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The amount of salt-affected agricultural land is increasing globally, so new crop varieties are needed that can grow in salt-affected soils. Despite concerted effort to develop salt-tolerant cereal crops, few commercially viable salt-tolerant crops have been released. This is puzzling, given the number of naturally salt-tolerant grass species. To better understand why salt-tolerance occurs naturally but is difficult to breed into crop species, we take a novel, biodiversity-based approach to its study, examining the evolutionary lability of salt-tolerance across the grass family. We analyse the phylogenetic distribution of naturally salt-tolerant species on a phylogeny of 2684 grasses, and find that salt-tolerance has evolved over 70 times, in a wide range of grass lineages. These results are confirmed by repeating the analysis at genus level on a phylogeny of over 800 grass genera. While salt-tolerance evolves surprisingly often, we find that its evolution does not often give rise to a large clade of salt-tolerant species. These results suggest that salt-tolerance is an evolutionarily labile trait in grasses.

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Sodium chloride reduces the growth of rice seedlings, which accumulate excessive concentrations of sodium and chloride ions in their leaves. In this paper, we describe how silicon decreases transpirational bypass flow and ion concentrations in the xylem sap in rice (Oryza sativa L.) seedlings growing under NaCl stress. Salt (50 mM NaCl) reduced the growth of shoots and roots: adding silicate (3 mM) to the saline culture solution improved the growth of the shoots, but not roots. The improvement of shoot growth in the presence of silicate was correlated with reduced sodium concentration in the shoot. The net transport rate of Na from the root to shoot (expressed per unit of root mass) was also decreased by added silicate. There was, however, no effect of silicate on the net transport of potassium. Furthermore, in salt-stressed plants, silicate did not decrease the transpiration, and even increased it in seedlings pre-treated with silicate for 7 d prior to salt treatment, indicating that the reduction of sodium uptake by silicate was not simply through a reduction in volume flow from root to shoot. Experiments using trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), an apoplastic tracer, showed that silicate dramatically decreased transpirational bypass flow in rice (from about 4.2 to 0.8%), while the apparent sodium concentration in the xylem, which was estimated indirectly from the flux data, decreased from 6.2 to 2.8 mM. Direct measurements of the concentration of sodium in xylem sap sampled using Philaenus spumarius confirmed that the apparent reduction was not a consequence of sodium recycling. X-ray microanalysis showed that silicon was deposited in the outer part of the root and in the endodermis, being more obvious in the latter than in the former. The results suggest that silicon deposition in the exodermis and endodermis reduced sodium uptake in rice (Oryza sativa L.) seedlings under NaCl stress through a reduction in apoplastic transport across the root.

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Bicellular microhairs are present on the surfaces of leaves of grasses with the exception of the Pooideae. In some halophytic grasses, these glandular hairs secrete salt, suggesting the intriguing question 'can the microhairs of grasses that do not normally encounter salinity also secrete salt?' Microhairs were counted in replicas of the adaxial and abaxial surfaces of leaves of various ages of maize plants growing either in the absence of salt or in the presence of 40, 80 or 120 mM NaCl. The number of microhairs per unit area of adaxial leaf surface of the youngest leaf almost doubled as the salinity increased from zero to 120 mM NaCl; on the abaxial surface, the number of microhairs increased by 50%. Spraying this leaf with benzyl adenine (BA) caused, when averaged across salinities and surfaces, a 32% increase in the number of microhairs. Salinity reduced leaf area but in all the salinity treatments, spraying with BA increased the total number of microhairs per leaf. Washing leaves of plants provided estimates of the loss of salt from those leaves. There were large differences between the Na:K molar ratios in the washing solution and the leaf tissue, indicating a high selectivity for sodium over potassium for loss from the leaf. BA did not influence the efficiency of salt loss, expressed per microhair, at any salinity level, but did increase loss per leaf. Thus, BA increased salt loss from plants due to its influence on the number of microhairs and leaf area, but not due to its effect on the efficiency of the secretion process per se.

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Salinity is an ever-present threat to crop yields, especially in countries where irrigation is an essential aid to agriculture. Although the tolerance of saline conditions by plants is variable, crop species are generally intolerant of one-third of the concentration of salts found in seawater. Attempts to improve the salt tolerance of crops through conventional breeding programmes have met with very limited success, due to the complexity of the trait: salt tolerance is complex genetically and physiologically. Tolerance often shows the characteristics of a multigenic trait, with quantitative trait loci (QTLs) associated with tolerance identified in barley, citrus, rice, and tomato and with ion transport under saline conditions in barley, citrus and rice. Physiologically salt tolerance is also complex, with halophytes and less tolerant plants showing a wide range of adaptations. Attempts to enhance tolerance have involved conventional breeding programmes, the use of in vitro selection, pooling physiological traits, interspecific hybridization, using halophytes as alternative crops, the use of marker-aided selection, and the use of transgenic plants. It is surprising that, in spite of the complexity of salt tolerance, there are commonly claims in the literature that the transfer of a single or a few genes can increase the tolerance of plants to saline conditions. Evaluation of such claims reveals that, of the 68 papers produced between 1993 and early 2003, only 19 report quantitative estimates of plant growth. Of these, four papers contain quantitative data on the response of transformants and wild-type of six species without and with salinity applied in an appropriate manner. About half of all the papers report data on experiments conducted under conditions where there is little or no transpiration: such experiments may provide insights into components of tolerance, but are not grounds for claims of enhanced tolerance at the whole plant level. Whether enhanced tolerance, where properly established, is due to the chance alteration of a factor that is limiting in a complex chain or an effect on signalling remains to be elucidated. After ten years of research using transgenic plants to alter salt tolerance, the value of this approach has yet to be established in the field.

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Production of grain legumes is severely reduced in salt-affected soils because their ability to form and maintain nitrogen-fixing nodules is impaired by both salinity and sodicity (alkalinity). Genotypes of chickpea, Cicer arietinum, with high nodulation capacity under stress were identified by field screening in a sodic soil in India and subsequently evaluated quantitatively for nitrogen fixation in a glasshouse study in a saline but neutral soil in the UK. In the field, pH 8.9 was the critical upper limit for most genotypes studied but genotypes with high nodulation outperformed all others at pH 9.0-9.2. The threshold limit of soil salinity for shoot growth was at ECe 3 dS m(-1), except for the high-nodulation selection for which it was ECe 6. Nodulation was reduced in all genotypes at salinities above 3 dS m(-1) but to a lesser extent in the high-nodulation selection, which proved inherently superior under both non-saline and stress conditions. Nitrogen fixation was also much more tolerant of salinity in this selection than in the other genotypes studied. The results show that chickpea genotypes tolerant of salt-affected soil have better nodulation and support higher rates of symbiotic nitrogen fixation than sensitive genotypes.

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Rice (Oryza sativa) is sensitive to salinity, which affects one-fifth of irrigated land worldwide. Reducing sodium and chloride uptake into rice while maintaining potassium uptake are characteristics that would aid growth under saline conditions. We describe genetic determinants of the net quantity of ions transported to the shoot, clearly distinguishing between quantitative trait loci (QTL) for the quantity of ions in a shoot and for those that affect the concentration of an ion in the shoot. The latter coincide with QTL for vegetative growth (vigor) and their interpretation is therefore ambiguous. We distinguished those QTL that are independent of vigor and thus directly indicate quantitative variation in the underlying mechanisms of ion uptake. These QTL independently govern sodium uptake, potassium uptake, and sodium:potassium selectivity. The QTL for sodium and potassium uptake are on different linkage groups (chromosomes). This is consistent with the independent inheritance of sodium and potassium uptake in the mapping population and with the mechanistically different uptake pathways for sodium and potassium in rice under saline conditions (apoplastic leakage and membrane transport, respectively). We report the chromosomal location of ion transport and selectivity traits that are compatible with agronomic needs and we indicate markers to assist selection in a breeding program. Based upon knowledge of the underlying mechanisms of ion uptake in rice, we argue that QTL for sodium transport are likely to act through the control of root development, whereas QTL for potassium uptake are likely to act through the structure or regulation of membrane-sited transport components.

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Secondary salinization and its relationship to irrigation are strong incentives to improve the tolerance of crops to salinity and to drought. Achieving this through the pyramiding of physiological traits (phenotypic selection without knowledge of genotype) is feasible. However, wide application of this approach is limited by the practicalities of assessing not only the parents, but also large numbers of individuals and families in segregating generations. Genotypic information is required in the form of markers for any quantitative trait loci involved (marker-assisted selection) or of direct knowledge of the genes. In the absence of adequate candidate genes for salt tolerance, a quantitative trait locus/marker-assisted selection approach has been used here. Putative markers for ion transport and selectivity, identified from analysis of amplified fragment length polymorphism, had been discovered within a custom-made mapping population of rice. Here it is reported that none of these markers showed any association with similar traits in a closely related population of recombinant inbred lines or in selections of a cultivar. Whilst markers will be of value in using élite lines from the mapping population in backcrossing, this has to be considered alongside the effort required to develop and map any given population. This result cautions against any expectation of a general applicability of markers for physiological traits. It is concluded that direct knowledge of the genes involved is needed. This cannot be achieved at present by positional cloning. The elucidation of candidate genes is required. Here the problem lies not in the analysis of gene expression but in devising protocols in which only those genes of interest are differentially affected by the experimental treatments.

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Five varieties of rice (Oryza sativa L.) of varying salinity resistance were grown in non-saline and in saline conditions, with and without a repeated exposure to ozone at a concentration of 83 nmol mol(-1) giving an AOT40 (cumulative exposure above 40 nmol mol(-1)) of 3600 nmol mol(-1) h. Salinity caused a substantial reduction in shoot and root dry weight in all varieties, but the effect on root growth was proportionately less than on shoot growth. Ozone reduced root dry weight but the treatment used did not significantly affect shoot dry weight. Both salinity and ozone reduced plant height. The potassium concentration in the leaves of all five varieties was reduced by salinity, and by ozone in both saline and non-saline treatments. Ozone reduced the sodium concentration in plants grown at 50 mM NaCl but had no effect upon the chloride concentration. Carbon dioxide assimilation, transpiration and stomatal conductance were all reduced by salinity and by ozone and there was close quantitative similarity between the effects of ozone and/or salinity upon assimilation, stomatal conductance and transpiration. There were some antagonistic effects but there were additive effects of salinity and of ozone on root dry weight, plant height, shoot potassium concentration, photosynthesis, transpiration and stomatal conductance. The possible basis of the additive effects of salinity and ozone on gas exchange and mineral uptake are discussed.

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A multiple cross was constructed with the aim of combining component traits for the complex salinity resistance character. The aim was to combine donors for physiological traits with the agronomically desirable semidwarf/intermediate plant type and with the overall salinity resistance of the traditional tall land races. We report a study of selection strategies in the resulting breeding population. The effects of early selection for agronomic traits and early selection for low sodium transport were compared with a control population in which minimal selection was practised. Conventional selection for agronomic characters at early generations selected against low sodium-transporting (and thus potentially salt-tolerant) genotypes. In contrast, mild early selection for low sodium transport enriched the population in potentially salt-resistant genotypes but did not select against agronomic (semi-dwarf/intermediate) genotypes. It is concluded that selection for agronomic traits should be made after selection for salt resistance and, ideally, should be delayed until the population has reached near-homozygosity.

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The determination of ion concentrations within cells and sub-cellular compartments remains a difficult procedure, as the volumes to be analyzed are rather small. X-ray microanalysis is sufficiently sensitive, and has adequate resolution, to measure these concentrations. The major difficulties are related to the preparation of material for analysis. We have compared the measurement of sodium, potassium, and chloride contents in a salt tolerant unicellular alga, Dunaliella parva, following either freeze-substitution (using two different resins) or molecular distillation drying. All three procedures gave similar results: after freeze substitution, ion contents were marginally (but not significantly) higher following embedding in Nanoplast MUV 116 resin than in Spurr resin. Since the Nanoplast can be polymerised at low temperatures, it has advantages over the Spurr resin.

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Phenotypic resistance of salinity is expressed as the ability to survive and grow in a salinised medium. Some subjective measure of overall performance has normally been used in plant breeding programmes aimed at increasing salinity resistance, not only to evaluate progeny, but to select parents. Salinity resistance has, at least implicitly, been treated as a single trait. Physiological studies of rice suggest that a range of characteristics (such as low shoot sodium concentration, compartmentation of salt in older rather than younger leaves, tolerance to salt within leaves and plant vigour) would increase the ability of the plant to cope with salinity. We describe the screening of a large number of rice genotypes for overall performance (using an objective measure based on survival) and for the aforementioned physiological traits. There was wide variation in all the characters studied, but only vigour was strongly correlated with survival. Shoot sodium concentration, which a priori is expected to be important, accounted for only a small proportion of the variability in the survival of salinity. Tissue tolerance (the cellular component of resistance reflecting the ability to compartmentalise salt within leaves) revealed a fivefold range between genotypes in the tolerance of their leaves to salt, but this was not correlated positively with survival. On the basis of such (lack of) correlation, these traits would be rejected in normal plant breeding practice, but we discuss the fallacies involved in attempting correlation between individual traits and the overall performance of a salt-sensitive species in saline conditions. We conclude that whilst overall performance (survival) can be used to evaluate the salt resistance of a genotype, it is not the basis on which parents should be selected to construct a complex character through breeding. It was the norm for varieties which had one good characteristic affecting salt resistance to be unexceptional or poor in the others. This constitutes experimental evidence that the potential for salt resistance present in the rice genome has not been realised in genotypes currently extant. The results are discussed in relation to the use of physiological traits in plant breeding, with particular reference to environmental stresses that do not affect a significant part of a species' ecological range.

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The ion content of compartments within cortical cells of mature roots of the halophyte Suaeda maritima grown at 200 mol·m(-3) NaCl has been studied by X-ray microanalysis of freeze-substituted thin sections. Sodium and Cl were found in the vacuoles at about four-times the concentration in the cytoplasm or cell walls, whereas K was more concentrated in the cell walls and cytoplasm than in vacuoles. The vacuolar Na concentration was 12- to 13-times higher than that of K. The Na concentration of cell walls of cortical cells was about 95 mol·m(-3) of analysed volume. The cytoplasmic K concentration within the mature cortical cells was estimated to be 55 mol·m(-3) of analysed volume.

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Rates of ion transport and transpiration were measured during the day and night in whole seedlings of Suaeda maritima growing over a range of salinities, in order to calculate concentrations of sodium and potassium in the xylem during these periods. Mean sodium concentration in the xylem was maximal at 56 mol m-3 Na with an external salinity of 200 mol m-3 NaCl. The sodium concentration in the xylem was greater in the dark than in the light at all external salinities investigated. Comparison of the external sodium with that in the xylem indicated that sodium was more strongly excluded from the transpiration stream as salinity increased. The mean concentration of potassium in the xylem declined as external NaCl concentration increased, although selectivity for potassium increased at higher salinities. Results are discussed in relation to osmotic adjustment in S. maritima.

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Significant varietal differences were apparent in the survival of seedlings of maize in saline conditions but only at relatively high external concentrations (200 mol m-3 NaCl), where there was a range from 0 to 66% survival, 25 d after salinization. For the varieties examined there was a strong negative correlation between Na concentrations in the third leaf and survival. Two resistant varieties (Across 8024 and Protador) and one salt-sensitive variety (LG11 ) were identified. The characteristics of ion accumulation were clearly different in salt-tolerant and salt-sensitive types, the difference becoming more pronounced with plant age. The distribution of ions, particular those of Na, K and Cl, was determined within subcellular compartments of roots cells using X-ray microanalysis of freeze-substituted tissue. Salinity induced a greater increase (about 1.7 times) in cytoplasmic Na concentration in the salt-sensitive variety (LG11 ) than in resistant varieties (Across 8024 or Protador). The mean K:Na ratio in the cytoplasm of the root cortical cells in the salt-resistant varieties grown for 15 d in saline conditions (100 mol m-3 NaCl) was twice that found for LG11 . Sodium and Cl concentrations in the vacuoles decreased radially inwards from the epidermal cells in salt-treated roots of LG11 .

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Osmotic potentials and individual epidermal cell turgor pressures were measured in the leaves of seedlings of Suaeda maritima growing over a range of salinities. Leaf osmotic potentials were lower (more negative) the higher the salt concentration of the solution and were lowest in the youngest leaves and stem apices, producing a gradient of osmotic potential towards the apex of the plant. Epidermal cell turgor pressures were of the order of 0.25 to 0.3 MPa in the youngest leaves measured, decreasing to under 0.05 MPa for the oldest leaves. This pattern of turgor pressure was largely unaffected by external salinity. Calculation of leaf water potential indicated that the gradient between young leaves and the external medium was not altered by salinity, but with older leaves, however, this gradient diminished from being the same as that for young leaves in the absence of NaCl, to under 30% of this value at 400 mM NaCl. These results are discussed in relation to the growth response of S. maritima.

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Addition of polyethylene glycol (PEG) as an osmotic agent (at -230 kilopascals) dramatically lessened the toxicity of NaCl (at 50 moles per cubic meter) to rice (Oryza sativa L.) seedlings. This was explained by a reduction in the uptake of NaCl. This reduction was much greater than could be accounted for by the lowered transpiration rate resulting from the solute potential changes due to the PEG.Low concentrations of PEG (-33 kilopascals and less) had no effect upon transpiration rate but reduced sodium uptake (from 10-50 moles per cubic meter NaCl) by up to 80%. PEG (at -33 kilopascals) also reduced chloride uptake but had no effect upon the uptake of potassium from low (0.5-2.0 moles per cubic meter) external concentrations. However, the increased uptake of potassium occurring between 2 and 10 moles per cubic meter external concentration was abolished by PEG. Similar concentrations of mannitol had no effect upon sodium uptake in rice. PEG, in similar conditions, had much less effect upon sodium uptake by the more salt-resistant species, barley.(22)Na studies showed that PEG reduced the transport of sodium from root to shoot, but had a long half time for maximal effect (several days).(14)C-labeled PEG was shown to bind to microsomal membranes isolated from rice roots; it is suggested that this is due to multipoint attachment of the complex ions of PEG which exist in aqueous solutions. It is argued that this reduces passive membrane permeability, which accounts for the large effect of PEG on sodium influx in rice and the different effects on sodium influx and (carrier-dependent) potassium influx.

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Absolute measurements of elemental concentrations within thin biological samples are often made by reference to a series of standards which resemble the samples in chemical and physical properties and the linear relationship between (p-b)/c and concentration. This principle requires that the chemical and physical properties of the matrix remain constant throughout a series of standards with different elemental contents and throughout different regions of the samples. Some of the changes undergone by specimens during X-ray microanalysis, e.g. loss of elements or organic mass loss, are also influenced by the composition of the matrix. A simple empirical modification to the linear (p-b)/c versus concentration relationship is presented to account for some of these effects and therefore improve quantitation of analyses.

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Grown under saline conditions, Suaeda maritima accumulates Na(+) and Cl(-) into its leaves, where individual mesophyll cells behave differently in their compartmentation of these ions. Measurements of ion concentrations within selected subcellular compartments show that freeze-substitution with dry sectioning is a valuable preparative technique for analytical electron microscopy of highly vacuolate plant material. Using this approach, absolute estimates were made of Na(+), K(+) and Cl(-) concentrations in the cytoplasm, cell walls, chloroplasts and vacuoles of leaf mesophyll cells.

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An attempt has been made to localize glycinebetaine in shoots of Suaeda maritima L. Dum. using a technique based on the formation of an iodoplatinate precipitate. Deposits were largely restricted to the cytoplasm of salt-grown plants and were analysed by transmission analytical electron microscopy. The results are considered to support the hypothesis that glycinebetaine acts as a cytoplasmic osmoticum to balance high vacuolar salt levels in certain halophytes.

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An amino acid-incorporating microsomal fraction has been isolated from the leaves of the halophyte Suaeda maritima and the characteristics of the incorporation described. There were no differences in the properties of the microsomes isolated from plants grown in saline and non-saline conditions. The incorporation was severely inhibited by high concentrations of sodium or potassium ions. The results are discussed in relation to the mechanism of salt tolerance in halophytes and the localization of salt in the cells.