Potassium nutrition and salt stress

Potassium (K + ) is the most abundant inorganic cation in plants, comprising up to 10% of a plant’s dry weight ( Leigh & Jones, 1984 ). Potassium is an important macronutrient for plants, as it carries out vital functions in metabolism, growth and stress adaptation. These functions can be classified into those that rely on high and relatively stable concentrations of potassium in certain cellular compartments or tissues and those that rely on potassium movement between different compartments, cells or tissues. The first class of functions includes enzyme activation, stabilisation of protein synthesis, neutralisation of negative charges on proteins and maintenance of cytoplasmic pH homeostasis ( Marschner, 1995 ). The optimal potassium concentration for enzyme activation and protein synthesis is in the range of 100 mM ( Wyn Jones & Pollard, 1983 ). Thus, for optimal metabolic activity, cells rely on controlled potassium concentrations of around 100 mM in metabolically active compartments, i.e. the cytoplasm, the nucleus and the stroma of chloroplasts, and the matrix of mitochondria. Other roles of potassium are linked to its high mobility. This is particularly evident where potassium movement is the driving force for osmotic changes - as, for example, in stomatal movement, light-driven and seismonastic movements of organs, or phloem transport. In other cases, potassium movement provides a chargebalancing counter-flux essential for sustaining the movement of other ions. Thus, energy production through H + -ATPases relies on overall H + /K + exchange ( Tester & Blatt, 1989 ; Wu et. al ., 1991) and transport of sugars, amino acids and nitrate is accompanied by K + fluxes ( Marschner, 1995 ). The most general phenomenon that requires directed movement of potassium is growth. Accumulation of potassium (together with an anion) in plant vacuoles creates the necessary osmotic potential for cell extension. Rapid cell extension relies on high mobility of the used osmoticum and therefore only few other inorganic ions (e.g. Na + ) can replace potassium in this role ( Reckmann et al ., 1990 ). Once cell growth has come to a halt, maintenance of osmotic potentials can be carried out by less mobile sugars and potassium ions can partly be recovered from vacuoles ( Poffenroth et al ., 1992 ; Marschner, 1995 ).