Plants and soil water

Absorption of water by roots The movement of water through the soil to the plant and then through the plant to the atmosphere is the main theme of this chapter. Water is taken in by roots through or past their epidermal cells. There is some uncertainty about the actual path that the water follows in its passage to the xylem system near the axis of the root. Many terrestrial plants possess a mechanism whereby the root cortex selectively admits solutes of the soil water required for their mineral nutrition. The electrolyte concentration of their xylem sap is usually much less than the concentration in the soil water whence it was derived and which, of necessity, fluctuates markedly with varying soil moisture conditions. Water in the protoxylem of the roots then moves axially along them to the stem xylem, which forms the main part of the vascular system of the plant. The leaves form the terminus for this transpiration system. The change from liquid water to vapour occurs in their substomatal cavities whence the water vapour diffuses out through the stomata and carbon dioxide diffuses in to its sites of uptake. When the loss of water lowers the total potential of the water in the leaves relative to that of the water in the root cortex a gradient is created that favours the upward movement of water through the soil-root-stem-leaf continuum (Slatyer, 1967; Kramer, 1983).