Plant & Soil Health: Is Hypocotyl Length Important?

Will soybean hypocotyl length become important when selecting varieties to place seed deeper?

The hypocotyl is that structure that lifts and pulls the cotyledons up through the soil surface. When soybean seeds germinate the hypocotyl elongates, pulling the two cotyledons (seed leaves) above the soil surface. The growing point is at the top of the hypocotyl where the two seed leaves are attached.

Corn has a similar structure called a mesocotyl and regardless of where you place the seed, at 1, 1.5, 2 or 2.5 inches, the growing point will be three quarters of an inch below the soil surface and the hypocotyl length will adjust accordingly.

Will soybeans compensate for depth as well by adjusting hypocotyl length? More producers still plant soybeans at 1 to 1.5 inches. But today some growers are planting soybeans deeper, 2 inches (almost as deep as corn), and succeeding.

Jim Beuerlein, retired soybean extension specialist at The Ohio State University wrote me an email several years ago stating, “Some of the (soybean) varieties we had 30 to 40 years ago could emerge from a depth of 4 inches or so, not because of vigor, but because they had long hypocotyl structures. Today, most current varieties have short hypocotyls and will not emerge if planted more than about 2 inches deep.”

He explained that seed germination and the depth seedlings can emerge from are affected by soil moisture, temperature and texture, and seed size. “Vigor and disease resistance are important, but the most important factor is the hypocotyl length of a variety when planting deeper.”

He explained that hypocotyl length isn’t really relevant until you plant deeper than 2 inches. Historically, growers have never wanted or needed to plant deeper than 2 inches in the Midwest. The shortening of a hypocotyl length could be the result of improving varieties and not thinking about selecting for longer hypocotyls.

But the mood is changing. Planting soybeans deeper means planting into more consistent soil conditions in terms of temperature, moisture and seed-to-soil contact, and away from surface residue and the risk of hair-pinning (seed placed in residue preventing seed-to soil-contact).

Seed companies have probably never intentionally bred for shorter hypocotyls and most probably prefer longer hypocotyls due to generally better emergence potential when the seeds are planted deep. So, if you are beginning to plant soybeans deeper than 1.5 inches and like the results, consider asking your seed dealer about hypocotyl length, because this is not a selection factor listed in seed catalogs.

Let us know in our forums how deep you plant soybeans and if you ever look at the hypocotyl length of the varieties you plant.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.