Q.If the concentration of salt in the soil is too high and the plants may wilt even if the field is thoroughly irrigated. Explain.

Water moves passively from a region of higher (less negative) water potential to lower (more negative) water potential; if soil salt concentration is very high, the soil's water potential drops so low that water cannot enter the root cells by osmosis (or may even be drawn out of them), causing wilting despite adequate soil water — this is called physiological drought.

Water movement into root cells from the soil occurs by osmosis, driven by the difference in water potential (Psi_w) between the soil solution and the cell sap inside root cells. Normally, soil water (being relatively dilute) has a higher (less negative) water potential than the more concentrated cell sap inside root cells, so water flows passively INTO the roots.

When the soil contains a very high concentration of dissolved salts, the solute potential of the soil solution becomes strongly negative, which correspondingly lowers the overall water potential of the soil water to a very negative value — potentially even MORE negative than the water potential inside the root cells.

In that situation:

• The normal water potential gradient (soil -> root) is lost or even reversed.
• Water cannot move into the root cells by osmosis, since there is no longer a favourable (higher-to-lower) potential gradient in that direction.
• In severe cases, water may actually move OUT of the root cells into the surrounding saline soil solution (since the soil's water potential is now lower than the cell's), causing the root cells to lose turgor. …