Mechanisms of nutrient uptake and absorption by the plant root system
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How nutrients reach the root: diffusion and water flow
The uptake of nutrients by the root reduces their concentration in the surrounding soil. This triggers the diffusion process: due to the thermal motion of molecules, elements move from zones of high concentration to the root surface. This gradient is maintained constantly as long as the root system continues to function.
The zone from which the root can passively "draw" elements through diffusion is very small. The distance over which this process extends ranges from 0.1 to 15 mm and depends on the diffusion rate of the specific ion. It is influenced by the soil type and the strength with which the soil adsorption complex holds the elements.
Although diffusion occurs continuously, the bulk of ions are transported to the roots along with the water flow. This means that without a sufficient level of soil moisture, plant nutrition is severely restricted, even if the reserves of elements in the solid phase are high.
Soil solution is the most accessible source of nutrition for plants, but it contains only a small fraction of the total element reserves. For normal nutrition, ions must constantly move from the solid soil phase into the solution. This process occurs at a high rate: for example, the rate at which phosphorus moves from the solid phase into the solution is 250 times higher than the rate at which the root absorbs it from that solution.
| Ion type by degree of soil adsorption | Ratio of ions in the solution to those on the particle surface (concentration gradient) |
|---|---|
| Non-adsorbed by soil | Up to 1 |
| Strongly adsorbed by soil | 10-4 |
Root "free space": the first barrier and nutrient reservoir
The uptake of mineral substances involves several stages. First, nutrients enter the cell walls and intercellular spaces of root tissues along with water or via diffusion — the so-called "free space." It occupies about 10% of the total root volume, is filled with water, and consists of two functional zones.
- Water space. Substances can migrate freely from here back into the soil, which is why the concentration of ions here and in the external solution is the same.
- Donnan space. Here, ions are adsorbed through exchange. Only ions from the external solution with the same charge sign and in an equivalent amount can displace them deeper into the root.
Exchange adsorption in the Donnan space is non-specific. Ions with a higher charge and lower hydration (according to the lyotropic series) are bound faster and in larger quantities. Due to the negative charge of cell walls, created by proteins and carboxyl groups of pectin substances (polygalacturonic acids), primary concentration of cations occurs here in the space near the plasmalemma.
The accumulation of elements in the free space of cell walls is a preparatory stage. It creates a kind of reservoir in the root tissues, from which the living part of the cell then selectively absorbs the necessary elements. The movement of ions into this reservoir occurs passively — via the convective flow of water and through diffusion.
- Ion diffusion distance — 0.1–15 mm
- Root free space volume — 10%
- Phosphorus release rate — 250 times higher than uptake
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