Question

Why hygroscopic water is not available to plants
A) Because its ꝕw is greatly increased.
B) Because its DPD is greatly increased.
C) Because its ꝕw is strongly negative
D) Because its ꝕw is increased and DPD is decreased

Answer 1

Hint:
Water which is absorbed from the atmosphere and held between the soil particles is known as hygroscopic water. Since it is held between the soil particles, it is available in very fewer amounts.

Complete step by step answer:
Water potential (ꝕw) can be understood as the potential energy possessed by the water molecules per unit volume, it is measured concerning pure water. The ability or the tendency of water to move from one area to another due to any processes such as gravity, osmosis, and capillary action is determined by considering the water potential. The movement of water molecules is from a region possessing higher water potential to the one possessing lower water potential. The water moving in capillary action, due to surface tension has a high water potential.
For plants to absorb the water present in the ground, the molecules should possess a high water potential. If the water potential value of the soil is greater than the value in the roots, then water molecules get passed on from the soil to the roots. Similarly, the water potential in the root should be greater than the values in the stem and leaves, then the upward motion to various parts takes place.
Since the hygroscopic water molecules being tightly bound, it has a very low water potential. That is water potential is very highly negative. Thus it is very hard to transport it into the roots, making it difficult to be available to plants.
Considering the other options given above:
DPD or Diffusion Pressure Deficit is the difference in pressure between pure water and a solution. The value is zero for a completely turgid cell and maximum for the cells which are flaccid or plasmolyzed cell. It has no role in assessing the absorption of hygroscopic water into the plants. It is important in the process of sap ascend in vascular tissues.
Thus by considering all the options, the most appropriate answer is C, the water potential is strongly negative.

Note:
The water and nutrients are absorbed by the plants, through the vascular tissues known as xylem. The absorption is due to the capillary action, where the water molecules present in the soil are attracted to it.