Ion Transport in Halophytes

The increasing problem of global land salinisation and associated multibillion dollar losses in crop production require a better understanding of key physiological mechanisms conferring salinity tolerance in crops. The effective way of gaining such knowledge comes from studying halophytes. Halophytes have always attracted the attention of plant physiologists, due to their remarkable ability to tolerate and even benefit from salt concentrations that kill most other plant species. At the very least, halophytes may provide genes that allow transgenic conference of salinity tolerance to crops. In addition, some halophytes have already been tested as vegetable, forage and oilseed crops in agronomic field trials, whilst others already show good potential to be developed as crops. Surprisingly, our knowledge of fundamental ionic and molecular mechanisms conferring salinity tolerance in halophytes is rather limited and, at best, is restricted to several model species. This chapter summarises the current knowledge of physiological mechanisms regulating ion uptake and sequestration in halophytes. The following topics are covered: specific anatomical and morphological features of halophytes; tissue- and organ-specific ion compartmentation; mechanisms of osmotic adjustment in halophytes; radial ion transport in halophytes roots; mechanisms of Na+ and K+ loading into the xylem; Na+ sequestration in vacuoles in roots and leaf cells; ion transport in guard cells; control of ion fluxes into salt glands and bladders; and oxidative signalling and damage repair in halophytes. It is concluded that more in-depth electrophysiological and molecular studies are needed to reveal the identity of membrane transport systems and better understand what appears to be the complex and highly orchestrated regulation of ion transport and sequestration in halophytes.