WATER PUMPS IN PLANT-ROOTS

Plants manage to raise large amounts of water to considerable heights. The precise mechanisms by which they do this have remained controvers ial over the centuries, with capillary forces, osmotic tensions, trans piration, and cohesion being alternatively invoked as the prime agents . We argue that root pressure is essential for most, if not all, plant s, and that it cannot be explained by any combination of these four fo rces: mechanical pumps are often required to achieve the reverse osmos is occurring in young root tips. The pumps are single-cell, osmosis-pl us contraction-pressurized, plasmodesma-valved chambers, located in th e endodermis (and sometimes exodermis). They act as heat pumps, coolin g the sap by less than 0.2 K. Their cell walls are strengthened by und ulating Casparian strips; their valves are pit fields, each traversed by a large number of plasmodesmata in the outer periclinal cell walls of the endodermis, whose sub-micron foldings allow them to serve as fl exible pistons. In this way, the root pressure of a large plant is est ablished by numerous cell-sized water pumps working at frequencies of about 1 Hz-like the human heart.