Coleoptile Length and Planting Depths

A greater coleoptile length enables wheat seedlings to emerge better from deeper planting.

by Jochum Wiersma, UM extension small grains specialist,
Northwest Research and Outreach Center
wiers002@umn.edu

The optimum planting depth for wheat and barley is around 1½ inches. This depth is ideal because:

1. It ensures that the crown roots will firmly anchor the plant, which in turn will help reduce root lodging.
2. It ensures that the young seedling can quickly emerge from the soil, while at the same time reducing the risk of moisture stress.

Seeding shallow will speed up emergence, but poses the risk that water becomes limiting, as the top surface layer can dry out before the root system of the young seedling is well established, and able to take up moisture from the soil below.  Conversely, deeper seeding increases the risk that the young seedling will not reach the soil surface.  The increase in depth already increases emergence time because:

1. The extra distance the coleoptile needs to overcome to reach the soil surface.
2. Growing degree days do not accumulate as fast at the greater depth.

When a wheat or barley seed is placed at greater depth, say for example 3 inches, the germinating seed will sense how deep it is seeded. If it senses it is planted deeper than 1 – 1 ½ inches, it will first extend the subcrown internode to a depth of 1 – 1 ½ inches.  This subcrown internode moves the whole crown (growing point from which the crown roots and all above ground stems originate) up toward the soil surface.  The need to extend the subcrown internode further delays emergence. The length to which the subcrown internode can extend is not infinite; the subcrown internode will extend to its maximum length, and the coleoptile will have to attempt to reach the soil surface from that depth the crown reached.

Both the length of the coleoptile and subcrown internode are genetically determined and thus different for each variety.  As a rule of thumb, you can assume that shorter stature varieties have shorter coleoptiles (and subcrown internodes). 

Tables 1 and 2 give an overview of average coleoptile length of the current HRSW and barley varieties, respectively. The coleoptile lengths were measured using a protocol described by Dr. Scott Haley, Colorado State University’s winter wheat breeder, in which 25 seeds were rolled into germination paper and vertically placed in glass beakers with adequate water to keep the germination paper wet but not have the seeds standing in water. The beakers were placed in a dark growth chamber for 4 days at ~ 40ºF and for 16 days at ~ 60ºF.  After the twenty days, the lengths of coleoptiles of the normal looking seedlings were measured from the crown to the tip of the coleoptile.  This test was replicated three times and the average length is reported in the tables. 

Table 1. Average coleoptile length of common HRSW varieties

Variety Coleoptile Length (inches) Marshall 1.8 Kelby 2.0 Hat Trick 2.3 Ulen 2.3 Trooper 2.3 Knudson 2.5 Ada 2.5 Freyr 2.6 Oxen 2.6 Steele-ND 2.6 Howard 2.7 Alsen 2.7 RB07 2.7 Rush 2.8 Walworth 2.8 Briggs 3.0 Banton 3.0 Fire-Ball 3.1 Bakker Gold 3.1 Granite 3.3 Bigg Red 3.3 Oklee 3.5 Glenn 3.5 Polaris 3.5 Reeder 3.6 Traverse 3.7 Granger 3.8

Table 2. Average coleoptile length of common six row malting barley varieties Variety Coleoptile Length (inches) Robust 2.1 Drummond 2.2 Stellar 2.3 Tradition 2.3 Lacey 2.4 Legacy 2.5

Why care about coleoptile lengths? Well, this spring may shape up to be dry, and you may feel the need to plant into moisture. A greater coleoptile length enables wheat seedlings to emerge better from deeper planting.

The information in Tables 1 and 2 shed some light on which varieties will tolerate planting deeper than the recommended 1½ inch depth.

Image: Nelson, J.E., K.D. Kephart, A. Bauer, and J.E. Conner. 1988. Growth Staging of Wheat, Barley, and Wild Oats. Misc. Bulletin 4387. Montana State University. Bozeman, MT.

The push pin at left indicates the coleoptile of a hard white wheat seed. The coleoptile is the sheath that encloses the first main shoot leaf and provides protection as it emerges from the soil, and as it does , its growth stops and the first true leaf pushes through the tip. To the right of the seed is the radical (first seminal root) and two other seminal roots, which form the basis of the plant’s root system.
Photo: Joel K Ransom, NDSU