sterility and failure of pollen to release from the anthers (dehiscence). It also led to flower abscission (basically the plant aborts the flower) and reduce pollen tube formation (how the pollen nucleus gets through the stigma to the ovule for pollination) when applied during the period of pollen sac and ovary development. And application during flower opening (anthesis) resulted in pollen injury (sterility) and reproductive organ abscission. All of these effects lead to reduced fruit/seed set in beans. (Interestingly, heat stress at the ovary development phase also led to parthenocarpy – basically the pods developed, sans seeds, without fertilization).

However, we get the most calls about tomatoes (they’re the top crop for most home gardeners). Is it the same issue? Yep. Numerous studies (Sato, et al., 2000; Pressman, et al., 2002; Abdul-BAki, 1992) show the same effect in tomatoes. Pressman, et al. (2002) linked the effects on pollen to changes in carbohydrates in the anthers (reduced starch storage and carbohydrate metabolism).

Tomato pollination and how to increase it in high tunnels — Tomato floral structures

To add insult to injury, high temperatures also slow down or stop ripening of crops like tomatoes. Picton and Grierson (1988) found that 35°C (95°F) temperatures altered the gene expression in tomato fruits – inhibiting the expression of polygalacturonase, which softens cells walls, allowing the fruit to ripen. Reduced photosynthesis would also reduce the availability of sugars for fruit development and ripening.

But there’s hope, both this season and in the long term! The effect on the plants is not permanent. When temperatures drop below that “high ambient temperature” threshold pollen production, and therefore fruit set, will return to normal (as long as the plant is healthy). Sato, et al. (2000) found that pollen release and fruit set resumed within a few days after heat stressed plants were “relieved” and temperatures dropped back into the optimal range of 26-28°C/22°C (78.8-82.4°F/71.6°F). So many of those plants will become productive again (good news for my own tomatoes and beans, which had an initial flurry of production then went on vacation), especially as we head into fall. And efforts are under way to develop and test heat stress resistant cultivars.

This last point may be more important than you realized. These production problems plague many areas around he world at current climactic norms. Many fear that increasing temperatures will limit the productive capacity of many areas of the world that are already struggling. It is easy to see how the difference in just of just a few degrees can take your veggie production from prolific to paltry.

You can also try to reduce the heat a bit yourself for an immediate fix. Shade cloth can help reduce temperatures a little bit, which may make all the difference in your garden if you’re just slightly over the “high ambient temperature” threshold.

Tomatoes under shade cloth | Tomatoes under shade cloth | Flickr — Tomatoes under shade cloth | Source: flickr.com

But in the meantime, if your vegetable garden has taken a summer siesta it will get around to producing again one day. You’ll just have to take good care of the plants in the meantime. And perhaps it’s a blessing in disguise – when its that hot I don’t want to be out working in the garden much, either.

Sources

Abdul-Baki, A. A. (1992). Determination of pollen viability in tomatoes. Journal of the American Society for Horticultural Science, 117(3), 473-476.Porch, T.G. and Jahn, M. (2001), Effects of high‐temperature stress on microsporogenesis in heat‐sensitive and heat‐tolerant genotypes of Phaseolus vulgaris . Plant, Cell & Environment, 24: 723-731. doi:10.1046/j.1365-3040.2001.00716.x
Pressman, E., Peet, M. M., & Pharr, D. M. (2002). The effect of heat stress on tomato pollen characteristics is associated with changes in carbohydrate concentration in the developing anthers. Annals of Botany, 90(5), 631-636.
Sato, S., Peet, M. M., & Thomas, J. F. (2000). Physiological factors limit fruit set of tomato (Lycopersicon esculentum Mill.) under chronic, mild heat stress. Plant, Cell & Environment, 23(7), 719-726.
Sharkey, T. D. (2005). Effects of moderate heat stress on photosynthesis: importance of thylakoid reactions, rubisco deactivation, reactive oxygen species, and thermotolerance provided by isoprene. Plant, Cell & Environment, 28(3), 269-277.

Sex and the Single Squash: A study in plant sex, sexuality, reproduction, and seed saving

In the 1960s, author and future Cosmopolitan magazine Editor Helen Gurley Brown scandalized the country with her book about independent single women called “Sex and the Single Girl.” Taking a page from Ms. Brown, we can have a discussion about “Sex and the Single Squash.” Here, we can talk about plant floral structure and reproduction and its effect on fruit production and even seed saving. A true discussion of the “birds and the bees” if you will. This is especially important in the vegetable and fruit realm, since reproduction is why we get tomatoes, peppers, apples, plums and such in the first place. It also is important for producing seeds, as those arise from the reproductive process as well.

Whether you knew it or not, flowers are not just different in appearance from plant to plant, but the ways in which they are pollinated and turn into fruit are different as well.

Some plants have what are called “perfect” flowers where both male and female parts are present, such as roses, apples and dandelions. In a way of speaking, you could say that these flowers are hermaphroditic. These flowers may or may not be self-pollinated. Depending on species genetics, some plants can self-fertilize (like tomatoes and beans) and others require cross-pollination (like apples).

Other flowers are “incomplete,” meaning that they have separate male and female flowers. Some plants with “incomplete” flowers are called dioecious (Greek, meaning “two households”), and have distinct male and female plants such as ginkgo trees, holly bushes and kiwi vines. Some “incomplete” plants are monoecious and have distinct but separate male and female flowers on one plant — like squash, cucumbers and corn.

So, here’s where the vegetable garden comes in — one of the questions that I get every year without fail has something to do with why most of the flowers on a squash or cucumber or other cucurbit (that’s what we call plants in this family) plant do not produce fruit.

There are a few explanations – high heat causing aborted flowers or fruits or improper pollination, absence of pollinators, or, most likely, the fact that some of those flowers were never going to set fruit because they were male. In answer, I have to explain that about half or more of the flowers on the plant are male and are, unfortunately, anatomically incapable of producing fruit.

There are a few ways to tell male and female flowers apart when it comes to members of the cucurbit family.

First, look at the base of the flower. If the base is swollen and looks like it is a tiny version of the mature fruit, then it is a female flower.

If the base is just a straight stem (in flowers, this stem is called a peduncle), then it is a male flower.

The second method is to look inside the flower. If there is one large central structure, called the pistil, that indicates the flower is female.

Male flowers will have several, smaller stamens inside. Female flowers also tend to be larger than male flowers.

In the world of the single, available female squash blossom, life revolves around attracting honey and other native bees that have also recently visited male flowers to assure pollen transfer.

All members of the cucurbit family require this pollination tango to make sure that the female flowers produce fruit.

Each species and even variety of squash have a different ratio of male to female flowers. The ratio is usually about 1-to-1, but it is not unusual to see varieties with many more males than females.

Many of the plants also produce an abundance of male flowers early in the season, sort of as a teaser to make sure bees are attracted to the plant later on to pollinate the female plants.

So if a majority of flowers die early in the season without setting fruit, or about half of the flowers die throughout the season, there is nothing to worry about.

If female flowers are dying throughout the season without producing fruit, though, there is a definite problem. This means that there are no bees available to pollinate the plants.

If fruits have shrunken parts or misshapen, then there could be an issue of incomplete pollination from not having bees around. This could result from not having enough food for them in the area to encourage their presence, or from weather being too cool or wet for bees to get out and pollinate.

The lack of bees could also be the result of improper use of pesticides in the area.

If it seems like the birds and the bees aren’t happening in your garden, there are ways that you can ensure fruitfulness by taking matters into your own hands.

Transferring pollen from male flowers to female flowers can be accomplished using a small artist’s paintbrush or by simply pulling off a male flower and using it to apply pollen directly.

Gardeners who want to save seeds from plants in this family should also pollinate flowers by hand, and actually go so far as to protect the female flower from outside pollen using some sort of cover.

In fact, this method is often used by plant breeders or those who want to save seeds of crops that easily cross-pollinate. Hand pollination followed by bagging the flower to keep pollen or pollinators away to avoid accidental unwanted pollen is often used to produce.

Believe it or not, several members of the squash family that look or taste nothing alike are the same species and can cross-pollinate. For example: Zucchini, summer squash, pumpkins, scallop squash, decorative gourds and acorn squash are all in the species Cucurbita pepo and can cross with each other.

A few years ago, one of my Master Gardeners came up to me at the end of a meeting and asked me what was wrong with her zucchini. She handed me an object roughly the shape of a zucchini, only a bit larger and splotched with orange. She had saved the seeds from the year before.

I immediately answered that her zucchini had crossed with a pumpkin. Both of these plants are the same species and can easily cross pollinate. Even if you don’t have pumpkins in your garden, bees can travel 2 miles or more in search of food. So she was left with what I would call a Puccini.

Easy cross-pollination of varieties is why the most common heirloom crop varieties you’ll find are tomatoes and beans. Both of these crops have closed flowers that help resist cross-pollination.

They are most likely to be self-fertile, meaning that the flower will pollinate itself without outside assistance. This helps the plant breed true — so next year you end up with something that’s roughly the same as what you had this year. These plants can be just a few feet away from a different variety and they will not cross pollinate.

If you want to save something that is bee-pollinated, like your squash, pumpkins or cucumbers, you might want to do the brush and bag technique. Otherwise you might end up with a surprise in the garden next year.

The heirloom varieties that we often save are open pollinated, meaning that when they cross with themselves their genetics are relatively stable and you won’t see a lot of difference from year to year. (There will still be some difference, so if you save seeds for a long time you can end up with your own strain of a variety suited to your garden and location.)

Hybrids, on the other hand, have less stable genetics than the open pollinated varieties. With the way genetics work, some of those offspring will have traits of the mother plant, some will favor the father and some will be similar to the plant you are trying to save (and some might look like the milkman).

When seed companies sell hybrid varieties, they have to maintain a population of the mother plant and father plant to cross them every year to get the specific hybrid variety.

While the results of saving seeds from hybrids will be unpredictable, it can also be fun. My friend, plant breeder Joseph Tychonievich, points out in his talks and his book, “Plant Breeding for the Home Gardener,” that you can save the seeds from plants most closely resembling the desired plant over several years.

Just keep planting your selected seeds and harvesting the closest one to what you want. After about three or four years, you can end up with a relatively stable, perhaps even open-pollinated variety, that is your very own based on that hybrid variety you love.

And if you end up with a cross-pollination, either purposeful or accidental, you won’t see a difference in the fruit from this growing season (except maybe in corn, but that’s another story) Those changes won’t be apparent until you grow out the seeds you saved. So you won’t know until next year if you have one of those pucchinis.

And don’t forget: If you do have an overabundance of male squash flowers, they are edible too. You can put them in a casserole, fry them, stuff them, and more.

Pollination strategies can lead to allergy woes

This article was originally published in my weekly newspaper column in the Charleston Gazette-Mail. Articles are archived at wvgardenguru.com.

A quick selfie with President Gee. He's perfected the art of the selfie. — A quick selfie with President Gee. He’s perfected the art of the selfie.

A few weeks ago I made my way to South Dakota for the annual meeting of the National Association of County Agricultural Agents (where fellow GP and I made the rounds at the trade show scrutinizing wacky products). It is a fun conference made even more special this year by the fact that WVU President E. Gordon Gee was in attendance as the conference co-keynote speaker and recipient of the Service to American/World Agriculture award. But I digress…..

Two days into the conference something wasn’t quite right. I kept feeling worse and worse, and by Wednesday I was confined to my hotel room (save for a venture out to the conference banquet for dinner). I would not have been functional for the rest of the trip save for the kindness of a co-worker who went through the pharmacy red tape to procure and deliver “the good stuff” to my hotel room.

I thought I had a sinus infection at best (I get them often) and the flu at worst (yes, it was really that bad). But guess what — I’m just really allergic to South Dakota. Two days after my return, I was nearly back to normal (well, my normal, anyway).

Those who know me know that I suffer from the occupational hazard of allergies. Irony dictates that my allergies are only to about two dozen plants and two molds (that occur in mulch/compost). Lucky me!

Lots of corn surrounding my hotel in the outskirts of Sioux Falls. — Lots of corn surrounding the outskirts of Sioux Falls.

My best guess is that I had a reaction to the corn pollen of South Dakota. It makes sense — while we do grow some corn here in West Virginia, the Mount Rushmore State boasts an estimated 4.75 million acres of corn. I don’t think I was tested for corn pollen allergies, but since corn is not a major crop here, it may not be part of the common test.

I tell this story not for sympathy (well, OK, maybe a little) but it brings up a good illustration about pollination strategies of plants.

Scanning electron microscope image of pollen grains from a variety of common plants: sunflower (Helianthus annuus), morning glory (Ipomoea purpurea), prairie hollyhock (Sidalcea malviflora), oriental lily (Lilium auratum), evening primrose (Oenothera fruticosa), and castor bean (Ricinus communis).

You see, plants like corn rely on chance and wind to spread their genes around. In corn, the pollen drops from the male flowers (the tassel on the top) to the stigma of the female flower (the end of the silk sticking out of the cob). The process relies on lots of pollen being released into the air, since there is a good chance that a lot of it will miss the target. Corn pollen is usually heavy, therefore it doesn’t blow too far from the plant (unless there is lots of wind).

This is why you don’t get a good corn crop if you don’t have a big block of corn in the garden — just one or two rows doesn’t drop enough pollen to pollinate all the flowers. When the silks don’t get pollinated, you’ll end up with incomplete cobs missing kernels. This can also happen if the corn is in bloom during a long period of rain — the rain washes all of the pollen off before pollination can occur.

Most of the major allergen-producing plants are wind pollinators — trees, grasses, ragweed. They all release copious amounts of pollen into the air hoping for it to land in the right place.

Some plants still rely on pollen getting moved from plant to plant or flower to flower, but they remove the chance involved with wind pollination. These plants have a stickier pollen that stays on the flower and waits for something to come along and move it — a bee, a butterfly, a moth, a hummingbird, etc. These plants hold on to their pollen and have the more directed approach of getting a courier to make a direct delivery of their pollen between flowers.

Since these plants don’t leave the pollination to chance, they generally produce less pollen. Some good examples are fruit trees (apples, peaches, pears), sunflowers, squash, goldenrod and roses. Since they don’t release it into the air, they usually aren’t considered major allergens.

Still yet, some plants want to take no chance with their next generation. Self-pollinating plants don’t rely on pollen being spread to different flowers — they take care of business themselves. These plants are perfectly fine without crossbreeding with other plants.

Sometimes, these plants are so dedicated to self-fertilization that they make it difficult for the pollen to leave the flower. Bean flowers have a lower lip that curves upward to protect the reproductive parts inside. Tomato flowers are nearly completely enclosed. You may see bees going from flower to flower, but their search for food is in vain — they can’t get into the flower. Their buzzing does help dislodge the pollen inside the flower, but they don’t have access to spread it around. Producers that grow tomatoes in greenhouses where there is no wind to knock the pollen loose either buy boxes of bumblebees to release in the greenhouse, or use something like a vibrating toothbrush to help the flowers self-pollinate (no joke).

This is why you can plant two different tomatoes just a few feet apart and not have them crossbreed, but you would have to plant squash up to two miles apart (or protect the flowers) to guarantee that you get the same variety if you plan on saving seeds. This is why the most commonly saved seeds, at least in this area, are tomatoes and beans — they are easy to guarantee that you won’t get something other than what you plant.

So if you learn anything from this article, check out how plants pollinate before you save their seeds, and take plenty of allergy meds with you if you go to South Dakota.