Storage of seed requires very slow metabolic activity, enough to keep membranes, but not enough to cause premature death of the embryo. Low seed moisture is mostly responsible for maintaining life in this semi-dormant condition. After planting, however, we want the seed to imbibe enough water to stimulate more activity. This process of imbibition was addressed in Corn Journal blog of 4/4/2017.
As the corn kernel is developing after pollination, embryo shoot and root cells are formed in a way that could quickly germinate with the moisture present in the tissue. A temporary dormancy prevents this from happening. Removal of the milky endosperm from the embryo as early as 10 days after pollination will allow the growth of root and shoots. Abscisic acid (ABA) in the endosperm appears to be the hormone involved in avoiding germination in seed before the full development. At least 10 single gene mutants are known to overcome dormancy in developing maize seeds, resulting in germination while on the ear (vipipary). Drying of the kernels initially by displacement with starch formation and then by air inactivates the dormancy.
Imbibition is the movement of water into the seed. It is a physical phenomenon, independent of the germination quality of the seed and of temperature. Movement of water into the seed is relatively fast, most occurring within a few hours. There is some evidence that slowing the imbibition by some seed treatments reduces the harm to membranes by giving them more time to repair. Rehydration of cellular tissue and chemicals cause swelling. Membranes, shrunken by drying, strengthen and activate but some solutes escape before damage from the drying is repaired. As the mitochondria are activated, ribosomes begin producing proteins needed for more cell growth, starches from endosperm are digested to form glucose molecules to be transported to the mitochondria in embryo cells. Oxygen uptake into the seed increases rapidly during imbibition - an indication of the active respiration occurring in cells. Heat, moisture and oxygen availability influence the speed in which imbibition initiates the germination of the corn seed.
Humans, without knowing cellular physiology, selected for these traits for Zea mays. And we get the benefit.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.