Question

If cohesion-tension transpiration pull theory is correct, a break in water column should
A. Increase water content of leaves
B. Increase rate of photosynthesis
C. Cause wilting of leaves
D. Have no effect at all

Answer 1

Hint: Water column xylem is created by cohesive-adhesive forces. Their columns are continuous and cannot be pulled away by cohesive and adhesive properties. They help in upward movement of water.

Complete Answer:
1) Cohesive tension theory:
(i) Mutual attraction between water molecules is called cohesion. The walls of tracheids and vessels of xylem are made up of lignin and cellulose and have strong affinity for water i.e. adhesion.
(ii) Cohesive and adhesive properties are such developed form and unbroken continuous water columns in xylem.
(iii) Xylem vessels are3 tubular structures, which extend from roots to the top of the plant. They are placed one above with their end walls perforated thus forming a continuous channel. This channel is filled with water. Water column is continuous and cannot be pulled away from xylem walls.
(iv) Cohesive force is also called tensile strength.
(v) Cohesive force is also called tensile strength.
(vi) Xylem tubes are strong enough to resist the collapse when water inside the tubes is under tension.

2) Tension:
(i) Transpiration pull develops a negative pressure or tension in xylem sap which is transmitted down to the root.
(ii) Due to transpiration of water DPD of water in mesophyll cells increases water is therefore absorbed by the cell walls from protoplasm which in turn takes water from the vacuole of the cell. DPD of xylem element increases. When the process of transpiration is rapid and absorption in low, water in xylem passes into a state of tension.
(iii) In a cohesion tension transpiration, the water column breaks, this will cause wilting of leaves as sufficient water required by plants will not be supplied.
(iv) There will be a loss of turgidity of leaves. Thus will result in dropping and folding of leaves.

Hence, the correct option is (C).

Note: Cohesion-tension transpiration pull theory was originally proposed by Dixon and Jolly in 1894.