Story last updated at 8/8/2012 - 1:07 pm
A recent discovery holding significant implications for the tomato industry may help plant breeders put the vintage flavor and quality of tasty heirloom tomatoes back into those insipid varieties we often find on our grocery store shelves. University of California, Davis plant scientists, studying genes that influence fruit development and ripening in tomatoes, had been looking for transcription factors, which affect fruit color and quality.
Transcription factors are proteins, which regulate genes or turn them on and off. Partnering with researchers at Cornell University and in Spain, the scientists discovered two transcription factors (GLK1 and GLK2) that control the development of chloroplasts and influence a tomato's flavor and color. Chloroplasts are structures in plant cells that enable photosynthesis by converting energy from the sun into sugars and other compounds.
In a June 2012 article on a University of California, Davis news website, Ann Powell, a biochemist in the Department of Plant Sciences stated, "This information about the gene responsible for the trait in wild and traditional varieties provides a strategy to recapture quality characteristics that had been unknowingly bred out of modern, cultivated tomatoes."
Researchers tested wild tomatoes that had turned unusually dark green just before ripening and found that the ones which naturally expressed GLK2, produced ripe fruit with higher levels of sugar, soluble solids and lycopene, a good thing for tasty, nutritious tomatoes. An increased understanding of the genes and processes responsible for each tomato characteristic will help researchers breed more desirable traits into tomatoes.
For decades tomato breeders have concentrated on developing more commercially viable tomatoes as the customary small farms serving local markets were gradually replaced by large agri-business corporations with more demanding production, shipping and storage requirements. Tomatoes were bred specifically to resist herbicides, insects and diseases and to allow mechanical harvesting by ripening all at once on compact plants with tough skins that could withstand the rough handling.
But genes aren't the only thing that affect tomato flavor. Long term storage and shipping at improper temperatures can cause severe loss of tastiness. Harvested at the mature green stage just before ripening begins, commercially grown tomatoes will be artificially ripened later (probably with ethylene gas which the tomato also produces naturally), weeks after being picked. If not handled gently, physical damage can also have a negative impact on flavor.
Home gardeners, luckily, can choose seeds for plants that grow in many shapes, sizes and types and harvest the fruit carefully by hand at the perfect state of ripeness. Many of the popular hybrid varieties are very good but for several years now I've been growing only heirloom vegetables with seeds from Seed Savers Exchange, a non-profit seed saving and distributing organization. Although I don't always save seeds from fruits I grow, I like the idea of planting old, standard varieties and plan to continue using them. The plants are vigorous, the flavor is excellent and the insect and disease problems are no worse than with the hybrids in my experience.
Tomatoes are interesting in that they begin life even before their flowers open. Cell division is virtually complete by pollination time after which cell enlargement happens until a sour, hard green fruit, the mature green stage, is produced. Tomatoes picked before they are mature green may not ever ripen or achieve full flavor.
Once tomatoes are full-grown, but green, complex chemical changes take place gradually as they ripen. Chlorophyll breaks down, carotene pigments are produced through different color stages and the fruit softens. The final color and texture will be determined by sugars, organic acids and volatile chemical compounds present in the fruit.
Here in Southeast Alaska it seems I always have plenty of green tomatoes when frost threatens and time is up for the season. Last year I put the mature green ones in a paper bag with a banana, which naturally releases ethylene to encourage ripening and it worked very well. Since then, I've come to understand that you need a banana that hasn't already been treated with ethylene (as the ones in the grocery store probably have been) so those tomatoes were apparently going to ripen anyway.
Lacking a banana tree in Thorne Bay, this year I believe I'll try the method of pulling the plants up by the roots with their green tomatoes and hanging them in a cool, dry place, checking regularly. When winter hits and homegrown tomatoes are long gone, it will be comforting to know that researchers are faithfully working on the flavor of tomatoes that grace our grocer's shelves.