Roots must breathe oxygen. There is a limit to the amount of dissolved gasses that can be contained in water. Aquatic plant roots do not need direct access to oxygen in the water or medium, because these plants are adapted to transport oxygen throughout its physiological system.
The rhizosphere is the narrow region around the root that is directly influenced by root secretions and associated microorganism. The rhizosphere contains many bacteria that feed on sloughed-off plant cells, termed rhizodeposition, and the proteins and sugars released by roots. Protozoa and nematodes that graze on bacteria are also more abundant in the rhizosphere. Thus, much of the nutrient cycling and disease suppression needed by plants occurs immediately adjacent to roots.
Secretions and endophytes
Plants secrete many compounds into the rhizosphere which serve different functions. Strigolactone, secreted and detected by mycorhizal fungi, stimulate the germination of spores and initiate changes in the mycorhiza that allow it to colonize the root. Symbiotic Nitrogen-fixing bacteria, such as the Rhizobium species, detect an unknown compound secreted by the roots of leguminous plants and then produce nod factors which signal to the plant that they are present and will lead to the formation of root nodules, in which the bacterium, sustained by nutrients from the plant, converts nitrogen gas to a form that can be used by the plant. Non-symbiotic (or "free-living") nitrogen-fixing bacteria may reside in the rhizosphere just outside the roots of certain plants (including many grasses), and similarly "fix" nitrogen gas in the nutrient-rich plant rhizosphere. Even though these organisms are thought to be only loosely associated with plants they inhabit, they may respond very strongly to the status of the plants. For example, nitrogen-fixing bacteria in the rhizosphere of the rice plant exhibit diurnal cycles that mimic plant behavior, and tend to supply more fixed nitrogen during growth stages when the plant exhibits a high demand for nitrogen.
- Template:Cite DOI
- "Microbial Health of the Rhizosphere". http://uwstudentweb.uwyo.edu/T/Twhite/. Retrieved 5 May 2006. [dead link]
- "The Soil Food Web". USDA-NRCS. http://soils.usda.gov/sqi/concepts/soil_biology/soil_food_web.html. Retrieved 3 July 2006.
- Sims GK, Dunigan EP (1984). "Diurnal and seasonal variations in nitrogenase activity (C2H2 reduction) of rice roots". Soil Biology and Biochemistry 16 (1): 15–18. doi:10.1016/0038-0717(84)90118-4.
- "The Soil Habitat". University of Western Australia. http://www.soilhealth.segs.uwa.edu.au/components/habitat. Retrieved 3 July 2006.
- Digging in the Dirt: Is the Study of the Rhizosphere Ripe for a Systems Biology Approach? - A review from the Science Creative Quarterly (retrieved 4 December 2006)