Cytosolic potassium homeostasis and the ability of various tissues to retain potassium under stress have emerged as important for salinity tolerance in plants, but recent evidence suggests that stress-induced K+ efflux may be equally important in mediating growth and development under hostile conditions. Here, the evidence is assessed, and the already-proposed concept of potassium efflux being a switch between metabolic and defence responses is developed. A new model is put forward which suggests signalling roles for cytosolic K+ changes, alongside well-known cytosolic Ca2+ and ROS ‘signatures’.