I harvested some corn plants at R1 stage 12 days ago. Around R1 stage, final kernel number would be determined. In corn, kernel row number (girth) and number per row (length) in an ear are main yield components. Girth determination of the uppermost ear (the ear harvested) begins shortly after the ear shoot is initiated (V5 to V6) and is thought to be complete by growth stage V12, while the potential number of kernels per row (length) is finally determined ca. one week before silk emergence from the husk. Previous studies indicates that around 40 days after emergence, the development of spikelet primordia on the uppermost maize ear (i.e. ovules) get started, and final ovule number is established several days before silk appearance (reviewed by Nasielski et al., 2019).

It would be different by temperature, but based on my note, ca. 40 days after emergence would be in V8-V9 stage. Since this stage, N demand in corn increases, and the development of spikelet primordia would be strongly affected by nitrogen conditions (in cereals, grain number is strongly source-limited). Therefore, in the stage, nitrogen stress will reduce final kernel number. This would be due to the below process.

reduced assimilate supply → reduced crop growth → less biomass → less dry weight partitioning to the developing ear → inhibited development of spikelet primordia (ovules) → less final kernel number.

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Reading Matter
Corn Development Stages- R1 (Silking)

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It seems that potential kernel number is determined before R1 stage. However, even though potential kernel number is determined before R1 stage, if pollination is not successful, kernels will be aborted. Therefore, the critical period to determine final kernel number (potential kernel number + successful pollination) would be in R1 stage.

Potential kernel weight

The new creation has come: The old has gone, the new is here. – 2 Corinthians 5:17

After silking stage, the period to determine kernel number has gone, and new period is here. It’s kernel weight. Approximately 12 days after silking, silks become dark, and will dry out rapidly over time.

In this stage, the developing kernels are white “blisters” (resemble a blister) on the cob and contain abundant clear fluid. Therefore, we call this stage R2 (Blister) stage. Usually, R2 (blister) stage occurs about 10 – 12 days after silking.

In R2 stage, each kernel develops an embryo (2n) and starch is beginning to accumulate in the endosperm (3n) by double fertilization. However, kernels still contain more than 85% moisture.

Therefore, abiotic stress (i.e. drought, heat) at this stage would reduce yield potential due to kernel abortion. In other words, if you want to drought/heat experiments on corn to elucidate how it affects kernel weight, R2 stage would be the good timing to impose such a stress.

Severe stress will cause kernel abortion from the tip of the ear downward. Some articles indicate kernel abortion will continue until the plant has has enough carbohydrates for the remaining kernels.

However, I doubt about this!! The abortion is really due to lack of resources? It might have been low weight potential itself?

In other words, does growing kernels are really affected by reduced resources? Does abiotic stress indirectly affects growing kernel by directly affecting resources (i.e. accelerated leaf senescence)? If so, I think that if we increase assimilate availability at abiotic stress conditions, kernel weight should be recovered. In this case, kernel weight would be source-limited.

If increased assimilate availability does not revert the reduction of kernel weight by abiotic stress, maybe abiotic stress directly affects growing kernels, indicating resources would be less relevant to growing kernels. In this case, kernel weight would be sink-limited.

So far, it’s not clearly elucidated whether corn is source or sink-limited during grain filling. I believe there are so many things we can study about that.

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Reference

Corn Growth Stages – Iowa State University Extension
Determining Corn Growth Stages – Bayer Crop Science
Effects of Severe Stress During Grain Filling in Corn – Purdue University Extension
Grain Fill Stages in Corn – Purdue University Extension
Staging Corn Growth – Pioneer
Nasielski, J. and Deen, B., 2019. Nitrogen applications made close to silking: Implications for yield formation in maize. Field Crops Research, 243, p.107621.