Transport of Water in Plants.

A sufficient balance in the supply and loss of water is an important prerequisite for the occurrence, growth and productivity of plants. For a higher plant, the water balance is given by the difference between water uptake across the root and water losses by transpiration in the shoot. Much is known about the mechanisms used by plants to minimize water losses across stomata. For technical reasons, much less is known about the input side of the balance, i.e. about the regulation of water uptake from the soil. Results are summarized on water uptake in great detail (cell, tissue, organ, and whole plant levels) . Experimental findings result in models of the water transport across roots which incorporate molecular mechanisms, i.e. the gating of water channels (aquaporins) by different external and internal factors. The recent criticism of the cohesion theory of the ascent of sap in plants is discussed. Recent results support the theory. They show that the evidence gainst the theory was based on artifacts. Overall, there is support that, to some extent, plants behave like“hydraulic machines”and that water flow within plants may be described by just a few physical principles.