Much of the attention we lavish on our indoor garden focuses on the above-ground portion of the plants. Healthy stems, leaves, flowers and fruits mean a good, productive crop and what goes on down in the root system often doesn’t get a thought, unless things start to go wrong.
The rhizosphere, or area immediately surrounding the roots, is an almost hidden part of the plant whose main functions include anchoring the plant and uptake of nutrients and water. However, roots are far more dynamic and complex than this and there is a whole sub-surface community affected by what roots excrete that plays a role in plant growth and disease resistance.
Root systems excrete a huge range of compounds into the surrounding growing medium or, in the case of NFT, raft or aeroponic systems, into the recirculating nutrient solution. The quantity and type of these root exudate compounds depends on a number of factors including plant age, species, health of the plant, exposure to certain stresses, population of microbes surrounding the roots, and presence and proximity to another plant’s root system, as well as unknown factors yet to be discovered.
What many underestimate is the actual volume of root exudates that are secreted, as it’s not possible to see or judge what may be occurring in the rhizosphere. The vast array of compounds plant roots excrete can account for between 5 and 21% of the photosynthetically fixed carbon, which is a significant cost to the plant.
For that reason, the plant must obtain some benefit from secreting compounds into its rhizosphere and this is something researchers are investigating. These root exudates consist of organic compounds such as amino acids, organic acids, sugars, a wide range of carbohydrates, phenolics, lignins, fatty acids, sterols, enzymes, mucilage and proteins, and also released ions and inorganic acids.
Root Exudates and Soilless Culture
In the early days of hydroponics there was concern that the organic matter, dead root cells and compounds released by roots would accumulate in the limited root zone and nutrient solution to the point where it promoted disease and retarded growth.
It was quickly proven that this isn’t the case, and we have come to understand in more detail how the dynamics of a restricted root zone works. Methods designed to disinfect and remove organic compounds or frequent nutrient solution changes were sometimes used to control the buildup of these exudates, but studies have shown this is not necessarily beneficial.
When microbial populations are permitted to develop in healthy hydroponic systems, they break down and metabolize carbon compounds in the root zone or nutrient solution, so this sort of buildup doesn’t occur when all is working well.
Plant roots develop and react in hydroponic systems similar to soil-based systems. Diverse and beneficial microbe species are found in a wide range of different soilless systems, and certain exudates released by roots are used by the plant to attract and select certain micro-organisms in the rhizopshere.
These microbes can then work, via different mechanisms, to influence plant health and growth. For example, root exudates act as signals that encourage and initiate a relationship with rhizobia and mycorrhizal fungi as well as rhizo-bacteria.
When plant roots sense an attack by pathogenic microbes, this triggers the release of certain exudates called phytoalexins or defence proteins and other unknown compounds in a process of underground chemical warfare.
What Can Go Wrong?
While many root exudate compounds create beneficial conditions in the rhizosphere by encouraging certain microbes or changing the physical or chemical conditions surrounding the roots, occasionally this process can go wrong.
Plant stressors, such as unfavorable growing conditions, high EC, low moisture levels, nutrient deficiency, lack of oxygen in the root zone and overcrowding, place pressure on the root system and promote the release of an increased amount of different types of root exudates.
The carbon compounds in these secretions can act as a food source for pathogens, tipping the balance in the rhizosphere from beneficial microbe populations to disease-causing fungi and bacteria.
Some plants, such as cucumbers, which produce large amounts of mucilage (polysaccharides and proteins) secreted from the rapidly growing outer layers of the root cap cells, may be particularly prone to this type of problem in systems that promote the buildup of organic compounds.
Studies found that root mucilage supports rapid growth of pythium root rot and serves as a food base from which the pathogen invaded the roots of cucumber plants. This was prevented when there were a sufficient number of other microbes present to break down the mucilage and prevent it from accumulating around the roots.
Researchers have determined that certain root exudates have the potential to be used in crop production as a defence against soil-borne disease pathogens, and that they also have the potential to boost growth and development by promoting relationships with beneficial microbes in the rhizosphere. In the future we may see a much different approach to root exudates: harnessing them for disease suppression as a way of boosting growth specific to hydroponic systems.