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The growth and metabolism of plants in response to different concentrations of tissue K is discussed in relation to current knowledge about the distribution and functions of this ion in plant cells. In the cytoplasm, K has an important role in providing the correct ionic environment for metabolic processes. The ionic requirements of protein synthesis seem to be particularly important in determining the composition of the cytoplasm. Potassium is not replaceable in its cytoplasmic functions and the plant probably needs to maintain the cytoplasmic concentration of K in the range of 100 to 200 mM. Potassium salts in the vacuole are involved in the generation of turgor but when unavailable they can be replaced by other solutes. Salts of other cations such as Na and Mg are often a readily available alternative to K but in their absence organic solutes must be accumulated. With these observations as a basis, a model is proposed in which, as the concentration of K in the tissue declines, the concentration in the cytoplasm is initially maintained constant, while that in the vacuole decreases. In order to maintain turgor, alternative solutes are accumulated in the vacuole as replacements for K. It is assumed that K in the vacuole can only drop to a certain minimum level and, once this is reached, any further decline of tissue K must be at the expense of that in the cytoplasm. This leads to a decrease in the rate of metabolic processes that depend on K and so to a decline in growth. The hypothesis explains the observed relationships between growth and concentrations of K in tissues, and their modification by Na and other cations.

The long arm of chromosome 4D of wheat (Triticum aestivum L.) contains a gene (or genes) which influences the ability of wheat plants to discriminate between Na+ and K+. This discrimination most obviously affects transport from the roots to the shoots, in which less Na+ and more K+ accumulate in those plants which contain the long arm of chromosome 4D. Concentrations of Na+ and K+ in the roots, and Cl- concentrations in the roots and shoots, are not significantly affected by this trait, but Na+, K+ and Cl- contents of the grain are reduced. The trait operates over a wide range of salinities and appears to be constitutive. At the moment it is not possible to determine accurately the effect of this trait on growth or grain yield because the aneuploid lines which are available are much less vigorous and less fertile than their euploid parents.

The concentrations of the major inorganic ions and glycinebetaine, choline and proline and the osmotic pressure of extract sap have been determined in eight salt marsh species and four sand dune species from local habitats. These results together with those previously reported on hydroponically grown plants and data assembled from the literature show that glycinebetaine accumulation is a feature of members of the Chenopodiaceae, Amaranthaceae, many Gramineae and some members of the Solanaceae and Compositae, particularly when exposed to conditions of low soil water potential. It is suggested that in these families betaine is employed as a non-toxic cytoplasmic osmoticum when decreased osmotic potentials are required. In some other plant species proline may fulfil a similar function. Another quaternary ammonium compound may be accumulated in the Plumbaginaceae in addition to proline. Some evidence suggests that the differences in the organic osmoticum used may relate to the different inorganic ion contents of the plants. The accumulation of nitrogen dipoles as cytoplasmic osmotica may make heavy demands on the nitrogen economy of the plants and this problem is discussed briefly.

A modification of the pressure probe is described which allows accurate routine recording of the turgor pressure of single cells at measured depth within a tissue. Measurements of radial profiles of turgor pressure in wheat roots grown in some simple salt solutions (0.5 mol m−3 CaCl2, 0.5 mol m−3 CaCI2 plus 10 mol m−3 NaCl, and 0.5 mol m−3 CaCl2 plus 10 mol m−3 KCI), are described. Turgor pressure was constant (approximately, 0.65 MPa) along a radius within the elongation zone irrespective of the nature of the bathing solution. In mature root tissue turgor pressure in the cortex was lower than that of the growing zone in all treatments and the pressure of the stele was on average 0.22 MPa higher than that of the cortex. Potassium in the medium bathing the root increased the turgor pressure in mature root (both cortex and stele) relative to low salt and sodium treatments. The results are discussed in relation to both root growth and ion accumulation.

Lone, M. I., Kueh, J. S. H., Wyn Jones, R. G. and Bright, S. W. J. 1987. Influence of proline and glycinebetaine on salt tolerance of cultured barley embryos.—J. exp. Bot. 38: 479–490. The addition of exogenous proline and glycinebetaine to cultured barley (Hordeum vulgare L. cv. Maris Mink) embryos increased shoot elongation under saline conditions. Inhibition of shoot elongation by NaCl was relieved by proline when plantlets were grown in deep crystallizing dishes but not in Petri dishes where shoots come into direct contact with the medium. The effect of proline could be related to a decrease in shoot Cl– and Na+ accumulation which was only observed in plantlets grown in crystallizing dishes. Proline but not betaine uptake into cultured plantlets was stimulated by NaCl while each organic solute inhibited the endogenous synthesis of the other solute under salt stress. Comparison of the effects of exogenously supplied proline with enhanced endogenous proline accumulation in the mutant line R5201 suggested that the increased proline accumulation in the mutant is an order of magnitude too low to have a significant physiological effect. The implications of the effect of proline on ion transport, discrimination and accumulation are discussed.

Inorganic cation concentrations were measured in shoots of hexaploid bread wheat (Triticum aestivum L.) and its presumed ancestors grown at 100 mol m−3 external NaCl. Aegilops squarrosa and T. aestivum had high K/Na ratios while T. dicoccoides and Ae. speltoides had low K/Na ratios. T. monococcum although having a high K/Na ratio, had the highest total salt load of the five species tested. The effect of the D genome (from Ae. squarrosa) was further investigated in seedlings of synthetic hexaploid wheats, and was again found to improve cation selectivity. Different responses were obtained from root and shoot tissue in this experiment. One synthetic hexaploid and its constituent parents were grown to maturity at 100 mol m-3 NaCl and the yields recorded. Despite complications due to increased tillering in the stressed hexaploid, it was possible to show that the addition of the D genome enhanced yield characteristics in the hexaploid wheat. An experiment with synthetic hexaploids derived from the tetraploid wheat variety “Langdon” and several Ae. squarrosa accessions revealed differences in vegetative growth rates between the different synthetic hexaploids in the presence or absence of 150 or 200 mol m−3 external NaCl. The possibility of transferring salt tolerance genes from Ae. squarrosa to hexaploid wheat using synthetic hexaploids as bridging species is discussed.

The growth rate of hydroponically grown wheat roots was reduced by mannitol solutions of various osmotic pressures. For example, following 24 h exposure to 0·96 MPa mannitol root elongation was reduced from 1· mm h−1 to 0·1 mm h−1 Mature cell length was reduced from 290 μm in unstressed roots to 100 μm in 0·96 MPa mannitol. This indicates a reduction in cell production rate from about 4 per h in the unstressed roots to 1 per h in the highest stress treatment. The growing zone extended over the apical 4·5 mm in unstressed roots but became shorter as growth ceased in the proximal regions at higher levels of osmotic stress. The turgor pressure along the apical 5·0 mm of unstressed roots was between 0·5 and 0·6 MPa but declined to 0·41 MPa over the next 50 mm. Following 24 h in 0·48 (200 mol m−3) or 0·72 MPa (300 mol m−) mannitol, turgor along the apical 50 mm was indistinguishable from that of unstressed roots but turgor declined more steeply in the region 5·10 mm from the tip. At the highest level of stress (0·96 MPa or 400 mol m−3 mannitol) turgor declined steeply within the apical 20 mm.

In the first part of this review the main features of salt tolerance in higher plants are discussed. The hypothesis of intracellular compartmentation of solutes is used as a basis for models of tolerance mechanisms operating in roots and in leaves. Consideration is given to the implications of the various mechanisms for the yield potential of salt-tolerant crop plants. Some work on the more salt-tolerant members of the Triticeae is then described. The perennial species Elytrigia júncea and Leymus sabulosus survive prolonged exposure to 250 mol m⁻³ NaCl, whereas the annual Triticum species are severely affected at only 100 mol m⁻³ NaCl. In the perennial species the tissue ion levels are controlled within narrow limits. In contrast, the more susceptible wheats accumulate far more sodium and chloride than is needed for osmotic adjustment, and the effects of salt stress increase with time of exposure. Two different types of salt tolerance are exhibited in plants capable of growing at high salinities. In succulent Chenopodiaceae, for example, osmotic adjustment is achieved mainly by accumulation of high levels of sodium and chloride in the shoots, accompanied by synthesis of substantial amounts of the compatible solute glycinebetaine. This combination of mechanisms allows high growth rates, in terms of both fresh and dry weight. At the opposite end of the spectrum of salt tolerance responses are the halophytic grasses, which strictly limit the influx of salts into the shoots, but suffer from very much reduced growth rates under saline conditions. Another variation is shown in those species that possess salt glands. The development and exploitation of crop plants for use on saline soils is discussed in relation to the implications of these various mechanisms.

Excision and subsequent incubation of the apices (1 cm) of wheat (Triticum aestivum L.) seedling roots in simple media severely reduced elongation from 28 mm·(24 h)-1 in intact roots to a maximum of 2 mm·(24 h)-1 in excised roots. The reduction in growth was accompanied by a loss of cell turgor in the growing zone but was correlated with a hardening of the cell walls in this region. Rheological properties were measured as percent extensibility (both plastic and elastic) using a tensiometer, and as instantaneous volumetric elastic modulus (εi) using the pressure probe. Excision decreased plastic and elastic properties with a half-time of some 60 min. Plastic extension was reduced from 2.5% to 0.9% and elastic from 4.8% to 2.6% for an 8-g load. By contrast, εi was increased by excision. The observed reduction in root elongation rate was accompanied by a reduction in mature cell length from 240 μm to 40 μm and a shortening of the zone of cell expansion.

A study has been made of the osmotic responses of the green intertidal alga, Ulva lactuca, under two fluctuating salinity regimes; sinusoidal and square-wave fluctuations between 30 and 100% sea water in a 12 h cycle. These regimes closely resemble the tidal fluctuation of salinity encountered by the alga in its natural estuarine habitat. Data on changes in the inorganic ions, potassium, sodium, chloride and sulphate; in the organic solute, dimethylsulphoniopropionate; in the total sugar levels and estimated osmotic and turgor pressures under the two salinity regimes are reported. Significant differences in the solute responses under these different conditions were detected. In general, better control of ion fluxes appeared to be exercised under the sinusoidal conditions which also buffered changes in dimethylsulphoniopropionate levels. Influxes of potassium were highly light-dependent. Chloride levels conspicuously failed to reach the steady-state levels in the 6-h-hyper-osmotic part of either the abrupt or gradual cycle. The possible significance of these data, which may better reflect osmotic changes in the natural environment, and some of the problems encountered, particularly in accounting for charge balance under some conditions, are discussed.