Effect of shading imposed by the algae Chaeotomorpha linum loads on structure, morphology and physiology of the seagrass Cymodocea nodosa.

In shallow coastal waters, seagrass and macroalgae occur together but under eutrophic conditions, bloom-forming algae can take over seagrasses causing an irreversible regime shift. Understanding the effect of macroalgae loads on seagrass meadows at an early stage can help prevent the loss of these ecosystems and the services they provide. In the present study, in situ experiments were conducted for 90 days in Bekalta (eastern coast of Tunisia) to assess the response of the seagrass Cymodocea nodosa when challenged with shading induced by filamentous macroalgae Chaetomorpha linum. Structural, morphological and physiological variables were regularly measured during the experiment. Shaded plants showed a sharp decline in shoot density, growth rate, and above-ground biomass, the impact being more pronounced on the physiological traits. Besides, shading by C. linum induced a significant increase in the contents of leaf photosynthetic pigments and phenolic compounds, whereas causing a decrease in soluble protein and sugar concentrations. Thus, shading imposed by C. linum loads appeared to induce a phoadpatative response in C. nodosa concomitant with carbon mobilization.

Interactive effects of salinity and phosphorus availability on growth, water relations, nutritional status and photosynthetic activity of barley (Hordeum vulgare L.).

The interactive effects of salinity and phosphorus availability on growth, water relations, nutritional status and photosynthetic activity were investigated in barley (Hordeum vulgare L. cv. Manel). Seedlings were grown hydroponically under low or sufficient phosphorus (P) supply (5 or 180 µmol KH(2) PO(4) plant(-1) week(-1) , respectively), with or without 100 mm NaCl. Phosphorus deficiency or salinity significantly decreased whole plant growth, leaf water content, leaf osmotic potential and gas exchange parameters, with a more marked impact of P stress. The effect of both stresses was not additive since the response of plants to combined salinity and P deficiency was similar to that of plants grown under P deficiency alone. In addition, salt-treated plants exposed to P deficiency showed higher salt tolerance compared to plants grown with sufficient P supply. This was related to plant ability to significantly increase root:shoot DW ratio, root length, K(+)/Na(+) ratio, leaf proline and soluble sugar concentrations and total non-enzymatic antioxidant capacity, together with restricting Na(+) accumulation in the upper leaves. As a whole, our results indicate that under concomitant exposure to both salt and P deficiency, the impact of the latter constraint is pre-dominant.

Differential response to salinity and water deficit stress in Polypogon monspeliensis (L.) Desf. provenances during germination.

The effects of provenance, salinity (0, 100, 200, 300 and 400 mm NaCl) and water deficit (0, -0.6, -1.1, -1.6 and -2.1 MPa mannitol solutions) on germination success of Polypogon monspeliensis were investigated. Eight Tunisian provenances from different bioclimatic origins were considered. Seed mass varied significantly between populations. Germination percentage was significantly affected by provenance, salinity and their interaction. Even at 300 mm NaCl, germination percentage of Tabarka, Kelbia and Kebili ranged from ca. 60% to ca. 85%, whereas Monastir, Gabes and El Haouaria succeeded in germinating in 200 mm NaCl. The 300 mm NaCl treatment highly reduced germination of Monastir and El Haouaria, and inhibited that of Gabes. Soliman and El Jem were the least salt-tolerant provenances. The severity of water deficit impact on seed germination was also provenance-dependent, especially at osmotic potentials of -1.1 to -1.6 MPa. At -1.6 MPa, germination percentage of Tabarka, Monastir and Kebili was close to 80%, while that of Gabes, El Jem and Kelbia was 0%, 5% and 20%, respectively. Regardless of provenance, germination was strongly impaired at -2.1 MPa. The variability of stress tolerance in P. monspeliensis could be of practical significance in programmes aimed at restoring arid and salt-affected lands since it allows use of provenances that germinate and establish successfully under unfavourable conditions prevailing in such zones.

Interactive effects of salinity, nitrate, light, and seed weight on the germination of the halophyte Crithmum maritimum.

Interaction of salinity, nitrate, light, and seed weight on the germination of Crithmum maritimum was investigated. Seeds of three size categories were germinated at 0-200 mM NaCl with either 0, 5 or 20 mM KNO 3 . Experiments were done under darkness, white light, or red light. Regardless of seed weight, germination was maximal in distilled water. Under salinity, the smallest seeds showed the highest germination percentage. Salt impact was amplified by darkness, but was mitigated by nitrate supply, red light and their combination. At the same PPFD, germination of T2 seeds was higher, when exposed to red light than under white light, suggesting that germination was more influenced by the light type than by the PPFD. As a whole, not only salinity, nutrient availability, seed weight, and light, but also their interaction may control the germination of this halophyte.