Ecophysiological plasticity of shallow and deep populations of the Mediterranean seagrasses Posidonia oceanica and Cymodocea nodosa in response to hypersaline stress

Abstract

The differential expression of the plant phenotypic plasticity due to inter- and intraspecific divergences can determine the plant physiological tolerance under stress. In this work, we examined the interspecific ecophysiological plasticity that the main Mediterranean seagrass species with distinct marine environmental distribution (Posidonia oceanica and Cymodocea nodosa) can exhibit in response to hypersaline stress. We also tested the potential implication of ecotypic intraspecific divergences in the development of such plasticities. To this end, plants from shallow (5-7 m) and deep (18-20 m) meadows of both were maintained under two salinity treatments (natural salinity level of 37, and hypersaline treatment of 43; Practical Salinity Scale) during a long-term experiment (i.e. 62 days) developed in a highly controlled mesocosm system. Hypersaline stress caused notable plastic physiological alterations in P. oceanica and C. nodosa, with appreciable inter- and intraspecific differences. Although both species were similarly able to osmoregulate by means of organic solute accumulation (proline and sugars) in response to hypersalinity stress, higher carbon balance reductions were detected in P. oceanica plants from the deep meadow and in shallower C. nodosa plants, due to both photosynthetic inhibition and enhancement of respiration. None of these deleterious effects were found in C. nodosa plants form the deeper meadow. Leaf photosynthetic pigments generally increased in P. oceanica from both depths, but light absorbance capacities by leaves and photosynthetic efficiency followed contrasting patterns, increasing and decreasing in plants from the deep and the shallow meadows, respectively, indicating distinct strategies to cope with photosynthetic dysfunctions. Despite the significant reduction of pigments in the shallower C. nodosa plants, their leaves were able to increase their light capture capacities under hypersaline stress, by means of particular leaf optics adjustments (pigment packaging reduction). The metabolic costs as a consequence of the physiological plasticity integration seemed to compromise the vitality of P. oceanica, but not in the case of C. nodosa. These results confirm that both the inter- and intraspecific divergences play a key role in the responses which both Mediterranean seagrasses could develop under hypersaline stress conditions, and that these were consistent with their distinct ecological strategies and salinity tolerance ranges.

Keywords: Cymodocea nodosa; Desalination discharges; Environmental impact; Mediterranean seagrass; Mesocosm; Phenotypic plasticity; Posidonia oceanica; Salinity stress.