Many plants accumulate organic osmolytes in response to the imposition of environmental stresses that cause cellular dehydration. Although an adaptive role for these compounds in mediating osmotic adjustment and protecting subcellular structure has become a central dogma in stress physiology, the evidence in favour of this hypothesis is largely correlative. Transgenic plants engineered to accumulate proline, mannitol, fructans, trehalose, glycine betaine or ononitol exhibit marginal improvements in salt and/or drought tolerance. While these studies do not dismiss causative relationships between osmolyte levels and stress tolerance, the absolute osmolyte concentrations in these plants are unlikely to mediate osmotic adjustment. Metabolic benefits of osmolyte accumulation may augment the classically accepted roles of these compounds. In re-assessing the functional significance of compatible solute accumulation, it is suggested that proline and glycine betaine synthesis may buffer cellular redox potential. Disturbances in hexose sensing in transgenic plants engineered to produce trehalose, fructans or mannitol may be an important contributory factor to the stress-tolerant phenotypes observed. Associated effects on photoassimilate allocation between root and shoot tissues may also be involved. Whether or not osmolyte transport between subcellular compartments or different organs represents a bottleneck that limits stress tolerance at the whole-plant level is presently unclear. None the less, if osmolyte metabolism impinges on hexose or redox signalling, then it may be important in long-range signal transmission throughout the plant.Key-words: betaine; cold stress; drought; fructans; mannitol; osmolytes; proline; salinity; sugar signalling; trehalose. Abbreviations INTRODUCTIONEnvironmental stress is the major factor limiting plant productivity . Abiotic stresses which cause depletion of cellular water (drought, high soil salinity and temperature extremes) are responsible for the greatest agricultural losses. Upon exposure to these prevalent stresses, many plants accumulate organic osmolytes, most commonly polyhydroxylic compounds (saccharides and polyhydric alcohols) and zwitterionic alkylamines (amino acids and quaternary ammonium compounds).Several recent reviews discuss osmolyte accumulation in plants (Ingram & Bartels 1996;Serrano 1996). It is generally accepted that the increase in cellular osmolarity which results from the accumulation of non-toxic (thus 'compatible') osmotically active solutes is accompanied by the influx of water into, or at least a reduced efflux from, cells, thus providing the turgor necessary for cell expansion. None the less, a conclusive demonstration that osmotic adjustment contributes to fitness in stressful environments has yet to be achieved (Munns 1993). Since all subcellular structures must exist in an aqueous environment, tolerance to dehydration also depends on the ability of cells to maintain membrane integrity and prevent protein denaturation. Hypotheses that attribut...

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Kinetics of P absorption were investigated in mycorrhizal (Gkmws fascadana) and nonmycorrhizal tomato (Lycopersicon escude_uu) roots to determine why increased ion absorption by mycorrhizae occurs. Initial rates of absorption of "2P were measured at 1 to 100 micromolar KH2PO4 (pH 4.6). Absorption rates of mycorrhizae were about twice those of control roots. Augustnsson-Hofstee analysis yielded two linear phases; V.. and K,. were calculated for each phase. In the low phase (I to 20 micromolar), V..,,S values for the mycorrhizal and nonmycorhizal roots were each 0.10 micromoles P per gram fresh weight per hour while K. values were 1.6 and 3.9 micromolar KH2PO4, respectively. For the high phase (30 to 100 micromolar), V,. vahes for mycorrhizal and nonmycorrhizal roots were 0.32 and 0.25 micromoles P per gram fresh weight per hour and K. values were 35 and 42 micromolar, respectively. These results indicate that at the lower phase concentrations, similar to those expected in most soil solutions, a major factor contributing to the increased uptake was an apparent greater affinty of the absorbing sites for H2PO4-(lower K.).Mycorrhizal plants commonly accumulate more P than do nonmycorrhizal plants, especially when P availability is limited (5,9). Increased absorption is usually attributed to increased surface area and increased soil exploration by the root-fungus association (7,17,19). To our knowledge, the kinetics of ion uptake by mycorrhizae has not been investigated. Information of this type is needed to determine whether enhanced absorption is in fact due solely to increased number of absorbing sites contributed by the fungus, or possibly to greater ion affinity of the fungal absorption sites, or a combination of both factors. Such a kinetic approach to examination of the absorption of P by mycorrhizae is reported here. Tomatoes, known to exhibit a mycorrhizal response in ion uptake (3,18), were used in the investigation. MATERIALS AND METHODS