What in the World is a Seedless Fruit?

Some years ago, I was given a refrigerator magnet with the quote, “Knowledge is knowing that a tomato is a fruit; wisdom is not putting it in a fruit salad.” I was reminded of Miles Kington’s quote last weekend when I enjoyed one of what I hope will be many bowls of fruit salad: cherries, strawberries, blackberries, blueberries, and (my favorite) home-grown peaches. Peaches are my summer passion! Last evening, I enjoyed a peach caprese salad at a local restaurant, and the combination of sweet and savory was heaven on Earth!

Isn’t this the best time of year to be a Michigander? Farmers’ markets, and our gardens, if we’re lucky, are a bounty of local produce. The long wait for a decent tomato, fresh corn, and just-picked salad fixins’ is over! Hooray! But, of course, it got me thinking. I’ve written recently about fruits. Botanically, fruits are defined as the ripened ovaries of plants, derived from flower structures, and contain one or more seeds. Think – apples. Culturally, we think of fruits as being sweet; they are usually, but not always, eaten raw and are frequently incorporated into the best part of a meal – dessert. According to my grandson Nolan, vegetables are none of those things, except sometimes eaten raw. Using the scientific definition, many foods we consider vegetables are actually fruits. Tomatoes (of course), peppers, cucumbers, squash, avocados – all fruits! Vegetables, according to plant folks, are other parts of edible plants. Common, “true” vegetables include potatoes, carrots, celery, broccoli, and many others. We don’t commonly think that vegetables are sweet - but savory, sometimes cooked, and rarely associated with dessert.

All this thinking about fruits and vegetables got me wondering about a special class of fruits – seedless. What kind of horticultural magic is necessary to make a fruit that can’t reproduce? Isn’t that against “nature”? Are they classified as GMOs (genetically modified organisms?) Rest easy. The science behind seedless fruits is varied and fascinating.

First things first. Seedless fruits are not genetically modified in the way that is defined as GMO. According to the Food and Drug Administration (FDA), genetic modification involves a specific change in an organism’s DNA, including the transfer of DNA from one type of organism to another. Seedless fruits are selectively bred, to be sure, but do not involve the genetic material of any other species. In other words, seedless grapes contain only DNA from grapes. Depending on the kind of fruit, seedless varieties arise from several different breeding strategies: grafting, cuttings (both of which produce clones), and selective crossbreeding. Although uncommon, plants that produce fruits with non-viable seeds are a rare, but naturally occurring event, sometimes arising from random mutation. Rarity makes sense since the purpose of a fruit is to house and help distribute the seed. A sterile fruit is useless to the plant; hence, they are not valuable in nature. But they do exist, for example, as discovered by primitive humans millennia ago, as the result of accidental crossbreeding of two different species of wild banana that produced a sterile fruit. That fruit was seedless compared to wild bananas, which have large, numerous seeds. Banana plants, fortunately, are easily grown, not from seeds, but by splitting off and planting small offshoots called pups, which grow at the base. These are genetically identical to the parent; clones that will also produce seedless fruit. Whether the new plant and its sterile offspring is propagated by shoots or sometimes by grafting a desired plant stock onto an existing plant, the result is a cloned fruit identical to others of its type. Another way to produce sterile, seedless fruit is by manipulating the number of sets of chromosomes in a plant. A hybrid variety of this type cannot make viable eggs and sperm that can be fertilized, and sets fruit with poorly developed seeds. This is how farmers grow seedless watermelons.

We all learned in basic biology that an individual - plant, or animal - contains one set of genetic material from each parent. Each cell, except for eggs and sperm, includes two copies of genetic material. That is true, mostly… Long ago, however, scientists discovered that it was possible, and even desirable, to produce plants with more than two complete copies of genetic material, and that breeding those plants together yielded commercially valuable offspring. Many of the beautiful flower varieties we purchase from the nursery and food crops we grow, including watermelons, result from these processes.

Seedless watermelons are grown in a farmer›s field where two different parent seeds were planted. One kind produces flowers containing ova (or egg cells) with not one but two complete sets of genetic material. The other parent seeds produce pollen, which generates sperm cells with only one DNA set. The flower cannot be successfully fertilized and the resulting fruit contains three sets of genes. It is sterile, and as the fruit develops into a mature melon, the seeds abort and stop development, resulting in a few tiny, soft, white, infertile remnants called traces. Voila, a seedless watermelon!

Certainly, these agricultural processes are the result of many years of scientific research and refinement. Market studies have concluded that, as consumers, we desire our fruit to be sweeter, have fewer seeds, and have longer shelf lives. These preferences are met through selective breeding and the manipulation of parent stock. Are seedless fruits «Frankenfoods?» No, but make no mistake. The days of simply finding a seed, planting it, and patiently waiting for a delicious treat are rare.

Dear readers, I haven›t even considered nuts and berries! Perhaps another month. In the meantime, enjoy the bountiful harvest of our hard-working farmers!

Cheryl Hach Retired Science Teacher Kalamazoo Area Math and Science Center

Reference:

Goldy, R. (2019, January 3).

Seedless fruit is not something new. MSU Extension. Retrieved August 31, 2022, from https://www.canr.msu.edu/ news/seedless-fruit-is-not-somethingnew