Whereas movement of water in corn is mostly a passive action through the dead cells of the xylem that form a narrow tube within the vascular system, distribution of carbohydrates from the sources of photosynthesis to elsewhere in the plant requires living cells of the phloem tissue.
The sugar sucrose is formed from the glucose product of photosynthesis and becomes the sugar for distribution within the corn plant. Movement of carbs between cells can be simple diffusion through those small ‘holes’ in cell walls, the plasmadesmata, as the molecules move from a high concentration to a low concentration. It is a little more complicated with travel through membranes by osmosis, but the basic principle is the same. Water is involved because It is the solvent of the sugar. Greater concentration of water equals less concentration of sugar. Water molecules are also affected by the principles of movement from high to low concentration, setting up dynamics for what is called turgor pressure within each plant tissue.
As the sucrose molecules move into a ‘sink’ such as newly formed cells of a growing point more complex molecules such as starch and thus maintains the osmotic pressure for more movement of sucrose into the kernels. Other sinks, such as biologically active tissue of all living plant cells, consume the sucrose in cellular respiration and formation of essential amino acids and cell structures. These various sinks are not all in the same direction from the carbohydrate sources where the photosynthesis occurred and consequently flow among phloem cells may not be in the same direction.
Whereas water movement in xylem tissue of the vascular bundles of a corn plant is mostly upwards from the roots, movement of the products of photosynthesis is affected by concentrations of various sugars in the sinks, allowing bi-directional flows.
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.