Seedless Fruits and Vegetables. How Does That Happen?

If flowering plants have evolved to complete their full life cycle and generate seed to continue the species, how can seedless fruits and vegetables exist? And how can they continue to be grown year in and year out without producing seed?

There are a few different forces at work. Let’s review a little bit of science, some natural processes, and the human interventions that are behind the growing of some of the seedless fruits and vegetables that we all enjoy.

The Science

Normally, a fruit forms after pollination of a flowering plant when the female ovary is fertilized by male pollen. Fertilization causes seed development. The fruit is actually the ovary that swells around the seed. Natural genetic mutations may cause fruit to develop in some plants without fertilization and therefore without producing seeds. This characteristic, development of fruit without fertilization, is called parthenocarpy.

Today’s seedless fruits and vegetables started with this natural mutation, followed by human recognition that seedlessness was an attractive trait. Then humans found ways to propogate that plant to preserve that characteristic.

Seedlessness has several attractive features. Seedless oranges, grapes and watermelons are less messy and more enjoyable to eat. Eliminating cucumber seeds eases digestion issues for many people.  And seedlessness lengthens shelf life since seeds tend to encourage the fruits’ deterioration to hasten their dispersal into the world.

Because seeds draw nutrients from the plant, seedlessness can affect fruit characteristics like nutrients, sugar content, size, and time to maturity. These characteristics are typically improved over time by breeders.

Let’s look at some examples.

Oranges

The seedless oranges we eat today derive from a seedless tree discovered by farmers in a Brazilian orange grove in the 1870s. They sent tree cuttings to the USDA which propagated the trees through grafting and rooting practices, starting the seedless navel orange boom in the US.

Today, seedless orange trees are grafted descendants of the original seedless orange tree– in essence, clones. When groves of these cloned trees are formed, and are self-pollinated, seedless oranges are the result. However, if a bee picks up pollen from a seeded orange tree in a neighboring grove and cross pollinates a “seedless” tree, seeds will develop. So, it takes both the parthenocarpic characteristic and self-pollination to produce seedless oranges.

Grapes

Seedless grapes have a similar but even longer history. They were originally discovered about 6,000 BC in the Mediterranean region. The seedless fruit appeared spontaneously, was recognized, and was propagated vegetatively through grafting and rooted cuttings.

With grapes, there are two different groups that we call seedless. One is parthenocarpic, meaning that the fruits develop without fertilization. The other is called stenospermocarpic, where fertilization actually occurs but seed development aborts and soft seed traces are left in the fruit. Stenospermocarpic varieties have the advantage of growing full-size fruit, while the parthenocarpic, truly seedless grapes, tend to be smaller.

Bananas

Seedless bananas have a similar history going back several thousand years. Seedlessness started as a natural mutation from heavily seeded wild bananas (see featured photo at the top). We tend to think of bananas as a tree fruit but actually they are an herbaceous perennial that is propagated vegetatively by transplanting side shoots or suckers from the main plant. Selected breeding over many years has developed the sweet, seedless fruits we enjoy today.

Bananas highlight the risks of cloning in this fashion. Through the 1900s, until the 1950s, the predominant banana cultivar that was imported into the US was called the Gros Michel.  Its lack of diversity made it susceptible to a fusarium fungus, called Panama disease, that destroyed banana plants in all growing regions. It was replaced with a disease resistant variety called Cavendish which now accounts for about 97% of banana exports. Unfortunately, there is a new variant of Panama disease called Tropical Race 4 (TR-4) that is harming Cavendish plants in several growing countries.

 The Cavendish banana is at risk. Breeders are working to develop replacements. While bananas are not currently genetically modified (GMO) fruits, the banana problem has raised an important question —  whether a genetic modification is a logical way to provide disease resistance while maintaining parthenocarpic traits and adding biodiversity.

Pineapples

Pineapples are similar to bananas in that they are herbaceous perennial members of the bromeliad family. Each segment on the exterior of the fruit produces a flower. Pineapples are called a syncarp, meaning that multiple flowers fuse into a single fruit. However, they are seedless because they are self-incompatible. This means that when a single pineapple variety is grown isolated from other varieties, and self-pollinates, fertilization does not occur. The plants are propagated vegetatively through root division, budding and runners.

Cucumbers

 Cucumbers and some squashes are different than the previous examples. For one thing they are annual plants that must be grown from seed every year. Parthenocarpy is induced into cucumbers in multiple ways. Breeders have developed hybrids that are naturally parthenocarpic, and grow fruits without fertilization. Also, parthenocarpy can be induced by applying growth hormones and pollen extracts.

To grow seedless fruits, female flowers of parthenocarpic varieties are covered or grown in greenhouses where pollination is prevented.

Interestingly, the small tender seeds that exist in some “seedless” cukes are viable and will germinate into productive plants.

Watermelons

Seedless watermelons have a different propagation process. Watermelon seed fertility is affected by varying the number of chromosomes in the seeds’ cells. Normal watermelons have 22 chromosomes, 11 coming from the ovary in the female flower and 11 from the male pollen. In 1939 a Japanese plant scientist determined that treating the leading ends of young vines with a chemical called colchicine, doubled the number of chromosomes in the plant to 44. When the female flowers from these plants are pollinated by pollen from a plant with the normal 22 chromosomes, the resulting plants have 33 chromosomes. The plants with 33 chromosomes produce fruit with a hard shelled seed that are collected and sold to produce seedless melons. When pollinated with pollen from a 22 chromosome variety, the result is melons with the soft white seed traces that are the “seedless” melons we enjoy.

This multi-step process for growing seedless watermelons is obviously quite expensive. It accounts for the higher prices for seedless vs seeded watermelons. However, since introduction to the US market in the 1990s, seedless watermelon breeders have steadily improved melon taste and consumers have steadily increased consumption. Currently, over 90% of commercial watermelon sales are seedless. Clearly, they are here to stay.

The Future: GMO Fruits?

Growers have been figuring out ways to produce seedless fruits by taking advantage of natural mutations for thousands of years. More recently we have developed ways to modify “normal” seeded plants through breeding and chemical application. The results are fruits that have practical and consumer appeal but are higher cost than natural fruits. In some cases, cloning has reduced diversity, increasing susceptibility to diseases and other environmental factors.

New technology provides a means to address many issues through genetic modification. To date, there is consumer pushback against fresh GMO products although GMO ingredients are widespread in processed foods. It will be interesting to see how this changes over time.

In the meantime, I hope that an increased understanding of how seedless fruits came to be increases your enjoyment of their various benefits.

Sources:

“Seedless Fruit is not Something New,” Michigan State Extension

“How do Seedless Fruits Arise and How are the Propogated?” Scientific American (2000)

“How are Seedless Grapes Grown,” Science ABC, https://www.scienceabc.com/eyeopeners/how-are-seedless-grapes-grown-parthenocarpy.html

“Producing Summer Squash Without Pollination,” Cornell University Extension, http://www.hort.cornell.edu/expo/proceedings/2014/Vine%20crops/Seedless%20squash%20Reiners.pdf

“Seedless Watermelon: How Do They Do That?” Michigan State University Extension, https://www.canr.msu.edu/news/seedless_watermelon_how_do_they_do_that

“Watermelon: A Brief History,” University of Missouri Extension, https://ipm.missouri.edu/MEG/2020/7/watermelon-DT/

“Growing Seedless Triploid Watermelons,” University of Nebraska-Lincoln Extension,  https://extensionpublications.unl.edu/assets/pdf/g1755.pdf#:~:text=Seedless%20watermelons%20are%20triploid%20%283X%29%20which%20causes%20them,from%20one%20parent%20and%202X%20from%20the%20other

“The Cavendish Crisis,” Evergreen.edu, http://blogs.evergreen.edu/terroir-jahni/the-rise-and-inevitable-fall-of-cavendish/

“A Banana-Destroying Fungus Has Arrived In The Americas,” Smithsonian Magazine, https://www.smithsonianmag.com/smart-news/banana-destroying-fungus-has-arrived-americas-180972892/

“List of Bioengineered Foods,” USDA Agricultural Marketing Service, https://www.ams.usda.gov/rules-regulations/be/bioengineered-foods-list

“The botany of seedless fruits,” Science and Plants for Schools, https://www.saps.org.uk/saps-associates/browse-q-and-a/322-from-which-part-of-the-flower-do-seedless-fruits-develop

“GMO Crops, Animal Food and Beyond,” US FDA, https://www.fda.gov/food/agricultural-biotechnology/gmo-crops-animal-food-and-beyond