The Evolution of Sweet Corn
Corn is a funny crop. It’s really a mutant grass, but as users of the grass’s seedhead (the cob), we’ve come to know it as both grain and vegetable, in varying genetic strains and stages of maturity. It is also uniquely genetically fragile–the only vegetable we grow whose current generation can be altered by the pollen of another cultivar, in terms of the sugar level, tenderness, and kernel color. In any other crop, we would have to wait until the seed produced by the cross-pollination were grown out to see the genetic changes. The gene to make sweet corn, the vegetable (with a higher sugar content), as opposed to field corn, the grain (with a higher starch content) is recessive, so different types of sweet corn, with variations on the sugar gene, must be constantly protected from the dominant “starch” genes in order to be apparent as traits.
Sweet corn was originally the result of a recessive mutation in genes that control the conversion of sugar to starch within the kernel’s endosperm. Field corn is always harvested in dent stage, when the kernels are dry and mature. Sweet corn, on the other hand, is harvested in milk stage, an immature growth phase where the kernel has a higher sugar content and is still soft, and the cob is eaten as a vegetable rather than a grain.
The problem with sweet corn is that the natural maturation process involves conversion of sugar to starch – even after it is picked. So the challenge for researchers has long been to come up with a cultivar with a better shelf life, which would be good for both storing fresh and preserved in canned form without too much added sugar or salt. Sweet corn has traditionally been thought of as an elusive taste, a crop that had to be picked early, often and consistently, and couldn’t be planted in greater numbers than could be used pretty quickly after harvest.
The milestone developments for sweet corn over the course of twentieth century began with hybridization, which allowed breeders to create cultivar crosses with more uniform maturity, improved quality, and better disease resistance. Following that, researchers at the University of Illinois discovered several different gene mutations for sweetness. Su was the original sugary gene and the natural mutation that first gave rise to sweet corn.
Then, sh2, the “shrunken gene” which was originally discovered when researchers noticed that it caused shriveled, smaller kernels – thanks to a greatly reduced proportion of starch in the kernel. The shrunken gene became the basis for Supersweet corn. Finally se, Sugary Enhanced, emerged, making its debut as ‘Everlasting Heritage.’ The “sugary enhancer” trait was originally developed from a cross between Illinois sweet corn and corn of Bolivian ancestry. All three types produced yellow, white, and bicolor cultivars, of which white needed the most genetic isolation to remain pure.
Although these breakthroughs were made in the 1950s and 1960s, Supersweet hybrids did not fully catch on until the 1980s. Popularity rose when grocers and processors discovered its longer shelf life and high sugar content, in comparison to conventional sweet corn. But there were several drawbacks.
Supersweet varieties have to be grown in isolation from other sweet corn varieties to avoid cross-pollination and the resulting increased starchiness. A quarantine space of at least 350 feet is needed to maintain the purity of the cultivar, or a staggering of planting times so that pollination of other corn occurs at least 10 days apart from that of the Supersweet corn.
Supersweet corn also needs twice the soil moisture to germinate as normal corn, so dry soil planting has to be avoided.
While Supersweet’s kernels tend to have tougher skin (pericorps), corn with the Sugary Enhanced gene has a thinner pericorps, which means that it has to be handled more gently and is more easily damaged by mechanical harvesting.
Sugary Enhanced (se) corn needs isolation as well but it is not as unequivocally altered by foreign cultivar pollen as are Supersweet varieties. Heterozygous versions are possible, meaning that it does not have to be entirely genetically pure for the traits to show. The “homozygous se” gene is preferable, since sugar levels are higher on 100 percent of kernels, while the “heterozygous se” gene means higher sugar levels on only 25 percent of kernels.
Supersweet varieties are the most vulnerable to contamination, with the most need for isolation. They are the least cold-tolerant, and also the least prone to consistent emergence – seeds have to be planted quite shallow, since the limited starch reserves in the kernel mean less fuel for new seedlings to germinate and emerge from the ground.
There is a continuum of many of these traits, running from the su gene to the sh2 gene, with Sugary Enhanced often falling in between, and striking a good balance – sweet enough, yet without the fragility and high-maintenance of many of Supersweet’s characteristics.
Sugary Enhanced varieties have proven ideal for the home gardener, since they have just enough cold-soil vigor while maintaining good eating quality for a long period of time. Although slightly less sweet than Supersweet, the kernels are more tender, with a creamy texture missing from most Supersweet hybrids.
Sugary Enhanced varieties, like all sweet corn, come in a variety of maturity times, so a single planting of several varieties – such as ‘Trinity,’ ‘Precious Gem,’ ‘Lancelot,’ and ‘Summer Delicious,’ can give home gardeners an ongoing harvest for 4-5 weeks. It’s important to remember that most SE varieties make up half their sugar content in the last couple days of maturity, and often look ready before their flavor has hit its peak. If you test for flavor and the taste doesn’t blow your mind, the National Gardening Association warns, just sit back and wait a few days; it’s sure to get better.
Supersweets were thought to be the corn of the future when they hit the market, but Sugary Enhanced varieties turned out to be the more enduring success story. Supersweets got most of the spotlight, however, because they represented the breakthrough of their time in corn breeding. Sugary Enhanced struck a more practical balance in growing needs, shelf-life, and long-lasting, yet more old-fashioned-flavored sweetness.
Corn breeding has certainly made great strides since the 1950s, but Warren Schultz of the National Gardening Association writes nostalgically of the changes corn has seen over the past century, “Maybe it’s the ephemeral sweetness and the urgent need for freshness that defines corn. By tempering that, breeders may have removed its very cornness.”
With cross after cross, in endless seeking of those few traits we crave, may we never lose that fundamental memory of corn’s essence – fresh from the late summer farm stand or the garden.
–Genevieve Slocum is a Contributor to The Free George.
The Free George is the online magazine and visitors’ guide of Upstate NY, covering things from Albany to Lake Placid, including Saratoga, the Lake George region and the Adirondacks. Check out our new City Blogs section for our extended coverage areas as well.
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