Breeding for consistent color in unripe fruit may have diminished flavor

By Susan Milius

Web edition: June 28, 2012
Print edition: July 28, 2012; Vol.182 #2 (p. 18)

It looks like 70 years of breeding for better color in unripe fruit has inadvertently helped create the wet-paper towel flavor of the modern tomato.

Growers care about the green of unripe tomatoes, explains biochemist Ann L. Thomas Powell of the University of California, Davis. Ripening globes that are each uniformly green let growers easily judge when a field will be ready for harvest. Over decades breeders have selected for this uniform green coloring instead of for tomatoes that turn a deeper shade around the stem end, Powell says.

The problem is, getting rid of that dark green zone, called green shoulders, turns out to have sabotaged a gene called SlGLK2 that boosts sugar and other sources of flavor in the ripe tomato, Powell and her colleagues report in the June 29 Science.

“It is a good illustration of unintended consequences,” says molecular biologist Harry Klee of the University of Florida in Gainesville, who studies tomato flavor.

For years, Powell says, breeders assumed that a ripe red tomato got all of its sugars from the little photosynthetic engines known as chloroplasts in the plant leaves. It turns out, however, that a green-shouldered tomato gets about 20 percent of its sugars from its own chloroplasts. Without a functional SlGLK2 gene, the ripening tomato forms fewer and punier chloroplasts that don’t deliver, Powell and her colleagues have found.

Skimping on sugars certainly could make a difference in flavor, says Klee, who routinely does taste tests in his lab. His tomato testing panels respond strongly to sugar content. “The more the better,” he says.

Volatile compounds wafting off a tomato’s flesh also play a big role in its appeal. The problem is, inadequate chloroplasts likewise won’t produce as much of the chemical precursors for some of those compounds. “It’s totally obvious you’re going to take a hit in some of the volatiles,” Klee says.

In the June 5 Current Biology, he and his colleagues highlighted the importance of a handful of volatiles — some of them mere whiffs — in seducing the nose and taste buds.

Exactly what the loss of the green-shoulders trait means for tomato flavor remains to be measured. But, Klee says, “it's not the whole story of why modern tomatoes are so bad, by a long shot.”

Picking them before they’re fully ripe diminishes flavor, as does refrigerating them. Chilling tomatoes below 13º Celsius kills metabolic processes still functioning in a picked tomato, Klee says, so the fruit never manages to replace the lovely volatiles that float away.

Even under ideal conditions, genetic differences will matter in flavor. When Klee and his colleagues pamper various commercial varieties, taste panels pan some of them and give the best ones decent but not brilliant ratings. And just because a variety is an heirloom doesn’t mean it tastes great, he cautions. For his taste panels, Cherry Roma is the reigning favorite.

When choosing among generic tomatoes, he recommends going for cherry tomatoes and other little types: “Breeders haven’t had as much time to mess them up,” he says.