Sweet corn breeders have their work cut out for them. Of the thousands
of crosses they make in the field each year, only a few ever result in
a commercial variety. Not only that, there are more and more genes being
used to provide corn with different degrees of sweetness, flavor, and eating
Many growers are already familiar with the three main genetic groups,
or genotypes, of sweet corn: sugary (SU), sugar enhanced (SE) and supersweet
or shrunken (SH2). What’s new is the development of varieties that contain
different combinations of these genes.
At the recent New England Vegetable and Berry Conference in Manchester,
New Hampshire, Blake Myers of Siegers Seed Company gave an excellent overview
of the genetics of sweet corn, and what that means for growers. I’ve based
this article on the information he provided.
Let’s start at the beginning. For the first 4,000 years or so that
people grew corn, there wasn’t any ‘sweet corn.’ Instead, field corn (dent
corn) was harvested when it was still immature, which wasn’t quite as bad
as eating ‘cow corn.’ Then, along came the ‘SU’ sugary gene that provided
more sugar than field corn. Sweet corn was first documented in the 1770’s
in Pennsylvania, although native Americans had probably cultivated it prior
to that. Sweet corn made its first appearance in seed catalogs in the 1820’s.
Sweet corn had primarily white kernels until 1902 when Golden Bantam,
a yellow variety, was developed. Then, crosses were made that resulted
in bi-color varieties. While the color of sweet corn kernels is important
with regard to consumer preference, it doe not have anything to do with
SU varieties have modest amounts of sugar in their kernels, and they
tend to have decent corn flavor. But, their conversion of sugar to starch
is rapid, so they have a narrow harvest window before flavor deteriorates.
They tend to good plant vigor. To avoid cross-pollination and poor quality,
SU varieties must be isolated from field corn and popcorn. They must also
be isolated from the supersweet group.
In the mid-1900’s, some very important new sweet corn genes were
developed: ‘SH2’ shrunken gene and the ‘SE’ sugary enhanced gene.
The SH2 gene greatly boosts the amount of sugar in the endosperm
(the part of the kernel where energy is stored for later use by the developing
embryo), and that’s why SH2 varieties are often called ‘supersweets’. The
seeds of these varieties have less starch in them, so they appear shriveled
and weigh less than other sweet corn seeds. (Because of the shriveled characteristic
of the seed, they are sometimes referred to as shrunken 2 or just SH2.)
They can be a challenge to grow, especially in the north, because they
need warmer soil for good germination. The SH2 gene slows the conversion
of simple sugars to starch, so supersweet varieties have a much wider harvest
window than other genotypes. SH2 varieties must be isolated or they will
become tough and starchy, and they will also ruin SU or SE varieties grown
next to them, if the pollen happens to cross.
The SE gene gives varieties improved eating quality over SU varieties
by slightly increasing the level and changing the types of sugars in the
kernels. SE varieties also have a very tender pericarp (the outer layer
of the kernel). The SE gene does not slow the conversion of sugars to starch
but the harvest window is slightly longer than with SU varieties because
of elevated sugar levels. SE varieties do not require isolation from SU
varieties, but they do require isolation from SH2 and field or popcorn.
Some SE varieties are sweeter than others, depending on whether one
or both of their parents were sugary enhanced. Varieties that get the SE
gene from both their parents are homozygous for that trait, or ‘double
SE,’ and all of their kernels have the SE characteristics. Varieties that
have just one SE parent are heterozygous, or ‘single SE,’ develop 25% of
their kernels with SE traits. Typically a homozygous SE will have better
eating quality than a heterozygous SE. For example, Tablesweet™ varieties
such as Parfait are homozygous SE with high sugar levels and slightly better
Within the SU and SE genotypes, modifier genes are responsible for
differences in flavor. A variety can be homozygous or heterozygous for
a modifier gene, just like the SE gene. Modifier genes are what allow breeders
to develop so many different varieties. They are why it’s possible to have
an 80-day, bi-color homozygous SE corn that has better eating quality that
another variety that shares those same features.
In recent years new varieties have been developed that have different
combinations of the three major genes (and their modifier genes) ‘stacked’
together. The goal of breeders is to put together the best characteristics
from SU, SE and SH2 genes.
‘Synergistic’ varieties are heterozygous or homozygous SE, with the
addition of some SH2 kernels. Sweet Breeds™ varieties such as Sweet Chorus
and Sweet Rhythm have about 25% SH2 kernels, 25% SE kernels and 50% SU
kernels. The seed has SU vigor but higher sugars. Triplesweet™ varieties
such as Providence have 100% SE kernels, 25% of which also have the SH2
trait. These are like double SE varieties in vigor, but sweeter.
‘Augmented SH2’ varieties are supersweet types that also have the
SE trait in all their kernels. Examples include Gourmet Sweet™, Multisweet™
and Xtra-Tender Brand™. These are high in sugars, slow to convert to starch,
and tender like an SE. Mirai has all three genotype traits in its kernels:
SH2, SE and SU.
In terms of isolation requirements, these newer varieties generally
behave as either a SU or SH2, depending on the combination of genes. The
table below shows which varieties need to be isolated from one another.
SU AND SE ISOLATION GROUP SH2 GROUP
Normal sugary (SU)
Sugar enhanced (SE)
Since there are so many sweet corn choices, it makes sense for growers
to consult with seed company representatives for help in deciding what
varieties will work best for the conditions on their farm, and their customer’s