Plant-pollinator balance is key for flower reproduction

The relationship between and plant and its insect pollinators is a major factor in each species unique biology. Did you know that approximately two-thirds of all flowering plants depend on insects for pollination? Once a flower is pollinated, it can even change its color and scent to lure insects to non-pollinated flowers!
Co-Authored by Christina Malinoski and Caroline Ailanthus

Let’s talk about pollination (basically, plant sex)! Unlike the animals we’re familiar with, plants can’t walk around to find a mate. Instead, they need some kind of intermediary to move their pollen from here to there. Some plants use water, and the wind is a popular option, as are certain bats and birds, but approximately two‐thirds of all flowering plants depend on insects for pollination5.

The relationship between a plant and its insect pollinators is a major factor in many aspects of each species’ unique biology, especially the shape and other aspects of the flower. For example, some plants can change the color and odor of their flowers after pollination in order to guide insects to flowers that have not yet been pollinated5.

So to understand plants, especially flowers, we have to understand the plant/pollinator relationship.

Two popular, but very different, groups of flowers are the roses and the orchids.

Stop and smell the roses! You can learn a lot with a single sniff!

Roses are perennial shrubs with pinnately-compound leaves—that means that each leaf is composed of many leaflets arranged on either side of a long stem (the petiole), a bit like the shape of a feather (“pinnate” means feather-like). At the base of the petiole, where it meets the twig, there are a pair of green, jagged-toothed wings, called stipules. All roses have toothed stipules, a characteristic not known to be found on other plants. It’s a reliable giveaway for roses, even the ones that don’t have thorns.

Roses have axillary buds that remain dormant in winter and bloom in spring. Short-day cultivars of roses flower in spring only, while long-day cultivars bloom consistently from spring through autumn2. Rose plants, especially the flower buds, must receive full-spectrum light, including both red and blue wavelengths, or they will not bloom2, a challenge for those attempting to grow roses under artificial light. Rose blossoms can be virtually any color except blue—the plants simply don’t make the flavanol responsible for blue color in plants2.

Roses are gorgeous, but in some ways, they are very simple flowers; they consist of all four basic flower parts (sepals, petals, stamens, and carpels) in very clear, unambiguous forms—except in the case of hybrid double roses, where the stamens are replaced by another ring of petals2. All members of the rose family, which includes apples and strawberries, have very similar flowers. Roses depend on a combination of a showy visual display and enticing scent to attract their pollinators, which include multiple species of bees. Unfortunately, and for unknown reasons, many popular cultivars lack the distinctive scent2.

Pollinators are rewarded by both protein-rich pollen (clearly they do not eat all the pollen they collect) and by carbohydrate-rich nectar. After the flowers fade, seeds develop in red, fleshy fruits, the rose hips. Animals distribute the seeds after eating the rose hips, which make an enjoyable, vitamin C-rich nibble for humans, too.

“The orchid is Mother Nature’s masterpiece.” ~ Robyn

Other than being gorgeous, orchids have almost nothing in common with roses. The flower parts are extremely specialized and often take strange, complex shapes, but they provide no nectar. Instead, they rely on trickery to attract their insect pollinators. Some have alluring scents, while others use their shape to mimic the insects themselves, who attempt to mate with the flowers and end up transferring pollen instead.

Orchids comprise an entire plant family, the second-largest in the world. They are an extremely diverse, and mostly tropical, group. One characteristic all orchids have in common is the production of huge numbers of tiny dust-like seeds. These seeds can travel long distances on the wind, but very few ever find the necessary conditions to germinate; orchid distribution may not be limited by how far the seeds can travel, but by how rare are the places where they can grow3.

The probability that an orchid plant will flower in any given year depends on both internal and external conditions such as age, light availability, and temperature. For varieties that bloom once and then die (monocarpic), it has been scientifically demonstrated that a certain threshold size and/or age needs to be reached before plants will flower3. For varieties that bloom multiple times over their lifespan (polycarpic), the plants’ ability to bloom may be dependent upon growing conditions from the previous year3.

Some orchids are grown commercially, either for their flowers or for other products (vanilla is an orchid, for example). Orchid-growing is also popular with home hobbyists. Other orchid species grow only in the wild, and many are extremely rare.

As different as they are…

Roses and orchids are very different groups of plants with very different reproductive strategies, yet they both depend on insect pollinators in the wild. But when it comes to pollination, not just any insect will do. In fact, for some plants, having the right insect partner—and not having the wrong one—can make all the difference in the world. But not just any insect will do. In fact, while roses can work with many different pollinators, some orchids are adapted to partner with just one insect species. Nor are orchids the only plants to specialize in that way.

The globeflower, Trollius europaeus, for example, has tightly-closed petals that exclude all potential pollinators except small flies of the genus, Chiastocheta4. In fact, when the flies were experimentally excluded, the plants produced very little viable seed4. Were anything to happen to the flies, globeflower would be lost, at least in the wild. Conversely, the flies’ larvae develop only inside globe flowers. Were anything to happen to the flower, the flies would be lost.

Even when plants are not dependent on a specific insect, the identity of the pollinator can still matter—and it’s possible that an extra pollinator is almost as bad as not having the right pollinator at all. For example, a newly introduced species could be capable of excluding the native pollinator, but not as good at actually pollinating the plants1. Generalist pollinators able to use many plant species can also be harmed by the replacement of native plant communities with exotic monocultures that provide only low-quality food1.

One such problem may be developing in South America, where a short-tongued bumblebee, Bombus terrestris, has established a foothold. B. terrestris is native to parts of Eurasia and Africa and has been intentionally introduced elsewhere to aid in crop pollination. Unfortunately, it is now breeding freely in parts of South America, where its population density is now three times that of the native bumblebee, B. dahlbomii1. The new bee could be capable of excluding the native bees as well as damaging the flowers.

As always is the case in science, further research is needed to be conducted, but that just shows how complex the interactions are between plants and their pollinators!

This article was co-authored by Caroline Ailanthus. Caroline is an established writer within the conservation community and has a degree in conservation biology.
References
1. Aizen, M., Morales, C., Vazquez, D., Garibaldi, L., Saez, A., & Harder, L. (2014, July 16). When mutualism goes bad: density’ dependent impacts of introduced bees on plant reproduction. Retrieved from https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.12924
2. Bendahmane, M., Dubois, A., Raymond, O., & Le Bris, M. (2013, February 23). Genetics and genomics of flower initiation and development in roses. Retrieved from https://academic.oup.com/jxb/article/64/4/847/436041
3. Jacquemyn, H., Brys, R., & Jongejans, E. (2010, August 4). Size’dependent flowering and costs of reproduction affect population dynamics in a tuberous perennial woodland orchid. Retrieved from https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2745.2010.01697.x
4. Klank, C., Pluess, A., & Ghazoul, J. (2010, August 30). Effects of population size on plant reproduction and pollinator abundance in a specialized pollination system. Retrieved from https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2745.2010.01720.x
5. Lucas-Barbosa, D., Sun, P., Hakman, A., Van Beek, T., Van Loon, J., & Dicke, M. (2015, July 7). Visual and odour cues: plant responses to pollination and herbivory affect the behaviour of flower visitors. Retrieved from https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2435.12509
The relationship between and plant and its insect pollinators is a major factor in each species unique biology. Did you know that approximately two-thirds of all flowering plants depend on insects for pollination? Once a flower is pollinated, it can even change its color and scent to lure insects to non-pollinated flowers!

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