Passive Absorption

a. Passive absorption : Passive absorption is the principal way water enters a plant through its roots, and by far the larger contributor — about 98% of all water absorbed by a plant is taken up passively. Crucially, the driving force behind passive absorption is not generated by the root at all; it is transpiration pull, generated up in the leaves, and the water moves strictly along the DPD (water potential) gradient this pull creates, so no expenditure of cellular energy (ATP) is required anywhere along the pathway.

Mechanistically, rapid transpiration from the leaves creates a tension (a negative pressure, or pull) within the water columns of the leaf xylem, because water evaporating from mesophyll cells lowers their water potential, which in turn is transmitted, cell to cell, down through the stem xylem all the way to the root xylem. This tension pulls water upward into the root passively, which in turn pulls more water in from the surrounding soil, continuing the whole chain of absorption described in sections 6.4 and 6.7. …