Plant and Water Relations (Class XII)

In a strawberry leaf, the neat droplet at the tip of every tooth of a leaf edge is guttation fluid.

Plants growing at high altitudes exhibit xeromorphy to reduce transpiration. Xeromorphy is indepedent of availability of water in the soil. It depends mainly on low air humidity. It is characterised by thick cuticle, several layers of epidermis and a few stomata confined in the lower epidermis.

Turgor pressure acts as a motive force for nyctinastic movement of leaves of mimosa pudica. It is exerted by the protoplasm against the wall.

J. Van 't Hoff proposed a law relating osmotic pressure to solute concentration. According to him, osmotic pressure is proportional to solute concentration.

In 1956, Stocking considered root pressure as an active process responsible for guttation and bleeding.

Addition of solute to the solvent lowers the free energy of the solvent molecules and increases the osmotic potential of the solution.

This is false. Solute potential can never be positive. The more the solute molecules are present, the more negative is the solute potential.

Root pressure is absent in gymnosperms. Transpiration causes negative water vapour pressure in surrounding cells of a leaf. Once this happens, water is pulled into the leaves from surrounding vascular tissues.

In 1884, Goldewski gave the relay pump theory. According to him, ascent of sap takes place due to rhythmic change in the osmotic pressure of living cells of xylem parenchyma and medullary ray.

Plants like cottonwood which draw water from the water table are called phreatophytes. These plants are supplied with surface water and often have their roots in touch with moisture.

Root pressure develops in the tracheary elements of xylem, i.e. tracheids and vessels. In plants, tracheary elements function as the primary water transport cells for which root pressure is an essential requirement.

The pressure which is required in opposite direction so as to stop the entry of water molecules across a semipermeable membrane is called osmotic pressure.

When a freshwater plant is transferred to marine water, it dies due to exosmosis.

The flow of water will take from A to B. 
Water potential of A = psi + p; = -16 + 10; = -6 bars
Water potential of B = -14 + 4; = -10 bars
Since osmotic entry always takes place from higher water potential to lower water potential, the direction of flow of water will be from cell A to cell B.

In symplast pathway, water moves through protoplasts connected by plasmodesmata. It is a slow pathway affected by the metabolic state of roots.

Field capacity is viewed as the optimal condition for plant growth and microbial activity. At field capacity, air is in macropores and water in micropores and soil potential is -33 kpa.

Flow of water through plants and soil is driven by gradient in hydrostatic pressure over macroscopic distances, by water potential across semipermeable membrane and by diffusion as water vapour from the leaves to the atmosphere.

According to Chambering theory (1884), ascent of sap takes place due to rhythmic change in the osmotic pressure of living cells of xylem parenchyma.

Rate of diffusion shares an inversely proportional relationship with the particle size. A smaller particle at a given temperature moves faster than the larger particle.  

The division of xylem into small vessel pipes helps in ascent of sap by reducing embolism. It is based on Murray's law.