The remaining dwarfs in the mid August garden

Reading this interesting addition to our newsletter reminds me that I need to reconnect with Tad Smith. We had occasional phone conversations back in the Off The Vine days, which I always enjoyed. Chats with him help fuel my own interest in amateur breeding.

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Breeding Tomatoes in the Home Vegetable Garden

Dr. Tad Smith

Breeding tomatoes adds another dimension to the joy of growing tomatoes in the home vegetable garden. In addition to the anticipation of tasting that first luscious fruit of the season, there is the unknown fascination about the size and taste of this “new” tomato. If the new tomato has characteristics that please the gardener, then it may become a permanent part of the yearly planting in the home garden. What could possibly be more satisfying than eating the fruits of one’s own handiwork?

There are several ways to breed tomatoes. For the gardener simply interested in planting some crossed-pollinated seed, let the bees and chance play the main roles. Tomatoes are normally self-pollinating, but 5% cross-pollination from insect activity is common in the home garden. If bees are not active or the weather is poor, cross-pollination is less likely to occur. However, this passive method places a major limit on the choice of parental tomato varieties because a method is needed to distinguish seedlings that developed from cross-pollinated seed.

Leaf shape can be an excellent marker to pick out the hybrid seedlings. For example, it is possible to cross a regular leaf tomato variety with a potato leaf tomato using no special skills. This can be useful for gardeners with unsteady hands or less than perfect vision needed to cross pollinate the flowers. Besides, botany class may have been years ago.

In the spring, plant one potato leaf tomato plant surrounded by several regular leaf plants. During the summer and early fall, collect a large amount of seed from 10 or more tomatoes from only the potato leaf plant. Next spring, plant all of the seed in several flats. As soon as the first true leaves develop, examine the true leaves carefully. Scattered throughout the flat of mostly potato leaf seedlings will be a few regular leaf seedlings. Since all of the seed came from potato leaf fruit, any regular leaf seedlings are the results of cross pollination. In this case, the regular leaf trait was dominant over the potato leaf trait. By choosing the potato leaf plants as the female parents, it was easy to spot the regular leaf hybrid seedlings.

This passive method requires working with tomato varieties possessing traits that can be differentiated in the seedling stage. Besides the potato vs regular leaf, the Woolly trait can be used. The Woolly gene is found in the Angora variety of tomato. If gardeners are not familiar with Angora, they should try it because it has a beautiful coat of white hairs over the entire plant surface. The Woolly trait is dominant over plants lacking a heavy coat of hairs. Therefore plant several Angora plants around one regular non-hairy plant. Examine seedlings derived from seed collected from the non-hairy plant for woolly seedlings. These Woolly seedlings will be the hybrids.

Active cross pollination is a more reliable method to breed tomatoes. There is no limitation on the parents used in the cross, except do not use hybrid plants. The technique of cross-pollinating tomatoes is relatively easy, but it requires practice and skill. It is well worth the effort to practice by crossing a potato leaf plant with pollen from a regular leaf plant. Then evaluate your skill by checking the leaf type of seedlings. If the seedling characteristics indicate that your skills are good, then attempt to cross pollinate plants that would lack visual differences at the seedling stage.

The inside method requires at least four large pots for two plants of each variety. Grow the plants in a southern window and augment the light with a bank of fluorescent lights set for 16 hours. Longer light regimes will damage tomato plants. Add a source of calcium to the soil mix to eliminate any chance of blossom end rot.

Once the plants form flower buds, it is time to plan the cross pollination. Just before or as the flower opens, remove the fused stamens from the flower that should form the future tomato. By removing these pollen producing parts of the flower (emasculation), self pollination is prevented. The stamens form a cone like structure around the center structure of the flower. A jewelers micro-forceps works well for this delicate task. The goal is to cleanly remove all of the stamens without damaging other parts of the flower. Practice removing the stamens on tomato flowers during the summer in the vegetable garden.

Generally, the emasculated flower requires a day or two to recover from the injury and to become fully receptive to pollen from another flower. The flowers that provide pollen should be at peak bloom, and the best time for cross pollination is around noon. The forceps tip is used to collect pollen from the inner surface of stamens from a different tomato variety. There are numerous grooves on stamens that contain large amounts of pollen. Of course, there is no reason to be gentle with the pollen source flowers. The pollen is transferred to the tip (the stigma) of the center part of the emasculated flower. This central part contains the stigma and long style located over the ovary. After pollination, the ovary of the flower swells and develops into a tomato.

Pollinate as many flowers as possible. Once the fruit sets, allow only two fruits to develop from each flower cluster. Otherwise, the weight of the fruits on plants raised inside may cause the tomatoes to pull loose from the stem.

The seeds from this cross at the F1 hybrids, and the F1 plants will be identical and express only the dominant traits of the parents. Since they are hybrids, they may be more vigorous than either one of the parents. Store the seed in coin envelopes that are carefully labeled and dated. Place the envelopes inside a plastic zip-lock bag and store it in the refrigerator. Tomato seeds stored at low temperature will be viable for decades. One tomato will yield anywhere from 10 to 200 seeds, depending on the variety and success of the pollination technique.

The genetics of tomatoes are well known. As a starting point, many common tomato traits are recessive. This includes the potato leaf shape, green stripes on fruit (seen in Tigerella), yellow flesh color, determinate plant shape, and unpigmented fruit epidermis (clear skin found on pink and white tomatoes). If any of these traits are matched with a typical red tomato variety, the dominant traits of the red tomato variety will be expressed in the F1 hybrids.

Only a few tomato traits are dominant over the characteristics of normal red tomatoes. The Woolly trait is one example. Another dominant trait is beta-carotene. In this case, the tomato is orange in color because of the dominant production of beta carotene pigment in the fruit. Caro Rich and Caro Red are two varieties with this gene.

The recessive genes do not disappear in the F1 hybrids. They simply are not expressed. However, these genes will be expressed in some plants of the next generation (F2) of tomatoes. The F2 generation produces tremendous variation from plant to plant. At this level, plant breeders start the long selection process of developing a new variety. This is a topic for another article.

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Even though I really enjoyed reading through this article when Tad submitted it, it was still great to have info I’ve absorbed through the years reinforced. Any of you who are interested in trying to cross some tomatoes will find lots of useful things in this article.