A general response of plants to high soil salinity relies on the cellular accumulation of osmolytes, which help the plant to maintain osmotic balance under salt stress condition and/or act as ‘osmoprotectants’ with chaperon or reactive oxygen species (ROS) scavenging activities. Yet the ecological relevance of this response for the salt tolerance mechanisms of halophytes in their natural habitats remains largely unknown. In this review, we describe and discuss published data supporting the participation of compatible solutes in those mechanisms, with especial focus on soluble carbohydrates. Evidence for a functional role of carbohydrates in salt tolerance include: (i) relatively high levels of specific sugars and polyols have been detected in many halophytic taxa; (ii) an increase in salt tolerance has often been observed in parallel with increased intracellular levels of particular soluble carbohydrates, in transgenic plants overexpressing the corresponding biosynthetic enzymes; (iii) there are several examples of genes involved in carbohydrate metabolism which are induced under salt stress conditions; (iv) specific sugars or polyols have been shown to accumulate in different halophytes upon controlled salt treatments; and (v) although very few field studies on environmentally induced carbohydrate changes in halophytes exist, in general they also support the involvement of this type of osmolytes in salt stress tolerance mechanisms. We also highlight the complexities of unequivocally attributing carbohydrates a biological role in salt tolerance mechanisms of a given tolerant species. It is proposed that research on halophytes in their natural ecosystems should be intensified, correlating seasonal changes in carbohydrate contents with the degree of environmental stress affecting the plants. This could be an important complement to experiments made under more controlled (but artificial) conditions, such as laboratory set-ups.

In their natural habitats, different mechanisms may contribute to the tolerance of halophytes to high soil salinity and other abiotic stresses, but their relative contribution and ecological relevance, for a given species, remain largely unknown. We studied the responses to changing environmental conditions of five halophytes (Sarcocornia fruticosa, Inula crithmoides, Plantago crassifolia, Juncus maritimus and J. acutus) in a Mediterranean salt marsh, from summer 2009 to autumn 2010. A principal component analysis was used to correlate soil and climatic data with changes in the plants' contents of chemical markers associated with stress responses: ions, osmolytes, malondialdehyde (MDA, a marker of oxidative stress) and antioxidant systems. Stress tolerance in S. fruticosa, I. crithmoides and P. crassifolia (all succulent dicots) seemed to depend mostly on the transport of ions to aerial parts and the biosynthesis of specific osmolytes, whereas both Juncus species (monocots) were able to avoid accumulation of toxic ions, maintaining relatively high K+/Na+ ratios. For the most salt-tolerant taxa (S. fruticosa and I. crithmoides), seasonal variations of Na+, Cl−, K+ and glycine betaine, their major osmolyte, did not correlate with environmental parameters associated with salt or water stress, suggesting that their tolerance mechanisms are constitutive and relatively independent of external conditions, although they could be mediated by changes in the subcellular compartmentalization of ions and compatible osmolytes. Proline levels were too low in all the species to possibly have any effect on osmotic adjustment. However—except for P. crassifolia—proline may play a role in stress tolerance based on its ‘osmoprotectant’ functions. No correlation was observed between the degree of environmental stress and the levels of MDA or enzymatic and non-enzymatic antioxidants, indicating that the investigated halophytes are not subjected to oxidative stress under natural conditions and do not, therefore, need to activate antioxidant defence mechanisms.