Résumé
Potassium (K+) can constitute up to 10 percent of total plant dry weight. It is the most abundant cation in the cytosol since it is compatible with protein structure even at high concentrations. At cellular level, this cation is used for basic functions, such as electrical neutralization of anionic groups, control of cell membrane polarization, osmoregulation and regulation of cell turgor (Clarkson and Hanson, 1980; Maathuis and Sanders, 1996). At whole plant level, K+ i s i nvolved i n h ighly c omplex and integrated functions. For instance, related to its involvement in turgor regulation, K+ plays a role in tropisms, or guard cell movements, which allow the plant to regulate the aperture of the stomatal pores present at the leaf surface. The importance of these functions probably explains why membrane transport of K+ has been studied more extensively than other nutrient ions, giving rise to breakthroughs and founding models, such as Epstein's dual mechanism, which proposes that membrane transport of K+, as well as of most other ion nutrients, results from the activity of both high affinity and low affinity transport mechanisms.