Question

Hygroscopic water is not available to plants because
A) Ψ is greatly increased
B) DPD is greatly decreased
C) Ψ becomes strongly negative
D) Ψ is greatly increased and DPD is greatly decreased

Answer 1

Hint: 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. 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 plants, it has to have a high water potential. Water potential of the soil has to be greater than that of the roots, which has to be greater than the stem, then the leaves, 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 Ψ. 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 in the soil and is 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. (A) Cannot be correct because Ψ is not increased in hygroscopic water, and B and D talk about DND which is not relevant to water uptake.

Hence the correct answer is option ‘C’.

Note:Several factors affect water uptake by plant. They include water potential, gravity, solute concentrations, and matrix effects.