Several factors can work together to result in corn leafing out prior to emergence.
Emergence. Visual signs of germination include swelling of the seed, elongation of the radical, then growth of the coleoptile.2 Roots grow down and the shoots (the coleptile and mesocotyl) grow up due to geotropism, which is plant growth in response to gravity (Figure 1). The coleoptile is a shield that protects the contained leaves as the shoot is pushed through the soil due to elongation of the mesocotyl, which is the white internode tissue between the seed and the coleoptile (Figure 1). When the coleoptile senses red wavelength light, hormones sent from the coleoptile to the mesocotyl are altered, halting growth of the mesocotyl.3 The coleoptile normally senses light approximately 3/4 inch below the surface, where the nodal roots form.
The processes of germination and emergence are highly dependent upon several types of plant hormones. One type thought to be instrumental in geotropism is auxins. Auxins are similar to the synthetic plant hormones in herbicides, such as 2,4-D, and cause cell elongation. Hormones can be greatly affected by temperature and other environmental conditions.
Leafing Out. If the coleoptile is damaged or mesocotyl has irregular growth prior to emergence, the leaves can break through the coleoptile (Figure 2). Without the protection from the intact coleoptile, it is very difficult for the leaves to penetrate the soil surface. There are often multiple factors that can contribute to problems with leafing out, including: chilling injury, soil compaction, soil crusting, planting depth, and saturated soil conditions.
Chilling injury can occur at different stages of germination and emergence. If kernels have adequate seed-to-soil moisture contact and imbibe cold water or experience cold temperatures in the 24 to 36 hours after planting, imbibitional chilling injury can occur, potentially resulting in failed germination and/or hindered growth of the radical or coleptile.4 Chilling injury during the emergence process can result in corkscrewed mesocotyls and/or leafing out below the soil surface.4 Environmental conditions that favor chilling injury include extended exposure to soil temperatures under 50º F, and/or large swings (25º to 30º F) in daily soil temperatures.3 Planting in dry soil conditions can result in inadequate seed-to-soil moisture contact within the seed furrow. These conditions can amplify the differences in emergence between seedlings. Planting deeper is often needed in dry soil conditions to reach soil moisture. However, planting deeper can increase leafing out underground as the length of time from germination to emergence is often greater, increasing the chances for coleoptile and/or mesocotyl damage.