This post is the summary of the article, 'Ear Initiation & Size Determination in Corn' by prof. Bob Nielsen in Purdue University, and therefore all storytelling in this post is based on the article.

In a corn plant, kernel number of per ear (KN) is an important contributor to the grain yield potential. Therefore, developmental stage of corn ear is the critical period to determine KN. Environmental stress during ear development may limit the potential ear size, and therefore final grain yield potential. This is because KN is strongly source-limited at pre-anthesis, and resources are important to developing corn ear.

1) Ear Shoot Development

An axillary meristem forms at each node (the base of the node) and growing toward the top, but it would be less chance at 6 – 8 nodes.

The apical meristem (growing point) finishes to generate leaf primordia, and initiate the tassel primordium around the V5 or V6 stage.

When the tassel primordium is initiated, the uppermost (final) ear is also initiated. This final ear is normally located at the 12^th to 14^th node (corresponding to the 12^th to 14^th leaf).

Around V10 (ten visible leaf collars), if we remove leaves (and leaf sheaths) from stalks, we can see the 8 – 10 ear shoots.

The ear shoots at the lower nodes are longer than the ones at the upper nodes because the lower ones were generated earlier. Each axillary meristem starts to generate ‘husk leaves’ at the nodes.

As time goes by, the upper 1 – 2 ear shoot would have priority over all the lower shoots to ultimately become the final ears to be harvested. This might be due to that upper ears are close to leaves of the upper canopy, and therefore it would easily use the photosynthetic carbon assimilation from the upper leaves.

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2) Ear Shoot Development

Kernel row number (girth) and number per row (length) in an ear are two main yield components in corn. Girth determination of the uppermost ear begins shortly after the ear shoot is initiated (V5 to V6) and is thought to be complete by growth stage V12. Girth determination on an ear proceeds in an acropetal fashion (from base to tip).

Girth first initiate as “ridges” of cells that eventually differentiate into pairs of rows. Thus, girth on ears of corn is always even (unless some sort of stress disrupts the developmental process). Girth is determined strongly by genotypic effect rather than by environmental effect. It means that girth for any given hybrid will be quite similar from year to year, regardless of growing conditions. In other words, girth shows less plasticity (and therefore greater heritability) in response to environments.

The potential number of kernels per row (length) is finally determined ca. one week before silk emergence from the husk. Kernel number (ear length) is strongly affected by environmental stresses. It means that ear length will vary dramatically from year to year according to various growing conditions. Severe stress can greatly reduce potential kernel number per row, indicating that kernel number is strongly source-limited as mentioned earlier.

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Reference

https://www.agry.purdue.edu/ext/corn/news/articles.03/EarSize-0609.html