Qurat-Ul-Ain Sadiq and Muhammad Nazim
Over the last century global food production (agricultural
production) has increased dramatically due to the application of better
agronomic practices, integrated pest control methods, classical plant breading,
and advance bioengineering technologies. In particular, cereal crop yields have
literally doubled during the last 50 years since the arrival of Green
Revolution. These achievements are attributed to the efforts of farmers,
agronomists, and plant biologists. It is likely, however, that both increased
yields and the acquisition of new arable land will be required to meet the
needs of the 21st Century. Whatever technologies are developed and used, they
must be sustainable in the long term. Food production is not uniformly
distributed across the globe due to the diversity of terrain, local climatic
conditions, and the available agricultural expertise. Clearly, there is a limit
to the amount of land available for food production and to the theoretical
limit on the maximum attainable yield of any given crop. At present, global
food production is unbalanced; 183 nations in the world depend on food from
outside their borders (food imports) and this food comes from those countries
with relatively low populations that practice intensive agriculture.
The greatest challenge for
humanity in the next few decades will be how to increase and sustain arable
production without degrading land. Land degradation is proceeding rapidly. The
Global Assessment of Land Degradation (GLASOD) estimate that a total of 1964
million hectares have degraded, 910 million hectare to at least a moderate
degree with significantly reduced productivity and 305 million hectare strongly
degraded (no longer suitable for agriculture).
Soil Salinity
Salinity affects 15% of the world’s land area, which amounts to 930
million hectares. However, land suffering from various degrees of salinity has increased from about 48 % of the
total irrigated lands in 1990, to 64% in 2000. Irrigation is important for agriculture;
irrigated land, which accounts for 15% of total arable land, produces at least
double that of rain-fed land. In total irrigated arable land produces 1/3 of
the world food supply. Salinization can cause yield losses of 10 to 25 percent
for many crops. Salinity also occurs through natural processes from the
accumulation of salts over long periods of time in the soil or groundwater. It
is caused by two natural processes: (1) the weathering of parent materials
containing soluble salts and (2) the deposition of oceanic salt carried in wind
and rain. Salinization caused by natural or human-induced processes also
results in the accumulation of dissolved salts in the soil water and
subsequently inhibits plant growth.
Effect Of Salinity
More than 99% of the world’s
food supply comes from land and less than 1% is from seas, oceans, and other
aquatic habitats. The total arable land on the earth is approximately 13
billion hectares of which 6 billion hectares are located in arid and semiarid
regions, and about 17 % of this is severely affected by salt. In irrigated
areas, which constitute 230 million hectares world-wide, 33 % is affected by
salt. Thus, the magnitude and the seriousness of the problem cannot be
understated. Moreover, 40,000 hectares of land world-wide is being lost every
year from agriculture due to salinity. The FAO also reported that 20-30 million
hectares of cultivated land is severely affected by salinity and an additional
60-80 million hectares are affected to some extent. Only about 10% of total
arable land on the Earth can be considered as free from salt stress. In a
survey on the distribution of 323
million hectares of saline soils throughout the World. In general, there is a
strong correlation between global agriculture yields and soil salinity.
Classification Of Plants According To Their Tolerance Of Salinity
Plants can be broadly classified into two groups according to their
tolerance of salinity:
(1) The salt sensitive plants, termed as ‘glycophytes’:
(2) The salt tolerant plants, or ‘halophytes’. Unfortunately, the
major crops of the world are glycophytes that can not grow in saline habitats
where salt concentrations are approximately above 100 mM NaCl. These plants do
not appear to possess mechanisms for adapting to the harmful effects of
salinity. These glycophytes have evolved in habitats with very low soil Na+
content, and may never have possessed the mechanisms or features to enable them
to cope with the water deficits and ion levels prevailing in saline habitats.
Some classifications categorize plants as follows: tolerant, moderately
tolerant, moderately sensitive and sensitive with respect of their response to
salinity. For instance barley, cotton, and sugar beet are considered tolerant
because they can grow in the salinity range of 6.9 to 8.0 dS m-1
(77-88 mM NaCl) without any apparent loss of yields, whereas most fruit trees,
carrot, and onion are considered sensitive with yield loss thresholds of less
than 2.0 dS m-1 (22 mM NaCl; Flowers, 2004). What is required is the
development of major crop varieties that can grow in saline soils without
losing their ability to produce high usable yields. The US Laboratory of
Salinity define a saline soil as one with a saturation extract (the solution
extracted from a soil at its saturation water content) electrical conductivity
(EC) of greater than 4 dS m-1, (equivalent to approximately 45 mM
NaCl). The growth of many glycophytes is significantly limited in
concentrations as low as 25-50 mM NaCl. In contrast, many halophytes grow well in
high concentrations of NaCl and complete their life cycle in full-strength sea
water (ca. 560 mM NaCl). Clearly, halophytes have the ability to avoid and get
rid of toxic ions by mechanisms preventing them from accumulating at metabolic
sites and impairing growth. NaCl inhibits the in vitro activity of many
enzymes. The cytoplasm of plant cells typically contains about 100 mM K+
and plant metabolism has, therefore, evolved to work efficiently at this
concentration. Increased levels of Na+ disrupt the ionic balance of
the cytoplasm: “the physicochemical properties of K+ and Na+
are similar, but not identical”. As Na+ levels in the cytoplasm
raise, the ionic interactions within and between proteins, their cofactors, and
substrates alters so that metabolism is no longer optimized. As a result, the
activities of many enzymes operating in different pathways are perturbed.
Deleterious Effects Of High Salinity On Plants
The primary effect of salt
on plants is an osmotic stress, which causes dehydration and loss of turgor
(within 1 hr). Subsequently, ingression of ions into cells can result in ion
toxicity. Wheat and barley have genotypes for salt tolerance and there were two
stages of growth response towards salt stress. Initially, they identified a
large decrease in growth rate, which arises from the loss of cell turgor. If
the plant can regain turgor there is the potential to resume growth, but there
is often a second reduction due to salt specific responses that originate from
the accumulation of salt at toxic levels within the cell. This may arise
through disruption in the normal hormonal signals from roots. Under salt stress
conditions, endogenous levels of a plant hormone, abscisic acid (ABA) increase,
which appears to act as a signal to promote tissue acclimation. Elevated ABA
levels have been correlated with increased tolerance to salt and exogenous
application of ABA accelerated the adaptation of cultured tobacco cells to
salt. Which provide further support for a role of ABA in the acclimation of
plants to salinity and osmotic stress. The correlation between osmotic stress
and change in the ABA levels have been well established at the molecular level.
In some plants for example citrus, salt toxicity is due to Cl-
instead of Na+. In plants chloride has two main roles: one as a
counter anion for cation transport (Ca+2, K+, Mg+2,
NH4+ etc.) for maintaining membrane potential; the second
as a major osmotically active solute which maintains both turgor and
osmoregulation. Chloride is also a micronutrient essential for healthy plant
growth. A minimal requirement for crop growth of 1gKg-1 dry weight
has been suggested, a quantity that can generally be supplied by rainfall.
Strategies For Coping With High Salinity
Recently, strategies for
solving the salinity problem in agriculture have tended to focus on soil
reclamation. However, this has proved to be extremely expensive and untenable.
In practice, land has been cultivated until salinity renders production
uneconomically, at the point where cultivation is switched to new areas. Lately,
with the development of breeding and bioengineering, the focus has turned more
towards developing salt-tolerant crops. There is a view that salt tolerance in
plants is a polygenic trait involving the co-ordinate expression of many genes,
and that the prospects for bioengineering are therefore remote. However,
recently this view has been challenged: salt resistant tomato and Arabidopsis
plants have been produced by transformation with a single gene. Therefore, the
prospects for overcoming salinity stress in crop plants using information
derived from model system such as Arabidopsis and rice (Oryza sativa L),
may not be as bleak as once thought. Plants exposed to saline environments
encounter three basic problems:
(1) Specific ion (Na+, Cl-, etc) toxicity;
(2) The need to maintain a favorable cell turgor pressure:
(3) The need to obtain essential nutrient ions (e.g. K+,
NO3-) in spite of the predominance of other chemically
similar and potentially toxic ions (e.g. Na+, Cl-) in the
growth medium.
Salt tolerance in plants not only varies considerably among species, but also depends very much on the conditions under which the crop is grown. There are several factors that influence salt tolerance in plants. These include temperature, the composition and levels of salts, the growth phase of the plant, and the Leaching Fraction.
Importance Of Turgor
Soft plant tissues
(non-lignified) are supported by the pressure of cell contents against the cell
walls. This is known as turgor pressure and is induced by the uptake of water
into cytoplasm of the cells so that pressure is exerted by the plasma membrane
on the cell wall. Water tends to move into the cell because of the osmotic
effect of the low molecular weight solutes in the cytoplasm and vacuole. Water
movement from the soil, through the plant, and into the air can be understood best
from the concept of water potential, which is measured in pressure units (bars,
Pascals etc.). Water always moves from high to low water potential whatever,
the cause of the difference in potential. By definition, pure water at S.T.P.
(standard temperature and pressure) and air at 100 % relative humidity has a
water potential of zero. Because the growth (expansion) of cells of plants
depend totally on turgor, decreased turgor is the factor most likely to inhibit
plant growth when they are exposed to high salt. As a result, transfer of a
salt-sensitive plants from their original habitat to a high salt medium will
produce a rapid water loss and wilting. Newly, the plant cell vacuoles have
gained a lot of attention because of their multifaceted role in plant
metabolism (e.g. recycling of cell components, regulation of turgor pressure,
detoxification of xenobiotics, and accumulation of many storage substances. Moreover,
the space-filling function of the vacuole is essential for cell growth, because
cell growth is driven by the expansion of vacuole rather than that of the
cytoplasm. Osmotic adjustment by halophytes and other salt-tolerant plants to
tolerate high saline conditions is a key strategy for survival and this can be
achieved by ion uptake from the soil solution and sequestration in the vacuole,
and by internal synthesis of compatible organic solutes in the cytoplasm. A
desiccated plant cell must reverse the water to potential gradient to survive,
by forcing water to flow back into the cell. Therefore, for plant cells to
thrive in concentrations above approximately120 mM NaCl (approximately 0.6 MPa,
the water potential of plants in a well-watered field), plants must develop a
strategy to re-establish turgor pressure. Halophytes achieve this by
accumulating enough osmotically-active solute in their vacuoles to reverse the
osmotic gradient so that water can be re-absorbed from the external medium. An
energetically cheap way of accomplishing this is to take up Na+ and
Cl- ions from the external medium and sequester them in the vacuole.
If the solute potential (Ψs, or osmotic potential) of the vacuole (vacΨs)
becomes more negative than that in the soil solution, water will flow into the
cell and turgor will rise. However, for the cytoplasm to absorb water, it is
necessary for the solute potential of the cytoplasm (cytΨs) to decrease in
parallel with vacΨs, and this can be achieved by the accumulation of non-toxic
compatible solutes (e.g. glycine betain, proline, and sugars).
Na+ Uptake Mechanisms
A high K+/Na+
ratio in the cytosol is a very important and essential feature for normal
cellular function in plants. Since living cells are not completely impermeable
to Na+, the low concentration of Na+ in the cytoplasm
requires its continuous exclusion normally against an electrochemical gradient.
Therefore, an active exclusion of Na+ occurs either by a primary
active Na+ pumping ATP ases or by a secondary active Na+/H+
antiporter mechanisms coupled to an electrochemical proton gradient. Due to the
physio-chemical similarity between K+ and Na+, it is
generally assumed that K+ and Na+ compete for common
absorption sites in the root. High affinity transporters are effective at very
low external K+ concentrations and saturate when external K+ concentrations
rise to 1 mM. Sodium, even in 20-fold excess, fails to compete significantly
with K+ for binding sites on High Affinity transporters. At higher
concentrations of K+ (> 1 mM), Low Affinity transporters become
important, and some of these transporters do not discriminate well between K+
and Na+ thus Na+ can competitively inhibit the absorption
of K+. Sodium uptake in plants is believed to be primarily through
Low Affinity transporters. (K+ channels) in different root cells,
including cortical, root hair, stellar and xylem parenchymatous cells, that can
sense external K+ concentrations. These ion channels transport at
rates between 106 and 108 ions per second per channel protein. Transport is
‘passive’, where the diffusion of ions through the channel is a function of
both the membrane voltage and the concentration difference across the membrane;
thus uptake is not directly coupled to the input of other forms of free energy.
Some argue, plants should be termed according to their ability to absorb Na+
and translocate it freely to the shoot. ‘Natrophiles’ take up and translocate
Na+ freely, whereas, ‘Natrophobes’ show a strong preference to
absorb K+ over Na+.
Salt Stress Sensing In Plants
Plants sense salt stress through ionic (Na+) and osmotic
stress signals. Therefore, excess Na+ can be sensed either on the
surface of the plasma membrane by a transmembrane protein or within the cell by
membrane proteins or Na+ sensitive proteins. In addition to its role
as an antiporter, the plasma membrane Na+/H+ antiporter
SOS1 (Salt Overly Sensitive 1), having 10 to 12 transmembrane domains and a
long cytoplasmic tail, may act as a Na+ sensor. This dual role would
be analogous to the sugar permease BglF in Escherichia
coli and the yeast ammonium transporter Mep2p.
Na+ Sequestration
A positive turgor is very important and essential for
expansion-induced growth of plant, and for stomatal functioning. When plants
are exposed to high salinity they desiccate, resulting in turgor loss. Plants
have evolved an osmotic adjustment mechanism (active solute accumulation) that
maintains water uptake and turgor under osmotic stress conditions. For osmotic
adjustment, plants use inorganic ions such as Na+ or K+
and synthesize organic compatible solutes such as proline and soluble sugars.
Vacuolar sequestration of Na+ is an important and cost-effective
strategy for osmotic adjustment, which also reduces the Na+
concentration in the cytosol. Na+ sequestration into the vacuole
depends on expression and activity of Na+/H+ antiporters
as well as on V-type V-H+ATPases and V-H+-PPases on the
tonoplast. These phosphatases generate the necessary proton gradient required
for activity of the Na+/H+ antiporters. Over expression
of AVP1, a gene that encodes the vacuolar H+-pyrophosphatase (H+PPase)
in Arabidopsis, enhanced sequestration of Na+ into the vacuole and
maintained higher relative water content in leaves. These plants also show
higher salt and drought stress tolerance.
Conclusions
Salt stress has heavily affected high yielding varieties in
comparison to salt tolerant varieties. Evidence was found for a genetic basis
for the observed differences in halo tolerance between the different genotypes,
with local landrace performing best at all stages of development. Halo-tolerance
appears to be partially related to the ability of genotypes towards Na+
exclusion from the plant and the regulation of Na+ transport from
the root to the shoot.
About the Author:
Qurat-Ul-Ain
Sadiq is a PhD Scholar ( Department of Soil and Environmeental
Sciences, Muhammad Nawaz Sharif, University of Agriculture Multan)
Muhammad Nazim
is an Assistant Agronomist in the office Director Agriculture (Extension)
Disvision Bahawalpur. He has pursued his degree of M.Sc.(Hons) in Crop
Physiology/ Agronomy from Department of Agronomy, MNS University of Agriculture
Multan, Pakistan.