1. Ion exchange:
The exchange of ions (anions or cations) from within the plant cells to the ions of soil is called ion exchange. By this method, the plant cell can exchange the unwanted ions for the required ones. Ion exchange is of two types. Contact exchange and Carbonic acid exchange.
In contact exchange, the ions that oscillate around the root hair overlap with the ions that oscillate around the soil particle and there will be mutual exchange of ions. According to carbonic acid exchange theory CO2 released during respiration combines with water to form carbonic acid disassociates into H+ and HCO3 – in soil solution. These H+ icons may be exchanged for K+ ions held by the soil particles. Since the exchange of ions here is mediated by carbonic acid, it is called carbonic acid exchange theory.
2. Mass flow:
According to this theory, when water enters into the plant passively through the influence of transpiration, salts also get in. Kramer (1956) is one of the supporters of this theory.
Initially when there is difference in concentration gradient (when the concentration of salts inside is less than outside) salts enter the plants through the simple process of diffusion.
4. Donan equilibrium:
Normally the total amount of cations and anions present inside the cell are balanced with the ions outside and they would be in constant motion. But there are certain fixed ions in the plant cell which are immobile. In order to balance this more ions have to enter the plant cell to maintain the equilibrium. This is called Donan’s equilibrium.