Ecophysiological Constraints Under Salinity Stress: Halophytes Versus Non-halophytes

By far high soil salinity is the most effective stressor for vegetation on globe, since about 20% of agricultural areas became saline by human activities even starting with ancient agriculture millennia ago. Moreover, most major crops are unable to tolerate even moderate salinity (<10% of ocean water salinity) without a serious setback in their biomass production. However, some species called halophytes have the ability to survive and compete their whole vegetation cycle under saline conditions as strong as seawater, not only brackish water. This is one of the most important systems for future uses of economic plants in arid countries. However, the differentiation of halophylic against non-halophylic species is somehow artificial and needs clarification. There is no strict definition available maybe also because of the diffuse use of the term stress neglecting the hormetic behaviour of plant stress response (eustress and distress). Moreover, many plants use the same general salt resistance effectors and regulatory mechanisms and the differences between halophilic and non-halophilic species seems tp be rather a quantitative that a qualitative nature. There is a big consensus that the stepless transition between both groups relies on a multiple network of adaptation mechanisms. Several ecophysiological parameters of various plant species growing on saline substrates exhibit a gradual behaviour according to hyperosmotic salinity. With these restrictions in mind, we give an up-to-date report on the halophilic plants and their defence responses to cope with salinity (with or without water deficiency and hyperosmotic salinity). We believe that a basic knowledge on ecophysiological performance of halophytes is an urgent need for the use of proper cultivation of of suited species. We conclude that all ecophysiological responses are influenced by hyperosmotic salinity, photosynthesis, and respiration as well as water relation, mineral nutrition, and ion transport. Growth and survival, dispersal, germination, and propagation are also strongly adapted. Moreover, classification of halophytes and their defence responses to stress within the framework of extremophiles are discussed. Some typical halophytes are illustrated from distinct hotspots of salinity.