Perennials, halophytes and reclamation plants

When land becomes salinized it is abandoned. Over the last decade, efforts have been made to reclaim salinized land for some sort of economic return. Halophytic plants have been investigated for this purpose. Halophytes are the native flora of saline soils, and can complete their life cycle at salinities above 250 mM NaCl. Most halophytes need at least 1 mM NaCl to grow well, many need 10-50 mM NaCl to reach maximum growth, and a few grow best at 200-300 mM NaCl (Flowers et al., 1986). Many halophytes are very slow-growing even at their optimal salinity, but a few are suitable for reclamation. For instance, tall wheat grass (Thinopyrum ponticum, a decaploid form of Agropyrum elongatum) is widespread in many continents and now planted as a fodder species in saline soil. Another species being planted for fodder is the dicotyledenous halophyte, Atriplex amnicola, a saltbush species native to Western Australia. Some halophytes are more tolerant than this, but A. amnicola shows the growth maximum at low salinity and the extended growth at very high salinities that is typical of many dicotyledenous halophytes.

Recently there has been strong interest in the salinity tolerance of trees that are not halophytes but have good salinity tolerance and high transpiration rates that can be planted in saline soils to lower water tables. Such species need also to be tolerant of waterlogging as secondary salinity occurs with rising water tables, so the ability to withstand periods of waterlogging and to continue high transpiration rates is essential. In Australia, river red gum (Eucalptus camaldulensis) has been the most widely used. However there are a number of other species that are better able to tolerate saline and waterlogged soils. Many Acacia, Casuarina and Melaleuca species are tolerant of high salinity; for example A. stenophylla did not suffer a 50% growth reduction until an ECe of 20 dS m-1, equivalent to about 200 mM NaCl (Marcar et al., 1995). Such species are recommended for reclamation of land that has become saline because of irrigation, as they provide some income to farmers as fodder and fuelwood.

Molecular approaches to achieve salinity tolerance

Gene transformation
Significant advances in the field of molecular biology technology have been made during the past decade. The use of molecular techniques to selectively introduce desired genes may provide alternative ways to classical plant breeding to achieve salinity tolerance. These techniques will benefit the development of salinity-tolerant cultivars based on specific traits that are controlled by one gene, eg a transcription factor or an important ion channel. The work of Blumwald and colleagues (e.g. Zhang and Blumwald, 2001) shows the progress made by using molecular technology. The authors reported the development of a salinity tolerant transgenic tomato plant in which over-expression of the vacuolar Na+/H+ antiporter shows dramatic improvement of vegetative growth and of fruit yield. This antiporter is the only known transporter that would compartmentalise Na+ in the vacuole, where Na+ has little chance of toxic effect on metabolism, or to be transported to younger leaves and fruits. These studies indicate great potential for transgenic methodology, but so far the evidence of the mechanism is not proven.

Flowers (2004) has questioned the current 'hype' (hyperbole) involving transgenics and concluded that so far no useful increase in salinity tolerance has been achieved. A number of genes, encoding proteins with known functions in ion transport or in synthesis of compatible organic solutes, as well as genes whose functions are not fully understood, have been used to transform a number of species in efforts to improve salinity tolerance. Despite numerous claims of improved salinity tolerance, poor experimental designs and choices of parameters measured to evaluate tolerance mean that much doubt remains. None of these transgenics has been proven in the field. Since salinity tolerance is a multigenic trait, large improvements based on modification of only one gene could only occur if the gene is a transcription factor and regulates a number of genes that control ion transport or some other process involved in salinity tolerance.
Given the natural diversity that exists, and given the current social antipathy to genetically-engineered crops, it might be more realistic to consider using the genes identified as perfect markers for naturally-occurring diversity.

Molecular markers
The development of molecular markers for physiological traits has made significant headway in recent years with the advancement of new technologies. Consequently, the use of molecular markers in breeding programs is increasing rapidly as they have been shown to greatly improve the efficiency of the breeding programs. Marker-assisted selection is non-destructive and can provide information on the genotype of a single plant without exposing the plant to the stress. The technology is capable of handling large numbers of samples. PCR-based molecular markers have the potential to reduce the time, effort and expense often associated with physiological screening. In order to use marker-assisted selection in breeding programs, the markers must be closely linked to the trait, and work across different genetic backgrounds.

The development of robust markers that are reliable across a wide range of backgrounds can be quite difficult, and is entirely dependent on an accurate phenotype screen. Understanding the physiology of sodium uptake is critical to the development of a reliable and accurate phenotype test, and thereby to the identification of a QTL (Quantitative Trait Locus) and a molecular marker linked to the locus.

QTL mapping and marker-assisted selection is a technique that has many advantages over phenotypic screening as a selection tool. The efficiency of genetic mapping has improved greatly in recent years, with the advent of high-density maps incorporating microsatellite markers, RFLP markers, and population-specific polymorphic fragments identified by the AFLP technique. The approach has been widely used to successfully map agronomic traits in a variety of cereal species. Although developing a suitable population for QTL analysis is laborious, and identifying a QTL is expensive, the markers that are linked to the QTL may be sufficient to use as the sole selection tool for a specific trait in a breeding program. QTLs for salinity tolerance have been described in several cereal species, including rice, barley and wheat. However, these studies have not yet yielded robust markers that can be used across a range of germplasm, significant associations between the trait and the marker being confined to the populations in which they were derived. The success of these studies could be limited by the small amount of genetic diversity present within modern cultivars, and the use of parental lines with small differences in the traits.

Conventional crop and pasture breeding for salt tolerance

As described in the companion article (Salinity Stress and its Impact), species vary in their capacity to tolerate salinity. Amongst the major crop species, barley, cotton, sugar beet and canola are the most tolerant; wheat and lucerne (alfalfa) are moderately tolerant, while rice and most legume species are sensitive. For a more complete list of crops see Richards (1969).

There is also variation in salinity tolerance within species, especially in outcrossing species like lucerne. The classical breeding approach is to (1) screen collected germplasm for salinity tolerance, (2) cross the identified tolerant types with the desired cultivars and (3) selecting the desired plant types having salinity tolerance as well as other desirable traits from the advancing and segregating generations. By exploiting the naturally occurring genetic variability that exists within a species, some relatively tolerant cultivars have been developed for crops including rice, wheat, lucerne, white clover and citrus.

There is probably a great diversity in salinity tolerance within species that has not been fully explored and exploited. One reason for this is the difficulty of screening large numbers of individuals for small, repeatable and quantifiable differences in biomass production, let alone yield. Obtaining a wide range of germplasm with potential genetic differences in salinity tolerance is not difficult, because international collections are usually available to any scientist. However, the difficulty lies in how to measure salinity tolerance.

How to measure salinity tolerance:

Salinity tolerance is difficult to measure because of its complexity. Not only are there a number of genes controlling salinity tolerance whose effect interacts strongly with environmental conditions, but there are two major and distinct components of salinity tolerance which can often be difficult to distinguish (Munns 2002).

Salinity imposes two major stresses on the plant: one is a high osmotic pressure in the soil solution making it harder for the plant to extract water, the second is a high NaCl concentration in the soil solution that makes it difficult for the plant to exclude the NaCl while taking up other cations and anions. The effect of these two stresses are seen in sequence. Salinity lowers the water potential of the roots, and this quickly causes reductions in growth rate, along with a suite of metabolic changes identical to those caused by water stress. Later, there may be salt-specific effects that impact on growth or senescence. This two-phase model is summarized in Fig. 1, and described in more detail in the companion article ‘Salinity Stress and its Impact’.

How to screen for differences in salinity tolerance within species based on growth:

Screening based on growth needs to allow for the two distinct mechanisms for salinity tolerance: tolerance to the osmotic effect of the saline soil solution, and tolerance to the salt-specific nature of the saline solution.

The osmotic effect alters the water relations of the plant, and reduces the rate of cell expansion. This leads to a reduction in the rate of development of new roots, leaves and lateral shoots. The osmotic effect also reduces stomatal conductance, which leads to reduced photosynthesis. It also causes accelerated senescence of older leaves. Thus there are three somewhat independent processes being affected (i.e. new leaf formation, old leaf death and photosynthetic activity) that all contribute to a reduction in the net assimilation rate of the plant. These effects are identical to drought.

The osmotic effect occurs instantly the soil water potential decreases, and recovers instantly it increases (Fig. 2). If the period of stress is short (hours) the recovery is complete. If the period of stress is long, the recovery is more limited, as stress may have already reduced the number of lateral shoots, and the number of cells in the dividing zones of growing roots and leaves, so that there are a reduced number of cells to respond.

Responses of leaf elongation are greater than of root elongation (Munns 2002). The reason for this is not known, but it means that leaf elongation or leaf expansion rates are a more sensitive indicator of osmotic stress than root elongation rates.

The genetic variation in the growth response to the osmotic effect of salinity is likely to be small, both within a species, and across similar species. For example, we found little difference between the effect of salinity on leaf elongation in 15 cultivars of bread wheat, durum wheat, barley and triticale, cultivars with established differences in reputation for salinity tolerance. All 15 cultivars had the same decrease when the salt was increased from 0 to 250 mM NaCl over 10 d. Even the most salinity-sensitive genotype, a durum wheat cultivar, had the same percentage reduction as the most salinity-tolerant genotype, a barley cultivar. In fact, there is a remarkable similarly between different species in the osmotic response in saline solution. For instance, 100 mM NaCl causes approximately 50% decrease in leaf elongation rate of the salinity-sensitive species maize and rice, and nearly as much in the salinity-tolerant species bread wheat and barley. Longer term growth responses clearly differ.

The osmotic effect has the same characteristics as drought, so genotypes could be screened for drought tolerance. But this is not easy either (see 'Drought Stress and its Impact).

The salt-specific effect causes increased uptake of Na+ and Cl-, and decreased uptake of essential cations particularly K+ and Ca2+. Uptake of Cl- may alter uptake of anions such as phosphate and nitrate but these effects are complex and vary between species. If the uptake of Na+ or Cl- exceed the plant's ability to partition the ions between different tissues or organs, or to sequester ('compartmentalise') the ions within the cell's vacuole, these ions build up in the cytoplasm to toxic concentrations.

The salt-specific effect shows up in old leaves that have accumulated excessive amounts of Na+ and Cl-. While it may be visible, in that old leaves die earlier, it may not affect the rate of new leaf production for some time. The salt-specific effect on growth is not seen until after the osmotic effect on growth (Fig. 1). The length of time required before the growth differences between genotypes due to the salt-specific effect can be seen depends on the salinity and the degree of tolerance of the species. This represents a 'second phase' of the growth reduction. The second phase will start earlier in plants that are poor excluders of Na+, and will start earlier when root temperatures are higher. For plants such as rice that are grown at high temperatures, two weeks days in salinity is sufficient to generate differences in biomass between genotypes that correlate well with differences in yield. However for temperate crops, as at least a month is needed for genotypic differences in the response of biomass production to take effect.

The labour and space demands of these long experiments makes them impractical for screening large numbers of genotypes, or selecting salinity-tolerant progeny resulting from crosses with cultivars. Not only do plants need to be grown for lengthy periods of time, but controls need to be included. Control plants consumes a large area of glasshouse space if they are to be grown at their optimum rate, with sufficient space so that radiation is not limiting. In the field, the major drawback is the heterogeneous nature of soil salinity and other soil constraints.

Screening for a trait associated with a specific mechanism is preferable to screening for salinity tolerance itself. Screening for specific traits can reduce the time needed to grow plants in salinity, and can eliminate the need to grow plants under control conditions, thus making savings on glasshouse space and labour.

The importance of traits:

Because of the complex nature of salinity tolerance, as well as the difficulties in maintaining long-term growth experiments, we recommend trait-based selection criteria for screening techniques. Specific traits are less subject to environmental influence than growth rates.
Further, this allows for different traits to be pyramided, especially when molecular markers for specific traits have been identified.

The most successful trait in terms of plant breeding relates to rates of Na+ or Cl- accumulation in leaves, measured as the increase in salt in a given leaf over a fixed period of time. This trait has a high heritability and has been used to develop cultivars of rice, white clover, and lucerne with increased tolerance to saline soil. Sometimes K+/Na+ discrimination instead of Na+ exclusion is used for screening, however the uptake of K+ and the resultant K+/Na+ discrimination may be the result of genetic differences in the regulation of Na+ uptake, and not independent of it. If so, there is nothing to be gained by measuring K+ as well as Na+.

A correlation between Na+ or Cl- accumulation and salinity tolerance is found in most species, however not all species contain significant genetic variation in Na+ or Cl- accumulation. Durum wheat is one species in which there is significant genetic variation in Na+ but not Cl- uptake. Genotypes with low Na+ uptake are more salinity tolerant, as indicate by greater biomass production over 1-2 months in saline solution, in comparison to non-saline solution (Fig. 3). In some species, Na+ is retained in roots and stems in exchange for K+, and only Cl- progresses through to the leaves, balanced by K+. In those cases, Cl- exclusion correlates with salinity tolerance.

Proving that Na+ (or Cl- ) exclusion confers salinity tolerance in terms of yield is not so easy. A comparison between durum landraces with very low and very high rates of Na+ accumulation showed that, at moderate salinity, the yield of genotypes with low Na+ was greater than those with high Na+, but at high salinity there was no yield advantage. The osmotic effect of the salinity then dominated (Fig. 4).

Tissue tolerance

Tissue tolerance, i.e. tolerance of high internal Na+ concentrations, cannot be measured directly, and is difficult to quantify. Yet it is clearly important; overexpression of the vacuolar Na+/H+ antiporter that sequesters Na+ in vacuoles (NHX1) improved the salinity tolerance of Arabidopsis, tomato and brassica (see next section).

Tolerance of high internal Na+ levels is evidenced by an absence of leaf injury despite high leaf concentrations of Na+. Concentrations of Na+ above 100 mM will start to inhibit most enzymes, so when tissue concentrations are over 100 mM, which corresponds to about 0.5 mmol g-1 DW (assuming a leaf water content of 5 g H2O g-1 DW), the Na+ must be compartmentalised in vacuoles, and be a higher concentration there than in the cytoplasm.

compartmentation Na+ in vacuoles to some extent, as levels of Na+ up to 1 mmol g-1 DW (200 mM) are quite common in photosynthetically active leaves of many species. In a study in wheat genotypes, Na+ became potentially toxic only when leaf concentrations exceeded 1.25 mmol g-1 DW (250 mM), as judged by the onset of non-stomatal reductions in photosynthesis in durum wheat at this concentration (James et al, 2002).

There may be genetic variation for tolerance of high internal Na+ concentrations in many species, as indicated by 'out-lyers' in correlations between leaf Na+ concentrations and salinity tolerance. This has been found in rice and wheat and probably many other species. However, quantitative assess of the genetic variation is difficult. The trait is characterised by leaf longevity, lack of necrosis, and prolonged growth despite very high accumulation of Na+. A recent paper describes the success or failure of various methods to screen durum wheat for genetic variation in tolerance of high internal Na+ (Munns and James 2003). The degree of leaf death was measured as an indicator of Na+ toxicity. Leaf injury, however could arise from a number of reasons. First there would be the osmotic effects of salt in the soil solutions, causing accelerated senescence due to leaf water deficit or hormonal effects arising from root signals. Second, there could be nutrient imbalances resulting in deficiencies or excesses of other ions. Third, there could be dehydrating effects of salts building up in the cell walls. A phenotype other than leaf injury is needed to distinguish between genotypes with different degrees of tissue tolerance. The onset of non-stomatal reductions in photosynthesis mentioned above is a possible phenotype.

Traits for the osmotic effect:

Measurements of growth, survival, leaf gas exchange and germination will indicate the osmotic effect of salinity. If the experiments are conducted for lengthy periods of time, measurements of growth, survival and gas exchange will also reflect toxic effects of salts in the leaves.

Growth can be measured as leaf elongation, root elongation, leaf area expansion, or shoot biomass. Of these indices of growth, leaf area expansion or shoot biomass are the most sensitive and comprehensive, as this includes production of tillers or lateral shoots. The number of lateral shoots is more sensitive to water stress than the elongation rate of any given leaf.

Gas exchange characters are stomatal conductance (including infra-red temperature), photosynthesis, transpiration efficiency (including carbon isotope discrimination), and chlorophyll fluorescence. For screening larger numbers of genotypes, traits related to stomatal conductance are the most feasible. Chlorophyll fluorescence as an indicator of photosynthetic activity was assessed by James et al (2002). They found that the simplest parameter, Fv/Fm, was no more sensitive than chlorophyll content itself. The quenching parameters, such as NPQ, did indicate a reduction in photosynthetic capacity before chlorophyll itself started to degrade (James et al. 2002), but these parameters require both light and dark-adapted readings and are not feasible for large numbers of genotypes.

Leaf injury can be quantified by a number of methods such as chlorophyll content, or electrolyte leakage of cut discs. Chlorophyll content can be measured with a SPAD meter.

Turgor and osmotic adjustment are also indicators of leaf injury and photosynthetic capacity.

All these methods are used also for drought tolerance.

Whole plant survival at high salinity may reflect the salt-specific effect rather than the osmotic effect, as the plant's ability to control salt uptake may break down at high salinity.

Germination is easy to measure, but little or no relation between salinity tolerance at germination and that of the seedling or adult plant has been found in any species examined.

Dryland Agriculture and Salinity

The causes of dryland salinity are in principle well understood. The replacement of perennial deep-rooted native vegetation by shallow-rooted annual crops or pastures results in wetter subsoils and accompanying larger deep drainage beyond the reach of shallow roots, leading eventually to rising water tables. If the ground water is saline, which it commonly is in semiarid environments, salt scalds appear when the water tables reach the soil surface. Even if the water tables are only brackish rather than saline the surface can become saline owing to the salt concentrating as water evaporates.

The essential difference, hydrologically, between native perennial vegetation and annual crops is that the perennials can use substantial amounts of water throughout the whole year. In a Mediterranean environment, for example, with its winter wet season, there is little difference during the winter in water use by annual or perennial vegetation. Rainfall that substantially exceeds evaporation during the winter may penetrate deeply into the subsoil under both types of vegetation. But during the summer, when the crops have been harvested, the perennials, enabled by their deep roots, can use water that may have penetrated well beyond the rooting depth of the crops during the winter.

Thus, mitigation of dryland salinity in cropping lands requires control of drainage beyond the reach of the crop roots. There is no single solution. There is, however, a range of options that farmers can select from, including growing longer-season crops, which tend to have deeper roots, and various techniques for incorporating some deep-rooted perennial species into cropping systems to tap the water in the deep subsoil that may have accumulated during a wet season(Black et al., 1981).

Phase farming is one effective way of incorporating perennials into a cropping system. It involves the tactical rotation of herbaceous perennial pasture, such as alfalfa (lucerne) which can be grazed or harvested for hay, with a series of annual crops. The perennial pasture dries the subsoil below the roots of annual crops, thereby creating a buffer zone in which water and nutrients that leak below the crops can be held for a few seasons, remaining largely accessible to the roots of the next phase of deep rooted perennials. A special issue of the Australian Journal of Agricultural Research (vol 52(2), 2001) contains several papers that discuss phase farming. One general conclusion from these is that the size of the buffer created by herbaceous perennials varies, reflecting soil type and local climate as well as species, from about 50 to 150mm. The larger values were due to lucerne and were typically achieved within two years of establishment. Phalaris was less effective than lucerne in developing the buffer. Danthonia and Eragrostis, were less effective still. With deep drainage under crops averaging about 30 to 50 mm per year, such buffers can deal with one to five years of drainage below the roots of crops.

Strips of woody perennials can also help, though their effectiveness is limited by the maximum lateral movement of water through unsaturated soil to their roots, which is typically no more than about 1 m. Even though surface roots may spread from the tree for a distance several times the spread of the canopy (and thereby reduce yields of adjacent crop), the deeper roots typically do not spread so far and the strips may do little more than control deep drainage only in a strip of soil little larger than the width of the canopy (Stirzaker et al., 2002). If the roots of such perennials can tap the groundwater, though, the lateral flow towards them can be large enough for them to take up a lot of water that might otherwise flow to lower points in the landscape, providing that the water is not too saline. The appropriate proportion of woody perennials to herbaceous species depends on the competitive interactions at the interface between trees and crop or pasture as well as on the ability of trees to extract groundwater and considerations of both help determine the area of cropland that needs to be sacrificed.

Another possibility is to identify, using georeferenced yield monitors, areas giving consistently poor yield. Such areas, which are especially prone to deep drainage, can be excluded from cropping, and put under either permanent perennial pasture or trees.

Determining the right mix of these various options, both in time and space, is hampered by the difficulty of estimating what the deep drainage is and how it may vary with season and treatment. Few long-term measurements are available. The development of a cheap drainage meter (Hutchinson et al., 2001) may help overcome this deficiency and help farmers find out what is happening on their own farms. Augmenting such measurements with reliable simulation models, of both phase farming and agroforestry, will also help. Essential input for such models includes the effective water-holding capacity of the soil as a function of depth and the effective depths of rooting of both the perennials and the crops. That the depths of rooting can depend markedly on season and cropping history is a considerable challenge. A reliable model can help estimate the impact of seasonal variability and management decisions on deep drainage by running the model through several decades of rainfall records; it should also be able to guide those management decisions, at least if the troublesome groundwater system is local. Where an outbreak of dryland salinity is a long way from where the accessions to groundwater are occurring, hydrologic models and measurements are needed, and although these can handle general flows of groundwater, they are as yet not able to guide specific actions on farms.

If these various options for reducing deep drainage are effective in lowering water tables so that any salt scalds dry out, there is still the problem that the salt remains in the root zone. Further rehabilitation requires a succession of plants, starting with halophytes, with can take up the water and thereby create space for rain to wash the salt deeper into the soil profile. Salt tolerant crops may then be able to grow there, and enable further leaching of the salt. If the soil has become sodic, chemical amelioration (say, with gypsum) may also be necessary.

Irrigated Agriculture and Salinity

All soils contain salts, and all irrigation waters, whether from canals or underground pumping, including those considered of very good quality, contain some dissolved salts. Hence, the process of soil salinization is dramatically exacerbated and accelerated by crop irrigation. The overall effect of irrigation in the context of salinity is that it “imports” large quantities of new salts to the soil that were not there before.

Removal of salts from the root zone (reclamation) is perhaps the most effective and longer lasting way to ameliorate or even eliminate the detrimental effects of salinity. However, in addition to being slow and expensive, the process requires large quantities of water and effective soil drainage. Consequently, it is not always possible or feasible to carry out a “true reclamation” operation. A number of different approaches involving removal or reducing the salts may be considered.

1. Soil Reclamation: The process of “true reclamation” involves replacing sodium ions in the soil with calcium. The released sodium ions are then leached deep beyond the root zone by using excess water and finally carried out of the field in the drainage water. The most commonly used method for replacing the sodium ions is by applying large quantities of gypsum (calcium sulfate) to the soil and followed by water ponding. The applied gypsum slowly dissolves in the water releasing calcium ions which replace sodium ions from the soil into the downward moving water. Lime (calcium carbonate) is not used as saline soils are sometimes already high in carbonate salts and are therefore alkaline. The reclaimed soils can become saline again unless appropriate management practices are followed.

2. Various management practices based on reducing the salt zone for seed germination and seedling establishment: The early seedling establishment and tillering phase are generally the most sensitive stages to salinity. Any management practice that could provide an environment of reduced salt concentration during these stages would mitigate the salinity effects and benefit the crop by promoting plant densities and early seedling growth. A number of approaches have been used.

2.1. Scraping and removal of surface soil: Due to continuous evaporation the salt concentration is the highest in the surface soil. The top soil can be scraped and transported out of the field. The practice has been used in many areas of the world (Qureshi et al., 2003).

2.2. Pre-sowing irrigation with good quality water: Where available, irrigation with good quality water prior to sowing helps leach salts from the top soil. This helps in promoting better seed germination and seedling establishment. The benefits of this practice were documented in a long-term study by Goyal et al (1999 a,b).

2.3. Appropriate use of ridges or beds for planting: The impact of salinity may be minimized by appropriately placing the seeds (or plants) on ridges. Where exactly the seeds should be planted on the ridge or bed will depend on the irrigation design. If the crop planted on ridges would be irrigated via furrows on both sides of the ridge, it is better to place plants on the ridge shoulders rather than the ridge top because water evaporation will concentrate more salts on the ridge top or center of the bed. If the crop is irrigated via alternate furrows, then it is better to plant only on one shoulder of the ridge closer to the furrow that will have water. For additional benefits, this approach may be combined with pre-irrigation (2.2) via furrows or sprinklers which will help reduce salt concentration in the area where seeds or plants are to be placed.

2.4. Planting into a pre-flooded field: An interesting approach has been widely used in the San Joaquin Valley of California to grow safflower crop on salt affected soils. Prior to planting, the field is flooded with good quality water. Just as most of the water has percolated into the soil and only a few millimetres of standing water is left, the seeds are flown over the field via an aircraft. The seeds traveling under the force of gravity get imbedded into the muddy soil surface where the salt concentration is expected to be the lowest. The approach has provided good seed germination and seedling establishment (Goyal et al., 1999 a,b).

3. General management practices to reduce the impact of soil salinity on crop performance: In addition to the management practices mentioned above, the following approaches may help reduce salinity impacts.

3.1. Mulching: Mulching with crop residue, such as straw, reduces evaporation from the soil surface which in turn reduces the upward movement of salts. Reduced evaporation also reduces the need to irrigate. Consequently fewer salts accumulate.

3.2. Deep Tillage: Accumulation of salts closer to the surface is a typical feature of saline soils. Deep tillage would mix the salts present in the surface zone into a much larger volume of soil and hence reduce its concentration and impact. Many soils have an impervious hard pan which hinders in the salt leaching process. Under such circumstances “chiseling” would improve water infiltration and hence downward movement of salts.

3.3. Incorporation of Organic matter: Incorporating crop residues or green-manure crops improves soil tilth, structure, and improves water infiltration which provides safeguard against adverse effects of salinity. In order for this to be effective, regular additions of organic matter (crop residue, manure, sludge, compost) must be made.

Irrigated agriculture can be sustained by better irrigation practices such as adoption of regulated deficit irrigation (RDI) or partial root zone drying methodology, and drip or micro-jet irrigation to optimise use of water. Current levels can be controlled by leaching fractions, where fresh irrigation water is available, and by drains.

The leaching fraction is the fraction of the applied water that passes through the root zone; this carries salts below the root zone. The smallest leaching fraction that maintains maximum crop productivity is called the ‘leaching requirement’. It depends on the salt content of the irrigation water and the salt tolerance of the crop.

If more than 30% passes through the root zone, the cost of drainage, or the risk of rising water tables, becomes too great. Hence, increasing the salt tolerance of crops is desirable. Salt tolerant crops are also needed if the drainage water is to be reused.

The disposal of saline drainage water from salt-affected irrigated land has been a controversial issue, and recycling of such waters has been considered for further crop irrigation. Feasibility studies indicate that re-use of drainage water is suitable for irrigation of moderately salt-tolerant crops. Up to 4 dS/m can be used for irrigation of moderately tolerant crops provided that the ground is leached with fresh water before sowing. With the more salt tolerant crops like sugar beet and cotton, the use of water up to 9 dS/m can be sustained for three years, but for a longer period the salinity must be reduced to 5 dS/m (Goyal et al. 1999 a,b).

A Review of Salinity Stress on Plants

All plants are subjected to a multitude of stresses throughout their life cycle. Depending on the species of plant and the source of the stress, the plant will respond in different ways. When a certain tolerance level is reached, the plant will eventually die. When the plants in question are crop plants, then a problem arises. The two major environmental factors that currently reduce plant productivity are drought and salinity (Serrano, 1999), and these stresses cause similar reactions in plants due to water stress.

These environmental concerns affect plants more than is commonly thought. For example, disease and insect loss typically decrease crop yields by less than ten percent, but severe environmental problems can be responsible for up to sixty-five percent reduction in yield (Serrano, 1999). There are global constraints on fresh water supplies, and this has led to a surge of interest in reusing water (Shannon and Grieve, 1999). However, in many cases the value of water has decreased because the water is salty. Salt stress can be a major challenge to plants.

It limits agriculture all over the world, particularly on irrigated farmlands (Rausch, 1996). To farmers, salt tolerance is important in vegetables because of the cash value of crops (Shannon and Grieve, 1999). As more land becomes salinized by poor irrigation practices, the impact of salinity is becoming more important (Winicov, 1998). This is creating the need for salt tolerant plants. Salinity resistance is a quantitative trait, and it has been resistant to plant breeding (Winicov, 1998).

Many factors interact with salinity, and this complicates studies on the effects of salinity. For example, humidity, temperature, light, irrigation, and soil fertility all alter the effect of salinity when present (Allen et al., 1994). The plants that grow in saline soils have diverse ionic compositions and a range in concentrations of dissolved salts (Volkmar et al., 1998). These concentrations fluctuate because of changes in water source, drainage, evapo-transpiration, and solute availability (Volkmar et al., 1998). Due to these varying conditions, plant growth depends on a supply of inorganic nutrients, and this level of nutrients varies in time and space (Maathius and Amtmann, 1999).

Either extreme condition concerning nutrients results in deficiency or toxicity in plants, and this is demonstrated by salt tolerance (Maathius and Amtmann, 1999). These conditions vary according to the plant species and growth conditions. Little is known about the genetic basis for diversity of salt tolerance in plants, and this could be partly explained through the definitions given for salinity.

Plants in natural environments are being exposed to increasing amounts of salinity. One-third of the land being irrigated worldwide is affected by salinity, but salinity also occurs in non-irrigated land (Allen et al., 1994).

There are large areas of primary salinity, but secondary salinity can be detected within one hundred years of settlement on an area of land. Drought and salinity are connected because in many regions, raising plants requires irrigation.

The irrigation water contains calcium, magnesium, and sodium (Serrano et al., 1999). As the water evaporates and transpires, calcium and magnesium transpire, leaving sodium dominant in the soil (Serrano et al., 1999).

At low salt concentrations, yields are mildly affected or not at all (Maggio et al., 2001). As the concentrations increase, the yields move towards zero. In fields, the salt levels fluctuate seasonally and spatially, and variation will occur due to the circumstances influencing each particular plant. This variability makes research difficult.

The uptake of ground water by plant roots can increase the salinity of ground water or the soil around the roots due to the exclusion of salt (Niknam and McComb, 2001). These variable conditions make research difficult, and this is compounded by the fact that each species has its own level of salt tolerance. Together, it will be a complicated process to match plants with their optimal growing conditions.

The response of plants to salt stress is based on the transcriptional action of many defense proteins, and research has not discovered the basis for them all (Serrano, 1999). Osmotic stress and ion toxicity are the problems stemming from salt stress, and the resulting decrease in chemical activity causes cells to lose turgor (Serrano et al., 1999). Cell growth depends on turgor to stretch the cell walls, and lack of turgor implies danger for cell survival.

The plant’s defense against this salinity attack requires osmotic adjustment, and, to a certain degree, this can be done through synthesis of intracellular solutes (Serrano et al., 1999).

Salinity creates the specific problem of ion toxicity, because a high concentration of sodium is bad for the cells. High salt concentrations inhibit enzymes by impeding the balance of forces controlling the protein structure (Serrano et al., 1999). The toxic effects of salt can occur at relatively low concentrations, depending on the plant species, so the homeostasis of sodium is important for the tolerance of organisms to salt stress.

The stress caused by ion concentrations allows the water gradient to decrease, making it more difficult for water and nutrients to move through the root membrane (Volkmar et al., 1998). In turn, the water uptake slows, and as the osmotic effect spreads from the root membrane to the internal membranes, the ion concentration inside the plant alters the solute balances (Volkmar et al., 1998).

Once high concentrations of salt have reached the inside of the plant, tissue and organs development is altered. The salt causes a slower rate or shorter duration of expansion of cells, and this compromises the size of the leaves (Volkmar et al., 1998). The overall effect of salinity on plants is the eventual shrinkage of leaf size, which leads to death of the leaf, and finally the plant. Salinity may also cause reduced ATP and growth regulators in plants (Allen et al., 1994).

There is a wide spectrum of salinity tolerance among higher plants (Robinson et al., 1997), but there is also variability in salt tolerance occurring in plants with lower salt tolerance, suggesting that there is the potential for improvements to be made in these plants (Allen et al., 1994). Because the responses in plants are not identical, those with better adaptations may be studied in an attempt to improve the other species. Halophytes are known for their adaptation to living in salty solution environments, and these plants adapt to salinity through altering their energy metabolism (Winicov and Bastola, 1997). These plants provide viable organisms to study the mechanisms they use to handle high concentrations of salt. By using these plants as models, research should be capable of improving the tolerance of non-halophytic plants. It is known that salinity induces a change in the signals of root origin, which changes the hormonal balance of the plant, and this affects root and shoot growth (Lerner et al., 1994). Through observation of root and shoot growth and response to salinity, the varying degrees of salt tolerance can be determined.

The ability of plants to survive and maintain their growth under saline conditions is known as salt tolerance. This is a variable trait that is dependent on many factors, including the species of the plant. There is a continuous spectrum of plant tolerance to saline conditions ranging from glycophytes that are sensitive to salt, to halophytes which survive in very high concentrations of salt (Volkmar et al., 1998). Unfortunately, most crops are not halophytic.

Studies in crops suggest that salt tolerance is a multigenic trait (Niknam and McComb, 2000), which makes it more difficult to study and improve. Although, it has also been noted that in some species the salt tolerance acquired can be passed along to offspring (Niknam and McComb, 2000). Tolerant species use more than one strategy to tolerate or avoid stress. It is important to keep the levels of ions low in the leaves, particularly in the young ones. This can be done by excluding the ions at the point of uptake and reducing the translocation of ions to the shoot (Niknam and McComb, 2000).

The capacity of the plant leaves to accommodate the export of salt from the root is linked to the growth rate, so the ability of the plant to continue to grow would indicate a high level of salt tolerance. Plants have morphological features in their roots that can prevent the uptake of large amounts of salt. If salt does enter the plant, there are physiological and metabolic events that can counteract salt at a cellular level (Winicov, 1998).

Specifically, there are two mechanisms commonly used by plants to tolerate high salt concentrations. Avoidance is the process of keeping the salt ions away from the parts of the plant where they are harmful (Allen et al., 1994). This can be done through the passive exclusion of ions by a permeable membrane, the active expelling of ions by ion pumps, or by dilution of ions in the tissue of the plant (Allen et al., 1994). Secondly, tissue tolerance occurs when ions have already accumulated in the tissue of the plant, and they are then compartmentalized into the plant’s vacuoles for storage (Allen et al., 1994). These two methods prevent the ions from accumulating and causing damage to the plant. These would be ideal targets for genetic manipulation of plants to become more tolerant of saline conditions.

In order to judge the tolerance of plants to salinity, the growth or survival of the plant is measured because this is the culmination of many physiological mechanisms occurring within the plant (Niknam and McComb, 2000). In low to moderate salinity conditions, salt exclusion is the strategy. Hence, the growth and yield are measured as determinants of salt stress (Niknam and McComb, 2000). However, under higher salinity conditions, ion toxicity becomes a cause of death, so survival is measured (Niknam and McComb, 2000). Researchers must decide whether to test for the ability to survive under mild salt stress and never know the full potential of the plant to grow. On the other hand, subjecting plants to concentrations beyond their capability results in death of the plant and little knowledge of the salt tolerance.

Plants have several processes to respond to salt stress. A basic two-phase model describes the overall growth response to salinity as an initial water deficit lasting for a few days or weeks. Then the second phase occurs, where the ion toxicity initiates leaf death (Rausch et al., 1996). This overview of plants’ response to salt stress broadly categorizes the cellular mechanisms, but there is more detail to the cellular reaction. The early response of plants reacting to salt that has reached their leaves is to exclude it from the cytoplasm (Volmar et al., 1998). One means of eliminating the salt that accumulates in plant cells is through storage of the salt ions in vacuoles. This is an important adaptation of plants to salinity. Another method is allowing the salt to build up outside the cells, in the intracellular space. This leads to a gradient of water moving out of the cells to accommodate the change in ion concentration, and eventually too much water leaves the cell and the cell becomes dehydrated (Volmar et al., 1998). This will lead to cell death.

The vacuoles comprise most of the cell volume making them good for storage, but the cytoplasm is only one percent of the cell volume (Volkmar et al., 1998). This makes the cytoplasm very sensitive to slight changes in rate of saline transport. The rate of salt passing through the membrane must not exceed the rate of salt being collected into the vacuoles, or there will be an imbalance in the cell (Volkmar et al., 1998). As older cells lose their capacity to grow and provide vacuoles, the new growth cannot handle the burden of collecting all the salt ions, this leads to premature death in the cells of leaves, and the plant will quickly succumb to the decreasing ability to compartmentalize the salt (Volkmar et al., 1998).

Exclusion of salt from the shoot is a prime form of tolerance in non-halophytic plants, and most of the sodium going from the roots to the shoots is via the xylem stream (Robinson et al., 1997). This means that the rate of accumulation is mostly determined by the rate of transpiration (Volkmar et al., 1998). Therefore the stomatal control of transpiration would control the uptake of sodium, and the inhibition of stomatal opening would regulate the salt level in the shoot (Volkmar et al., 1998).

This inhibition combined with the compartmentalization in vacuoles would help achieve a tolerable level of salt within the cell. These two mechanisms also provide feasible pathways to genetically manipulate for more salt tolerant plants. Additionally, the ability of plants to counteract stress will depend on the levels of potassium available to the plant (Maathius and Amtmann, 1999). Potassium is important to all plants as a balancing charge, and the plant must maintain a high potassium level to counter balance the excess salt. Alternatively, sodium is only essential for some C4 species, where it functions as a micronutrient (Maathius and Amtmann, 1999). For most other species, sodium is not necessary for growth. The availability of some sodium is beneficial to the plant, but too much will cause damage.

Another means of salt stress damage is found in relation to potassium within the cell. Due to the similar structures of sodium and potassium, the competition for binding sites causes potassium deficiency within the cell (Maathius and Amtmann, 1999). The sodium competing for potassium binding sites in the cytoplasm inhibits metabolic processes that depend on potassium, and this is another pathway that mandates cellular levels of sodium must be kept to a minimum. Some studies have shown that plants able to maintain a high level of potassium are also associated with salt tolerance (Volkmar et al., 1998).

The ratio of sodium and potassium in a cell is controlled by transport systems on plasma and vacuolar membranes, and there are three processes that transport these ions. Pumps are transporters fueled by energy and transported across an electrochemical gradient, but there are no pumps found in higher plants (Maathius and Amtmann, 1999). Next, carrier proteins undergo conformational change during transport, and finally ion channels are proteins that catalyze the dissipation of transmembrane ionic gradients (Maathius and Amtmann, 1999; Yeo, 1998). All of these mechanisms transport ions across membranes, and they could all potentially be useful in altering the salt tolerance in plants by over-expression of these genes. There is not an extensive amount of understanding surrounding these transporters, but it is thought that they activate long distance signaling pathways (Maathius and Amtmann, 1999).

Similarly, a sodium-hydrogen antiport has been reported in salt tolerant species, but it is absent in salt sensitive species (Maathius and Amtmann, 1999). This demonstrates that it may be implicated as a factor influencing sodium accumulation. There may be specific processes or individual enzymes that are especially sensitive to salinity, and if these processes are overcome, tolerance may be achieved for a wider variety of plants at a higher concentration of salinity (Yeo, 1998). There would also need to be a high level of specificity of expression in a gene engineered to pump out sodium (Yeo, 1998). If the pump was active on a continuing basis, it could be lethal to the cell. In reality, many processes will have to work together to achieve tolerance.

Concentrating on larger scale methods of dealing with salt stress, plants have several mechanisms to adjust to a saline environment. Lots of information states that roots play a crucial role for short-term adaptation to salt tolerance. The concentrated salt surrounds the root membrane, and it is thought that the morphology of the roots affects the amount of salt taken into the plant (Maggio et al., 2001).

Some features of the root must be advantageous because they help the root take in water. Because salinity is first perceived in the root, the root sends the signal hormone abscisic acid, which directly or indirectly down regulates the leaf expansion rate (Rausch et al., 1996). Salt exclusion from the root is likely to be part of the salt tolerance found in plants. However, when salt ions make it into the plant, they accumulate in the leaf. As stated above, it is beneficial to the cells of the leaves to compartmentalize the salt ions into the vacuoles. Leaf cell growth is sensitive to salt, because the energy used for compartmentalization takes energy away from cell growth (Volkmar et al., 1998).

The root signal tells the shoot to stop growing to conserve energy as well. Growth could be considered a means of regulating the concentration of salt, although high concentrations of salt induce inhibition of growth when the plant needs to continue growth to dilute salt concentrations and find space for vacuoles. All of these broad reactions to salt stress could be target systems to regulate tolerance by the plants: the structural components of the roots, ion transporters, or cell wall and membrane components (Winicov and Bastola, 1997). These mechanisms are the only way that plants can adapt to saline conditions themselves, but there have been suggestions of external maneuvers to counteract the salinity.

Some scientists have suggested that trees could be planted to take up some of the excess salt. Trees have high water use and can lower water tables to reduce salt discharge into streams (Niknam and McComb, 2000). This would prevent secondary salinization of the surrounding areas, and benefit plants living near the tree. It has not been proven to what extent the tree planting would assist in preventing salt stress in plants.

Many other studies have shown that salt stress can also be alleviated by an increased supply of calcium to the growth medium (Rausch et al., 1996). Depending on the concentration ratio, sodium and calcium can replace each other from the plasma membrane, and calcium might reduce salt toxicity (Rausch et al., 1996). If none of these mechanisms are available to the plant, eventually the leaf death rate will overcome the leaf growth rate and plant death will occur. The differences found in salt tolerant plant species are related to the time it takes salt to reach its maximum accumulation and cause plant death. By studying plants with varying tolerance, eventually scientists will discover the differences in the plant genome that are causing sensitivity or resistance.

A new strategy to study salt sensitive plants involves selecting root mutants with high sensitivity (Maggio et al., 2001). This is hard to study because there are not many species that have root mutants other than Arabidopsis. If there are gene sequences that are similar, then this method should be helpful in discovering the genes responsible for salt sensitivity.

Although there is not enough knowledge on the specifics of salt tolerance in every plant species, there are numerous options for genetic modification of plants to make them more tolerant to salt stress. Some progress has been made with the tomato plant, and transgenes have been successfully inserted into its genome (Allen et al., 1994).

The tomato plant was recently made to harbor excess sodium in its leaves while leaving the fruit tasting the same. Many studies have been done on yeast because of the ease with which they are studied, and there are many similarities at the cellular level between fungi, plants, and animals (Yeo, 1998). Hopefully the studies with yeast will soon prove fruitful in gaining a better understanding of the cellular processes involved in salt stress reactions. Some studies have shown that acquired cellular salt tolerance can be achieved in the laboratory for some species (Winicov and Bastola, 1997). This has been achieved through over-expression of genes that become limiting under salt stress. Consistent with the multigenic characteristics of the salt tolerant trait, these findings imply that small improvements could be made from enhanced expression (Winicov and Bastola, 1997). These transgenic activities may be successful in over-expressing the transcription of a gene, but many of the processes are dependent on more than one pathway (Winicov, 1998). This would require the complete understanding of all pathways to have a strong impact on salt tolerance.

Another option for genetic modification is assistance to the cell in achieving ion homeostasis under salt stress. This would include altering ion channels or other transporters. Several of these mechanisms need to be changed because altering one gene may not be sufficient to optimize adaptation to salinity (Winicov, 1998), but altering an ion pump may be a viable option to explore. As researchers are able to understand the developments happening within the plant, there will be more evidence to support the responses of the plant to the genetic modification. Some optimistic discussion of salt tolerant plants includes the notion of plants that are able to live virtually in seawater. Scientists are most likely far away from that ability, but they are working to improve the growing conditions and yield for the crops that are affected by minor secondary salinization (Winicov, 1998). Maximizing the root growth would also provide relief from the salt stress, and this could be modified within the genome of plants. It is most likely that multiple modifications will have to occur to overcome the multigenic trait of salt sensitivity.

Many scientists have become discouraged by the fact that salt tolerance remains largely unexplained due to the many processes that are affected by the stress. This presents difficulties when transgenic genes are inserted into plants, and the results are not apparent. As they learn more about the cellular mechanisms and what pathways are explored, then it will be easier to use genetic modification. Most likely the modification will have to tackle multiple aspects of the salt sensitivity. Furthermore, the aim of this modification is to assist crop growing, but not to formulate plants that can grow in abnormally high concentrations of salt. The future of plants looks bright, and this is aided by continual research on the topic. One day soon, crops will be altered to survive and produce maximum yield grown under minimal conditions. The problem of salt stress will be alleviated and farmers will be satisfied.