Salt tolerance and crop potential of halophytes

Although they represent only 2% of terrestrial plant species, halophytes ar e present in about half the higher plant families and represent a wide dive rsity of plant forms. Despite their polyphyletic origins, halophytes appear to have evolved the same basic method of osmotic adjustment: accumulation of inorganic salts, mainly NaCl, in the vacuole and accumulation of organic solutes in the cytoplasm. Differences between halophyte and glycophyte ion transport systems are becoming apparent. The pathways by which Na+ and Cl- enters halophyte cells are not well understood but may involve ion channel s and pinocytosis, in addition to Na+ and Cl- transporters. Na+ uptake into vacuoles requires Na+/H+ antiporters in the tonoplast and H+ ATPases and p erhaps PPi ases to provide the proton motive force. Tonoplast antiporters a re constitutive in halophytes, whereas they must be activated by NaCl in sa lt-tolerant glycophytes, and they may be absent from salt-sensitive glycoph ytes. Halophyte vacuoles may have a modified lipid composition to prevent l eakage of Na+ back to the cytoplasm. Because of their diversity, halophytes have been regarded as a rich source of potential new crops. Halophytes have been tested as vegetable, forage, a nd oilseed crops in agronomic field trials. The most productive species yie ld 10 to 20 ton/ha of biomass on seawater irrigation, equivalent to convent ional crops. The oilseed halophyte, Salicornia bigelovii, yields 2 t/ha of seed containing 28% oil and 31% protein, similar to soybean yield and seed quality. Halophytes grown on seawater require a leaching fraction to contro l soil salts, but at lower salinities they outperform conventional crops in yield and water use efficiency. Halophyte forage and seed products can rep lace conventional ingredients in animal feeding systems, with some restrict ions on their use due to high salt content and antinutritional compounds pr esent in some species. Halophytes have applications in recycling saline agr icultural wastewater and reclaiming salt-affected soil in arid-zone irrigat ion districts.