I understand the reason we want seedless watermelons – because of the big seeds, but seedless tomatoes? Do we really need them? Why would you grow them? If they don’t make seed, how can they be grown from seed? Are these some kind of new GMO franken-toms?
The story of seedless tomatoes is quite interesting and can teach us a lot about plants. In this post I’ll squash out the current knowledge about them and provide you with the background you need to make the proper choice when buying plants for your garden.
Why Do We Need Seedless Tomatoes?
When I first discovered seedless tomatoes I thought it was just another useless product, but there are some good reasons to have them and also some health myths we need to debug.
Many sites about tomatoes suggest that some people can’t eat the seeds because of a condition called diverticular disease, were seeds get stuck in the folds of the intestine. This is old and incorrect information. The medical references I found suggest that there is no evidence seeds cause this problem, ( also this reference and this one that explains diverticular disease well).
Some people have trouble digesting tomatoes, or are allergic to them, but this is the whole tomato, not just the seeds. I found no medical reason why anyone would benefit from seedless tomatoes.
The tomato juice and ketchup industry would benefit from using seedless tomatoes. The seeds just get in the way of processing the fruit and producing seedless juice.
What is a Seedless Tomato?
You might think that a seedless tomato is any tomato that does not have seeds in it, but it’s not that simple. There are a number of tomatoes that are considered seedless because they have very few seeds in them.
There are also tomatoes that produce 100% seedless fruit – some of the time. Usually fruit produced early in the season is seedless and fruit produced late contains small amounts of seed. The problem is that you can’t tell which is which until you cut them open.
Some tomatoes that have seeds that are not fully developed. You might have noticed this is so-called seedless watermelons that have small, white undeveloped seed. The same thing can happen in tomatoes and these types are usually called seedless.
There is also a true seedless tomato that does not even have small immature seeds.
Some Tomato Biology
The tomato – the part we eat – is a fruit. In biology, the term fruit is the part of a plant where the seeds develop. The seed capsules on flowers in the garden are also fruits even though they don’t have a fleshy part like a tomato or apple.
When a flower is pollinated, the pollen fertilizes the ovule (the egg) inside the ovary of the flower. Once this happens the ovule develops into a seed and the ovary develops into a fruit. The important point for this discussion is the fact that the fruit does not develop until the ovule is fertilized. In a normal plant, if there is no fertilization there will be no fruit.
If fruits develop only if seeds are forming, how do we ever get seedless tomatoes?
One way is to cross a diploid with a tetraploid to produce triploid (3 sets of chromosomes). If pollen from a normal diploid plant reaches the tetraploid, fertilization takes place, fruit develops, but proper seeds are rarely formed. This is the process used in some watermelons which produce small, white dysfunctional seeds. In order to grow these in the garden you need some normal plants (the diploids) growing beside your tetraploids.
This may work well for watermelons, but most tomato flowers are fertilized with their own pollen, either by bumble bees or by wind. This explains why different heirlooms can be grown quite close together without contaminating the lines. A mix of both diploids and tetraploid tomatoes would be mostly self pollinated producing few seedless triploid fruits.
There is another condition in plants called parthenocarpy. Such plants have one or more mutations that allow fruit to develop even if there is no fertilization. In tomatoes, one such mutation increases the auxin level so that fruit is produced with no fertilization.
Low Seed Tomatoes
Low seed tomatoes have been around for a long time and many of these are heirloom tomatoes. These varieties produce a small number of seeds but are not really seedless.
The paste tomato is an example of this. Over many years people have selected a tomato that is very fleshy and dry. In the process they have also selected for a low seed count.
Over the years people also found other types of tomatoes with few seeds and preserved them as well. The following are some of the cultivars that claim to have few seeds. In my casual reading, it seems as if the actual seed count depends on where the plants are grown, and maybe even the purity of the seed line. Not everyone reports low seed count for these varieties.
Some varieties that are reported to have low seed count include, ‘German Pink’, ‘Valencia’, ‘Ashleigh’, ‘Amish Paste’, ‘German Johnson’ and ‘Gold Nugget’. My friend at Seeds of Imbolc has also suggested; ‘Sicilian Saucer’, ‘Marizol Gold’ and ‘Italian Heirloom’.
Almost Seedless Tomatoes
Dr. James Baggett from Oregon State University did a lot of work to develop new varieties of vegetables that produced earlier in the cool Oregon climate. His work in tomatoes resulted in several almost seedless tomatoes.
What is the connection between producing fruit in cold climates and being seedless? One of the consequences of parthenocarpy is that fruit development does not need to wait until warm temperatures for proper fertilization. Fruits develop as soon as flowers are formed. So selecting for fruit development in cool conditions indirectly selects for seedlessness. Many of Dr. Baggett’s tomatoes are parthenocarpic.
Some of the varieties in this group include, ‘Oregon Spring’, ‘Santiam’, and ‘Siletz’.
The First Seedless Tomato
Burpee introduced a tomato called ‘Sweet Seedless’ around 2004 and they claim that it is the first seedless tomato. This appears to be an F1 hybrid but the details of the parents have not been made public. Unlike the heirlooms mentioned above, most people find that ‘Sweet Seedless’ is 100% seedless or very close to it.
Some of the promotion for ‘Sweet Seedless’, including Burbee, says, “And precisely because there are no seeds, all the sweetness goes into the fruit itself and is immediately available for you to enjoy”. That logic does not make sense. Just because seeds are not produced does not mean the tomato is extra sweet. In fact, reports on discussion forums indicate it has an average sweetness.
Interestingly, Burbee has been selling this tomato for more than a dozen years and yet they have not introduced other seedless tomatoes.
More Seedless Tomatoes
Once you understand parthenocarpy it becomes obvious that gene editing could be used to produce seedless tomatoes, and that is exactly what has happened. Keishi Osakabe from Tokushima University has used the gene-editing technique called CRISPR to produce flowers which contain high levels of auxin. These tomatoes produce fruit without being pollinated.
In order to grow these new tomatoes you would need to obtain cuttings. They can’t be grown from seed since the plants make no seeds.
In normal breeding it is a lot of work to migrate a new mutation into existing varieties, but not so with the CRISPR technique. Seedlessness could be easily added to any variety. This is one of the big benefits of CRISPR technology.
Since the fruit is developed without pollination, bees are no longer required. This is a benefit for closed areas like greenhouses, or in geographic locations that do not have enough wind or bumblebees to naturally pollinate tomatoes.
This research is very new and these tomatoes are not yet available on the market.
- How Do Seedless Fruits Arise? ; https://www.scientificamerican.com/article/how-do-seedless-fruits-ar/
- Gene Editing Produces Seedless Tomatoes; https://www.newscientist.com/article/2127640-gene-editing-opens-doors-to-seedless-fruit-with-no-need-for-bees/