Hypoxically induced tolerance to anoxia in roots of tomato (Solanum lycopersicum) was previously shown to depend on sucrose and the induction of sucrose synthase. In contrast to maize, root hexokinase (HXK) activities did not increase during hypoxia and glucose was unable to sustain glycolytic flux under anoxia. In this paper, we asked whether hypoxic metabolism in roots would be altered in transgenic tomato plants overexpressing either a plant (Arabidopsis) or a yeast (Saccharomyces cerevisiae) HXK and whether such modifications could be related to improved energy metabolism and consequently root tolerance under anoxia. Tomato plants grown hydroponically with shoots always maintained in air were submitted to a 7 d hypoxic treatment applied by stopping air bubbling. A combination of techniques including 1 H-nuclear magnetic resonance spectroscopy, RT-PCR and enzyme analyses was used to obtain a broad picture of hypoxic root metabolism. In normoxic conditions, HXK overexpression resulted in higher ADP and AMP levels only in roots of AtHXK1 transgenic plants. During hypoxic treatment, oxygen levels in the hydroponic tank decreased rapidly to 5 kPa within the first 2 d and then remained at 5 kPa throughout the 7 d experiment. Oxygen levels were similar at 5 and 20 cm below the water surface. A decline of the adenylate energy status was observed after 2 d of hypoxic treatment, with a further decrease by 7 d in roots of non-transgenic (WT) and ScHXK2, but not in AtHXK1 transgenic plants. Sucrose synthase activity increased to comparably higher levels at 7 d of hypoxic treatment in WT and ScHXK2 compared with AtHXK1 roots. Differences between WT and the transgenic plants are discussed with respect to the metabolic response to low (hypoxia) but not zero (anoxia) oxygen.

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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 response.

. Rôle des champignons mycorhiziens à arbuscules de zones arides dans la résistance du trèfle (Trifolium alexandrinum L.) au déficit hydrique. Agronomie, EDP Sciences, 2000, 20 (3) Résumé -La tolérance du trèfle mycorhizé (Trifolium alexandrinum L.) à la sécheresse diffère selon les isolats de champignons mycorhiziens associés à la plante hôte. Cinq isolats fongiques ont été isolés à partir des sols de palmeraies marocaines et étudiés pour leur aptitude à améliorer la tolérance du trèfle au stress hydrique. L'application d'une contrainte hydrique de 30 % de la capacité au champ réduit sévèrement la progression des champignons mycorhiziens dans le cortex racinaire du trèfle. Les isolats provenant de la palmeraie d'Aoufous sont moins affectés par le dessèche-ment du sol que les Glomus et Sclerocystis d'Agdz. L'effet de la mycorhization quant à l'amélioration de la production de biomasse du trèfle est bien apparu dans le cas d'infection par les isolats fongiques d'Aoufous et le Glomus mosseae (souche de référence provenant de l'INRA de Dijon, France). Ces mêmes champignons mycorhiziens ont permis au trèfle de maintenir sa teneur en eau, son potentiel hydrique et sa transpiration foliaire à des valeurs élevées par rapport au témoin non inoculé. Les valeurs de la résistance des stomates et du déficit de saturation en eau sont restées plus faibles chez les plantes mycorhizées que chez les plantes non mycorhizées. On note également, que les isolats du complexe d'Aoufous et le Glomus mosseae se sont montrés les plus agressifs dans la colonisation des racines du trèfle et ce quel que soit le niveau du traitement hydrique imposé au sol. Les isolats fongiques provenant des sols de la palmeraie d'Aoufous se sont montrés aussi efficaces que Glomus mosseae. mycorhize / croissance / relation hydrique / sécheresse / zone aride Abstract -Effect of arbuscular mycorrhizal fungi on drought resistance of clover. Tolerance of mycorrhized clover (Trifolium alexandrinum L.) to drought depends on the arbuscular mycorrhizal fungi associated to the host plant. Five arbuscular mycorrhizal fungi isolates were collected from five Moroccan date palm grove's soils and were investigated for their ability to improve the plant tolerance to water deficit stress. Applying a constraint of 30% field capacity reduces severely the mycorrhizal rate in the root cortex of most of the tested isolates. Fungi collected from Aoufous