Frank Morton

Lettuce is just a beautiful plant. We have a long relationship, people and lettuce. Recent genomic research has illuminated Lactuca’s prehistory of domestication, that is, its domestication prior to historical records. Apparently, all of the lettuce eaten today comes from a single domestication event that occurred 10,829 years before the present in the land we now call Iraq, which seems like an absurd level of accuracy for an event like “domestication.” Nevertheless, both the age and the genomic technology used to determine it are impressive. Researchers have tracked down a wild population of Lactuca serriola (the progenitor of garden lettuce) that has more genetic similarity to Lactuca sativa than it does other populations of L. serriola—clear evidence that our dinner lettuce evolved from this very population of the wild species which still grows in Iraq today.

The historical record of lettuce begins 4800 years ago on the walls of Egyptian tombs where we see oilseed lettuce being threshed, looking a lot like bolted romaine. A leafy lettuce type arrived in China via the Silk Road about 1900 years ago, and 1000 years later stem lettuce diverged from the leafy progenitor to become the most popular type of lettuce in Asia. The ancestral leaf lettuce also went to Europe for oilseed use, and about 500 years ago butterhead and romaine types diverged from that ancestral form. The Spanish brought these to the Americas in the 1600s, where crisphead lettuce would eventually be developed. Our history with lettuce is just about as long as our history with any domesticated plant.¹

One aspect that I love about lettuce is how it comes to us ready to eat—raw, unadorned—at nearly any stage of life, until afflicted by the bitterness of overmaturity. Stem lettuce even overcomes this last limitation, having been selected for consumption when the bolting stem itself has bulked up to be a toothsome vegetable. Salad greens, and lettuce in particular, are some of the few vegetables where the grower gets full credit for the flavor and quality that reaches the plate. We always appreciated that appreciation as salad growers for fine restaurants, where chefs generally get the glory. Even chefs express appreciation for salad growers. I was once told by a chef that our Wild Garden Salad made his restaurant look exceptional and all he had to do was put it on a plate. He said it with some irony.

The form of a lettuce rosette is not far from that of a rose, and so I find breeding new kinds of lettuce as satisfying as breeding flowers. Like flowers, everyone seems to favor lettuce, which isn’t something you can say for chicory, or even cabbage. I know a lot of people have disdain for certain vegetables—beets, tomatoes, beans, peas, brussels sprouts—but I never hear or see people hating on or removing lettuce from their salad. The diversity of distinctive and acceptable traits in lettuce gives the breeder a lot of colors, shapes, architectures, textures, and tastes to work with. But more important possibly, breeding for color and pigment intensity is breeding for nutrition. Red pigments and other flavonoids are antioxidants, as are all plant pigments and related compounds, and antioxidants in green and red lettuce are significant in our diets (lettuce is the third most consumed vegetable in the US, after tomatoes and potatoes). Vegetable colors are good for us.

More than 20 years ago I became a fan of the red spotted trait found in a few heirlooms at the time. ‘Amish Speckled’ butterhead, ‘Forellenschluse’ romaine, and ‘Sanguine Amelioree’ butterhead were the ones I could access, but I noted dozens of red or brown spotted kinds in the 1885 Vilmorin catalog of vegetables, The Vegetable Garden. I started putting spots, speckles, and splashes of red onto everything lettuce. I’m pretty sure I made the first speckled oakleaf (‘Flashy Butter Oak’) and soon I plagued the lovely gems with way too many ruby spots. Lately, I’m out to put color into the hard white hearts of crispheads, which is also a new combination, and a contribution to the nutrient profile of the much-maligned “iceberg.” Despite an undeserved reputation for nutritional emptiness, crisphead lettuce is favored by many for its crunch and juicy thickness, not to mention how the leaf fits and enlivens a sandwich. The sheer volume of its consumption makes it a significant source of choline in the common American diet, and worthy of attention in regard to nutritive improvement.

Color in lettuce is controlled by 7 genes that determine the color and placement of pigments. Other genes further regulate color intensity and hue. The five basic leaf color traits are for red leaves, pink tinged margins, spotted, dark green, and light green leaves. Multiple regulation genes create the differing shades and intensities of red and green we are familiar with and, in the case of the spotted gene, create different types and degrees of spotting. Sometimes this looks like fine red dustings of spray paint, and other times like splatters, or irregular blocks, or long red streaks against a green background of various shades. But most interesting of all (and strangely unmentioned in the literature), the red of the spotted trait can be expressed without exposure to UV light and thus we can get a very pleasing pink/red patterning on blanched leaves inside the head. The common red-leaf gene is only expresse when turned on by UV light, drought stress, or cold temperatures, and so the familiar pattern of red lettuce is a head that’s red on the outside, but blanched green on the inside. While anthocyanin is a pigmented form of antioxidant flavonoid, it is worthwhile to remember that many colorless flavonoids are also important antioxidant phytonutrients produced by the same biosynthetic pathway.

I have found that by mixing plants with varying spotted patterns and red leaf traits, red plants can be produced that are “red to the heart.” Strikingly so. But they can also become more than that—highly susceptible to disease, crisp to a fault, prone to tipburn, lightweight—and with all that, have terrific taste. This is what I love about lettuce. Even the imperfect or the failures can be delicious. Seldom so with squash.

I’ve read (in a plant patent introduction) that lettuce breeders consider the red spotted trait unpredictable, undesirable, and something to be avoided lest the lettuce appear sick with spots. For me, that would have been an enticement, but I was already besotted when I read it.  One of the surprising expressions of the red spotted trait is a “zoning” pattern that appears unpredictably, but often enough to be familiar. Rather than spotting, a “pie slice” of the head will be intensely solid red, often bisecting a leaf down the midrib. If that isn’t enough to scare off a lettuce breeder, the rest of the plant might be unspotted. Amazing really. Mendel’s Laws have a difficult time explaining it and, in fact, cannot.

When I began growing zinnia, I noted that the red spotted pigment pattern in a variety called ‘Peppermint Stick’ behaved much like the red spotted trait in lettuce. The spotting could be uniform across the flower head or it could be heavily or lightly spotted, or sometimes there were no spots at all. Sometimes there was that pie wedge of solid red, just like the lettuce heads I’m familiar with. I’ve convinced myself lately that these zinnia and lettuce spotting genes are homologous, that is, they are evolutionarily the same genes expressed in a different species of the same plant family, the Asteraceae.

I have a working hypothesis that the behavior of the red spotting gene is like that caused by “jumping genes” in maize. The kernels of some indigenous maize are marked by red or other colors in stunning patterns of lines or spots or splotches that geneticists of the mid-20th century could not explain. Mendel’s Laws of Inheritance suggested no mechanisms for such patterns of pigmentation. Geneticist Barbara McClintock, who had a knack for observing maize chromosomes under a light microscope, concluded that genes were moving themselves around the chromosomes, breaking up normal gene expression for color in developing tissues. Those red-streaked kernels should be, would be, solid red in a Mendelian world. She called the disrupters “jumping genes,” later called transposable elements, or transposons. Her colleagues (men, of course) nearly laughed her out of the academy. In 1983, some three decades later, she was awarded a Nobel Prize for her discovery. That must be some sweet affirmation.

I’ve lately searched around for some confirmation of my hypothesis, but can find nothing suggesting that transposons are active in spotted lettuce. But I did find that transposons are responsible for the unusual streaking and zonal (wedge-shaped) pigment patterns in some morning glories (‘Grandpa’s Carnival,’ ‘Flying Saucers,’ et al.), 2 in ‘Broken Colors’ four o’clocks,3 and in bicolor snapdragons. In these flowers, transposons can cause fine speckling, streaking, elimination of red altogether, altered pigment patterns, intensity, and hue, as well as loss of leaf pigments (variegation).4  I also found images of the very spotted zinnia color patterns I have seen in the garden presented as illustration of transposon effects in flowers.  In a 1938 publication by USDA geneticist Ross C. Thompson,5 the author mentions observations made on chlorophyll loss (variegation) in lettuce, and notes that these changes do not follow Mendelian rules. He comments on similar observations made about four o’clocks and snapdragons (between 1921 and 1933), which dovetail neatly with the recent findings of transposon activity in these species, cited above.

Finally, aside from its beauty, ubiquity, unassertive character, longstanding favorability and inner mystery, lettuce is just easy. Easy to grow, easy to have almost anytime, anywhere. There is no trellis, no trench, no hilling to add to its perfection. Soil temperature, nighttime lows, calendar dates—nothing like this is very critical to some degree of success with lettuce, nothing that a piece of row cover or shade cloth can’t helpfully moderate outside of frozen winter. Lettuce is agreeable, affirming our ability to feed ourselves something, and in short order. Perhaps lettuce taught people to garden 10,000 years ago, a lesson in regeneration and short-term gratification. Maybe lettuce was the first small step toward the long term project that would become wheat and peas, and 6000 years later, maize. And being so like a rose, perhaps lettuce quietly suggested that kind of beauty waiting in the garden as well.

Originally published in the 2020 Wild Garden Seed Catalog.

Lei Zhang, et al. RNA sequencing provides insights into the evolution of lettuce and the regulation of bioflavonoid biosynthesis. Nature Communications 8, Article Number 2264 (2017)
Shigeru Iida et al. Genetics and epigenetics in flower pigmentation associated with transposable elements in morning glories. Advances in Biophysics, Volume 38, 2004. Pages 141-159
M Suzuki, et al. Transposon-mediated mutation of CYP76AD3 affects betalain synthesis and produces variegated flowers in four o’clock (Mirabilis jalapa). Journal of Plant Physiology 2014 Nov 1:171(17):1586-90.
Da Luo, et al. Pigmentation mutants produced by transposon mutagenesis in Antirrhinum majus. The Plant Journal (1991), 59-69
Thompson, Ross C., Genetic Relations of some Color Factors in Lettuce, USDA Technical Bulletin No. 620, June 1938.