A greenhouse investigation of responses to different water stress regimes of Laurus nobilis trees from two climatic regions

Plants from two populations of Laurus nobilis (Tunisia issued from a semi-arid inland site and Algeria originating from a coastal sub-humid area) were exposed during three months under similar controlled conditions to two stress intensities of permanent stress (60% (S1) and 20% (S2) of Held capacity) or to cyclic water stress, plants being re-watered when the soil moisture dropped to 60% (S11) or 20% (S22) of field capacity. One-year old plants displayed contrasting physiological strategies to cope with water stress. Algeria exhibited a higher decrease in osmotic potential (psi s) in relation to stress-induced proline accumulation. Glycinebetaine accumulated in response to drought in response to permanent stress (Algeria) or cyclic stress (Tunisia). The two populations had similar net photosynthesis (A) but Algeria exhibited higher water use efficiency (WUE) than Tunisia. A drought-induced increase in the apoplastic water content (AWC) was noticed in response to mild stress intensities (S1 and S11) in Tunisia and in response to higher stress intensities (S2 and S22) in Algeria in relation to a stress-induced accumulation of pectin and proportion of arabinose within the pectic fraction. Bulk modulus of elasticity (E) increased in Tunisia in response to permanent drought and in Algeria in response to cyclic stress, as a result of a stress-induced increase in cellulose (Algeria) or hemicellulose (Tunisia). It is concluded that water stress tolerance could be achieved by both osmotic and elastic adjustment in the coastal population which did not exhibit a prodigal water use comparatively to the inland population. Differences between populations are strongly influenced by the kinetics of water stress application. (C) 2009 Published by Elsevier Ltd

Structural changes induced by NaCl in companion and transfer cells of Medicago sativa blades

Medicago sativa var. Gabes is a perennial glycophyte that develops new shoots even in high salinity (150 mM NaCl). In the upper exporting leaves,K+ is high and Na+ is low by comparison with the lower leaves, where Na+ accumulation induces chlorosis after 4 weeks of NaCl treatment. By secondary ion mass spectroscopy, a low Na+/K+ ratio was detected in the phloem complex of blade veins in these lower leaves. By transmission electron microscopy, the ultrastructural features were observed in the phloem complex. In the upper leaves of both control and NaCl-treated plants, companion cells in minor veins were found to be transfer cells. These cells may well be involved in the intravenous recycling of ions and in Na+ flowing out of exporting leaves. Under the effect of NaCl, companion cells in the main veins develop transfer cell features, which may favor the rate of assimilate transport from exporting leaves toward meristems, allowing the positive balance necessary for the survival in salt conditions. These features no longer assist the lower leaves when transfer cells are necrotized in both minor and main veins of NaCl-treated plants. As transfer cells are the only degenerating phloem constituent, our observations emphasize their role in controlling nutrient (in particular, Na+) fluxes associated with the stress respons