|
The Impact of Salinity Stress |
Dr.
Rana Munns, CSIRO Division of Plant Industry |
1. The Environmental and
Physiological Nature of Salinity
Introduction
According to the FAO Land
and Plant Nutrition Management Service, over
6% of the world's land is affected by either salinity or sodicity (Table 1). The term salt-affected
refers to soils that are saline or sodic, and these cover over 400 million
hectares, which is over 6% of the world land area (Table 1) . Much of the
world’s land is not cultivated, but a significant proportion of cultivated land
is salt-affected. Of the current 230 million ha of irrigated land, 45
million ha are salt-affected (19.5 percent) and of the 1,500 million ha under
dryland agriculture, 32 million are salt-affected to varying degrees (2.1
percent).
|
Table 1: Regional
distribution of salt-affected soils, in million hectares |
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|
Regions |
Total area |
Saline soils |
Sodic soils |
||
|
|
Mha |
Mha |
% |
Mha |
% |
|
Africa |
1,899 |
39 |
2.0 |
34 |
1.8 |
|
|
3,107 |
195 |
6.3 |
249 |
8.0 |
|
Europe |
2,011 |
7 |
0.3 |
73 |
3.6 |
|
Latin America |
2,039 |
61 |
3.0 |
51 |
2.5 |
|
Near East |
1,802 |
92 |
5.1 |
14 |
0.8 |
|
North America |
1,924 |
5 |
0.2 |
15 |
0.8 |
|
Total |
12,781 |
397 |
3.1% |
434 |
3.4% |
Salinity
occurs through natural or human-induced processes that result in the
accumulation of dissolved salts in the soil water to an extent that inhibits
plant growth. Sodicity is a secondary result of salinity in clay soils, where
leaching through either natural or human-induced processes has washed soluble
salts into the subsoil, and left sodium bound to the negative charges of the
clay.
A saline soil is defined as having a high
concentration of soluble salts, high enough to affect plant growth. Salt
concentration in a soil is measured in terms of its electrical conductivity, as
described in the section below on measurements. The USDA
Salinity Laboratory defines a saline soil as having an ECe of 4
dS/m or more. ECe is the electrical conductivity of the ‘saturated
paste extract’, that is, of the solution extracted from a soil sample after
being mixed with sufficient water to produce a saturated paste. However, may
crops are affected by soil with an ECe less than 4 dS/m. The
moisture content of a drained soil at field capacity may be much lower than the
water content of its saturated paste. Further, under dryland agriculture, the
soil water content might drop to half of field capacity during the life of the
crop. The actual salinity of a rain-fed field whose soil had an ECe
of 4 dS/m could be 8-12 dS/m. As described below, this would severely limit
yield of most crops.
Types and Causes of Salinity
Natural or primary salinity
Primary
salinity results from the accumulation of salts over long periods of time,
through natural processes, in the soil or groundwater. It is caused by two
natural processes.
The
first is the weathering of parent materials containing soluble salts.
Weathering processes break down rocks and release soluble salts of various
types, mainly chlorides of sodium, calcium and magnesium, and to a lesser
extent, sulphates and carbonates. Sodium chloride is the most soluble salt.
The
second is the deposition of oceanic salt carried in wind and rain. 'Cyclic
salts' are ocean salts carried inland by wind and deposited by rainfall, and
are mainly sodium chloride (Table 2). Rainwater contains from 6 to 50 mg/kg of
salt, the concentration of salts decreasing with distance from the coast. If
the concentration is 10 mg/kg, this would add 10 kg/ha of salt for each 100 mm
of rainfall per year. Accumulation of this salt in the soil would be
considerable over millennia. The amount of salt stored in the soil varies with
the soil type, being low for sandy soils and high for soils contain a high
percentage of clay minerals. It also varies inversely with average annual rainfall.
For example, in Western Australia, the salt content of a 40 m profile ranges
from 170 to 950 tonne/ha for rainfall averaging from 1000 mm to 600 mm per
year.
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Table 2: Concentration
of salts in rain and seawater. |
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|
The composition of rainwater varies
greatly depending on prevailing winds and distance from the coast, and the
table gives the composition of rainwater from a northern hemisphere source
(Encyclopaedia Britannica). It is measured as mg/kg or ppm (parts per million).
The composition of seawater is uniform around the globe, and is expressed as
g/kg or ppt (parts per thousand). The electrical conductivity of rainwater is
about 0.01 dS/m, and of seawater is 55 dS/m. |
||||
|
Ion |
rainwater
(local) |
seawater
(global) |
||
|
mg/kg (ppm) |
(µmol/L) µM |
g/kg (ppt) |
(mmol/L) mM |
|
|
Sodium (Na+) |
2.0 |
86 |
10.8 |
470 |
|
Chloride (Cl-) |
3.8 |
107 |
19.4 |
547 |
|
Sulfate (SO4 2-) |
0.6 |
6 |
2.7 |
28 |
|
Magnesium
(Mg2+) |
0.3 |
11 |
1.3 |
53 |
|
Calcium (Ca2+) |
0.1 |
2 |
0.4 |
10 |
|
Potassium (K+) |
0.3 |
8 |
0.4 |
10 |
|
Total |
7.0 |
|
35.0 |
|
Secondary or human-induced salinity
Secondary
salinisation results from human activities that change the hydrologic balance
of the soil between water applied (irrigation or rainfall) and water used by
crops (transpiration). The most common causes are (i) land clearing and the
replacement of perennial vegetation with annual crops, and (ii) irrigation
schemes using salt-rich irrigation water or having insufficient drainage. Prior
to human activities, in arid or semi-arid climates, the water used by natural
vegetation was in balance with the rainfall, with the deep roots of native
vegetation ensuring that the water tables were well below the surface. Clearing
and irrigation changed this balance, so that rainfall on the one hand, and
irrigation water on the other, provided more water than the crops could use.
The excess water raises water table and mobilises salts previously stored in
the subsoil and brings them up to the root zone. Plants use the water and leave
the salt behind until the soil water becomes too salty for further water uptake
by roots. The water table continues to rise, and when it comes close to the
surface, water evaporates leaving salts behind on the surface and thus forming
a ‘salt scald’. The mobilised salt can also move laterally to water courses and
increase their salinity.
Irrigated
lands of the world in 1987 totalled 227 Mha (Table 3). In many irrigated areas,
the water table has risen due to excessive amounts of applied water coupled
with poor drainage. In most of the irrigation projects located in semi-arid and
arid areas, the problems of waterlogging and soil salinity have reached serious
proportions even before the full potential of the irrigation project could be
realised. Most of the irrigation systems of the world have caused secondary
salinity, sodicity or waterlogging. Table 3 shows that the proportion of
salt-affected irrigated land in various countries ranges from a minimum of 9%
to a maximum of 34%, with a world average of 20%. Irrigated land is only 15% of
total cultivated land, but as irrigated land has at least twice the
productivity of rainfed land, it may produce one-third of the world’s food.
|
Table
3: Global estimate of secondary salinisation in the world's irrigated lands |
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|
Country |
Total land area cropped Mha |
Area irrigated |
Area of irrigated land that is salt-affected |
||
|
Mha |
% |
Mha |
% |
||
|
|
97 |
45 |
46 |
6.7 |
15 |
|
|
169 |
42 |
25 |
7.0 |
17 |
|
Soviet Union |
233 |
21 |
9 |
3.7 |
18 |
|
|
190 |
18 |
10 |
4.2 |
23 |
|
|
21 |
16 |
78 |
4.2 |
26 |
|
|
15 |
6 |
39 |
1.7 |
30 |
|
|
20 |
4 |
20 |
0.4 |
10 |
|
|
3 |
3 |
100 |
0.9 |
33 |
|
|
47 |
2 |
4 |
0.2 |
9 |
|
|
36 |
2 |
5 |
0.6 |
34 |
|
|
13 |
1 |
9 |
0.1 |
9 |
|
Subtotal |
843 |
159 |
19 |
29.6 |
20 |
|
World |
1,474 |
227 |
15 |
45.4 |
20 |
|
Source:
Ghassemi et al. (1995) compiled from FAO data for 1987 |
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Irrigation
water adds appreciable amounts of salt, even with good quality irrigation water
containing only 200-500 mg/kg of soluble salt. Irrigation water with a salt
content of 500 mg/kg (i.e. 500 mg/L) contains 0.5 tonnes of salt per 1,000 m3.
Since crops require 6,000-10,000 m3 of water per hectare each year,
one hectare of land will receive 3-5 tonnes of salt. Because the amount of salt
removed by crops is negligible, salt will accumulate in the root zone, and must
be leached by supplying more water than is required by the crops. If drainage
is not adequate, the excess water causes the water table to rise, mobilising
salts which accumulate in the root zone. When the crop is unable to use all the
applied water, waterlogging occurs.
Land
clearing also changes the hydrological balance. In its natural state, native
deep-rooted and perennial vegetation use almost all the rainwater that falls on
the land. In arid or semi-arid climates the growth rate of the natural
vegetation is limited by the availability of fresh rainwater. Salts will be
flushed down by rain, and accumulate at the bottom of the root zone to the
limiting concentration for roots to extract water, at approximately 50 dS/m.
Clearing the deep-rooted native vegetation, and replacing it with
shallow-rooted annual species that do not use all the rainfall, allows
rainwater to escape below the roots, and 'recharge' the groundwater. Clearing
of native vegetation for dryland agriculture can increase the rate of drainage
by 100 times. In the Mallee Region of southern
Soil sodicity and sub-soil salinity
Sodic soils
have a low concentration of soluble salts, but a high percent of exchangeable
Na+; that is, Na+ forms a high percent of all cations
bound to the negative charges on the clay particles that make up the soil
complex. Sodicity is defined in terms of the threshold ESP (exchangable sodium
percentage) that causes degradation of soil structure. The negatively charged
clay particles are held together by divalent cations. When monovalent cations
such as Na+ displace the divalent cations on the soil complex, and
the concentration of free soluble salts is low, the complex swells and the clay
particles separate ('disperse'). The USDA Salinity
Laboratory defines a sodic soil as having an ESP greater than 15, but in
If the concentration of soluble salts is sufficiently
low, hydrolysis of the sodic clay will occur, creating a highly alkaline soil. Alkaline soils are a type of sodic soil
with a high pH due to carbonate salts, and are defined as having an ESP of 15
or more with a pH of 8.5-10.
|
Additional
reading on Soil Salinity and Sodicity Ghassemi
F., Jakeman A.J., Nix H.A. 1995.
Salinisation of land and water resources: Human causes, extent, management
and case studies. UNSW Press, Naidu
R., Sumner M.E., Rengasamy P. (Eds.). 1995. Australian Sodic Soils. CSIRO
Publications. Rengasamy
P. 2002. Transient salinity and subsoil constraints to dryland farming in
Australian sodic soils: an overview. Aust.
J. Exp. Agric. 42, 351-361. |
The process of sodicity is
complex and occurs over a long period of time. Initially, salts that have
accumulated within the soil profile, from either airborne deposition or mineral
weathering, cause the clay fraction of the soil to become saturated with
sodium. Subsequently, leaching of the profile, either by rainwater over
prolonged periods, or by irrigation with fresh water, lowers the electrolyte
concentration and the clay particles disperse. Further leaching washes the
dispersed clay particles deeper into the profile where they block pores and
hinder infiltration of water. The soil then is very slow
to drain, and is readily waterlogged.
In semi-arid environments, soil profiles are commonly saline/sodic, where the salt has
accumulated due to the low permeability of the sodic subsoil. In theory, if
sufficient salts accumulate, the threshold electrolyte concentration for
flocculation will be exceeded and the clay will flocculate and take on
pseudo-structure. However, given that permeability and leaching will then
increase, the subsequent dilution of salts will cause colloids to disperse.
Consequently, a quasi-steady state between flocculation and dispersion
processes is maintained.
Saline/sodic soils are
widespread in arid and semi-arid lands of the world (Table 3), with a large
component in
2. Measuring soil salinity
Soil
salinity is measured by its electrical conductivity. The SI unit of electrical
conductivity (EC) is dS/m. Table 4 shows the relationship to other units of
conductivity, and to NaCl concentration (10 mM NaCl has an EC close to 1 dS/m).
Originally the conductivity was measured in a saturated paste extract (ECe),
but the method is tedious as first the saturated paste has to be made, second
the water needs to be extracted by a powerful vacuum pump, and third a very
sandy soil does not make a saturated paste. A more convenient and universal
method is a '1:5 extract'. The soil is dried or compressed, shaken with 5 g
deionised water per 1 g soil, the probe of a hand-held conductivity meter is
placed in the suspension, and the EC measured. The EC of the soil that was
sampled can be calculated if its water content is known at the time of
sampling. Alternatively, the measurement can be related to the field capacity
of the soil using conversion factors. As a rough guide, a sandy soil will have
a field capacity of 0.2 g water per g soil, and a clay soil about 0.4 g/g.
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Table 4: Units for
measuring salinity, and conversion factors. |
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|
Conversion
factors relating total dissolved salts or pure NaCl to an electrical
conductivity (EC) of 1 dS/m (1 deciSiemen/metre) are given, along with
equivalent units of various types, old and new. The
conversion of EC of 1 dS/m to total dissovled salts (640 mg/L) assumes a
composition of salts that is common in groundwater across the world. The
exact factor varies from 530 (if the salt is predominantly NaCl) to 900 (if
the salts are formed predominantly from divalent ions). |
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|
Measurement and units |
Application |
1 dS/m is equal to: |
Equivalent units |
|
Conductivity (dS/m) |
soils |
1 |
1 dS/m = 1 mS/cm = 1 mmho/cm |
|
Conductivity (µS/cm) |
irrigation and river water |
1000 µS/cm |
1 µS/cm = 1 µmho/cm |
|
Total dissolved salts (mg/L) |
irrigation and river water |
640 mg/L (approx.) |
1 mg/L = 1 mg/kg = 1 ppm |
|
Molarity of NaCl (mM) |
laboratory |
10 mM |
1 mM = 1 mmol/L |
|
Table 5: Electrical conductivity (EC) of
pure solutions at 20°C (dS/m). |
|
|
The solutions represent those of salts found in soils
or in seawater. Data from the Handbook of Physics and Chemistry (CRC Press,
55th editition, 1975). (Note that 1 dS/m = 1 mmho/cm). |
|
|
Solution |
EC (dS/m) |
|
10 mM NaCl |
1.0 |
|
100 mM NaCl |
9.8 |
|
500 mM NaCl |
42.2 |
|
10 mM KCl |
1.2 |
|
10 mM CaCl2 |
1.8 |
|
10 mM MgCl2 |
1.6 |
|
50 mM MgCl2 |
8.1 |
The
electrical conductivity of irrigation or river water is measured with the same
hand-held conductivity meter as above, but is expressed in units 1000 times
magnified, as channel or river water would normally have a very low
concentration of salts. River water quality is often expressed as dS/cm (1000 x
dS/m). Irrigation water quality is often expressed as total soluble salts, an
international convention being that 1 dS/m is equivalent to 640 mg/L of mixed
salts (Table 4).
Some
data on the electrical conductivity of pure solutions relevant to saline soils
or to seawater are given in Table 5. As 10 mM NaCl has an EC of almost exactly
1.0 dS/m, and as it is the most common salt in solution in ground or river
water, a 'rule of thumb' is to muliply the EC by ten to equate it to salt
molarity.
Soil
salinity on a large scale is mapped with an electromagnetic (EM) conductivity
meter. This instrument estimates the bulk electrical conductivity of the soil,
which depends on the salinity of the soil solution, its water content, and the
type and amount of clay in the soil. The output needs to be calibrated by
chemical measurements of cores taken from the field. The ground EM conductivity
meters consist of a small transmitter coil, energised with an alternative
current. This current generates a primary magnetic field in the ground, which
induces small currents which generate their own secondary magnetic field, which
is smaller and proportional to the soil conductivity. A receiver coil close by
measures the primary and secondary magnetic fields. The EM38 meter (Geonics Limited) is designed for
agricultural surveys and measures to 1.5 m depth in the vertical mode and 0.75
m in the horizontal mode. An EM38 survey can be used to map the extent of
subsoil salinity as well as discharge areas. Fig 1 illustrates the
heterogeneity of soil salinity as shown by the EM meter.
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|
|
Figure 1.
Example of an EM survey taken on a paddock basis at ground level. (Image
courtesy of P. Rampant, Department of Primary Industry, |
Airborne
EM mapping is feasible now that global positioning systems have become
available for accurate and rapid mapping. In the airborne method, the
transmitter is slung below a plane flying at 150 m, and the receiver is towed
behind. The plane generates a magnetic field, and the moving field passsing
through the ground creates a secondary electric field whenever it hits
something conductive. The receiver trailing behind the plane collects the data
(For example, the Australian
National Airborne Physics Project).
3. The repercussions of salinity
The Effect of Salinity on Plants
Salts
in the soil water may inhibit plant growth for two reasons. First, the presence
of salt in the soil solution reduces the ability of the plant to take up water,
and this leads to reductions in the growth rate. This is referred to as the
osmotic or water-deficit effect of salinity. Second, if excessive amounts of
salt enter the plant in the transpiration stream there will be injury to cells
in the transpiring leaves and this may cause further reductions in growth. This
is called the salt-specific or ion-excess effect of salinity (Greenway and
Munns, 1980). The definition of salt tolerance is usually the percent biomass
production in saline soil relative to plants in non-saline soil, after growth
for an extended period of time. For slow-growing, long-lived, or uncultivated
species it is often difficult to assess the reduction in biomass production, so
percent survival is often used.
As
salinity is often caused by rising water tables, it can be accompanied by
waterlogging. Waterlogging itself inhibits plant growth and also reduces the
ability of the roots to exclude salt, thus increasing the uptake rate of salt
and its accumulation in shoots. More information on waterlogging stress can be
found on this web site under ‘The
Stresses’.
Variation in Salt Tolerance between Species
The
three most important crops in the world are wheat, rice and maize. Differences
in the growth response of various species are shown in Fig. 2. Wheat is one of
the more salt-tolerant crop species, and many cultivars that have been selected
for yield in water-limited conditions do not suffer a 50% reduction in biomass
until salinities reach 15 dS/m (approximately 150 mM NaCl). Rice is more salt-sensitive, and many
cultivars suffer a 50% reduction in growth at half this concentration of salts.
Maize falls in between these two species in terms of salt sensitivity.
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|
Figure 2.
Biomass production of four diverse and important plant species in a range of
salinities. Wheat is one of the more salt-tolerant crops, and rice is one of the
more salt-sensitive crops. Two halophytes: a saltbush species Atriplex amnicola and a grass Diplachne (syn. Leptochloa) fusca or
Kallar grass. Both halophytes show outstanding salt tolerance with high
growth rates and are being used in |
Another
criterion of salt tolerance of crops is their yield in saline versus non-saline
conditions. A survey of salt tolerance of crops, vegetables and fruit trees was
made by the USDA Salinity Laboratory.
This shows for each species a threshold salinity below which there is no
reduction in yield, and then a regression for the reduction in yield with
increasing salinity (Fig. 3). Full details are available on line. The
data in some cases are for a single cultivar of the species, or a limited
number of cultivars at a single site, so they are not necessarily
representative of the species. Further, the data are related to an ECe
value, which is not an appropriate reference point for a sandy soil, or for
many current soil salinity estimates that based merely on a 1:5 extract.
However, the data are useful in that they show the wide range of tolerance
across species, and also show that yield has a different pattern of response
than does vegetative biomass (compare with Fig. 2) Yield always shows a
threshold in response to a range of salinities (Fig 3), but with young plants a
threshold is rarely seen. With plants exposed to salinity at an early stage of
seedling development there are linear reductions in both leaf area expansion
and total plant biomass with increasing salinity, as shown in Fig. 2.
|
|
|
Figure
3. Categories for classifying crop tolerance
to salinity according to the USDA Salinity Lab. Note that the ECe is more
applicable to an irrigated than a rainfed field, in the latter the soil
moisture content might be 2-4 time less than in a saturated paste. |
Causes of the Growth Reduction under Saline Conditions
The
effects of a saline soil are two-fold: there are effects of the salt outside
the roots, and there are effects of the salt taken up by plants.
The
salt in the soil solution (the “osmotic stress”) reduces leaf growth and to a
lesser extent root growth, and decreases stomatal conductance and thereby
photosynthesis (Munns, 1993). The cellular and metabolic processes involved are
in common to drought-affected plants, and described under Drought Stress and Its Impact on
this site. The rate at which new leaves
are produced depends largely on the water potential of the soil solution, in
the same way as for a drought-stressed plant. Salts themselves do not build up
in the growing tissues at concentrations that inhibit growth: meristematic
tissues are fed largely by the phloem from which salt is effectively excluded,
and rapidly elongating cells can accommodate the salt that arrives in the xylem
within their expanding vacuoles. So, the salt taken up by the plant does not
directly inhibit the growth of new leaves.
The
salt within the plant enhances the senescence of old leaves. Continued
transport of salt into transpiring leaves over a long period of time eventually
results in very high Na+ and Cl- concentrations, and they
die. The rate of leaf death is crucial for the survival of the plant. If new
leaves are continually produced at a rate greater than that at which old leaves
die, then there might be enough photosynthesising leaves for the plant to
produce some flowers and seeds. However, if the rate of leaf death exceeds the
rate at which new leaves are produced, then the plant may not survive to
produce seed. For an annual plant there is a race against time to initiate
flowers and form seeds, while the leaf area is still adequate to supply the
necessary photosynthate. For perennial species, there is an opportunity to
enter a state of dormancy, and thus survive the stress.
The
two responses occur sequentially, giving rise to a two-phase growth response to
salinity. The first phase of growth reduction is quickly apparent, and is due
to the salt outside the roots. It is essentially a water stress or osmotic
phase, for which there is surprisingly little genotypic difference. Then there
is a second phase of growth reduction, which takes time to develop, and results
from internal injury. The two-phase growth response is illustrated in Fig 4. The experiment was conducted with two
genotypes with contrasting rates of Na+ uptake, and known
differences in salt tolerance; previous experiments had shown that the genotype
with the low Na+ uptake rate had a higher survival of high salinity.
Fig 4 shows that during the first 3-4 weeks after the soil was salinised, there
was a large growth reduction in both genotypes. This is called the 'Phase 1'
response, and is due to the osmotic effect of the salt. Then after 4 weeks, the
genotypes separated; the one with the low Na+ uptake rate continued
to grow, although still at a reduced rate compared to the controls in
non-saline solution, but the one with the high Na+ uptake rate
produced little biomass and many individuals died. This is the 'Phase 2'
response, and is due to genotypic differences in coping with the Na+
or Cl- ions in the soil, as distinct from the osmotic stress.
|
|
|
Figure 4.
Two accessions of the diploid wheat progenitor Ae. tauschii grown in supported hydroponics in control solution
(closed symbols) and in 150 mM NaCl (open symbols). Circles denote the
tolerant accession, triangles the sensitive one. The arrow marks the time at
which symptoms of salt injury could be seen on the sensitive accession; at
that time the proportion of dead leaves was 10% for the sensitive and 1% for
the tolerant accession (Munns et al., 1995). |
These
results illustrate the principle that the initial growth reduction is due to
the osmotic effect of the salt outside the roots, and that what distinguishes a
salt-sensitive plant from a more tolerant one is the inability to prevent salt
from reaching toxic levels in the transpiring leaves, which takes time.
To
grow in saline conditions, plants must maintain a high water status in the face
of soil water deficits and potential ion toxicity. A plant can only grow or
survive in a saline soil if it can both continue to take up water and exclude a large proportion of the
salt in the soil solution.
The Required Extent of Salt Exclusion
Roots
must exclude most of the Na+ and Cl- dissolved in the
soil solution or the salt will gradually build up with time in the shoot and
become so high that it kills it. To prevent salt building up with time in the
shoot, roots should exclude 98% of the salt in the soil solution, allowing only
2% to be transported in the xylem to the shoots. This value of 2% can be
calculated from the following equation:
The
concentration at which NaCl accumulates in the shoot depends on the salt
concentration in the soil solution, the percentage of salt taken up by roots,
and the percentage of water retained in the leaves:
|
[NaCl]shoot = |
[NaCl]soil x |
% salt taken up |
(Eqn.
1) |
|
%
water retained |
|
Plants
retain only about 2% of the water they transpire, ie they take up about 50 time
more water from the soil than they retain in their shoot tissues. The
percentage of transpired water that is retained in the shoot can be calculated
from the product of the water use efficiency (wue; mass of shoot produced per mass of H2O transpired)
and the shoot water content (wc;
shoot H2O per shoot mass):
Water
use efficiency (WUE) of plants growing at moderate evaporation demand are
usually in the range of 3-6 mg g-1, the variation due to extremes of
evaporative demand, rather than a peculiarity of the species. For a water use
efficiency of 4 mg g-1 and a shoot H2O:DW ratio of 5:1,
about 20 mg of water is retained in the shoot for every g of water transpired
(Eqn. 2). That is, the shoot retains only 2% of the water transpired. In order
to prevent the salt concentration in the shoot increasing above that in the
soil, then only 2% of the salt should be allowed into the shoot, i.e.
98% should be excluded.
%
water retained = wue x wc x 100 (Eqn.
2)
A
soil salinity of 100 mM NaCl or 10 dS m-1 is about as high as most
crops will tolerate without a significant reduction in growth or yield (see
Figs. 2 and 3), and a concentration of 100 mM NaCl on a whole shoot basis is
about as high as is desirable because it will include some old leaves with much
higher salt concentrations, as well as younger leaves or other tissues with
lower concentrations. So for plants to grow for extended periods of time in
soils with salinity of this order of magnitude, roots should ensure that no
more than 2% gets to the shoots. Roots
themselves do not accumulate excessively high concentrations of salt. The Na+
and Cl- concentration in roots is rarely higher than in the external
solution, and often is lower.
The Relationship between Transpiration and Salt Uptake
|
|
|
Figure
5. The relation between ion concentration in
the xylem (A), ion flux to the shoot (B), and transpiration (water flow)
(Munns, 1985). |
The
fundamental processes governing the relationship between water and ion flow
through roots are complex and not well understood. NaCl does not move passively
with the transpiration stream, neither is its movement entirely independent of
it, at least in some species, or over certain ranges of transpiration. Fig. 5
shows the relationship between water and salt flow in the xylem of barley
plants (Munns, 1985). As water flow increased from a very low to a moderate
rate, there was an increase in Cl- flux, showing that the movement
of the ion through the root was enhanced as water flow started to increase.
However, when the water flow increased from moderate to high rates, there was
little or no further increase in Cl- flux, showing that the movement
of the ion was independent of further increases in water flow. This
relationship also holds for Na+ and K+.
Measurements
of ion concentrations in leaves of most plants grown at different humidities
are consistent with this pattern, that salt transport to leaves is
substantially affected only if transpiration is greatly affected. An effect
might be seen more with species that are very poor excluders, such as lupin as
mentioned above, and rice, which carry much more salt in an apoplastic or
transpirational "bypass" pathway than other species. In rice, the
percentage of water moving through a bypass pathway from roots to shoots was
estimated as 5.5% of the total water transpired, and could account for all the
Na+ transported to the shoots, whereas in wheat only 0.4% of the
water moved along a bypass pathway and could not account for most of the Na+
transported (Garcia et al., 1997).
Mechanisms of Control of Salt Transport
Here
we look at differences between species in the ability to tolerate the
salt-specific component of salinity. Differences in tolerating the osmotic
stress itself are in common with drought tolerance.
The
rate at which old leaves die depends on the rate at which salts accumulate to
toxic levels. Thus, control of the rate at which salt arrives in leaves is
essential, as are mechanisms that reduce the toxicity of the salt. For species
lacking the ability to compartmentalise salts in the vacuoles to high
concentrations (see below), continued transport of salt to the leaves will
eventually result in either excessive build-up of salts in the cell walls or in
the cytoplasm. The former will cause death through dehydration, and the latter
will cause death through poisoning of metabolic systems such as photosynthesis
or respiration.
Control at the whole plant level
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Figure
6. Control points at which salt transport is
regulated. These are: 1. selectivity
of uptake from the soil solution, 2. loading of the xylem, 3. removal of salt
from the xylem in the upper part of the plant, 4. loading of the phloem and
5. excretion through salt glands or bladders. For a salt tolerant plant
growing for some time in a soil solution of 100 mM NaCl, the root
concentrations of Na+ and Cl- are typically about 50
mM, the xylem concentration about 5 mM, and the concentration in the oldest
leaf as high as 500 mM (Munns et al., 2002). |
Control
of salt transport into and through the plant takes place at five sites in the
plant (Fig. 6). Control occurs in the root cortex, at the loading of the xylem,
at the retrieval from the xylem in upper parts of the roots. These three
processes serve to reduce the transport to the leaves. Control in the shoot
occurs by the exclusion of salt from the phloem sap flowing to meristematic
regions of the shoot. An additional mechanism occurs in most halophytes:
specialised cells to excrete salt from leaves. However, halophytes also rely on
the first four mechanisms to reduce the flux of salt to the leaves – excretion
is an additional backup for plants growing in very saline site, and for
perennial species.
Exclusion
is particularly important for perennial species whose leaves may live for a
year or more. For these species there is greater need to regulate the incoming
salt load than for annual species whose leaves may live for only one month.
There
are contributory features that function to maintain low rates of salt
accumulation in leaves. High shoot/root ratios and high intrinsic growth rates
(Pitman, 1984), and absence of an apoplastic pathway in roots (Garcia et al.,
1997) all will serve to reduce the rate at which salt enters the transpiration
stream and accumulates in the shoot.
Ion concentration in shoot (mol g-1) = Ion uptake rate (mol g-1d-1) (Eqn 3)
Relative growth rate (g g-1 d-1)
Two principles emerge from this equation. First, a
fast-growing plant will have a lower concentration in the shoot than a slow
growing plant, for the same uptake rate. Note that it is the relative growth
rate (RGR) not the size itself that influences the ion concentration in the
shoot. The notion of 'vigour' as affecting ion concentration therefore needs to
be defined carefully, as RGR is independent of plant size. Second, any increase
in shoot ion concentration with increasing salinity may not be due to increase
ion uptake rate – it could be due to decreased RGR due to the osmotic effect of
the salt. This also means that an additional stress such as waterlogging that
depress RGR will also cause an increase in concentration in the shoot for the
same uptake rate
Control at the cellular level: ion compartmentation
There
is no evidence of adaptations in enzymes to the presence of salt, so mechanisms
for salt tolerance at the cellular level involve keeping the salt out of the
cytoplasm, and sequestering it in the vacuole. That this occurs in most species
is indicated by the high concentrations found in leaves that are still
functioning normally, concentrations well over 200 mM, which are known to
completely repress enzyme activity in
vitro (Munns, 2002) Generally, Na+ starts to inhibit most
enzymes at a concentration above 100 mM. The concentration at which Cl-
becomes toxic is even less well defined, but is probably in the same range as
that for Na+. If Na+ and Cl- are sequestered
in the vacuole of the cell, K+ and organic solutes should accumulate
in the cytoplasm and
organelles
to balance the osmotic pressure of the ions in the vacuole. The organic solutes
that accumulate most commonly under salinity are proline and glycinebetaine,
although other molecules can accumulate to lesser degrees.
Salt
tolerant species have transport systems on the tonoplast that can sequester Na+
and Cl- at high concentrations within the vacuoles, while
maintaining much lower concentrations in the cytoplasmic compartments. The ion
channels and transporters that regulate the net movement of salt across cell
membranes were described in several recent reviews (see under Salinity Stress).
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Additional
reading on plant responses to salinity (see
also 'update' section under
‘Salinity Stress’) Flowers,
T.J., Hajibagheri, M.A., Clipson, N.J.W. 1986. Halophytes. Quart.Rev.Biol.
61:313-337. Hasegawa,
P.M., Bressan. R.A., Zhu, J-K, Bohnert, H.J. 2000. Plant cellular and
molecular responses to high salinity. Ann.Rev.Plant Physiol.Plant
Mol.Biol.51: 463-499. Läuchli,
A., Lüttge, U. (Eds.) 2002. Salinity: Environment - Plants - Molecules.
Kluwer Academic Publishers. Pessarakli,
M. (Ed.) 1999. Handbook of Plant and Crop Stress. Second edition, Marcel |
In summary, roots do most
of the work in protecting the plant from excessive uptake of salts, and filter out most of the salt in the soil while
taking up water. Even so, there are mechanisms for coping with the continuous
delivery of relatively small amounts of salt that arrive in the leaves, the
most important being the cellular compartmentalisation of salts in the vacuoles
of the mesophyll cells. This strategy allows plants to minimize or delay the
toxic effects of high concentrations of ions on important and sensitive
cytoplasmic processes. The rate at which leaves die is the rate at which salts
accumulate to toxic levels, so genotypes that have poor control of the rate at
which salt arrives in leaves, or a poor ability to sequester that salt in cell
vacuoles, have a greater rate of leaf death.
Publications cited
Garcia A, Rizzo C.A., Ud-Din J., Bartos S.L., Senadhira
D., Flowers T.J., Yeo A.R. 1997. Sodium and potassium transport to the xylem
are inherited independently in rice, and the mechanism of sodium:potassium
selectivity differs between rice and wheat. Plant Cell Environ. 20:1167-1174.
Ghassemi F.,
Jakeman A.J., Nix H.A. 1995. Salinisation of land and water resources:
Human causes, extent, management and case studies. UNSW Press,
Greenway H., Munns R. 1980. Mechanisms of salt
tolerance in nonhalophytes. Annu. Rev. Plant Physiol. 31:149-190.
Munns, R. 1985. Na+, K+ and C1- Xylem sap flowing to
shoots of NaCl-treated barley. J. Exp. Bot. 36:1032-1042.
Munns R. 1993. Physiological processes limiting plant
growth in saline soil: some dogmas and hypotheses. Plant Cell Environ.
16:15-24.
Munns R (2002) Comparative physiology of salt and water
stress. Plant Cell Environ. 25,
239-250.
Munns. R., Schachtman D.P. Condon A.G. 1995). The
significance of a two-phase growth response to salinity in wheat and barley.
Aust. J. Plant Physiol. 22:561-569.
Munns, R., Husain, S..Rivelli, A.R James, R.A. Condon,
A.G. (Tony) Lindsay, M.P. Lagudah, E.S. Schachtman, D.P. Hare R.A. 2002.
Avenues for increasing salt tolerance of crops, and the role of physiologically
based selection traits. Plant and Soil 247:93-105).
Rengasamy P 2002. Transient salinity and subsoil
constraints to dryland farming in Australian sodic soils: an overview. Aust. J.
Exp. Agric. 42:351-361.
