Seedless Fruits and Vegetables

Background

The fruits that are grown, sold, and eaten are essentially the ripened ovary of a plant. In the wild, fruit-bearing plants spread their seeds either by dropping their ripe fruits to the ground or by being eaten by animals, who then excrete the seeds. The tasty fruit is merely the mechanism by which the plant passes its seeds along. But from the human consumer's point of view, the seeds can be a nuisance. Spitting out hard, bitter seeds lessens the pleasure of eating grapes, for example. As a result, horticulturists have developed seedless varieties of popular fruits and vegetables. Seedless varieties make up over half the United States grape market, the seedless navel orange is a mainstay of the orange industry, and the seedless watermelon saw increasing popularity since its introduction in the 1990s. Seedless fruits and vegetables are produced by meticulous cross-breeding, and it can take decades to bring a new strain to commercial viability.

History

Careful breeding of plants to yield desired results, such as small seeds or bigger fruits, has been done since the dawn of agriculture. The scientific underpinning of plant breeding began to be understood in the mid-nineteenth century, with the work of Gregor Mendel. In 1856, Mendel, the father of genetics, was the first to publish his findings on the statistical laws governing the transmission of plant traits between generations. Mendel studied how specific traits in the pea plants in his garden were passed down to succeeding generations, and he formulated the idea of some sort of unit within plants that was responsible for heredity. His work lay fallow for some time, then was rapidly extended in the early twentieth century. By the middle of the twentieth century, researchers had established that inheritance is transmitted by genes, which express chemical information resulting in characteristic traits. For seedless fruits, it is important to understand more of the details of genetic transmission. Genes in plants and animals are usually deployed in pairs, called an allele. One gene in the allele is usually dominant, and the other recessive. This means that typically only one trait is expressed in the biological makeup of the organism, though there is still a second gene for that trait. This is important because every cell in an organism carries a complete genetic map of itself, called the chromosomes, in its nucleus. When a cell divides, the chromosomes double, and then a copy goes into the new cell. The exception is the sex cells, the ovum and sperm. These cells only carry half the genetic material, which is one chromosome, or one half of each gene pair. When the ovum and sperm meet, the gene pairs recombine, and the new individual created through sexual reproduction has a new full set of genetic material, with half inherited from each parent. In traditional plant breeding, the horticulturist tries to optimize a trait by breeding together plants that both have, for example, small seeds. If the new generation of plants has inherited the small seed gene from both parents, it should also have small seeds, and be able to pass this trait to its offspring in turn. Many factors complicate the picture, so that in real circumstances only a small percentage of the offspring may show the desired trait.

Seedless oranges and seedless grapes are the result of cultivation of naturally occurring seedless plants. The navel orange is descended from a seedless orange tree found on a plantation in Brazil in the nineteenth century. This tree was a mutation, that is, something in its genetic material had spontaneously changed, resulting in this unique plant. Orange growers propagated new trees from the original navel, so that all the navel oranges available in markets today are descended from that Brazilian tree. The common supermarket green seedless grapes are descended from a European seedless grape strain that probably originated between the Black and Caucasus Seas. Grape growers spread this variety all over the world, and the same species exists under many different names. It has been grown in the United States since at least 1872 under the name Thompson. Other seedless grape varieties, even red and black varieties, are also descended from the Thompson. The Thompson has a genetic abnormality that causes the seeds to arrest development. Though the flower is pollinated and the ovum fertilized, the seeds stop growing after a few weeks. So, the grape is not entirely seedless; rather, the seeds are aborted, and exist as tiny specks inside the fruit. Commercial growers treat the plants with a growth hormone called gibberillin, which is normally secreted by developing seeds. The flowers are dipped or sprayed with the hormone so that the grapes grow big and juicy despite the arrested seeds.

Seedless watermelons began to be a big seller in United States markets in the 1990s. Besides the convenience of having few or no hard black seeds when consuming the fruit, the new variety has a hard shell, making it easy to ship and giving it longer shelf life. Seedless watermelons are sterile, that is seedless, because they have three sets of chromosomes. This condition is called triploid. Standard watermelons, like Thompson grapes and most other organisms, have two sets of chromosomes, and are called diploid. To produce triploid watermelons, a diploid parent is pollinated by tetraploid watermelon, which has four chromosomes. During sexual reproduction, the new organism inherits half of each parent's genetic material. As a result, the new watermelon gets one chromosome from the diploid parent, and two from the tetraploid, making it triploid. The triploid hybrid is virtually seedless. It produces a very few seeds, and these can be planted to grow new watermelons. But the new plants must be pollinated by standard diploid watermelons in order to produce fruit.

Research & development

The development of a new strain of seedless fruits or vegetables is a painstaking process. Research is typically carried out by horticulturists working at an agricultural development laboratory or government research station, where they can devote years to the work. A researcher studies thousands of seedlings to find ones with the desired characteristics. In searching for a seedless variety, other factors have to be taken into account, as well. The seedless fruit will not be commercially viable if it does not have good flavor, if it is prone to disease, if it is misshapen, etc. The fruit must be as good as seeded varieties, with seedless as an added advantage. So the researcher breeds likely plants, studies the offspring, and breeds these with other likely plants. The developer of the Flame Seedless, a red seedless grape, experimented with over 100,000 seedlings in the course of the quest. The plant that produced the Flame was a cross of five different varieties.

The traditional process for breeding seedless fruits was to cross a seeded female plant with a strain of seedless male. The offspring were seedless about 15% of the time. Then a successive generation could be produced from this 15%. Starting in the 1980s, horticulturists found ways to speed up the process by culturing the tissue of seedless plants. With grapes, the aborted seeds of the seedless strain are grown in a petri dish or test tube. Then these seedless strains can be crossed with other seedless strains, resulting in offspring that is 50-100% seedless. This technique has been used with great success with grapes, speeding up the time it takes to bring a new seedless variety to market. With watermelons, the sprouting tip of a seedless plant is placed in a petri dish filled with growth regulators and nutrients, and the one tip will sprout as many as 15 clone plants. This technique has also been used to produce seedless tomatoes.

Cultivation

Scaling up

• 1 After a new strain is developed, commercial seed growers license the stock and
  

  Mendel's Second Law.
  begin producing enough seed on a large enough scale to sell to farmers. The following process given is for seedless watermelons. The commercial seed grower obtains seed or seedlings from the developer, and plants them in an isolation field. These are the tetraploid (four chromosomal) plants. The isolation field must be three miles from any other watermelon field, so that bees do not pollinate the seedless plants with other varieties. Or, the isolation field may be surrounded by another plant, such as corn. The isolation field is typically one to five acres. The grower carefully maintains the plants, probably treating them with a growth hormone. Bees pollinate the plants, which produces possibly hundreds of pounds of stock seed. Next, this stock seed is planted in a seed production field along with a variety of regular diploid watermelons. When the plants flower, the cultivators pollinate them by hand. Pollen from the male diploid plant is transferred to the female tetraploid flower. The fruit that results has triploid seeds. These seeds are sold to farmers.

Germination

• 2 Because the triploid plants are generally fragile and easily killed by improper temperatures, they are usually germinated in a greenhouse and transferred to the field as seedlings. Triploid seeds also have an unusually thick coating, so the farmer may nick the rounded end of the seed before planting, to speed germination. The farmer buries the seeds in light potting soil and adds a little water. The greenhouse then needs to be maintained at around 85°F (29°C). After most of the seeds have sprouted, the temperature is lowered to between 70-80°F(21-27°C).

In the field

• 3 When the seedlings are three to four weeks old and have developed two or three leaves, the farmer transplants them into the field. The farmer also must plant a standard diploid watermelon in the field, one row for every two or three of the triploid. These may have been directly seeded into the field earlier, or transplanted at the same time. The diploid variety is chosen to mature at the same time or earlier than the triploid. To avoid confusion at harvest time, the diploid should also have a different rind color than the triploid, so it is clear which one is seedless. Both varieties grow and mature, and bees pollinate the flowers. The triploid plants produce seedless watermelons. These are harvested and sold. To grow more triploid watermelons the next year, the farmer must start over with new seed.

The Future

Because of the success of sophisticated tissue culture methods, the time it takes to develop seedless fruits and vegetables is lessening. This means horticulturists can plan varieties to fill specific market gaps, such as a seedless black grape that matures in August, when few black grapes are available. Another technique that may quicken the production of seedless varieties is gene transfer. Biologists can fuse a new gene into a fruit plant that tells the plant to produce a growth hormone. The growth hormone stimulates the growth of the fruit even without pollination. The unpollinated plants produce no seeds. In the late 1990s, this method was successfully conducted on tomatoes and watermelons. This kind of biotechnology is one of the fastest growing areas of plant science. So the future may produce many more seedless fruit and vegetable varieties, without the long testing and development time needed in the past.