Mechanism of Water Absorption

Kramer (1949) identified two genuinely separate mechanisms by which roots take up water, operating independently of one another. Active absorption relies on forces generated within the root itself, and comes in two forms: an osmotic form (proposed by Atkins in 1916 and Priestley in 1923), in which hypotonic soil water is first imbibed by the root-hair cell wall and then drawn in osmotically because the cell sap inside is hypertonic, with the resulting pressure that builds up at the protoxylem recognised as root pressure - though this idea runs into trouble because xylem sap is not always highly concentrated and root pressure is not universal, especially in large trees; and a non-osmotic form (identified by Bennet-Clark in 1936, Thimann in 1951 and Kramer in 1959), in which water is absorbed even against an unfavourable concentration gradient, powered instead by energy released from root respiration - confirmed by the fact that respiratory poisons like KCN or chloroform measurably reduce water uptake. Passive absorption, by contrast, gives the root itself no active role at all: water uptake here is driven entirely by transpiration in the shoot pulling water out of leaf cells, raising their DPD and transmitting that tension all the way down the xylem to the root, where it pulls in soil water. Passive absorption can use eithe …