Fruit is the miracle weapon of prevention: colourful, sugary sweet and pure nature. Seedless fruits are the most popular, i.e. those with which the plant can no longer reproduce in the wild. They are just "imitations" of a fruit, so to speak. Udo Pollmer explains how profound the technology is that produces such artificial products.
by Udo Pollmer / November 4th, 2012, new version January 2024
Seedless fruit fetches higher prices than "normal" fruit. This is why plant breeders are working to expand the supply. In addition, it is not so easy to grow a new plant from fruit without seeds. This secures licence fees for new varieties.
But seedlessness is not a new achievement - at least in the case of grapes. The common grape variety "Thompson Seedless" is...
...even ancient. This is what the trade calls the good old seedless sultana; it is probably of Turkish origin. When dried, they are called sultanas - hence the name. The same applies to currants from the Greek grape variety Korinthiaki. They are probably the ancient small-fruited graecula already described by Pliny.1 In the wild, these mutants would have quickly perished due to a lack of offspring. However, generations of winegrowers have grafted the grapevines onto other vines and have continued to cultivate them to this day.
Both varieties are a biological curiosity. In order for berries to form, they must first be pollinated. Then even small seeds develop. But these disappear again. This has consequences: If the seeds are missing, the hormones that stimulate the growth of the berries are also missing. This is why seedless grapes are often very small. To make them marketable, winegrowers spray their vines with gibberellins or brassinosteroids.2,3 This increases the size of the berries and gives the fruit a fuller appearance.4 Thanks to genetic engineering, there are now new seedless varieties that bear attractive fruit even without hormone sprays.5 However, the result can also be significantly improved with hormones.6,7
Before hormones were available, winegrowers resorted to defoliants such as phenoxyacetic acids (2,4-D, 2,4,5-T).8 These cause some of the berries in the bunch to die, making the remaining ones fuller. 9 An even older method is crushing. The winegrower removes the bark around the trunk with a ring-peeling knife in order to block the transport of nutrients between the crown and the rootstock. As a result, the photosynthesis of the leaves only benefits the grapes: they become larger, heavier and sweeter. In return, the rootstock is weakened. This method is still common today, often in combination with herbicides or hormones.10-12 Customers value grapes prepared in this way above all else.
Away with the hummingbirds
The Navel Orange is older, although not quite as old as the Sultanina or the Korinthiaki: this mutant was found in Bahia in Brazil a good two centuries ago. Its trademark is the "navel" in the form of a second very small orange (syncarpy). It is still propagated today by grafting - i.e. by clones. As clones cannot fertilise themselves, seedless oranges can be harvested as long as no other clone is spreading its pollen far and wide. In the meantime, new seedless varieties have been created using protoplast fusion by electric shock and gamma rays from the cobalt-60 cannon, some even without a navel. 13-15
We only know the large, fleshy pineapple as seedless. It is the result of the seamless union of hundreds of individual berries. In contrast to the sultana grape, the pineapple tree also bears fruit without pollination. This phenomenon is called parthenocarpy. In the wild, however, pollination is unavoidable, where it forms small, rock-hard and rough seeds, thousands of which are distributed in the fruit flesh. To prevent this from happening with cultivated pineapples, only a single clone is grown in the plantations, as with the navel orange. This clone cannot fertilise itself. Because the pineapple is most effectively fertilised by hummingbirds in the wild and thus becomes inedible, Hawaii has taken the precaution of banning the import of these cute feathered creatures.16
Even pineapples cannot do without hormone therapy: In order to be able to harvest the fruit at the same time, flowering is synchronised, for example with calcium carbide, ethephon, etacelasil, naphthylacetic acid or aminoethoxyvinylglycine.16-19 Fruit size can also be optimised with other growth hormones such as cytokinin and gibberellin. 20
Until now, pineapples have mostly been propagated by cuttings. To remedy the painful lack of suitable planting material, tissue culture with hormonal all-round supply is on the rise.21,22 New varieties are being bred to improve resistance to cold so that they can also be grown in cooler climates. In addition, happy-growing perennials are desirable, because with the clones available today, a pineapple field is ready for harvest only after a year and a half.23
Cell toxins & hormone cocktails
The most popular seedless fruit in world trade is undoubtedly the banana. Like the pineapple, it is parthenocarpic and its fruit thrives without pollination. Like the pineapple, the banana is propagated by offshoots and cuttings. The sweet banana cannot form seeds because it contains 3 sets of chromosomes (triploid) instead of the usual 2.
The (diploid) wild form (Musa acuminata) of today's edible banana still contained two sets of chromosomes. It was still full of large, black, hard seeds. How our sweet, nutritious and seedless dessert banana came about is a mystery.24 In the past, people did not eat the fruit of the original banana, but rather its flowers and rhizome, also known as "corm".25 Both are still part of the Filipino cuisine today.
How the seedless watermelon got its three sets of chromosomes is no secret. In contrast to the banana, seedless watermelons are paradoxically grown from seeds. It starts with the usual black seeds, which are abundant in the flesh of "normal" melons. These are treated with colchicine, the poison of the autumn crocus. This doubles the number of chromosome sets in the genetic material from 2 to 4. This 4-seed is grown the following year together with the normal 2-seed. After pollination, the seeds of the new harvest contain the intersection - namely 3 sets of chromosomes.26
They are now triploid like the banana. But unlike bananas, they still need to be fertilised for seedless melons to ripen. (As with the seedless sultana grape.) For pollination, the vegetable grower plants a few rows of normal 2 varieties in between. In the seedless melons, only a few small white edible pods remain as a reminder of their elaborate production.27,28 You just have to know how to do it! Every plant works differently, many paths lead to seedlessness.
Seedless aubergines demanded a comparatively simple exercise. Here, spraying the flower with suitable hormones was sufficient. Gradually, genetic engineering is making this step superfluous.29,30 Either way, the fruit grows larger and without the seeds, the flesh remains light in colour and does not brown.29,31 This makes it appear "fresher" and "unspoilt" to the customer.
In the case of another nightshade plant, the tomato, the seedless varieties are not intended for the end consumer. They would be suspicious and smell genetic engineering. This time, rightly so, these fruits are not historical mutants, but mostly the result of modern CRISPR/Cas9 or Cas-CLOVER breeding.32,33 The processing industry appreciates this: The production of ketchup eliminates the need to separate and dispose of the seeds.34,35
Strawberry time
Hormones are now the alpha and omega for the sensitive strawberries. In this specific case, however, not to produce "seedless" fruit. The tiny brown nuts (achenes) on the shiny red surface hardly bother the end consumer. The situation is different when the fruit is industrially processed into juice or jelly. The seeds are a nuisance. However, as hormone producers, these are just as important for the development of the fruit as they are for grapes. To date, it has not been possible to grow strawberries without achenes.
Apart from that, strawberry cultivation offers everything that has a name in the hormone scene: Gibberellins interrupt seed dormancy. Indolebutyric acid stimulates root growth,36 thidiazuron promotes the growth of axillary buds,37 prohexadione calcium reduces shoot growth and increases fruit set.38 6-Benzyladenine increases the number of flowers,39 Auxin enlarges the flower base,40 Triacontanol, cytokinin and chlormequat control the ripening, size and quality of the fruit.39-41 (Incidentally, chlormequat is a stalk shortener for cereals under the name CCC, or Cycocel). Abscisic acid develops the colour of the berries.40 Newer varieties owe their appetising fruity aromas to genetic engineering.42-44
A total of 28 different ripening hormones are known from strawberries alone, some of which are produced synthetically.45 The list is therefore not exhaustive, nor are the uses clear-cut. Depending on the strawberry variety, different hormones are active, which in turn can have different effects. It should also be noted that different legislation and authorisations apply in each country, although this should not be overestimated in view of the global trade in fruit.
Rapid cooling after harvesting and packaging in a modified atmosphere (low oxygen, high carbon dioxide) have proven effective in preventing losses during storage and transport. In addition, nitrogen monoxide, UV-C rays, sodium hydrogen sulphide and calcium baths with subsequent high-pressure treatment keep the delicate harvest appetisingly fresh. This allows us to enjoy strawberries from distant subtropical regions without any worries. 40, 46-48
Relatively new preservatives for strawberries are hexanal and ethyl formate. Ethyl formate is often used as a solvent, fungicide and larvicide. It is best known as a rum flavouring in baked goods, spirits and confectionery. Hexanal, on the other hand, smells intensely of green fruit and is also a popular flavouring agent. In food packaging, however, it is considered a harmful substance because it can migrate into the product as a component of printing inks and paints. Hexanal is also a component of exhaust fumes from engines and pellet heaters as well as a fragrance for perfumes.46,49,50
Life cycle assessments: As you like it
Many eco-balancers praise seedless fruit: it requires fewer nutrients, which are now available for the formation of fruit flesh. This reduces the burden on the environment. Seedless varieties improve the economic efficiency of fruit processing: the consumption of energy is reduced, as is the proportion of residual materials. Best of all, seedless fruit varieties can also be grown in regions where suitable pollinators are lacking.31 But there are still disadvantages: If they are identical clones, pests and pathogens have an easy game with it. Bananas in particular are at high risk from fungal diseases such as Fusarium head blight or Black Sigatoka.51
Without mutation breeding, without genetic engineering and without hormonal all-round care, our all-year-round supply of fresh fruit is not possible. While the whole world speculates that hormones are commonplace in stables, their most important place of use is the orchard. Don't think that the choice of active ingredients is limited to pure plant hormones. Animal hormones such as cortisol or melatonin are also an option.52,53 Oestrogen and progesterone have even been recommended for strawberries.54 The uses go far beyond seedlessness, fruit colour and aroma; even the angle of inclination of the branches of fruit trees is hormonally regulated to enable maximum exposure to sunlight.
Delicious fruit is usually the opposite of "pure nature". Large, flavoursome, sweet fruit is always the result of sophisticated "high-tech". The benchmark for naturalness is the tiny wild strawberry and not a large-fruited organic strawberry from Spain. Fruit has developed in exactly the same way as our livestock: the ancestor of our chickens was the red junglefowl, which laid a dozen small eggs a year. Today, a laying hybrid produces over 300 large eggs a year.
Even in climatically favoured regions with optimal conditions for the growth of fruit, farmers use the same methods. To quote the Indian National Research Centre for Grapes: "The use of growth regulators, especially GA3 [gibberellin A3], has become common practice among Indian grape growers producing for export. Phytohormones are also used for rooting, termination of dormancy, flowering, fruit set, delaying abscission [e.g. dropping of ripe fruits] and senescence [e.g. ripening but also ageing of the crop] as well as to increase growth. Plant hormones are extremely important magical chemicals [...] The cultivation of grapes is almost impossible without the use of growth regulators."55
Delicious fruit on the fresh produce shelves is usually the opposite of "pure nature". Large, flavoursome, sweet fruit is usually the result of sophisticated "high-tech". The benchmark for naturalness in the case of grapes, for example, is the wild form, with its small, tart berries but large seeds, and not juicy, plump organic grapes from India. The long journey from the inedible wooden pear to the large, juicy butter pear is no different.
Fruit developed in the same way as our livestock: the ancestor of our chickens, the red junglefowl, laid a dozen small eggs a year. Today, a laying hybrid produces over 300 large eggs. Some see this as a sign of man's alienation from nature. Others are proud of it, because it feeds everyone, both with animal and plant-based food.
Father Frost sets the table in winter
Seedless fruits are no strangers to the wild. Botanists have long puzzled over their purpose. What is such a thing good for? It may sound paradoxical, but they can increase reproductive success.56 Not only humans, but also other predators such as insects or birds favour the seedless specimens.57 To attract them, the plants even reduce the proportion of defence substances in them, i.e. "secondary plant substances", whose main task is to reduce digestibility and thus nutritional value.
Thanks to the fruit imitations, the real fruits with their valuable cargo of seeds initially remain undisturbed. They are nevertheless distributed, albeit later. During the winter, long after the migratory birds have left, they are eaten by hungry animals and the indigestible seeds are deposited in a dung heap. The frost improves the digestibility and nutritional value of the fruit, just as we humans know from sloes, Brussels sprouts and kale.
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Copyright: EU.L.E. e.V.
English editor: Josef Hueber, Eichstätt
