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.
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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 |
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|
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
links Salinity
Tolerance Reference Database – From USDA Salinity Lab Salinity tolerance. (2001) Salinity
Tolerance –mostly molecular genetics (2001) Salinity Tolerance
–mostly molecular genetics (2002) Improving crop salt tolerance –
(2003) Developing
salt-tolerant crop plants: challenges and opportunities (2005) |
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.
References
Black AL, Brown PL, Halvorson AD, Siddoway FH
(1981) Dryland cropping strategies for efficient water-use to control saline
seeps in the Northern Great Plains, U.S.A. Agricultural Water Management 4,
295-311.
Flowers TJ (2004)
Improving crop salt tolerance. Journal of Experimental Botany 55,
307-319.
Flowers TJ, Troke PF, Yeo, AR (1977) The mechanism
of salt tolerance in halophytes. Annual Review of Plant Physiology, 28, 89-121.
Goyal SS, Sharma SK, Rains DW, Lauchli A (1999a)
Long term reuse of drainage waters of varying salinities for crop irrigation in
a cotton-safflower rotation system in the San Joaquin Valley of California - A
nine year study: I. Cotton (Gossypium
hirsutum L.). Journal of Crop Production 2, 181-213.
Goyal SS, Sharma SK, Rains DW, Lauchli A (1999b)
Long term reuse of drainage waters of varying salinities for crop irrigation in
a cotton-safflower rotation system in the San Joaquin Valley of California - A
nine year study: II. Safflower (Carthamus
tinctorius L.). Journal of Crop Production 2, 215-227.
Hutchinson PA, Bond WJ (2001) Routine measurement
of the soil water potential gradient near saturation using a pair of tube
tensiometers. Australian Journal of Soil
Research 39,
1147-1156.
Husain S, Munns R, Condon AG (2003) Effect of
sodium exclusion trait on chlorophyll retention and growth of durum wheat in
saline soil. Australian Journal of Agricultural Research 54, 589–597.
James RA, Rivelli AR, Munns R, von Caemmerer S
(2002) Factors affecting CO2 assimilation, leaf injury and growth in
salt-stressed durum wheat. Functional Plant Biology 29, 1393-1403.
Munns R (1993) Physiological processes limiting
plant growth in saline soils. Some dogmas and hypotheses. Plant Cell and
Environment 16, 15–24.
Munns R, James RA (2003) Screening methods for
salinity tolerance: a case study with tetraploid wheat. Plant and Soil 253,
201-218.
Munns R, Schachtman DP, Condon AG (1995) The
significance of two-phase growth response to salinity in wheat and barley.
Australian Journal of Plant Physiology 22, 561–569.
Passioura JB, Munns (2000) Rapid environmental changes that
affect leaf water status induce transient surges or pauses in leaf expansion
rate. Australian Journal of Plant Physiology 27, 941-948.
Qureshi
R H, Aslam M, Akhtar J (2003) Productivity enhancement in the salt-affected
lands of Joint Satiana Pilot Project area of Pakistan. In SS Goyal, SK Sharma, DW Rains (Eds) Crop Production in Saline
Environments: Global and Integrative perspectives. The Food Products Press,
Binghamton, New York.
Richards L A (Ed) (1969) Diagnosis and improvement
of saline and alkali soils. Agriculture Handbook No. 60, United States
Department of Agriculture, Washington DC.
Stirzaker RJ, Vertessy RA, Sarre A (Eds) (2002)
Trees, Water and Salt: an Australian guide to using trees for healthy
catchments and productive farms. Rural Industries Research and Development Corporation:
Canberra.
Zhang HX, Blumwald E (2001) Transgenic
salt-tolerant tomato plants accumulate salt in foliage but not in fruit. Nature
Biotechnology 19, 765-768.
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