The cell is surrounded by the plasma membrane containing ion channels that restrict Na+ from entering passively; influx of Na+ is passive as the cytoplasm is negatively charged with regard to the cell wall.Excess Na+ that leaks in is pumped back out by SOS1.This adjustment allows cells to maintain turgor and volume, so that the plant can continue to function.With osmotic adjustment, all cells in roots and shoots can continue to grow and expand, and leaves continue to carry out photosynthesis.Plants vary in their ability to control the uptake of Na+ and Cl for osmotic adjustment, in their efficiency at excluding or transporting ions across membranes, and on how they partition and transport ions within cells, tissues, and organs.There is a delicate balance between excluding salts to avoid excessive concentration in the leaves, while taking up sufficient ions for osmotic adjustment.Too little salt and the plant may suffer from water deficit or will have to use energy-expensive organic solutes , while too many salts will cause salt toxicity and kill the leaves.The more salt-tolerant crop species like barley, sugar beet and cotton have Na+ and Cl concentrations in leaves and roots that are close or equal to that of the external solution, thus allowing energy-efficient osmotic adjustment.If the cell Na+ and Cl concentration is not equal to the external solution,low round pots energy-rich organic solutes such as sucrose balance the external osmotic potential.Cell turgor and volume is maintained; however, these sugars are then no longer available for the synthesis of new cell walls and cell constituents such as proteins and so the plant grows slower.

A plant avoids salt toxicity by two independent mechanisms that exist in all plant species but are effective to varying extents in different species.These are ion exclusion by roots and ion compartmentation within all cells throughout the plant.Ion exclusion by roots—Roots exclude nearly all the salt in soil solution while taking up water.All plants, including halophytes, exclude 90–95% of the salt in the soil solution from the transpiration stream.Salts concentrate in leaves because plants evaporate about 95% of the water taken up by roots through leaf surfaces.It therefore follows that approximately 95% of the salt in the soil solution must be excluded by the roots to maintain a steady concentration in the leaves.Some species exclude up to 99% from their leaves via additional control points in the upper part of the root system, and the leaf bases and stems.Cellular ion compartmentation and “tissue tolerance”—The concept of “tissue tolerance” was based on observations that halophytes can accumulate NaCl in their leaves at very high concentrations , yet their enzymes that play a key role in essential metabolic processes are as sensitive to salt as are non-halophytes.Therefore, halophytes must effectively compartmentalize salt in the cell vacuole , thereby preventing their interference in key metabolic compartments within the cell.The strategy of sequestering Na+ and Cl in vacuoles and keeping concentrations low in the cytoplasm is critical to tissue tolerance.Membrane transporters responsible for the control of Na+ and Cl movement were studied intensively in the 1990s and their functions determined by electrophysiologists, as summarized in reviews focusing on membrane transport of Na+ in relation to salt tolerance by Apse and Blumwald , Munns and Tester , and Ismail and Horie.

In the following section we consider tolerance for saline and sodic soils separately and devote special attention to boron toxicity.Saline soils—Realizing that salt tolerance is determined by the control of Na+ and Cl uptake by roots and the transport of these ions within the plant, research has focused on identifying and cloning the genes responsible for this control.The main two approaches are to look for natural variation within crop species and create mutants in a model species that is amenable for genetic transformation.The model plant Arabidopsis has been used extensively, as its small genome speeds up gene discovery and its short life cycle and ease of transformation speeds up functional analysis of a candidate gene.Membrane transporters that are important in controlling Na+, Cl and K+ transport in relation to improving crop salt tolerance via molecular breeding have been the subject of several extensive and authoritative reviews, most recently by Ismail and Horie.For the control of Na+ transport within plants, three membrane transporter genes have received the most attention.These are SOS1, NHX1, and HKT1 family members, summarized in Table 1.The first two transporters are highly conserved across all species, and little natural genomic variation has been found.Although there are clear differences between species in the ability to accommodate these ions in vacuoles in their leaves, it remains unknown whether this is due to genetic variation in NHX1 leading to differences in levels of activity, to variation in the leakiness of the tonoplast, or to the efficiency of the proton pumps or ATPases that energize these transporters.The third transporter exhibits a degree of natural genetic variation especially in rice and is known to affect Na+ accumulation in leaves and hence salt tolerance in rice and wheat.

It may not be as toxic to metabolism as Na+ but this is difficult to know as we cannot measure the concentrations of Na+ or Cl in the cytosol where most of metabolism takes place, or in the mitochondria.All the same, Cl exclusion is especially relevant for perennials such as citrus and grapevine, which exclude Na+ well but over time Cl can accumulate to high levels in leaves.Grafting scions with stocks for Cl exclusion has been shown to improved yield for saline soil.Candidate genes for the control of Cl transport in salt-affected plants are reviewed by Ismail and Horie.Sodic soils—Sodic soils are those that have a high exchangeable sodium percentage and are described in more detail in Section 12.Soil sodicity can directly affect plant growth, such as by sodium-induced Ca2+ deficiencies , as well as indirectly due its adverse effect on soil structure.Under sodic conditions, soil aggregates are dispersed, leading to reduction of large soil pores thus affecting water flow and gas diffusion,plastic pots 30 liters increased soil strength and soil crusting.The increased soil strength reduces root proliferation and seedling emergence, and promote water logging thereby affecting plant growth by reducing oxygen diffusion to the roots and CO2 away from the root.Water logging notonly creates anoxia, but also reduces Fe3+ to Fe2+ and Mn4+ to Mn2+, sulfate to sulfide, and promotes denitrification, produces toxic constituents, and aggravates waterborne diseases.Therefore, a plant under sodic stress is likely encountering additional abiotic or biotic stresses.A survey of genetic variation in water logging and salt tolerance of many fodder plants for salt-affected soils is given by Rogers et al..Boron toxicity—Boron is often present in saline environments in excess amounts and can cause injury to susceptible crops.While an essential element, there is a small concentration range in the soil solution between what is deficient for plant growth and what is excessive.Boron uptake by plant roots occurs by passive diffusion across the plasma membrane,facilitated transport through intrinsic proteins in the membrane, and energy-dependent transport through a high affinity uptake system.Boron transporter genes that control the uptake of B have been identified in wheat as alleles of the transporter Bot-B5.Boron remains immobile in most species after it enters the leaf but in some, particularly stone-fruits, it can remobilize via the phloem to fruits and growing parts of the plant.

Boron forms complexes with polyols that allow for its mobility , therefore making it difficult to use tissue diagnosis for B deficiency and toxicity.Salinity-boron Interactions—Despite the common occurrence of salinity with boron, very little research has addressed the complex interaction of these two abiotic stresses on plant growth, which can be antagonistic or synergistic.Wimmer et al.found that combined salinity and boron stresses significantly increased the B-soluble fractions and that these soluble fractions were an indicator of B-toxicity.Soil pH can also influence the salinity-B interactions , and could affect membrane transport characteristics.Roots do most of the work protecting the plant from excessive salt uptake by excluding salts in the soil solution while taking up water, but we do not know whether this occurs in all parts of the root system, or whether it is confined to young roots or lateral branch roots.A more directed molecular breeding approach needs certainty about which cells or cell layers within the root anatomy are the site of Na+exclusion.Recent analysis suggests that the epidermis is the main site of Na+ exclusion, not the endodermis as was traditionally thought.Efflux of Na+ that has leaked into the root is expensive, possibly consuming over 10% of the total ATP produced by root respiration so it is important to know where in the root this occurs, and whether this is due entirely to SOS1 or other transporters.The other expensive process could be the maintenance of high concentrations of Na+ and Cl
in cell vacuoles, and we need to know the “leakiness” of the tonoplast membrane, which could result in a major costs to cells that need to keep pumping Na+ back into the vacuole.Sodic soils, which often have high pH, have received little attention by physiologists, yet they are more widespread than saline soils of neutral pH.Many sodic soils have very high pH of 9–10 or more, which alters the speciation and thereby solubility and uptake of many minerals including Al.Research is needed to better understand multiple stress interactions including the effects of soil compaction or water logging.In addition, B toxicity like many other abiotic stresses, causes the formation of reactive oxygen species, yet little is known about the actual mechanism of B toxicity in plants or how B toxicity affects the plant’s antioxidant defense system.Salinity-B interactions are complex and merit further scientific investigation.Summary: Mechanisms of adaptation to saline soils of neutral pH have been thoroughly studied by physiologists.A plant avoids salt toxicity by two independent mechanisms that are effective to varying extents in different species.These are ion exclusion by roots, and ion compartmentation within all cells.It is essential to keep Na+ concentrations low in the cell cytoplasm but at the same time minimize the energy costs of accumulating high concentrations of organic solutes for osmotic adjustment.Key genes for control of Na+ and Cl
uptake from a saline soil, their transport throughout the plant, and their sequestration within cells have been identified.Genomic variation exists but has not yet been fully explored and its usefulness exploited.Despite significant biotechnological efforts toward the development of crop plants that can increasingly tolerate salinity and water stress, progress has been slow and remains a huge challenge.In most countries, genetic modification of staple food products is not acceptable.Advances in “gene editing” have the potential to overcome the objections to previous GM technology but gene editing is still not widely accepted by regulators.Therefore, genetic approaches should continue looking for natural variation within species, rather than introducing genes from other species.Vast natural variation exists within the genome of crop species and their close relatives which is underutilized for breeding salt tolerance.This biodiversity is contained in large international seed collections and should be used to provide new germplasm with improved yield on salt-affected land.Why is this genetic variation not used more by breeders? To answer this question, we need to understand plant breeding methods and the requirements for commercial release of a new cultivar.The over-riding criteria for a new release are yield potential and quality of product.If yield on the best soil is reduced by the introduction of a salt-tolerant gene, it will be of no interest to breeders, even though yield on saline soil might be improved.There are two practical reasons for this:breeding trials are always conducted on typical soils of the regions, not on the more saline and saline soil is always heterogenous within a field, so total yield is largely determined from yield of the less saline parts of the field.These authors concluded that the most efficient way to increase yields at high salinity was to select for the best performing lines at low salinity.Not all breeders agree with this, but for most commercial breeding companies, yield on saline soil is subordinate to yield potential.Conventional breeding for salt tolerance starts with new germplasm with known variation for a specific quantitative trait, crossing into current breeding lines to introduce the trait, then a number of rounds of back-crossing to the elite parent to remove the unintended and undesirable traits that have been introduced with the new germplasm.