Roots — so often out of sight and out of mind — have four essential functions. They anchor a plant in the ground, they uptake water and nutrients, they store excess food, and they synthesise some phytohormones (plant hormones) and other compounds.
The rhizosphere is the narrow 1–2 mm region that surrounds each and every root, in which plant secretions are found and in which microbial life is richest. That microbial population is also known as the root microbiome.
The strength of anchorage a plant has in the soil is solely down to its root structure, or the root morphology. This morphology is a combination of genetics and environment, and depends on the depth of rooting, the degree of root branching, the number of root hairs and tips, and ultimately the soil the roots grow in and the nutrients, water and oxygen available to those roots.
Genetics determines root depth and branching potential, while the environment influences how that potential is expressed.
Grasses have a fibrous system of many thin roots that grip the upper soil layer tightly, and these thin multi-branched roots readily penetrate the narrowest channels within the soil to extend their reach. While some tree species have a genetically-determined shallow but dense root system, many other species are genetically able to grow a less dense root system of thicker, stronger roots better able to penetrate deeper.
The soil environment has the final say in a root morphology — any species, even grass, will be compromised when energy needed for growth is diverted into a root system which has to push through hard, dry claypans or rock. Similarly, friable soils will still lead to stunted growth if oxygen, water and nutrients are in short supply. (As in deserts.)
Water and Nutrient Uptake
Roots are accessing water and nutrients as they grow out and stabilise the plant. Plant nutrients are only available to plants when dissolved in the soil water — this combination of water and nutrients is called the soil solution. Nutrients reach roots by one of two ways: mass-flow and diffusion.
Mass-flow is where nutrients are carried by water that roots uptake. This is a passive process, and the amount that enters the plant is dependent on the nutrient concentration in the water, how much of that water flows to the roots, and how much water is subsequently uptaken by the roots. Concentrations of these nutrients around the roots may further increase, decrease, or remain the same depending on a balance between supply rate (water flow) to the roots and rate of uptake.
Diffusion is the movement of a nutrient (or any molecule or atom) from a region of high concentration to one of lower concentration through random movement. Diffusion can be towards a root if this is the area of lower concentration, or away from a root if this is the area of higher concentration. However, plants can create a ’sink’ by taking in high amounts of a nutrient, which depletes the surrounding region and draws more nutrients inwards through diffusion.
Roots further develop root hairs, tiny extensions which increase a root’s surface area significantly, just as the microvilli in animal guts do. This increased surface area of both roots and guts enables far more water and nutrients to be absorbed.
Photosynthesis is the harnessing of light energy to produce the carbohydrates that are needed to fuel growth and development. Plants produce their own food, in other words. (And another word for this is autotroph, or ’self-nourishment’.) These carbohydrates, called photosynthates, are either used immediately or are stored for future use in the main storage organs: stems and roots.
Synthesis of Hormones and Other Compounds
Roots are continually growing and it makes sense that hormones needed for that growth are produced where needed. Growth hormones produced by roots include auxins, cytokinins and giberellins.
Roots also produce other organic compounds known as exudates. These are typically polysaccharides, sugars, amino acids, and organic acids excreted into the rhizosphere and which attract microbes or otherwise make some nutrients available.
The rhizosphere is an ecological system in its own right, often involving plant-microbe interactions beneficial to both. Microbes feed on the root exudates in this region, and the breakdown products produced in turn benefit the plants in other ways.
One reason many plants suffer from transplantation is because their rhizosphere has been disrupted and time is needed to reestablish it. (Another reason is due to the mechanical action of physical removal destroying the numerous, tiny, delicate root hairs.)
Plants, in feeding soil microbes with root exudates, naturally draw these populations closer to them. Any microbes here which are involved in the nitrogen cycle thus produce ammonium and nitrates closer to the roots. This in turn can alter the immediate pH and influence the uptake of other nutrients.
Plant exudates also feed mycorrhizae (plural form of mycorrhiza, ‘root fungus’). These fungi have long, exploring hyphae which act as an extension of a plant’s roots, and provide the plant with phosphorus, water and other nutrients in return.
Some exudates directly help the plant without the need for microbes. Availability of phosphate increases upon the release of malic and citric acids which lower surrounding pH and release phosphate adsorbed to organic matter. Release of iron-carrying phytosiderophores increases the availability of iron by chelating with iron and moving to the roots via diffusion.