Plant Salinity Tolerance

Abstract

Soil salinity is one of the major abiotic stresses limiting plant growth and resulting in crop yield reductions. The adverse effects of salinity stress on plant growth happens in two phases: the osmotic phase, which occurs immediately after salt stress and results in a rapid inhibition in plant growth; and the ionic phase, which occurs after several days or weeks of salt stress, when ions accumulate to high toxic concentrations in the shoot and affects shoot function. Plants have evolved several mechanisms to deal with salt stress, which can be divided into three mechanisms: osmotic tolerance, ion exclusion and tissue tolerance.

Key Concepts

  • Osmotic stress causes a rapid and immediate inhibition of plant growth as a result of high salt concentration around the roots.
  • Ionic stress occurs after several days or weeks of salt exposure, when Na+ accumulates in the shoot to toxic concentrations.
  • Osmotic tolerance is the ability of plants to maintain growth immediately after salt stress; it appears as an ability to better maintain growth and stomatal opening.
  • Ion exclusion is the ability to exclude Na+ from the shoots by the roots, to prevent accumulation of Na+ in the shoot; it depends on the up‐ and downregulation of specific ion transporters to control the amount of Na+ being transported to the shoot.
  • Tissue tolerance is the ability of plants to tolerate elevated levels of Na+ in the shoot; it requires Na+ compartmentalisation into the vacuole and accumulation of compatible solutes in the cytoplasm.
  • Halophytes are plants native to saline environments and that have high salinity tolerance.
  • Glycophytes are salt‐sensitive plants, being a large majority of plants.
  • Stomatal closure is closing of the stomatal pores to prevent water loss during salt and drought stress; it is stimulated by ABA, among other triggers
  • Vacuolar Na+ sequestration is an important tissue tolerance mechanism; It involves Na+ transport from cytosol into vacuole to help maintain low cytosolic Na+.
  • Salt bladders arise from epidermal cells; they are modified trichomes and provide an important mechanism of salt exclusion from the main part of leaves in halophytes.

Keywords: salt stress; osmotic stress; ionic stress; osmotic tolerance; ion exclusion; tissue tolerance; soil salinity; halophytes; glycophytes; compatible solutes

Figure 1. The effects of salt stress on plants and salinity tolerance mechanisms. Left side: the effect of salt stress on plants, which is thought to occur in two phases: the osmotic phase, which starts immediately after first exposure to high salt concentrations and may continue throughout exposure to salt; and the ionic phase, which occurs after several days of salt exposure, once ion concentrations have built up to high levels in the shoot and started to affect shoot function. The right side of the figure summarises the three main mechanisms of salinity tolerance in plants: osmotic tolerance, where long‐distance signals are transmitted from root to shoot to minimise the effect of salt stress on the reduction of shoot growth; tissue tolerance, which involves salt compartmentalisation in the vacuole (mediated by ion transporters and proton pumps) and accumulation of compatible solutes; and ion exclusion which involves several mechanisms that prevent the accumulation of toxic concentrations of Na+ and Cl within leaves, which involves, in particular, Na+ retrieval from xylem and Na+ efflux back to the soil.
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Further Reading

Deinlein U , Stephan AB , Horie T , et al. (2014) Plant salt‐tolerance mechanisms. Trends in Plant Science 19 (6): 371–379. DOI: 10.1016/j.tplants.2014.02.001.

Hanin M , Ebel C , Ngom M , Laplaze L and Masmoudi K (2016) New insights on plant salt tolerance mechanisms and their potential use for breeding. Frontiers in Plant Science 7 (1787). DOI: 10.3389/fpls.2016.01787.

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Wu H (2018) Plant salt tolerance and Na + sensing and transport. The Crop Journal 6 (3): 215–225. DOI: 10.1016/j.cj.2018.01.003.

Sánchez‐Barrena MJ , Martinez‐Ripoll M and Albert A (2018) Structural biology of salt and drought tolerance in plants. In: eLS. Chichester: John Wiley & Sons Ltd. DOI: 10.1002/9780470015902.a0027628.

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Al‐shareef, Nouf Owdah, and Tester, Mark(Mar 2019) Plant Salinity Tolerance. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001300.pub3]