Principles and strategies in breeding for higher salt tolerance

Summary Salinity is an environmental component that usually reduces yield. Recent advances in the understanding of salt effects on plants have not revealed a reliable physiological or biochemical marker that can be used to rapidly screen for salt tolerance. The necessity of measuring salt tolerance based upon growth in saline relative to non-saline environments makes salt tolerance measurements and selection for tolerance difficult. Additionally, high variability in soil salinity and environmental interactions makes it questionable whether breeding should be conducted for tolerance or for high yield. Genetic techniques can be used to identify the components of variation attributable to genotype and environment, and the extent of genetic variation in saline and nonsaline environments can be used to estimate the potential for improving salt tolerance. Absolute salt tolerance can be improved best by increasing both absolute yield and relative salt tolerance.     

Salt tolerance in the halophyte Suaeda maritima L. Dum.

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.     

Isolation of a copper complex and its rate of appearance in roots of Mimulus guttatus

A Cu-complex was isolated from the roots of copper-tolerant Mimulus guttatus. The elution volume of the complex determined by gel permeation chromatography was similar to that of rat-liver cadmium thionein. The complex was heat stable, had a relatively high ratio of absorbance at 254 nm: 280 nm and incorporated ³⁵S. The complex, purified using a combination of gel permeation chromatography and anion-exchange chromatography, contained more glutamine/glutamic acid and glycine residues than mammalian metallothioneins. The amount of the complex in roots increased after 5 h growth in a solution containing 16 M Cu. Induction was preceded by an increase in the concentrations in root tissue of unknown compounds containing sulphur which may serve as precursors. The availability of these compounds appeared to regulate the rate of synthesis of this Cu-complex.Abbreviations CuBP copper-binding protein - HPLC highperformance liquid chromatography - MT metallothionein - Th thionein - Tris 2-amino-2-(hydroxymethyl)1-3-propanediol     

Water deficit enhancement of proline and α-amino nitrogen accumulation in potato plants and its association with susceptibility to drought

Potato plants ( Solanum tuberosum L. cvs 'Up-to-Date', 'Desiree', 'Alpha', 'Spunta', 'Elvira' and 'Troubadour') were exposed to cycles of water stress and relief during growth. Severe water deficit induced increased proline content 6- to 7-fold in nonturgid leaves which just started to wilt, and 8- to 27-fold in fully wilted leaves of potatoes. However, proline content was not affected during the early stages of stress development over a range of osmotic potentials in the leaves. The rising proline content was related to turgor loss of leaves independent of changes in the osmotic potentials, which indicates that proline involvement in osmoregulation of potato leaves is unlikely.
Repeated cycles of water stress and relief resulted in increased proline and α-amino nitrogen content in the tuber tissue of some of the cultivars. The smallest increase in proline content was obtained in 'Alpha' tubers and the content of α-amino nitrogen remained unaffected by the water stress. Concomitantly, 'Alpha' was the most drought-tolerant cultivar, as determined by its capacity to accumulate dry matter in tubers under stress conditions. On the other hand, in tubers of cultivars which were more susceptible to drought, a marked increase in proline and α-amino nitrogen was observed in response to water stress. The possible association of these findings with tolerance of potatoes to repeated short periods of drought is discussed.     

An observation of an association between adaptive pigmentation and thermal tolerance of fish

Replidated experiments indicate an association between thermal tolerance of juvenile bluegill sunfish and adaptive pigmentation to background color. Bluegills exposed to black backgrounds were able to survive higher temperatures longer than those exposed to blond backgrounds, when water temperatures were raised at a rate of 0.1°C/h from 24 to 36°C. Plausible explanations are considered which involve heat radiation effectiveness (by the fish) related to the black body concept, and stress associated with adaptive pigmentation.     

Electrical tolerance (breakdown) of theChara corallina plasmalemma: II. Inductive property of membrane and effects of pH o and impermeable monovalent cations on breakdown phenomenon

Summary Changes in the chord conductanceG and the membrane electromotive forceE _m in the so-called breakdown region of large negative potential of theChara plasmalemma were analyzed in more detail. In addition to the increase inG, the voltage sensitivity of the change inG increased, which was the cause of marked inductive current in the breakdown region. The breakdown potential, defined as a critical potential at which both low and high slope conductances of theI–V _m relationship cross, almost coincided with the potential at which an inductive current began to appear. This breakdown potential level changed with pH _o in a range between 5 and 9. TheChara plasmalemma was electrically most tolerant around pH _o 7.In some cellsE _m shifted to a positive level as large as +50+70 mV during the breakdown phenomenon. Such a large positive shift ofE _m is caused mainly by the increase in conductance of Cl^– and partly Ca²⁺ and K⁺.     

Notes on the use of wood refugia by Physella heterostropha (say) in a thermally disturbed swamp

Physella heterostropha size distributions and densities were measured in Pen Branch delta, a thermally altered swamp of the Savannah River, South Carolina. During cessation of thermal input, measurements were taken in a highly impacted area and more natural area. Of the three substrates sampled, logs, small woody material and benthos, snail density was highest on small pieces of submerged wood and < 20% of the snails on logs were out of the water. Following resumption of thermal input, snail density was four times higher on logs than previously measured. Snails responded to elevated water temperatures by congregating in a narrow band slightly above the water. Snail size did not appear to affect their response. However, the thermal history of the snails influenced their behaviour and survival rates. Logs may be a potential refuge for snails when water temperature are greater greater than 38°C. However, our results indicate that long-term survival in this manner may be impossible.     

Mechanical impedance increases aluminium tolerance of soybean (Glycine max) roots

The effect of aluminium (Al) on root elongation was studied in solution culture and sand culture. Compared to solution culture, in sand culture a ten times higher Al supply was necessary to inhibit root elongation to a comparable degree. This was due to a much lower Al uptake into the 5 mm root tips in sand culture. Fe concentrations in root tips were also lower in sand culture. Ca concentrations were higher and less depressed by Al, whereas Mg and K concentrations were not affected by the culture substrate. Regressions of Al concentrations in root tips versus inhibition of root elongation by Al revealed root damage at lower Al concentrations in sand culture. The effect of culture substrate on Al tolerance was independent of N source and could also be shown in flowing solution culture with and without sand. The results indicate that mechanical impedance in sand culture decreased Al uptake. This may be due to enhanced exudation of organic complexors thus reducing activites of monomeric Al species.     

Partial characterization of the trait for enhanced K+−Na+ discrimination in the D genome of wheat

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.     

Salinity tolerance of Kosteletzkya virginica. II. Root growth, lipid content, ion and water relations

Abstract. Kosteletzkya virginica (L.) Presl., a dicot halophyte native to brackish tidal marshes, was grown on nutrient solution containing 0. 85, 170 or 255 mol m ³ NaCl, and the effects of external salinity on root growth, ion and water levels, and lipid content were examined in successive harvests. Root growth paralleled shoot growth trends, with some enhancement observed at 85 mol m ³ NaCl and a reduction noted at the higher salinities. Root Na⁺ content increased with increasing external NaCl, but remained constant with time for each treatment. K⁺ content, although lower in salt-grown plants after 14 d salinization, subsequently increased to levels comparable to unsalinized plants. A strong K⁺ affinity was reflected in the increased K⁺/Na⁺ selectivity of salt-grown plants and by their low Na⁺/K⁺ ratios. Cl levels rose in salinized plants and values were double or more those for Na⁺, indicating the possibility of a sodium-excluding mechanism in roots. Root phospholipids and sterols, principal membrane constituents, were maintained or elevated and the free sterol/phospholipids ratio increased in salinized K. virginica plants, suggesting retention of overall membrane structure and decreased permeability. This response, considered in light of root calcium maintenance and high potassium levels, suggests that salinity-induced changes in membrane lipid composition may be important in preventing K⁺ leakage from cells.