Salinity Stress and its Mitigation
Rana Munns1,
Sham S. Goyal2, and John Passioura1
1CSIRO
Plant Industry, GPO Box 1600, Canberra ACT 2601,
2Department
of Agronomy and Range Science, University of California,
Soil
salinity (accumulation of salts in the surface zone) has a number of causes,
which differ in different geological and climatic regions. The causes can be
natural, be due to clearing of native vegetation (‘dryland salinity’), or due
to irrigation (Table 1). Mitigation of soil salinity and its impact on plants
must therefore be considered somewhat differently in the context of these three
scenarios. Salinity is often accompanied by other soil properties, such as
sodicity, alkalinity, or boron toxicity, which exert their own specific effects
on plant growth. Waterlogging often accompanies salinity due to clearing or to
irrigation. This article will consider only salinity.
|
Table 1. Classes of soil salinity and
expected land use. (*ECe of a saturated extract which
approximates
saturated field water content). |
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|
Degree of salinity |
Site
characteristics |
Use |
Species |
|
Low
salinity (ECe*=2
to 4 dSm-1) |
Natural
salinity; often seasonally dry Irrigation
salinity; can be waterlogged after irrigation |
Cropping
|
Low-moderate
salt tolerance |
|
Moderate
salinity (ECe=
4 to 8 dSm-1) |
Dryland
salinity; often seasonally waterlogged |
Crop-pasture rotations |
Moderate-high
salt tolerance |
|
High
salinity (ECe
> 8 dSm-1) |
Discharge
areas; can be seeping or dry according to season |
Grazing
or revegetation |
Halophytes |
Salts
in soils are primarily chlorides and sulfates of sodium, calcium, magnesium,
and potassium. Symptoms of soil salinity include slow and spotty seed
germination, sudden wilting, stunted growth, marginal
burn on leaves (especially lower, older leaves), leaf yellowing, leaf fall,
restricted root development, and sudden or gradual death of plants.
This
article describes two ways in which salinity stress of crops can be mitigated - by changing farm
management practices, and by breeding for increased salt tolerance in crops.
The article accompanies the previous one "Salinity Stress and its
Impact" which outlines the mechanisms of salt tolerance in different
species. Here we describe various traits
for salt tolerance that can be used to select for tolerant genotypes in
breeding programs
Mitigation of
Salinity Stress by Management Practices:
|
Some relevant links Saline
and Alkali Soils – Handbook No. 60; Diagnosis and Improvement. Use of
Saline Water for Crop Production Dryland
Salinity web site – contains ample information on salinity management |
The
management solution is different in the three types of salinity.
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.
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Table 2. Leaching requirement for typical
irrigation waters in California as related to salt tolerance of crop (taken
from J.D. Oster, G.J. Hoffman and F.E. Robinson, California Agriculture,
October 1984). |
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|
Salinity of
applied water (dS/m) |
Leaching
requirement |
|||
|
Sensitive crop |
Moderately
sensitive crop |
Moderately
tolerant crop |
Tolerant crop |
|
|
0.05
(freshwater canal) |
0.01 |
0.01 |
0.01 |
0.01 |
|
0.3
(California aqueduct) |
0.05 |
0.02 |
0.02 |
0.02 |
|
1.3
(Colorado River) |
0.14 |
0.07 |
0.04 |
0.02 |
|
5.0
(reused drainage) |
not possible |
0.27 |
0.15 |
0.10 |
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).
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.
Natural Salinity
Natural
salinity occurs in the low rainfall zones, of 300-400 mm and below, where even
an annual crop can use all the rain, so water tables are not disturbed.
Can
salt be removed from the soil ? In the cases of
natural salinity, with no rising water tables and no movement of salt to the
surface, it may be envisaged that salt could be removed from the soil by crops
that accumulate salt. However, calculations do not support this idea. First,
the grain does not contain any salt, so the normal harvesting methods do not
remove any salt. The straw contains salt, but if it is reincorporated or used
for animal fodder, the salt will still be returned to the soil
. Fruit of horticultural species contains little salt as well, so in
both cases, the straw or trash would need to be transported away from the site
and into catchments
that empty directly into the ocean.
If
we presume that the trash can be removed to a safe site, how much salt will it
remove from the field? If the vegetative harvest is 5 tonnes per Ha per year,
and if it contains 10% salt per unit dry matter, then the harvest of salt would
be 0.5 tonne/Ha/year. However the amount of salt in
the topsoil could be over 15 tonnes/Ha (for the top 1 m of
soil containing salt at 10 dS/m in the soil water). So
30 years would be required to remove the salt from the top 1 m. This
supposes that a vegetative harvest of 5 tonnes can be sustained (which is
unlikely) and does not take into account that salt is continually added to the
soil in the rain and wind, or in any irrigation water that might be used to
produce a 5 tonne crop.
So
for natural salinity, as well as for irrigated and dryland, improving the salt
tolerance of crops is a useful approach.
Mitigation of
Salinity Stress by Exploiting Plant Salinity Tolerance:
|
Some
relevant resources Salinity
Tolerance Reference Database – From USDA Salinity Lab Salinity tolerance.
(2003) Salinity
Tolerance (2006) Salinity Tolerance (2008) |
1. 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).
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Fig.
1. Schematic illustration of the two-phase
growth response to salinity for two genotypes that differ in salt tolerance.
They differ in the rate at which salt reaches toxic levels in leaves.
Timescale is days or weeks or months, depending on the species and the
salinity level. During Phase 1, growth of both genotypes is reduced because
of the osmotic effect of the saline solution outside the roots. During Phase
2, leaves in the more sensitive genotype die and reduce the photosynthetic
capacity of the plant. This exerts an additional effect on growth. Adapted
from Munns (1993). |
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.
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Fig. 2. The effect of
changes in salinity of soil solution on elongation rate of a barley leaf, (a)
with no control of leaf water status, and (b) with the plant maintained at balancing
pressure, i.e. at constant leaf water status, throughout the changes. The
vertical broken lines mark the times at which the light was turned on or off, and the broken horizontal line in (a) marks zero
elongation rate. At the changes, full strength nutrient solution was
exchanged with the same solution containing 75mM NaCl, or vice versa.
Reproduced from Passioura and Munns (2000). |
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.
Traits for salt-specific effect
Salt exclusion
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Fig.
3.
Relationship between salinity tolerance and leaf Na+ concentration in
subspecies durum selections. Na+
concentrations were measured on leaf 3 after 10 d in 150 mM NaCl and biomass
after 24 d in control and salt treatments. All values are means (n=5). Munns
and James (2003). |
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Fig.
4. Effect of different salinity levels on
grain yield of low and high Na+ uptake durum landraces (133 d in salt).
Husain et al. (2003). |
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.
Halophytes
have the ability to compartmentalise Na+ to very high concentrations in vacuoles.
Halophytes have no special metabolic adaptation to high salinity. Early studies
showed that in vitro activities of
enzymes extracted from the halophytes Atriplex
spongiosa and Suaeda maritima were
just as sensitive to NaCl in the assay media as were enzymes extracted from the
common bean or green pea. So their ability to tolerate Na+
levels as great as 3.5 mmol g-1 DW (about 700 mM) is due to their ability
to compartmentalise most of the Na+ in vacuoles (Flowers et al., 1977).
Glycophytes
are able to 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.
Table
3 summarises the features of salt-specific traits, along with traits for the
osmotic effect of salinity.
|
Table 3. List
of possible techniques used to screen for salinity tolerance (adapted from
Munns and James 2003) |
|||||
|
TechniqueA |
Osmotic
or salt-specific effect measured |
Controls
needed |
Advantages
(speed, dependence on controlled conditions etc.) |
Length
of time of experiment |
Correlation
with tolerance in field |
|
Whole
plant growth :measurements -
biomass -
RGR -
RGR in salinity alone |
Osmotic
or both |
Yes Yes No** |
Closest
measure of salinity tolerance in the field, and an indicator of yield. |
4-6
weeks |
Moderate |
|
Root
elongation |
Osmotic |
Yes |
Quick,
does not require controlled environment. |
1-2
weeks |
Low |
|
Leaf
elongation |
Osmotic |
Yes |
Non-destructive |
2
weeks |
Low |
|
Germination |
Osmotic |
Yes |
Large
numbers easily handled |
1
week |
Low
or none |
|
Survival |
Either
or both |
No* |
Limited
experimental period, as can adjust the salinity. Highly tolerant genotypes
stand out. |
2-8
weeks, depending on salinity |
Uncertain,
and counter-productive for annual crops |
|
Leaf
injury -
leakage from discs -
chlorophyll content |
Either
or both |
No* |
Non-destructive |
2-4
weeks |
Unproven |
|
Chlorophyll
fluorescence |
Either
or both |
No* |
Non-destructive,
can do in field |
2-8
weeks |
Unproven |
|
Tissue
culture |
Either
or both |
Yes |
|
Months |
None |
|
Salt-specific traits - Na+ exclusion - K+/Na+ discrimination -
Cl- exclusion -
tissue tolerance |
Salt-specific |
No No No No** |
Non-destructive |
1-2
weeks 1-2
weeks 1-2
weeks 2-6
weeks |
High High High Unknown |
|
*
assumes plants do not vary in the control treatment, either between genotypes
or between experiments. There should be 100% survival, no leakage, and
typical F parameters of healthy plants. **
assumes that RGR and leaf longevity in controls conditions does not vary
between genotypes – this is likely, but needs to be verified |
|||||
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.
These
methods are summarised in Table 3
2. 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.
3. 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.
Conclusions
There is abundant unexplored and
unexploited genetic variation that can be harnessed to improve the salt
tolerance of crop and horticultural species. This avoids the need for genetic
engineering involving transformation with a single gene, which has both
sociological problems in the current attitude to transgenic food, and also
physiological limitations as salt tolerance is a complex process and many genes
are controlling it.
Increases in the salinity tolerance of
crops and pasture plants can overcome some of the impacts of salinity in areas
not associated with rising water tables. Where there is a high water table,
plants need to be tolerant of waterlogging as well as of salinity.
Solutions to salinity depend on
lowering water tables, which means that plants should use as much water as
possible, and use all the rain or irrigation water. Management of salinity
includes planting perennials in areas identified as recharge areas. Salinity
tolerance is not an important character for these plants, rather deep roots and
efficient water extraction is more important.
Long-term solutions to salinity
involve changed farming systems and a changed attitude to using water.
Water-inefficient crops such as rice might give way to water-efficient crops
such as alfalfa (lucerne). In catchments with rising water tables, solutions
will also depend on social, economic, and political factors, and an acceptance
that old ways have to change if agriculture is to be sustained.
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