Classical physicochemical theories of plant nutrient uptake

To effectively manage plant nutrition, it is necessary to understand the mechanisms by which elements from the soil solution penetrate the root. The physicochemical properties of plant cell membranes determine how quickly and in what volume the applied fertilizer is absorbed. Based on many years of research, scientists have formulated three classical theories that explain this process.

Physicochemical mechanisms of nutrient absorption

According to the diffusion-osmotic theory, proposed in 1837, the cell functions as a simple osmotic system. Water and salts dissolved in it penetrate through porous membranes due to the difference in concentrations between the external solution and the cell sap. It is assumed that water and nutrients enter the roots simultaneously, and the roots themselves play a passive role.

The lipoid theory, substantiated in 1899, is based on the separate absorption of water and nutrients. The rate at which substances penetrate the cells of root hairs depends directly on their ability to dissolve in the lipids of the plasmalemma. Research has confirmed that the higher the affinity of a specific substance for fats and fat-like compounds, the faster it crosses the cell barrier.

The ultrafiltration theory likens the absorptive apparatus of roots to a fine sieve. The penetration of substances through the plasmalemma depends on the diameter of its pores and the size of the penetrating molecules. Small molecules pass through wide pores faster, while large ones are retained.

The structure of pores in the membrane is explained by three pathways:

• Permanent channels lined with protein. Proteins on the walls of the channels are capable of recognizing molecules, which ensures high selectivity of absorption.
• Aqueous gaps between lipoprotein complexes of the membrane. Through these gaps, substances penetrate the cell together with water.
• Temporary openings in the lipid layer. They arise randomly as a result of the thermal motion of the "tails" of phospholipid molecules.