Sodium as nutrient and toxicant

Background: Sodium (Na⁺) is one of the most intensely researched ions in plant biology and has attained a reputation for its toxic qualities. Following the principle of Theophrastus Bombastus von Hohenheim (Paracelsus), Na⁺ is, however, beneficial to many species at lower levels of supply, and in some, such as certain C4 species, indeed essential. Scope: Here, we review the ion's divergent roles as a nutrient and toxicant, focusing on growth responses, membrane transport, stomatal function, and paradigms of ion accumulation and sequestration. We examine connections between the nutritional and toxic roles throughout, and place special emphasis on the relationship of Na⁺ to plant potassium (K⁺) relations and homeostasis. Conclusions: Our review investigates intriguing connections and disconnections between Na⁺ nutrition and toxicity, and concludes that several leading paradigms in the field, such as on the roles of Na⁺ influx and tissue accumulation or the cytosolic K⁺/Na⁺ ratio in the development of toxicity, are currently insufficiently substantiated and require a new, critical approach.