Salt toxicity comprises osmotic and ionic components both of which can severely affect root and shoot growth. Uptake of Na+ across the plasma membrane is very fast resulting in physiological effects on extracellular as well as intracellular sites. Sodium reduces binding of Ca2+ to the plasma membrane, inhibits influx while increasing efflux of Ca2+, and depletes the internal stores of Ca2+ from endomembranes. These changes in the cell Ca2+ homeostasis are suggested here to be the primary responses to salt stress that are perceived by root cells. Salt would almost instantly reduce the amount of Ca2+ being transferred to the leaf cells, with Ca2+ activity dropping and Na+ activity rising in the apoplasm of leaf cells. This Ca2+ signal would be transported to leaves together with, if not preceding, the signal of limited water supply. Hormonal signals are likely to be secondary in nature and caused by the Na+‐related disturbance of the root cell Ca2+ homeostasis. Ameliorative effects of supplemental Ca2+ on salt stress are exerted through preventing Na+‐related changes in the cell Ca2+ homeostasis.

Quantitative information on the uptake and distribution of Al at the cellular level is required to understand mechanisms of Al toxicity, but direct measurement of uptake across the plasma membrane has remained elusive. We measured rates of Al transport across membranes in single cells of Chara corallina using the rare 26 Al isotope, an emerging technology (accelerator mass spectrometry), and a surgical technique for isolating subcellular compartments. Accumulation of Al in the cell wall dominated total uptake (71-318 g m Ϫ2 min Ϫ1 ), although transport across the plasma membrane was detectable (71-540 ng m Ϫ2 min Ϫ1 ) within 30 min of exposure. Transport across the tonoplast was initially negligible, but accelerated to rates approximating uptake across the plasma membrane. The avacuolate protoplasm showed signs of saturation after 60 min, but continued movement across the plasma membrane was supported by sequestration in the vacuole. Saturation of all compartments was observed after 12 to 24 h. Accumulation of Al in the cell wall reflected variation in {Al 3ϩ } induced by changes in Al supply or complexing ligands, but was unaffected by pH. In contrast, transport across the plasma membrane peaked at pH 4.3 and increased when {Al 3ϩ } was reduced by complexing ligands. Cold temperature (4°C) reduced accumulation in the cell wall and protoplasm, whereas 2,4-dinitrophenol and m-chlorocarbonylcyanidephenyl hydrazone increased membrane transport by 12-to 13-fold. Our data suggest that the cell wall is the major site of Al accumulation. Nonetheless, membrane transport occurs within minutes of exposure and is supported by subsequent sequestration in the vacuole. The rapid delivery of Al to the protoplasm suggests that intracellular lesions may be possible.

summary Aluminium is the most important growth‐limiting factor in many acid soils throughout the world. Physiological effects of Al toxicity and mechanisms of tolerance are not well understood. An initial uptake of Al is confined to the apoplasm. Aluminium complexes (whose exact identification is beyond current experimental techniques) enter the cytosol slowly and only after prolonged exposure. Electrochemical properties of the cell wall Donnan free space as well as the plasma membrane of root cells are altered by the presence of Al ions that are polyvalent cations in acidic environments. The primary Al effects are very fast (taking only seconds to several minutes to develop) and may therefore occur while Al is still in the Donnan free space and on the apoplasmic side of the plasma membrane. Resumption of root growth upon removal of Al ions supports such a claim. Aluminium affects membrane permeability for both electrolytes and non‐electrolytes; it reduces accumulation of divalent cations (especially Ca and Mg) by interfering with the membrane transport. Aluminium alters the pattern of Ca2+ fluxes across the plasma membrane, thus supposedly disturbing symplasmic Ca2+ homeostasis. Supplemental Ca2+ can greatly alleviate deleterious Al effects. Frequently observed changes in the secretory activity of root cells exposed to Al are mediated through altered cell Ca2+ homeostasis; they result in cessation of cell wall growth and stoppage of root elongation. Involvement of calmodulin in the Al‐related phenomena is suggested to be indirect, at least in the initial stages of the Al treatment when Al is likely to be confined to the apoplasm. The role of growth substances, at least auxins and cytokinins, in the Al toxicity syndrome appears to be related to the Ca‐Al interactions that may alter the pattern of auxin transport as well as cytokinin biosynthesis and transport. Disturbance of the cell Ca2+ homeostasis appears to be an important feature of ion‐related environmental stresses in general (salt, heavy metals, aluminium).

SUMMARY The question of intracellular or extracellular primary lesion in the Al toxicity syndrome is unresolved. One of the crucial points for answering that question is quantification of Al fluxes across the plasma membrane within seconds or minutes of exposure of roots to Al, i.e. concurrent with or preceding the first symptoms of Al toxicity. A review of the available literature on Al uptake shows that there is abundant information on Al accumulation in root tissues only after the relatively prolonged uptake period, ranging from 30 min to over 24 h. Most of these reports either assume or claim explicitly that intracellular Al was measured, even though Al ions are bound strongly to negative charges in the apoplasm. Therefore, an effective and complete desorption of apoplasmic Al after the uptake period is crucial for measurements of intracellular Al. However, published studies do not seem to have desorbed cell‐wall‐bound Al appropriately. Recent studies with giant algal cells of Chara corallina, where physical separation of cell wall and cytoplasm after the uptake period can be achieved surgically, showed that desorption of Al from the apoplasm, even employing a wider variety of desorbents and more stringent conditions than in any of the previously published reports, cannot be achieved completely within 5 h. Consequently, measured rates of uptake of Al across the plasma membrane of intact Chara cells employing the cell wall/cytoplasm separation technique are up to several orders of magnitude lower than previously published values in which cell‐wall Al was attributed to the transmembrane uptake component. Although there is no doubt that Al does cross the plasma membrane, no information exists about which Al species or complexes take part in the transmembrane flux, mainly because of complexities inherent in Al speciation at the solution/ion exchanger (i.e. the cell wall and the membrane) interface. Similarly, it is not known which membrane transporters are involved in transport of Al across the plasma membrane. The Al‐resistant plant genotypes generally accumulate less Al in the root tips than do the Al‐sensitive genotypes. No direct relationship, however, appears to exist between increased organic acid extrusion as a mechanism of resistance to Al and decreased transmembrane flux of Al. Al‐accumulator plant species accumulate relatively large amounts of Al in their tissues without having a greater Al uptake rate than non‐accumulator genotypes. Progress in deciphering structural and functional aspects of transport of Al across the plasma membrane of intact plant cells will rely on using giant algal cells in which physical separation of the cell wall and the cytoplasm can be achieved, because, at present, there is no reliable quantitative method which can overcome problems presented by a relatively large apoplasmic Al pool remaining after desorption of intact root cells of higher plants.

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