Question

Why hygroscopic water is not available to plants
a. Because it's $\Psi_\omega$ is greatly increased
b. Because it's DPD is greatly decreased
c. Because it's $\Psi_\omega$ is strongly negative
d. Because it's $\Psi_\omega$ is increased and DPD is decreased

Answer 1

Hygroscopic water is tightly held water on soil particles surfaces as a result of adhesion; this water is essentially unavailable to vegetation. Water held by soil under specific relative humidity and temperature conditions (usually 98% relative humidity and 25°C). Water flows from regions of high water potential to regions of low water potential. Capillary water has a high water potential.

Complete answer:
Hygroscopic water is that which surrounds the soil particles and is held tightly to them as a thin film. It is not available to plants, because it has very low water potential (strongly negative). In other words it has very less water molecules or it is present as an extremely thin film. Water present in capillary spaces of soil is freely available to plants.

Other water present in soil is that which is present in capillary spaces, and gravitational water that flows down to the bed rock. For water to be available to the plants, it has to have a high water potential. Water potential of the soil has to be greater than of the roots, which has to be greater than the stem, then the leaver, and finally the water potential of transpiration. The problem with hygroscopic water is that it is a very thin film, and tightly bound to the soil, and therefore has a very low water potential or $\Psi$. This means that the water potential is highly negative, because of which it is not available to plants.

Capillary water is present in the spaces of soil and freely available for uptake by plants. On the other hand, gravitational water that flows down to the bedrock is not available unless pumped up to the surface for irrigation. Option (A) cannot be correct because $\Psi$ is not increased in hygroscopic water, and DND is not relevant to water uptake, so option B and D are not correct.

Hence, the correct answer is option (C).

Note: Hygroscopic water is absorbed from the atmosphere and the soil particles hold on them tightly. Several factors affect water uptake by plant. They include water potential, gravity, solute concentrations, and matrix effects.