The control of Na(+) and Cl(-) uptake from soils, and the partitioning of these ions within plants, is an essential component of salinity tolerance. Genetic variation in the ability of roots to exclude Na(+) and Cl(-) from the transpiration stream flowing to the shoot has been associated with salinity tolerance in many species. The maintenance of a high uptake of K(+) is also essential, so measurements of Na(+), K(+) or Cl(-) are frequently used to screen for genetic variation in salinity tolerance. As these ions are not bound covalently to compounds in cells, they can be readily extracted with dilute acid. Na(+) and K(+) can be measured in a dilute nitric acid extract using a flame photometer, by atomic absorption spectrometry or by inductively coupled plasma (ICP)-atomic emission spectrometry. Cl(-) can be measured in the same acid extract with a chloridometer or colorimetrically using a spectrophotometer.