I had never heard of root exudates before creating this website, but in fact their production is so important to plants that they “invest up to 20–40% of their photosynthetically fixed C”ref in this process.
Root exudates are basically substances created by root cells and sent out into the nearby environment – known as the rhizosphere. These can be waste products which diffuse across the cell wall, or manufactured compounds which serve a specific purpose in response to the environment.ref
There are many different exudates produced by plants, including carbohydrates (sugars), organic acids (such as acetic, citric or malic acid), amino acids, flavonols (molecules which can have protective effects on cells), enzymes (such as amylase, which helps to digest carbohydrates), plant growth regulators (substances which stimulate cell growth, such as auxins), phenolic acids (which have anti-oxidant properties), flavonoids, terpenoids, tannins, steroids and an assortment of other substancesref1,ref2.
The roles they play are just as diverse, including:
- producing food in the form of carbon metabolites to support beneficial bacteria (such as nitrogen fixing bacteria), fungi, nemotodes and protozoaref1,ref2, which in turn assist with nutrient uptake and the production of their own compounds such as phytohormones
- producing phytotoxins (plant poisons) such as terpenes (also found in conifer sap) to repel pathogenic microbes, invertebrate herbivores and parasitic plantsref1,ref2
- changing the pH of the surrounding soilref
- detecting ‘kin’ (related plants) and avoiding competing plants which are not relatedref
- changing the soil chemistry to allow for better nutrient uptake – for example exuding chelating substances which allow for better uptake of metallic micronutrients, or organic acids which enable better phosphorus uptakeref
So it seems that the roots of a tree can act like a sort of pharmacy, creating compounds that protect and nurture the tree and its beneficial partners via the rhizosphere. In my opinion what this means for bonsai is that firstly you don’t want to damage the tree’s ability to produce exudates, and secondly you don’t want to remove too much of the soil from a healthy root ball.
You can damage the tree’s ability to produce exudates by underwatering/drought – and this may not be recoverableref, but also by failing to provide all of the nutrients and micronutrients needed for healthy growth (by not fertilising enough).
What’s in the bonsai pot is clearly more than just roots and soil – it’s an entire ecosystem delicately managed by the tree itself. So perhaps being less heavy-handed during repotting would be a good idea – replacing a good amount of the soil back into the pot along with its microbes, exudates and adjusted chemical makeup.