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.


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.