Josep Llinares

Responses of five Mediterranean halophytes to seasonal changes in environmental conditions

We have studied the responses to changing environmental conditions of five halophytes in a Mediterranean salt marsh, during a 2-year period. Salt tolerance in succulent dicotyledonous halophytes is mostly dependent on compartmentalisation of toxic ions in vacuoles and biosynthesis of osmolytes for osmotic adjustment – mechanisms that appear to be constitutive in the most tolerant taxa – while monocots avoid excessive ion transport to the plant aerial parts. Contrary to what has been described for salt treatments under artificial conditions, the selected halophytes are not affected by oxidative stress in their natural habitat, and do not need to activate antioxidant defence mechanisms.

Environmentally induced changes in antioxidant phenolic compounds levels in wild plants

Different adverse environmental conditions cause oxidative stress in plants by generation of reactive oxygen species (ROS). Accordingly, a general response to abiotic stress is the activation of enzymatic and non-enzymatic antioxidant systems. Many phenolic compounds, especially flavonoids, are known antioxidants and efficient ROS scavengers in vitro, but their exact role in plant stress responses in nature is still under debate. The aim of our work is to investigate this role by correlating the degree of environmental stress with phenolic and flavonoid levels in stress-tolerant plants. Total phenolic and antioxidant flavonoid contents were determined in 19 wild species. Meteorological data and plant and soil samples were collected in three successive seasons from four Mediterranean ecosystems: salt marsh, dune, semiarid and gypsum habitats. Changes in phenolic and flavonoid levels were correlated with the environmental conditions of the plants and were found to depend on both the taxonomy and ecology of the investigated species. Despite species-specific differences, principal component analyses of the results established a positive correlation between plant phenolics and several environmental parameters, such as altitude, and those related to water stress: temperature, evapotranspiration, and soil water deficit. The correlation with salt stress was, however, very weak. The joint analysis of all the species showed the lowest phenolic and flavonoid levels in the halophytes from the salt marsh. This finding supports previous data indicating that the halophytes analysed here do not undergo oxidative stress in their natural habitat and therefore do not need to activate antioxidant systems as a defence against salinity.

Effects of Salt Stress on Three Ecologically Distinct Plantago Species.

Comparative studies on the responses to salt stress of taxonomically related taxa should help to elucidate relevant mechanisms of stress tolerance in plants. We have applied this strategy to three Plantago species adapted to different natural habitats, P. crassifolia and P. coronopus–both halophytes–and P. major, considered as salt-sensitive since it is never found in natural saline habitats. Growth inhibition measurements in controlled salt treatments indicated, however, that P. major is quite resistant to salt stress, although less than its halophytic congeners. The contents of monovalent ions and specific osmolytes were determined in plant leaves after four-week salt treatments. Salt-treated plants of the three taxa accumulated Na+ and Cl- in response to increasing external NaCl concentrations, to a lesser extent in P. major than in the halophytes; the latter species also showed higher ion contents in the non-stressed plants. In the halophytes, K+ concentration decreased at moderate salinity levels, to increase again under high salt conditions, whereas in P. major K+ contents were reduced only above 400 mM NaCl. Sorbitol contents augmented in all plants, roughly in parallel with increasing salinity, but the relative increments and the absolute values reached did not differ much in the three taxa. On the contrary, a strong (relative) accumulation of proline in response to high salt concentrations (600–800 mM NaCl) was observed in the halophytes, but not in P. major. These results indicate that the responses to salt stress triggered specifically in the halophytes, and therefore the most relevant for tolerance in the genus Plantago are: a higher efficiency in the transport of toxic ions to the leaves, the capacity to use inorganic ions as osmotica, even under low salinity conditions, and the activation, in response to very high salt concentrations, of proline accumulation and K+ transport to the leaves of the plants.

Is salinity the main ecologic factor that shapes the distribution of two endemic Mediterranean plant species of the genus Gypsophila

AimsResponses to salt stress of two Gypsophila species that share territory, but with different ecological optima and distribution ranges, were analysed. G. struthium is a regionally dominant Iberian endemic gypsophyte, whereas G. tomentosa is a narrow endemic reported as halophyte. The working hypothesis is that salt tolerance shapes the presence of these species in their specific habitats.MethodsTaking a multidisciplinary approach, we assessed the soil characteristics and vegetation structure at the sampling site, seed germination and seedling development, growth and flowering, synthesis of proline and cation accumulation under artificial conditions of increasing salt stress and effect of PEG on germination and seedling development.ResultsSoil salinity was low at the all sampling points where the two species grow, but moisture was higher in the area of G. tomentosa. Differences were found in the species’ salt and drought tolerance. The different parameters tested did not show a clear pattern indicating the main role of salt tolerance in plant distribution.ConclusionsG. tomentosa cannot be considered a true halophyte as previously reported because it is unable to complete its life cycle under salinity. The presence of G. tomentosa in habitats bordering salt marshes is a strategy to avoid plant competition and extreme water stress.