Unravelling the unseen world of pollination in a garden 

Bottle gourd vine in bloom

Introduction: At Willowgate Community Garden there are approximately 130 gardeners who grow a vast selection of the ever-popular Solanaceae (tomatoes, peppers, eggplants etc) and Curcurbitaceae (squashes, gourds, melons, cucumbers etc) vegetables close to each other. At this time of the year, the garden is abloom with all sorts of insects, bees, butterflies and some less-seen pollinators flitting from flower to flower in search of nectar. This creates a very interesting microcosm to study pollination in nature.

Why do flowers need pollination?

Fertilization occurs only after pollination. If a flower is not pollinated, no fertilization takes place. As such fruits, as well as seeds, will not form. The flower will wilt and then die without forming any fruit or seeds since it does not contribute to the survival of the plant.

Floral scents, one of the most powerful ways of attracting pollinators, are a complex mixture of highly volatile compounds and essential oils that flowers emit into the atmosphere to act as a powerful signal to guide particular pollinators to the flowers. The target pollinator detects the flower's scent as far as 1 km away and follows the concentration gradient of the chemical producing the scent and homes in on the flower. Sadly pollution especially from hydrocarbons drastically reduces the efficacy of these scents.

Flowers that use scents to attract pollinators are generally drab in appearance, white or purple-brown to dark red-brown, and exude very strong scents that can be detected at distances over one kilometre.

Plants' scent levels tend to be highest when the flowers are ready for pollination and when potential pollinators are active. Bees or butterflies pollinate plants whose scent is high during the day, while moths and bats pollinate plants whose fragrance is greatest at night. 

Newly opened and young flowers, not ready to function as pollen donors, produce fewer odours and are less attractive to pollinators than older flowers. Once a female flower has been sufficiently pollinated, changes occur with the floral bouquets that lead to these flowers being less attractive and thereby direct pollinators to unpollinated flowers, maximizing the reproductive success of the plant.

Aren't pollinators attracted to many different flowers? What are the consequences?

Pollinators are indeed opportunistic feeders and there is no telling the sequence in which they visit flowers - this in turn means that visiting pollinators can potentially carry pollen from many plants. 

But plants have a way of dealing with this random cocktail of pollen. All incompatible pollen falls by the wayside - only those of a specific shape, texture and form attach themselves to the style of the flower resulting in a successful pollination.

But that's not all, there is another evolutionary trick that plants use - the placement of the style within the flower. Let me explain:

• A key factor in true-to-type breeding is an inserted style, that is, a style enclosed entirely within the anther covering. This prevents cross-pollination and is seen more often in plants selectively bred for specific traits ie most modern hybrid tomatoes 
• By contrast, flowers in the wild have exserted style with tips longer than the protective anther casing. This makes sense because in the wild there were native pollinators that required exposed styles for pollination (albeit with the added risk of cross-pollination). 

Pollination of Cucurbits

Cucurbits are edible vegetables of the Curcurbitaceae family that include squashes, gourds, melons, cucumbers etc. Their flowers have exserted styles, hence cross-pollination by insects is very likely when these plants within the same family of cucurbits are in present close proximity

• Cucurbita pepo: Pumpkins and some gourds like Zucchini, Yellow Crookneck, Acorn, Spaghetti, Patty Pan, and Delicata etc all may cross with each other but not with melons, pumpkins, or cucumbers.
• Cucurbita moschata: Squashes like Butternut, Yellow crookneck, Cabasa and Buttercup squashes may cross with each other but not with melons, pumpkins, or cucumbers.
• Cucumis melo:  Melons like Muskmelon, Cantaloupe, Charentais, Honeydew, and Armenian Cucumber may all cross with each other, but not with squashes, pumpkins, or Cucumis sativus cucumbers.

If you wonder which two different plants can cross-pollinate just look at their botanical names; for example take Cucumber aka Cucumis sativus, Butternut squash aka Cucurbita moschata, Zucchini aka Cucurbita pepo and Tromboncino squash aka Cucurbita moschata. 

The first part of the scientific name is the genus or family name (Cucumis and Cucurbita), and the second part is the species name.

While different families of cucurbits (for example zucchini squash aka Cucurbita pepo and a cucumber aka Cucumis sativus) cannot cross-pollinate, two different varieties of the same squash species can cross-pollinate (Butternut and Tromboncino since both are Cucurbita moschata) 

The curious case of the FrankenSquash

Xenogamy (Greek xenos=stranger, gamos=marriage), is the scientific term for the transfer of pollen from the anther of one plant to the stigma of a different but compatible plant bringing genetically different types of pollen grains to the stigma. 

FrankenSquash is a term I coined, to describe this exact same experience I had a few years ago after germinating seeds from a particularly delectable variety of butternut squash, that made the most delicious soup;  hoping to enjoy it for another season. And boy, was I in for a surprise...

The following year, the squash seeds resulting from a xenogamous cross-pollination of Butternut and Tromboncino produced a surprise hybrid squash that looked like a tromboncino squash (so named because it looks like a trombone) but tasted like a butter squash. This is all part of the fun of gardening at Willowgate. 

The elusive pollinator of Bottle Gourds

This finally brings us to something I learned just this week. when I decided to do a deep dive into the pollination of bottle gourds (Lagenaria siceraria) for the following reasons:

• A fellow gardener who has a luscious bottle gourd vine with plenty of white flowers was very puzzled to notice that the flowers were mostly closed during the day and was worried if this was a problem that could result in lower yield.

• Furthermore, the bees that normally buzz around other cucurbits flowers seemed most disinterested in the bottle gourd blooms. 

My field observations of the bottle gourd in bloom

So I mosied over to his garden plot to take a closer look and just like he said there was an abundance of delicate white male flowers though they weren't all open for business (just the ones in the shady parts were open).

I wasn't expecting to see white flowers, as almost all other cucurbits in the garden have yellow blooms, this anomaly can be explained when you realise that the bottle gourd is not native to the Americas but comes from East Africa.

Female bottle gourd flower with an immature fruit at the base of the stem

Another thing, I noticed was that the male flowers seem to vastly outnumber the female blooms, distinguishable by a swollen section just below the flower that is an unfertilized fruit. 

What science says about pollination in bottle gourds

Male bottle gourd flowers remain closed during the day

Bottle gourd (Lagenaria siceraria), also known as Calabash or White flower gourd, is monoecious with unisexual pollen-bearing male and seed-bearing female flowers found separately on the leaf axils of the same creeping vine - the downside of this arrangement is the need of pollinators to carry pollen from male flowers to female flowers. If no pollination takes place, the immature fruit at the base of the female flower will rot and fall off.

Statistically, there is a strong male-biased floral ratio (20 male flowers to every 1 female flower) and the male flower lifespan is much smaller, which matches what I saw.

Studies have shown that both male and female bottle gourd flowers open at dusk and close the next morning, with the male flowers blooming and closing slightly earlier than female flowers. This explains why most of the flowers were closed during the day.

The mystery night pollinator

The floral biology traits of curcubits in general seem to be more suitable for pollination by Apoidea (a family of bees that also includes wasps and ants). By contrast, the flowers of bottle gourds are particularly successfully pollinated by Lepidoptera (family of butterflies and moths) or more specifically the Hawk Moths or Sphinx Moths (Sphingidae) which seem to prefer flowers that are nocturnal with highly fragrant long floral tubes concealing pools of thin but abundant nectar. 

Let me introduce you to the Hawk moth, with the world’s longest proboscis of any other moth or butterfly (some up to 14 inches long) - perfect to reach the nectar at the bottom on the big white bottle gourd blooms that stay open all night for business. 

Most moths are nocturnal coming out after dusk, unlike their “respectable cousins” the butterflies, which are out during the daytime, and glorified in prose, poetry, and art. Unfortunately, moths have been vilified because of their association with the dark of night and our innate fear of things that come alive at night.

Sadly, in recent times pollinators have declined due to various reasons such as changes in farmland use, intense use of pesticides, diseases, environmental pollution, climate change etc

So there you have it, there is a lot of activity in the garden but there is also a hidden science that is equally fascinating.

References

1. Scent. (n.d.). US Forest Services - United States Department of Agriculture. Retrieved August 1, 2024, from https://www.fs.usda.gov/wildflowers/pollinators/Plant_Strategies/scent.shtml
2. Burrows, R. (2019, September 19). https://extension.sdstate.edu/saving-seed-pumpkins-squash-cucumbers-melons-and-gourds. South Dakota State Unversity Extension.
3. Ravichandran, M., R, S. M., Ganapathy, N., & Geethanjali, S. (2022). Plant-pollinator interaction, pollinator diversity and relative abundance in bottle gourd (Lageneria. . . ResearchGate. https://www.researchgate.net/publication/361951626_Plant-pollinator_interaction_pollinator_diversity_and_relative_abundance_in_bottle_gourd_Lageneria_siceraria_in_Coimbatore
4. Maundu, P. (2004, March 1). Pollinators of the bottle gourd (Lagenaria siceraria) were observed in Kenya. International Journal of Tropical Insect Science.
5. Buchman, S. (n.d.). Hawk moths or sphinx moths (Sphingidae). US Forest Service - United States Department of Agriculture. Retrieved July 31, 2024, from https://www.fs.usda.gov/wildflowers/pollinators/pollinator-of-the-month/hawk_moths.shtml