Inner parts of a corn stalk are composed of vascular bundles and large living cells called parenchyma cells. Movement of sugars produced by photosynthesis in leaf cells move to the phloem cells. Concentration of sugars causes water to move via osmosis into those phloem cells, putting pressure for movement of the soluble sugars through the small connections (plasmadesmata) between phloem cells. Digestion and respiration in the growing points of the roots and eventually the newly pollinated ear ovules, with the assistance of cytokinins, directs the pressure towards the root through the stalk.
Large pith cells adjacent to the phloem cells become storage locations for sugars as the plasmadesmata connect between the parenchyma cells. These glucose and sucrose sugars accumulate in the stalk pith tissue. This storage reaches its peak shortly after pollination when the direction of flow switches to the newly formed kernel embryos. Sugars in stalk pith cells become a reserve that allows a constant movement to developing kernels, despite short-term reduction in photosynthesis because of cloudy weather or even the more long-term damage from leaf disease or insects. Although some of the sugars are utilized in cellular metabolism of the pith cells, the excessive accumulation becomes essential to maintaining life in root tissues as well as warding off potential stalk pathogen invaders.
The 50 days of grain fill causes a drain on the sugars stored in the stalk parenchyma cells. Genetics and environment affect the corn plants ability to produce sufficient photosynthates to meet the demand for sugars to flow to the ear. The genetic complexity of this is huge. It must involve factors affecting photosynthetic rates, efficiency of movement of sugars, storage capacity of pith cells, size and structure of roots and size and number of kernel embryos.
Given the variable environments and multiple genetics for corn grain production we should not be surprised that no single corn hybrid is perfect every year in every field
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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.