A reassessment of the function of the so-called compatible solutes in the halophytic plumbaginaceae Limonium latifolium.

The compatible solute hypothesis posits that maintaining osmotic equilibrium under conditions of high salinity requires synthesis of organic compounds, uptake of potassium ions, and partial exclusion of NaCl. To assess whether osmotic adaptation in Limonium latifolium proceeds according to this hypothesis, a comprehensive analysis of solute accumulation during NaCl treatments was conducted. Determination of prevailing inorganic ions and establishment of the metabolic profiles for low M(r) organic substances revealed that contrary to the mentioned hypothesis the major contributors to osmolarity were constituted by inorganic solutes. Independent of salinity, only 25% of this osmolarity resulted from organic solutes such as Suc and hexoses. Proline (Pro), beta-alanine betaine, and choline-O-sulfate were minor contributors to osmolarity. Compatible inositols also occurred, especially chiro-inositol, characterized for the first time in this species, to our knowledge. Principal component analysis showed that only a limited number of metabolic reconfigurations occurred in response to dynamic changes in salinity. Under such conditions only sugars, chiro-inositol, and Pro behave as active osmobalancers. Analysis of metabolic profiles during acclimatization to either mild salinity or nonsaline conditions showed that organic solute accumulation is predominantly controlled by constitutive developmental programs, some of which might be slightly modulated by salinity. Osmolarity provided under such conditions can be sufficient to maintain turgor in salinized seedlings. Compartmental analysis of Pro and beta-alanine betaine in leaf tissues demonstrated that these solutes, mainly located in vacuoles under nonsaline conditions, could be partly directed to the cytosol in response to salinization. Thus they did not conform with the predictions of the compatible solute hypothesis.

Seasonal changes in the levels of compatible osmolytes in three halophytic species of inland saline vegetation in Hungary.

Seasonal changes in the leaf concentration of compatible osmolytes were investigated in three halophytic species (Lepidium crassifolium, Camphorosma annua and Limonium gmelini subsp. hungaricum) native to a salty-sodic grassland. The investigated species were shown to accumulate both carbohydrate- and amino acid-derived osmolytes. The leaf tissues of C. annua (Chenopodiaceae) preferentially stored glycine betaine and pinitol, while in L. gmelini (Plumbaginaceae) beta-alanine betaine, choline-O-sulphate, and pinitol were accumulated. In the leaves of L. crassifolium (Brassicaceae) a very high amount of proline, associated with a high level of soluble carbohydrates was found. Not only the biochemical nature of the osmolyte, but also the seasonal pattern of osmolyte accumulation showed significant species-specific fluctuations. In addition, the cellular levels of the observed osmolytes changed with the growth period and according to the environmental parameters. The highest concentrations of osmolytes were found in March, when low temperatures, hypoxic conditions and high salt concentrations were the main constraints to plant growth. The high structural diversity of osmolytes combined with their multifunctionality and the seasonal flexibility of the metabolism in plants facing multiple stresses is discussed.