In many cases herbicide injury is assumed to be the culprit, but as we will discuss later, leafing underground from herbicide has distinctly different symptoms. Crusting is another common cause of leafing underground. Crusts can restrict the movement of the coleoptile, but the five-leaf miniature seedling inside the coleoptile can continue to grow and expand. This causes the side of the coleoptile to rupture, allowing the leaves to exit while underground. When the coleoptile ruptures on the side, you can generally rule out premature light interception as the problem.
Cold conditions can greatly aggravate a crusting problem, causing even more leafing underground.
Some herbicides can cause corn to leaf out underground. Both cold and crusting can aggravate this situation by delaying emergence. The longer the shoot is under the soil surface, the more herbicide the coleoptile will absorb, weakening it. Whenever conditions delay emergence of a germinated seedling, physiological changes begin to occur within the seedling. Compounds which are toxic to the seedling can build to levels that will cause the death of the seedling.