Although the movement of sugars to the various growing points of a corn plant requires energy from photosynthesis to move through the living cells of phloem tissue, water movement from roots to leaves through the non-living xylem tissue of the vascular bundle is mostly due to the physical character of water molecules.
Sugar solubility of water favors the initial movement of water into root hairs. This process of movement of water across root hair cell membranes is a physical phenomenon of osmosis, as water moves from a higher concentration outside the root hair to a lower concentration in the sugar (and other molecules) dissolved in water within the cells. This osmotic pressure also promotes water movement into the xylem tubes within the vascular bundles of the roots.
Water cohesiveness keeping the water molecules together along with the removal of water molecules from transpiration through leaf stomata, essentially pulls the water up the plant, carrying with it the minerals dissolved in the water.
Corn stems and leaves with multiple vascular bundles in the stem contribute to stability of water uptake and distribution throughout the leaves. The veins are parallel to each other in the corn leaves. At the base of the leaf, where it connects to the stem at the node, the system becomes much more complex. The vascular tissue goes horizontal with fusions between the individual veins. Also, the xylem ‘tubes’, (vessels) have end walls, forcing the water moving up from roots and stem through small pores that act as filters. The pores are sufficiently small to filter out any particles being carried upward with the water. Many bacteria and even some viruses are too large to pass through the pores. Each node of corn, even in the small seedling, has this complexity of the vascular tissue. Root vascular tissue connects with the stem vascular system at the first leaf node. Whereas an individual leaf may have up to 20 main veins, the node may have 100 horizontal vascular bundles and with fusion of vessels at the nodes. This redundancy protects the plants from a problem in single vascular bundle or one root branch from blocking transport of water and minerals to the leaves. Likewise, the movement of carbohydrates from leaves to roots gets distributed to all roots. Water soluble substances such as minerals and toxins can move freely up the plant with water through the xylem, but most fungal spores and bacteria are filtered out by the pores.
Movement of water into stem and leaf cells also is physical, water moving from higher water concentration in the xylem tubes through membranes of the living, metabolically- active cells, allowing direct utilization of water in photosynthesis and other activities. Cohesiveness allows more water to follow.
We are apprehensive that corn plants lose water through transpiration but also should appreciate that because of the loss of water through the stomata, not only is CO2 allowed in the plant, and oxygen escapes, the process allows uptake of water and transport of minerals also is occurring.
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.