Corn Development Stages- R3 (Milk)
After 18 – 20 days from R1 stage, the kernels become mostly yellow and contain “milky” white fluid (i.e. starch accumulates). However, moisture content of kernel at R3 is around 80%.
Starch accumulation continuously occurs in the endosperm, and cell division of endosperm is almost complete, indicating that growth of kernels since R3 would be mostly due to cell expansion and starch accumulation rather than cell division of endosperm. It also means that potential kernel size would be finally determined at this stage. Severe environmental stresses would be still able to abort kernels, even though not as easily as at the R2 stage. However, the effect of environmental stresses in R3 would be more relevant to shrinking kernels (due to moisture≈ 80%) rather than aborting kernels.
Also, severe environmental stresses at this stage would cause remobilization of assimilates from stalks to kernels, eventually loading. Since R1, grain filling will take ‘primary’ priority over other plant functions, and starch accumulation will actively occur around R3. Corn plants will remobilize sugars from the stalks to growing kernels to fill the kernels, and therefore stalks might be weakened due to reduced carbohydrate content.
Some articles indicate that nutrient deficiencies (or leaf diseases) which reduce green leaf tissue (=less photosynthetic activity) can result remobilization of assimilates from stalks to kernels. Drought stress during grain fill can result in remobilization to support grain filling. Stalks weakened by remobilization would result in lodge.
However, I assume that remobilization would not be the main sources to fill growing kernels during grain filling. In wheat, grain weight is usually sink-limited during grain filling, indicating assimilates are surplus during grain filling (Case I in the below figure A). When severe environmental stresses occur, growing grains will use reserved carbohydrate in stem (=remobilization) up to 50% (Case III in the below figure A).
Therefore, in wheat, it hardly becomes source-limited during grain filling. When assimilate availability increases, the change of grain weight would be minor, showing all data points lie on 1:1 ratio line (Figure B).
However, corn is slightly controversy to clearly state it’s sink- or source-limited. Borrás et al, (2004) indicates that kernel weight shows some degree of source-limited as grain weight is changed according to the change of assimilate availability.
However, other studies also indicate that kernel weight show sink-limited during grain filling.
I suggest two methods to elucidate whether kernel weight in corn is source- or sink-limited.
First, we might be able to figure out through the amount of water-soluble carbohydrates (WSCs) in stalk.
We would be able to decrease assimilate availability during grain filling. For example, corn canopy is shaded to reduce photosynthetic activity during grain filling. Then the amount of water-soluble carbohydrates (WSCs) in stalk will be measured. If there are no differences in WSCs between control and shading treatment, we might assume that assimilates are not the critical factor to growing kernels as assimilates seems surplus during grain filling. However, only remobilization would not be able to clearly support it’s sink- or source- limited. In wheat, up to 50%, remobilization occurs, but it would be still considered as sink-limited.
Second, we can manipulate source or sink strength around R1 – R2 stage.
When we increase sink strength (i.e. de-graining, above figure), the change of kernel weight is minor, we might suggest that kernel is sink-limited during grain filling, indicating that potential capacity of kernel would be more important to determine final kernel weight rather than resources during grain filling.
Reference
- Determining Corn Growth Stages – Bayer Crop Science
- Grain Fill Stages in Corn – Purdue University Extension
- Stalk Cannibalization in Corn – Bayer Crop Science
- Effects of Severe Stress During Grain Filling in Corn – Purdue University Extension
- Borrás, Lucas, Gustavo A. Slafer, and Marıa E. Otegui. “Seed dry weight response to source–sink manipulations in wheat, maize and soybean: a quantitative reappraisal.” Field Crops Research 86.2-3 (2004): 131-146.
- Ordóñez, R.A., Savin, R., Cossani Rial, C.M. and Slafer, G.A., 2018. Maize grain weight sensitivity to source-sink manipulations under a wide range of field conditions. Crop Science, 2018, vol 58, p. 1-16.