Environmental & other issues
The coleoptile is the embryonic leaf that penetrates the soil so the seedling can reach the surface. Deep planting, deeper than the coleoptile's ability to elongate, can slow emergence or cause stand establishment problems. Varieties differ in their coleoptile lengths. Semi-dwarf varieties tend to have greater problems because they have shorter coleoptiles than taller varieties. Early planting in hot soils reduces the coleoptile's ability to elongate.