Uneven emergence

​Much of the USA corn growing areas are experiencing a wet, cool spring.  It is likely that corn seed already planted in these soils will experience temperature and excessive water stress and that some soils will become compacted, leading to not all seedlings emerging at the same time.  PSR's seed tests also detect some seed samples that have a biological problem in which not all the seed germinate at exactly the same time.
 
It is not clear why some individual seeds within a seed lot are slower to elongate the shoot and root, allowing the seedling to emerge from the soil.  One hypothesis is that the slower seedlings had deterioration of the membrane structure in embryo cells, possibly in the mitochondria. This is enhanced with imbibition swelling the membranes, resulting in tearing the tissue.  Membranes in mitochondria are the sites of cellular respiration in which sugar molecules are enzymatically changed into ATP molecules, the energy source for almost all cellular activity, including cell elongation.  Other cellular membranes are major locations for transport of components needed for cell elongation as well.
 
There are multiple cells in the corn embryo, each with many mitochondria.  Complete death of the seed occurs when too many membranes are destroyed for the seed to grow.  Sufficient amount of functioning cell organelles, however, can lead to repair of damaged membranes, eventually gaining enough momentum for cell elongation in the shoot and root tips.  This can result in eventual emergence from soil surface.  Heat energy and oxygen in soil are important to this process as well.
 
Seedlings emerging later than adjacent seedlings can struggle for the whole season, as they are shaded from light and at competitive disadvantage for minerals in the soil.  This can also result in plants with delayed flowering, possibly because of insufficient moisture to push out the female silk, missing the normal pollination.
 
Detection of these individual seeds with internal membrane damage is not easy and may not be detected with standard germination testing.  When planted in artificial soil mixes, most seed lots with high germination percentages emerge uniformly.  Those with non-emerging seedlings will also tend to have more late emerging seedlings as well.  Rarely one finds a seed sample in which the percent emergence is 95% but includes most at three leaves exposed while some only show the coleoptile. Even in these cases, there will be some with each level of development, showing only the ‘spike’, or one leaf, or two or three.  How to classify germination of these seed?  They are alive and they germinated but how will the delayed emergence of these plants detract from field performance of the seed lot?  
 
Uneven emergence in the field could have several causes.  Analysis of main factors is difficult.  Seed biology is one of those factors.