In river valleys of the world’s driest desert (The Atacama of South America) large stands of giant horsetail ( Equisetum giganteum) are found to tolerate soil water salinity up to at least half that of seawater. This Free—open access paper reports how roots selectively exclude Na and take-up K in response to salinity while stomatal conductances and photochemical efficiency of PhotosystemII remain unaffected.