Signs of Fall 3: Tomatoes!

Photo by Softeis, Wikimedia Commons

(Click on the following link to listen to an audio version of this blog … Tomatoes!

This is the time of year when tomatoes, REAL GARDEN tomatoes, are available for every meal. They go on top of breakfast bagels. They go in between slices of bread on sandwiches at lunch. They pile up on bruschetta for hors d’oeuvres. They help to deliver the chopped garden onions and hot peppers in homemade salsas. They brighten up dinner salads and make spaghetti and pizza sauces sharp and flavorful. I am surprised that we all don’t turn orange from all of the tomatoes we are consuming!

Tomatoes!! Fruits or vegetables? Depends on your context. Botanically, tomatoes are fruits (berries, actually) that develop from a fertilized ovary. Culinarily, tomatoes are vegetables primarily because they are oozing with umami flavors and are typically eaten as part of a main entrée rather than as part of a desert. Memorably, Ronald Regan’s administration tried to classify ketchup as a vegetable to make school lunches appear to be more nutritious! Fortunately, tomatoes have avoided political classifications!

Currant tomato. USGS. Public Domain

Tomatoes are the edible fruits of the plant Solanum lycopersicum.  This domesticated plant is the product of thousands of years of careful selection and observation by native people of southern Mexico and northern-western South America. The wild progenitor of the tomato is a plant called the “currant tomato” or the “pimp” (scientific name, S. pimpinellfolum). The natural range of the currant tomato is in the mountains of Peru and Ecuador, but it has spread widely primarily with help of humans. The currant tomato has even become naturalized in the Galapagos Islands! The tiny fruits of the currant tomato are edible and are even richer in lycopene, vitamin C, phenolic acids and assorted antioxidants than modern, domesticated tomatoes.

Yellow currant tomato flower. Photo by TomRPoole, Wikimedia Commons

The classic model of the development of the domesticated tomato describes native people of Peru, Ecuador and Mexico carrying out selective breeding and cultivation of varieties of the currant tomato beginning around 500 BC. A new origin hypothesis, however, from a research team at the University of Massachusetts, describes a wild form of the domestic tomato evolving from natural mutations in the currant tomato some 80,000 years ago in Ecuador. These larger fruited, weedy tomatoes were them collected and selectively cultivated by native peoples beginning 7000 years ago. These early tomato varieties then spread throughout Central America and Mexico.

In the early 1500’s, just after the Aztec Empire fell to the Spanish Conquistadors, descriptions of the food markets in the Aztec capital city (now, Mexico City) are rich with long lists of types, shapes and colors of tomatoes. Bernardino de Sahagun’s detailed descriptions of Aztec society and culture not only has extensive descriptions of tomatoes but also of the many of sauces and dishes that featured tomatoes as main ingredients.

Hernan Cortez, the conqueror of the Aztecs, is thought to have sent a tomato plant back to Spain in 1521. Tomatoes were distributed all across the Caribbean and to also to many distant Spanish colonies. Tomatoes also traveled to countries in Europe and grew readily in the climate around the Mediterranean Sea. By the mid-1500’s, tomatoes were growing all around the world!

Photo by Wilfredor, Wikimedia Commons

At first, though, people were reluctant to eat tomatoes. The obvious close relationship of tomato plants to well-known, highly toxic plants (like deadly nightshade), led some to believe that the fruit of the tomato plant was also poisonous. Tomato plants, at first, were grown as ornamentals. By the early 1600’s, though, Spanish cuisine had adopted the tomato as an important food component, and Italy, by the late 1600’s or early 1700’s had done the same. In Britain and in the British colonies in North America the superstitious avoidance of the tomato persisted, and even as late as 1897 “experts” were advising that tomatoes should not be eaten. They were referred to as “globes of the devil” and that they were “sinisterly dangerous.”

The wild, currant tomato and the early varieties of domesticated tomatoes were probably all pollinated by small, halictid (“sweat”) bees. When the tomato spread around the world, though, it did not take these sweat bee pollinators with it. Other mechanisms for pollination, then, had to be developed. In most domesticated tomato varieties, the pollen is dispersed by wind or by the physical shaking of the tomato flowers by passing animals or by large pollinators like bumblebees clinging to the outside of the flowers while buzzing their wings. This “buzz pollination” occurs readily out in garden plots but must be done artificially when tomatoes are grown in greenhouses. The purposeful shaking of the flowering tomato plants is often accomplished by vibrating the plants with a wand called an “electric bee!”

Photo by User:FoeNyx, Wikimedia Commons

The domesticated tomato in spite of all of its varieties and hybrids has a very narrow range of genetic diversity. Compared to the wild currant tomato, cultivated tomatoes only have 5% of the wild species’ genetic variation. The cause of this genetic constriction was probably the two “bottleneck” events that occurred in the development of the domesticated tomato: the initial transport of limited numbers and diversities of wild tomatoes to their Central American and Mexican sites of breeding, and the transport of another small sampling of types of tomatoes from the Mexican markets out to Europe and the rest of the world. Ongoing research into the wild tomatoes of the Andes Mountains may be a way to expand the gene base of the tomato and increase its viability and survivability in our changing world.

Commercial tomato breeders in the 20^th Century grappled with a number of problems and repeatedly found that their breeding efforts to correct these problems had unexpected consequences. For example, a big problem in commercially grown tomatoes is the uneven and, sometimes incomplete, ripening of the fruit. The discovery of the “uniform ripening phenotype” (also called the “U-phenotype”) corrected this production/harvesting difficulty. These U-phenotype tomatoes ripened evenly and completely and generated round, red, healthy looking fruit that were easy to pick and transport. Unfortunately, though, this U-phenotype changed the sugar metabolism and flavor chemistry of the fruit and made them much less pleasing to eat. This modern adaptation generated tomatoes that looked like tomatoes but did not taste like them! Many other breeding efforts (like changes that increased yields, or increased fruit size, or increased tolerance to diseases and pests) had similar, unexpected impacts on flavor qualities and taste.

Most tomatoes are classified as either “heirlooms” or “hybrids.” Heirlooms are tomato varieties have been grown for at least 40 years with any crossbreeding. Generally, heirloom tomatoes have distinct (and wonderful!) flavors and textures, but may suffer from environmental stresses and/or disease susceptibilities.

Tomato and pepper harvest, 2023. Photo by D. Sillman

Hybrids are formed from crosses of two distinct varietal lineages. The ideal characteristics of each parental strain are combined and expressed in the hybrid. Currently, there are about 700 tomato hybrids being commercially grown. Most commercially grown tomatoes, in fact, are hybrids. A great advantage to the seed companies who develop these hybrids is that the seed of the hybrid fruits are typically random mixes of the genes from the parental strains. It is not possible for a grower to harvest seed from their plants to grow a reliable crop the next year. Thus, the seed company continues to have a market in which they can sell their seeds!

Hybridization initially dealt with production/transport problems (as described in the development of the U-phenotype). More recently, though, improved taste and improved nutritional features have become the goals of hybridization. The competition to produce hybrids is intense, and most hybrids only remain on the market for 4 or 5 years.

Use of laboratory techniques to modify the genetic sequences of tomato varieties and to insert sometimes non-tomato genes into a variety’s genome generates plants that are referred to as “genetically modified organisms” (“GMO’s”) or “transgenic varieties.” Toward the end of the 20^th Century a number of transgenic tomato varieties were developed. Some of these GMO tomatoes are still on the market, but others (like the long, shelf-life variety “Flavr-Savr”) are not. Cost and consumer resistance are often the factors cited in the failures of these transgenic fruits.

Early Girl tomato patch. Photo by D. Sillman

In my garden here in hot, dry Northern Colorado one type of tomato has been the most successful. It is the Early Girl hybrid that has been around since the early 1970’s. Early Girl is bred to tolerate high temperatures and low, soil moisture. In fact, in a process referred to as “dryland farming,” Early Girl plants can be encouraged to grow roots deep into the soil profile and rely more on deep soil moisture rather than irrigation. The fruit of the dryland Early Girls is also said to be exceptionally sweet!

August and September are the tomato months! Enjoy!