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.

The role of calcium in salt toxicity

Thirty crop species provide 90% of our food, most of which display severe yield losses under moderate salinity. Securing and augmenting agricultural yield in times of global warming and population increase is urgent and should, aside from ameliorating saline soils, include attempts to increase crop plant salt tolerance. This short review provides an overview of the processes that limit growth and yield in saline conditions. Yield is reduced if soil salinity surpasses crop-specific thresholds, with cotton, barley and sugar beet being highly tolerant, while sweet potato, wheat and maize display high sensitivity. Apart from Na+ , also Cl- , Mg2+ , SO4 2- or HCO3 - contribute to salt toxicity. The inhibition of biochemical or physiological processes cause imbalance in metabolism and cell signalling and enhance the production of reactive oxygen species interfering with cell redox and energy state. Plant development and root patterning is disturbed, and this response depends on redox and reactive oxygen species signalling, calcium and plant hormones. The interlink of the physiological understanding of tolerance processes from molecular processes as well as the agronomical techniques for stabilizing growth and yield and their interlinks might help improving our crops for future demand and will provide improvement for cultivating crops in saline environment.

Salinity and crop yield

Plants acquire mineral nutrients from their native soil environments. Most crop plants are glycophytes and have evolved under conditions of low soil salinity. Consequently, they have developed mechanisms for absorbing, transporting, and utilizing mineral nutrients in nonsaline soils. Under saline conditions, which are characterized by low nutrient-ion activities and extreme ratios of Na / Ca , Na /K , Mg /Ca , and Cl /NO3 , nutritional disorders can develop and crop growth and quality may be reduced. This is not surprising, since under saline conditions, Na and/or Cl often exceed macronutrient concentrations by one or two orders of magnitude and even more in the case of micronutrients. Halophytes, native to saline environments, may also develop nutrient disorders despite their remarkable ability to absorb nutrients selectively from soil solutions dominated by Na and Cl . Nutrient imbalance can develop in salt-stressed plants in different ways. It may result from the effect of salinity on nutrient availability, uptake, transport, or partitioning within the plant or may be caused by physiological inactivation of a given nutrient resulting in an increase in the plant’s internal requirement for that essential element. It is likely that salinity may affect one or more of these processes at the same time, but whether or not this imbalance results in loss of crop yield or quality depends on its severity, which is influenced by a number of environmental factors. Nutrient availability and uptake by plants grown in saline environments is related to (a) the activity of the nutrient ion in the soil solution which depends on pH, pE (the negative log of

Mineral Nutrient Acquisition and Response by Plants Grown in Saline Environments

Root-zone salinization presents a challenge to plant productivity that is effectively countered by salt-tolerant halophytic plants, but unfortunately, much less successfully by major crop plants. The way in which salt affects plant metabolism is reviewed. Cellular events triggered by salinity, namely salt compartmentation, osmotic adjustment and cell wall hardening are connected to the whole plant responses, namely leaf necrosis, altered phenology and ultimately plant death. The roles of ion exclusion and K/Na discrimination in mediating crop response to salt appear to be central to the tolerance response, but they are by no means essential. The processes involved in regulating ion uptake at the membrane level are considered. Recent work elucidating the interaction between calcium and salinity tolerance is reviewed. Key words: Cell growth, cell turgor, ion regulation, K+/Na+ discrimination, osmotic adjustment, salt tolerance

Physiological responses of plants to salinity : a review

Acknowledgements. Introduction. A: Environment. 1. Global impact of salinity and agricultural ecosystems M.G. Pitman, A. Lauchli. 2. Salinity in the soil environment K.K. Tanji. 3. Salinity, halophytes and salt affected natural ecosystems S.-W. Breckle. B: Organisms. 4. Adaptation of halophilic Archaea to life at high salt concentrations A. Oren. 5. Adaptation of the haloterant alga Dunaliella to high salinity U. Pick. 6. Mangroves U. Luttge. C: Mechanisms. 7. Ultrastructural effects of salinity in higher plants H.-W. Koyro. 8. Intra- and inter-cellular compartmentation of ions - a study in specificity and plasticity G. Wyn Jones, J. Gorham. 9. Salinity, osmolytes and compatible solutes D. Rhodes, et al. 10. Sodium-calcium interactions under salinity stress G.R. Cramer. 11. Salinity and nitrogen nutrition W.R. Ullrich. 12. Pressure probe measurements of the driving forces for water transport in intact higher plants: effects of transpiration and salinity U. Zimmermann, et al. 13. Salinity, growth and phytohormones R. Munns. 14. The adaptive potential of plant development: evidence from the response to salinity G.N. Amzallag. D: Photosynthesis. 15. Influence of salinity on photosynthesis of halophytes C.E. Lovelock, M.C. Ball. 16. Performance of plants with C4-carboxylation modes of photosynthesis under salinity U. Luttge. 17. Induction of Crassulacean acid metabolism by salinity -- molecular aspects H.J. Bohnert.E: Molecules. 18. Function of membrane transport systems under salinity: plasma membrane L. Reinhold, M. Guy. 19. Function of membrane transport systems under salinity: tonoplast M. Binzel, R. Ratajczak. 20. Genetics of salinity responses and plant breeding J. Gorham, G.Wyn Jones. 21. Halotolerance genes in yeast R. Serrano. 22. The long and winding road to haloterance genes A. Maggio, et al. Index.

Salinity : environment - plants - molecules

Salinity-calcium interactions, which have been shown to be important in plants grown in dryland saline soils of the Canadian prairies, were studied in two species differing in salt tolerance. In solution culture, wheat showed a greater reduction in growth and a higher incidence of foliar Ca deficiency symptoms than barley when grown under MgSO4 or Na2SO4 plus MgSO4 salt stress. Amendment of the saline solution with Ca to increase the Ca/(Na+Mg) ratio ameliorated the effects of salt, but more so in wheat than in barley. At least part of the difference in salt tolerance between the two species must therefore relate to species differences in the interaction of salinity and Ca nutrition. The greater response of wheat to Ca was not due to a lower Ca status in leaf tissue; on the contrary, although Ca amendments improved tissue Ca/(Na+Mg) ratios in both species, salinized wheat had equivalent or higher Ca content, and higher Ca/(Na+Mg) ratios than did barley. The higher Ca requirement of wheat is apparently specific to a saline situation; at low salinity, wheat growth was not reduced as extensively as that of barley as Ca/(Na+Mg) ratio was decreased. High night-time humidity dramatically improved wheat growth under saline conditions, but increasing the Ca concentration of the saline solution had no effect on growth in the high humidity treatment. Membrane leakage from leaf tissue of wheat grown under saline conditions was increased compared to tissue from non-saline plants. Plants grown in Ca-amended saline solutions showed no increase in membrane leakage. These results confirm the importance of Ca interaction with salinity stress, and indicate differences in species response.

A. Cipywnyk

Papers

Salinity-induced calcium deficiencies in wheat and barley