Cohesion-Tension Theory (Transpiration Pull Theory)

c. Cohesion-tension theory (Transpiration pull theory) : The cohesion-tension theory, also known as the transpiration pull theory, is the theory most widely accepted today for explaining ascent of sap. It was put forward by Dixon and Jolly in 1894, and rests on two linked physical principles: cohesion and adhesion of water, and transpiration pull.

Cohesion and adhesion, and transpiration pull : Cohesion is the strong force of mutual attraction between water molecules themselves, arising from hydrogen bonding, while adhesion is the strong force of attraction between water molecules and the lignified walls of the xylem vessel lumen. Acting together, these two forces maintain a single, continuous, unbroken column of water inside the xylem, running the entire distance from the roots, up through the stem, all the way to the tip of the topmost leaf.

Transpiration pull : The actual pulling force that moves this column comes from transpiration in the leaf. Excess water is lost as vapour, mainly through the stomata on the leaf surface; as this water leaves, the DPD (water potential deficit) of the mesophyll cells rises, and these cells respond by withdrawing water from the nearby xylem in the leaf veins. Because transpiration is continuous, a continuous gradient of suction pressure (DPD) is set up, running from the guard cells at the leaf surface all the way down to the xylem in the leaf. This creates a genuine tension — a negative pull, called the transpiration pull — inside the leaf xylem. Because the xylem water column is cohesive and continuous, this tension is transmitted all the way down through the stem to the roots, pulling the entire column of water upward, passively and against gravity, and this is what accounts for the ascent of sap.

Objections / limitations of transpiration pull theory:

i. The mechanism depends on the water column in the xylem remaining unbroken and continuous for the pull to be transmitted properly; but temperature fluctuations between day and night can allow gas bubbles to form and enter the water column, breaking its continuity (a phenomenon now generally called cavitation or embolism). …