4.3.1  Water relations

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Sections 4.1 and 4.2 dealt with acquisition of ions by plant cells, emphasising control of ion fluxes and input of energy to maintain electrochemical gradients. Pumps concentrate selected solutes within cells to form a sap in which metabolic reactions can occur. Pumps also raise osmotic pressure (Π) within cells above that of the external solution, setting up a gradient in free energy that draws water across plasma mem-branes into cells. Rising concentrations of common solutes such as sucrose, potassium ions or nitrate ions inside (or outside) plant cells produce approximately equivalent rises in Π.

As water enters a plant cell the cell swells, causing the plasma membrane to exert force on the adjacent cell wall. When tension develops within this wall, an opposing wall pressure raises the energy of water within the cell until it equals that of water outside. Water ceases to cross the plasma membrane, hydraulic equilibrium is reached and the cell has developed a turgor pressure (P) equal to Πinside – Πoutside.

The energy of water is therefore an entity described by two variables, P and Π. It is defined experimentally by the term water potential (Ψ) according to the relationship:

Ψ = P – Π (4.9)

This equation holds only when no water is flowing across the plasma membrane, an assumption that is not applicable to cell walls during expansion. However, it is widely used to link solute acquisition with hydrostatic pressure, thereby capturing the concept of osmotically driven growth.