At about the V5 (5 visible leaf collars) the growing point differentiates into a tassel. It is all cell growth now. Cell elongation in the stem cells as well in the leaf cells is greatly affected by water pressure within the cells. Warm wet conditions consequently result in taller plants and larger leaves. Kernel row number is set by V6 but the number of kernel ovules per ear is affected by the water pressure. Dry conditions resulting in few kernels and smaller tassels. Drought also can reduce the opening of the stomata and consequently less CO2 intake and reduced photosynthesis. Energy for the cell growth is provided by photosynthesis, the sugars guided to the various sinks in roots, leaf tips, shoots and tassel. The uppermost leaves get the direct sunlight allowing the highest photosynthetic rate. As the canopy closes in, the lowest leaves receive only a fraction of the light, sometimes not producing enough carbohydrate to meet respiratory needs for normal metabolism. These lowest leaves often become susceptible to relatively weak pathogens, develop yellow spots and drop off.
Meanwhile some pathogens, such as Sclerophthora macrospora, cause of crazy top of corn, that probably reached the growing point at the V1 or V2 stage is carried along, thriving on the movement of corn carbohydrates being moved to the tassel and ear shoots. Viruses, usually carried to the growing corn plant by insect vectors move through the phloem tissues in the direction of carbohydrate flow. Various resistance systems can ward them off but susceptible varieties can be damaged greatly.
By V6 stage the corn plant growing point is not putting out new cells, differentiation is over and now it is up to the dynamics of cell elongation to determine final plant development.
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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.