Water and salt-specific effects on plant growth
1. Introduction
Abiotic stress limits crop productivity [1], and plays a major role in determining the distribution of plant species across different types of environments. Abiotic stress and its effects on plants in both natural and agricultural settings is a topic that is receiving increasing attention because of the potential impacts of climate change on rainfall patterns and temperature extremes, salinization of agricultural lands by irrigation, and the overall need to maintain or increase agricultural productivity on marginal lands. In the field, a plant may experience several distinct abiotic stresses either concurrently or at different times through the growing season [2].
In reference [3] are showed some common examples of the abiotic stresses a plant may encounter which include a decreased availability of water, extremes of temperature including freezing, decreased availability of essential nutrients from the soil (or conversely the build-up of toxic ions during salt stress), excess light (especially when photosynthesis is restricted) or increased hardness of the soil that restricts root growth.
In the face of a global scarcity of water resources and the increased salinization of soil and water, abiotic stress is already a major limiting factor in plant growth and will soon become even more severe as desertification covers more and more of the world's terrestrial area. Plants are often subjected to periods of soil and atmospheric water deficits during their life cycle. Moreover, the faster-than-predicted change in global climate [4] and the different available scenarios for climate change suggest an increase in aridity for the semiarid regions of the globe. Together with overpopulation, this will lead to an overexploitation of water resources for agriculture purposes and increased constraints on plant growth and survival aid, therefore, on realizing crop yield potential [5]. Thus, if a single abiotic stress is to be identified as the most common in limiting the growth of crops worldwide, it most probably is low water supply [1].
Like the water stress, salinity is one of the major severe abiotic factors affecting crop growth and productivity [6]. Salt’s negative effects on plant growth have initially been associated with the osmotic stress component caused by decreases in soil water potential and, consequently, restriction of water uptake by roots.
The literature shows that drought and salinity are already widespread in many regions. Therefore, in reference [7] the authors have presented that in world where population growth exceeds food supply, agricultural and plant biotechnologies aimed at overcoming severe environmental stresses need to be fully implemented.
1.1. Plant stress definitions
The term stress is most often used subjectively and with various meanings. The physiological definition and appropriate term for stress are referenced as responses to different situations. The flexibility of normal metabolism allows the development of responses to environmental changes, which fluctuate regularly and predictably over daily and seasonal cycles [8]. Thus, stress plays a pivotal role in determining interaction outcomes because it strongly influences the strength of underlying positive and negative interactions.
Stress is defined as any external abiotic (heat, water, salinity) or biotic (herbivore) constraint that limits the rate of photosynthesis and reduces a plant’s ability to convert energy to biomass [9]. The strength of positive interactions increases with increasing stress except at the most extreme levels. In contrast, the strength of negative interactions is either unrelated to stress and remains consistently high, or alternatively, decreases with increasing stress [10].
Environmental stress could be defined in plants as any change in growth condition(s), within the plant’s natural habitat, that alters or disrupts its metabolic homeostasis. Such change(s) in growth condition requires an adjustment of metabolic pathways, aimed at achieving a new state of homeostasis, in a process that is usually referred to as acclimation [11]. However, the concept of plant stress is often used imprecisely, and stress terminology can be confusing, so it is useful to start our discussion with some definitions. Stress is usually defined as an external factor that exerts a disadvantageous influence on the plant. In most cases, stress is a measured in relation to plant survival, crop yield, growth (biomass accumulation), or the primary assimilation processes (CO2 and mineral uptake), which are related to overall growth. In addition, the concept of stress is intimately associated with that of stress tolerance, which is the plant’s fitness to cope with an unfavourable environment. In the literature the term stress resistance is often used interchangeably with stress tolerance, although the latter term is preferred [12].
According with the literature [13], changes in strength of these underlying processes drive shifts between competition and facilitation. The stress gradient hypothesis (SGH) predicts that facilitation and competition vary inversely along stress gradients with facilitation more frequent and stronger when stress is high and competition more frequent and stronger when stress is low. The SGH also predicts that the strongest facilitation should occur with competitive species at the upper limits of their stress tolerance while the strongest competition should occur with stress tolerant species located at their ecological optimum.
Shifts in the structure of interaction outcomes, i.e. a shift from competition to facilitation, along stress gradients are likely to have profound implications for community stability [14, 15]. Community compositional instability can be defined as changes in species abundances that drive directional changes in community composition. There is growing evidence that changes in the structure of species interactions can reduce such stability [16].
In both natural and agricultural conditions, plants are frequently exposed to environmental stresses. In [17] the work presents how some environmental factors, such as air temperature, can become stressful in just a few minutes; others, such as soil water content, may take days to weeks, and factors such as soil mineral deficiencies can take months to become stressful.
1.2. Concepts and consequences of water and salt stress on plants
Water-deficit stress can be defined as a situation in which plant water potential and turgor are reduced enough to interface with normal functions. Water stress is considered to be a moderate loss of water, which leads to stomatal closure and limitation of gas exchange. Desiccation is a much more extensive loss of water that can potentially lead to gross disruption of metabolism and cell structure and eventually to the cessation of enzyme catalyzing reactions. Water stress is characterized by reduction of water content, turgor, total water potential, wilting, closure of stomata, and decrease in cell enlargement and growth. Severe water stress may result in arrest of photosynthesis, disturbance of metabolism, and finally death [8, 18].
The term ‘salinity’ refers to the presence in soil and water of electrolytic mineral solutes in concentrations that are harmful to many agricultural crops. Except along seashores, saline soils seldom occur in humid regions, thanks to the net downward percolation of fresh water though the soil profile, brought about by the excess of rainfall compared with evapotranspiration. In arid regions, on the other hand, there may be periods of no net downward percolation and hence no effective leaching, so salts can accumulate in the soil. Hence the combined effect of meager rainfall, high evaporation, the presence of salt-bearing sediments, and – in many places, particularly river valleys and other low-lying areas – the occurrence of shallow, brackish groundwater which gives rise to saline soils [19].
Salinity in soil or water is one of the major stresses and, especially in arid and semi-arid regions, can severely limit crop production. The Figure 1 is showing that deleterious effects of salinity on plant growth are associated with low osmotic potential of soil solution (water stress), nutritional imbalance, specific ion effect (salt stress), or a combination of these factors [20].
1.3. Mechanisms of acclimation or adaptation to water and salt stress
Drought and soil salinity are among the most damaging abiotic stresses affecting today’s agriculture. It is understandable that plants are under periodic water stress because of the unpredictable nature of rainfall. Salt stress may also occur in areas where soils are naturally high in salts and/or where irrigation, hydraulic lifting of salty underground water, or invasion of seawater in coastal areas brings salt to the surface soil that plants inhabit. Plants have evolved mechanisms that allow them to perceive the incoming stresses and rapidly regulate their physiology and metabolism to cope with them. Very often such regulations and responses include feed-forward mechanisms for stress reduction that are in addition to the responses that are seen after stresses have caused irreversible damage to physiological functions. A good example of such a feed-forward mechanism the ability of plants to regulate their water loss through partial closure of stomata and/or reduced leaf development, long before there is a substantial loss of their leaf turgor or some irreversible damage to inner membrane systems. [22-24].
In this way, talking specific about water deficit, the physiological responses of plants to water stress include leaf wilting, a reduction in leaf area, leaf abscission, and the stimulation of root growth by directing nutrients to the underground parts of the plants. Plants are more susceptible to drought during flowering and seed development (the reproductive stages), as plant’s resources are deviated to support root growth. In addition, abscisic acid (ABA), a plant stress hormone, induces the closure of leaf stomata (microscopic pores involved in gas exchange), thereby reducing water loss through transpiration, and decreasing the rate of photosynthesis. These responses improve the water-use efficiency of the plant on the short term [25]. The Figure 2 reveals physiological, biochemical and molecular responses to this abiotic stress in plants. It is very important highlight that most of these responses are similar in salt-stressed plants.
In salt-stressed plants, in addition to osmotic effects it is also affected by toxic damages as function of nutritional disequilibrium and high salt levels uptake for plants. Thus, salinity inhibition of plant growth is the result of osmotic and ionic effects and the different plant species have developed different mechanisms to cope with these effects [6]. The osmotic adjustment, i. e., reduction of cellular osmotic potential by net solute accumulation, has been considered an important mechanism to salt and drought tolerance in plants. This reduction in osmotic potential in salt stressed plants can be a result of inorganic ion (Na+, Cl-, and K+) and compatible organic solute (soluble carbohydrates, amino acids, proline, betaines, etc) accumulations [27]. However these changes are only any few initial responses of many others occurred from salt-stressed seedlings. For instance this behavior, in the Figure 3 has been showed a schematic summary of the stresses that plants suffer under high salinity growth condition and the corresponding responses that plants use in order to survive these detrimental effects.
2. Water stress x salt stress effects on plant growth and development
Plant responses to drought and salinity are complexes and involve adaptive changes and/or deleterious effects. The decrease in the water potential occurred in both abiotic stresses results in reduced cell growth, root growth and shoot growth and also causes inhibition of cell expansion and reduction in cell wall synthesis [29]. According these authors, drought (likely to salinity) affects the regular metabolism of the cell such as carbon-reduction cycle, light reactions, energy charge and proton pumping and leads to the production of toxic molecules.
The literature has affirmed that plant responses to salt and water stress have much in common. For example, according reference [30], salinity reduces the ability of plants to take up water, and this quickly causes reductions in growth rate, along with a suite of metabolic changes identical to those caused by water stress. Therefore, most mechanisms were development by plants to tolerate abiotic stresses like water deficit and salinity, which are schematically showed in Figure 4.
2.1. Salinity and water deficit problem in arid and semi-arid regions
Arid and semiarid regions of the world such as Brazilian’s Northeast, mostly faces with inadequate, irregular and erratic nature of rainfall. Moreover, according [3], in the field, drought can cause a number of plant stresses including temperature, light and nutrient stresses. However, the stress component that defines drought is a decrease in the availability of soil water. For [32], in addition with recurrent drought lack of efficient use of scarcely available water amplified the impact of water scarcity in agricultural production and productivity. These authors affirm the reduction of agricultural production results from a combination of many factors, such as crop management, crop genetics and biotic stress. Therefore, limited and irregular rainfall directly and indirectly leads to low production levels and consequently food insecurity in developing regions of the world such as Brazilian’s semiarid. In addition, [33] have related it is widely recognized that land use–cover changes, such as desertification in arid and semiarid regions and deforestation in tropical zones, may exert an influence on regional or even global environmental change by changing the hydrological cycle and surface energy balance.
Looking for avoid the water stress and to improve the productivity at arid and semiarid regions generally adopt the use of irrigation, principally the drip. In this way, drip irrigation has the potential to increase crop yields with less water and it can maintain relatively constant soil water content over time near the drip lines. However, [34] have related a disadvantage of drip irrigation is that salt accumulates near the periphery of the wetted area. According them, this salt accumulation can be a matter of concern if the emitter placement does not coincide reasonably well with the location of the plant row, particularly for crops that are sensitive or moderately sensitive to soil salinity.
2.2. Salinity in soil is increased by the water deficit and aridity conditions
In arid regions, there may be periods of no net downward percolation and hence no effective leaching, so salts can accumulate in the soil. In reference [19], the combined effect of meager rainfall, high evaporation, the presence of salt-bearing sediments, and – in many places, particularly river valleys and other low-lying areas – the occurrence of shallow, brackish groundwater which gives rise to saline soils. In addition, the irrigation has been one of the major practices that more contributes with the soil salinization, like showed in Figure 5.
Less obvious than the appearance of naturally saline soils, but perhaps more insidious, is the inadvertent induced salination of originally productive soils, caused by human intervention. Irrigation waters generally contain appreciable quantities of salts. (For example: even with relatively good-quality irrigation water containing no more than 0.3 kg salts m3, applying 10,000 mm water per season would add as much as 3 000 kg salts ha-1 per year!) Crop plants normally extract water from the soil while leaving most of the salt behind. Unless leached away (continuously or intermittently), such salts sooner or later begin to hinder crop growth [19].
These deleterious effects previously showed caused by salinization can be managed by changed farm management practices. In [30] has been indicated in irrigated agriculture, better irrigation practices, such as drip irrigation, to optimize use of water can be employed. In rain-fed agriculture, this researcher suggest practices such as rotation of annual crops with deep-rooted perennial species may restore the balance between rainfall and water use, thus preventing rising water tables bringing salts to the surface. All such practices will rely on a high degree of salt tolerance, not only of the perennial species used to lower a saline water table, but also of the crops to follow, as some salt will remain in the soil.
2.3. Salt stress and irrigation and bad water uses
Irrigation water quality can have a profound impact on crop production. In reference [36], the work affirms that all irrigation water contains dissolved mineral salts, but the levels and composition of the salts vary depending on the source of the irrigation water. Salinity from irrigation can occur over time wherever irrigation occurs, since almost all water (even natural rainfall) contains some dissolved salts. When the plants use the water, the salts are left behind in the soil and eventually begin to accumulate. Since soil salinity makes it more difficult for plants to absorb soil moisture, these salts must be leached out of the plant root zone by applying additional water. This water in excess of plant needs is called the leaching fraction [37].
Salination from irrigation water is also greatly increased by poor drainage and use of saline water for irrigating agricultural crops. Therefore, inefficient or bad irrigation water and drainage systems are a major cause of excess leakage and increase the risk of salinity and waterlogging in irrigation areas. Poor water distribution on paddocks results in some areas being under-irrigated, causing salts to accumulate (where watertables are high) and other areas being over-irrigated and waterlogged. Groundwater mounds can develop under irrigation areas as a result of leakage from inefficient systems and restrictive layers. This puts pressure on the regional groundwater system forcing saline groundwater into waterways. Irrigating with saline water adds salt to the soil and increases the need for applying more irrigation water to leach salts past the plant root zone [38].
From studies of FAO [39] we can affirm that prevention and reclamation of salt-affected soils require an integrated management approach, including consideration of socioeconomic aspects, monitoring & maintenance of irrigation schemes and reuse and/or safe disposal of drainage water. Implementation of efficient irrigation and drainage systems and good farming practices can prevent and, in some cases, reverse salinization. If appropriate management practices are not applied in time, it may be necessary to take the land out of production altogether. Moreover, actions to fight or mitigate salinization can be implemented by local institutions and research stations, while research and technology transfer can play a crucial role in providing tools, setting up management strategies or spreading water-saving techniques.
2.4. Growth and plant development affected by multiple stresses as water and salt stress
Plant growth can be limited by water deficit and by excess water. Water deficit occurs in most natural and agricultural habitats and is caused mainly by intermittent to continuous periods without precipitation. Excess water occurs as the result of flooding or soil compaction [12].
When plant cells experience water deficit, cellular dehydration occurs, this promotes reductions of cell turgor (
Water deficit not only decreases turgor pressure, but also decreases wall extensibility and increase yield threshold. The water deficit effects on yield threshold are presumably involved in complex structural changes of the cell wall that may not be readily reversed after relief of water deficit [12].
Salinity can have similar aspects to water stress in plant growth, except for the addition of ion cytotoxicity, which appears with salt excess in soil. [12].
The toxicity of high Na+ and Cl- in the cytosol is due to their specific ion effects. High salt concentrations, outside the cell, can result in osmotic stress. Once in the cytosol, however, certain ions act specifically, either singly or in combination, affecting whole plant because ions move to the shoot in the transpiration stream [6].
The increase of solutes on root medium, mainly the ions, can promote a reduction of water absorption by plant root system, contributing to reduction of root conductivity. As consequence, the plant absorbs less water, and if the transpiration rate is higher than water absorption rate, and the result is a water deficit, which could result in photosynthetic rate reduction and growth rate reduction [40]. According to the figure below, salt stress effects in timescale alters initially, absorption of water and nutrients, and membranes permeability [41].
These previous changes reflect in nutritional and hydric balance of plants and promote changes in metabolism, in hormonal balance, gas exchange and ROS production. All these changes undertake cell expansion and division, vegetative and reproductive growth and acceleration of leaf senescence, which result in plant death [12].
An important strategy to avoid ion toxicity is the accumulation of ions during osmotic adjustment in vacuoles, where the ions are kept out of contact with cytosolic enzymes or organelles. Many halophytes utilize vacuolar compartmentalization of Na+ and Cl- to facilite osmotic adjustment that sustain or enhances growth in saline environments. When ions are compartmentalized in vacuoles, at the same time other solutes must accumulate in the cytoplasm to maintain water potential equilibrium within the cell, as example of these compatible solutes, can be cited the quaternary ammonium compounds, sugar alcohols, quaternary ammonium compounds (QACs) and tertiary sulfonium compounds (TSCs) [12].
According to [6], the growth responses to salinity stress occurs in two phases: a rapid and intense response to the increase in external osmotic pressure related to NaCl increase in the medium, which contribute to a stronger reduction in growth. Followed, it can be observed, a slower response due to toxic ion accumulation in tissues (Ionic phase), which is related to a severe toxicity in leaves, represented in most of times by chlorosis and can be originated necrosis in these tissues (See figure 8). Leaf injury and death is probably due to the high salt load in the leaf that exceeds the capacity of salt compartmentation in the vacuoles, causing salt to build up in the cytoplasm to toxic leaves [6,30]. These responses vary strongly between genotypes, salinity levels, soil and other abiotic factors [6].
Plants undergo characteristic changes from the time salinity stress is imposed until they reach maturity [30]. Salinity inhibition of plant growth is the result of osmotic and ionic effects and the different plant species have developed different mechanisms to cope with these effects [30]. The plant responses to salinity are different between the species, the salt levels and also depends of the time exposition. In the table below could be observed the plant response to salinity at different time scales [Table modified from 30].
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As can be observed in this table, in the first few seconds or minutes, it has been observed that cells lose water or shrink. When the hours are passing, cells regain the original volume but cell elongation is reduced, contributing to lower rates of leaf and root growth. After some days, cell elongation and cell division promote alterations in leaf size appearance [30]. In plants, with high salt absorption, the oldest plants become senescent, and some leaves can dead. After months, differences between months, with high and low salt uptake become very pronounced, with a large amount o leaf injury and complete death in some cases if the salinity level is high enough [30].
The tolerance or plant response to salt can vary with cultivars, the plant age and growth stage, environmental conditions, cultural practices, irrigation management, soil fertility, and the intensity or other plant stress. Another, stress factor is wind that can be injurious to plants as salt. When the two are combined near the ocean, plants suffer even greater damage. Some crassulaceae and succulent plants are highly salt and wind tolerant, in different way, moderate salt tolerant plants of soil salinity usually tolerate light salt spray but should not be used in exposed locations [42].
The comparative effect of salinity and water stress on growth was analyzed in
Pinus seed germination was evaluated in different conditions: water stress simulated by PEG agent experiments, salt stress using sodium chloride (NaCl) and diluted sea water, associated to different temperatures (heat stress). Seed germination and the activities of the main enzymes involved in Pinus seed reserve utilization (glyoxylate cycle enzymes) decreased with increasing PEG, NaCl and seawater concentrations. In addition, the seawater treatment was the less severe on seed germination. As regard to heat treatments, the maximum germination percentage (80%) was obtained at 80°C and short exposure time (3 min) [44].
Another study involving water and salt stress effects on plant growth was verified in Tamarisk (
Salinity affects both vegetative and reproductive plant development, which promotes negative implications depending on the harvested organ, stem, leaf, root, shoot, fruit, fiber or grain [46]. Salinity generally reduces shoot growth more than root growth and can reduce the flowering and increase sterility. Considering the salt-tolerance importance from an agronomic or horticulturist perspective is based on the yield of the harvestable organ, relative to that in non-stressed environments, understanding how salinity affects vegetative and reproductive development is important for developing management strategies that can minimize stress at critical times [46]. Salinity stress could delay germination, although most plants are tolerant and there may be no difference in the percentage of germinated seeds.
2.5. Salt tolerant plants “Halophytes” and water stress tolerant plants
Halophytes are plants that can survive and reproduce in environments with high salt concentration (200 mM NaCl), these plants constitute about 1% of the world’s flora. Halophytes can be classified as “natural” and plants that tolerate salt but do not normally live in saline conditions [49]. Other classifications of halophytes are based on the characteristics of naturally saline habitats [49] or the chemical composition of the shoots (‘physiotypes’, [50] or the ability to secrete ions (recreto-halophytes, [51]).
The effect of salinity on growth varies amongst halophytes, which can be observed in figure bellow for different species [49]. According this figure,
The salt requirement by halophyte during their growth and development were evaluated in two halophytic species (
Studies involving extreme dicotyledonous halophytes show that optimal growth in the presence of low or moderate salt concentrations; on the other hand, this stimulatory effect is not commonly detected in monocotyledonous halophytes, nor in other salt-tolerant dicots, which grow best in the absence of salt [53]. The salt tolerance degree usually varies among halophytes and, for a specific species, also at different developmental stages [53], but inhibition of plant growth is always observed at sufficiently high salinity levels, in all investigated taxa [52, 53].
In study about growth parameters and anatomical changes in the halophyte
In another study also with A.
The development of thick cuticular membranes generally is interpreted as an adaptation to drought with regard to the formation of an efficient transpiration barrier. The capacity of
The NaCl differential tolerance of different maize genotypes were evaluated through some growth and physiological parameters and it was concluded that the SDM/RDM ratios, leaf Na+ content or leaf soluble organic solute content had no relation with salt tolerance. On the other hand, Na+ and soluble organic solute accumulation in roots due to salinity appeared to play an important role in the acclimation of maize genotypes, being that these characteristics could be used as physiological markers to salinity [62].
2.6. Studies involving adaptations or acclimations of tropical plants to water and and salt stress
Plants under salt stress can occasionally tolerate alterations in the environmental conditions. Salt tolerance in plants is determinate by multiple biological traits that will determine their water retentions and/ or acquisition capacity in safeguarding photosynthetic functions and ion homeostasis. The plant ability of eliminating free radical under salinity and water stress using active osmolytes, usually demand a lot of energy. Many plant species that are salt tolerant to salt stress show the ability to produce and accumulate osmoprotectants [63].
Water deficit and saline soils of the Brazilian semi-arid northeastern region are limiting factors and most of the times negatively affect the plant development.
Some studies evaluate the plant acclimation to abiotic stresses, and it is very common the pretreatment use for alleviates the negative effects on plants. The H2O2 pretreatment effect in maize seeds during germination and seedling acclimation to salinity, and it was observed that H2O2 pretreatment of seeds induced acclimation of the plants to salinity. It decreased the salinity deleterious effects on the maize growth. In addition, it was also verified differences in antioxidative enzyme activities, which may explain the increased tolerance to salt stress of plants originated from H2O2 pretreated seeds [65]. Another plant that is cited as water tolerant is
The water relations in six adult species of Caatinga, a typical vegetation from Brazilian semi-arid, in the middle of the dry season. Based on results, the trees were classified into four groups: (I)
In relation to commom bean plants, mild water deficit affected the photochemical apparatus in these plants, probably due to by down-regulation since plants did not show photoinhibition. The photochemical apparatus of A222 and A320 genotypes was more sensitive to drought stress. On the other hand, even after 10 d of water withholding, the maximum efficiency of photosystem 2 was not affected, what suggest efficiency of the photoprotection mechanisms [78].
The seasonal variations of physiological aspects of caatinga species demonstrated the high levels of water potential, even in the dry period, a situation in which commonly occurs water stress due to absence of soil water. It is related to water potential decrease during the absence of water in soil. The fluorescence data showed that the photosynthetic states were good with no apparent deficiency of water. In addition, it was verified that the survival strategies of these species to water deficit are efficient and result from a highly complex evolution [79].
In a study with
In relation to
In the figure below, it is resumed long-term or acclimations responses to drought stress and short-term responses that reach the plant perceive the water stress and develop the following responses against this condition [82]. The majority of traits related to plant drought adaptations, as example root size and depth, hydraulic conductivity and seed storage reserves are associated to plant development and structure and are constitutive rather than stress induced. In addition, a large part of drought plants resistance to drought and its ability to get rid of to excess radiation. The natural mechanisms responsible to leaf photoprotection, especially to related to thermal dissipation and oxidative stress. Plants can also endure water drought conditions by avoiding tissue dehydration, while maintaining tissue water potential as high as possible, or tolerating lower water potential. Dehydration avoidance is usually observed in annual and perennial plants, which has been associated to proper characteristics as example: capacity to minimize water loss and maximize water uptake [82]. The water loss could be done by stomata closer, reducing light absorbance by leaf or through reflectance increasing due to the development of a trichome layer, or by decresing of leaf surface with reduced growth or by shedding of young by old leaves. On the other hand, the maximizing water could be reaching by increasing in root system, which is resulting of alteration in allocation of plant sources [82].
3. Conclusion
Among the great number of abiotic stress affecting plants, drought and salinity are the most severe and stronger ones that limit plant growth and crop productivity in agriculture worldwide. These stress could have damages exceeding the sum of that attributed to all other natural disasters, and when they happen together promoted devastating changes in plants subjected to them. Plant responses to drought and salt stresses have much in common, the water limitations in water stress is very difficult to plant tolerate and maintain its normal growth and development. Salt stress occurs from both osmotic stress due to low water potentials and salt-specific effects, for this beyond the water restrictions, salt-stressed plants have to develop conditions to tolerate the toxic effects caused by ion accumulation, which could affect all aspects of plant metabolism. In the first view, we could speculate that salt stress in plants are more severe than water stress, by the fact that the first condition show two components, however, it is early to conclude this. This is because studies on the comparative physiology of plants to water and salt stress are few, and it is necessary perform more studies involving different plant species, as cultivate and as native, subjected at the same time to both stress to conclude what is the most severe for plant growth and development. Moreover, the development of new tools and strategies to evaluate the combination of water and salt stress on plant are also necessary, but the perspectives in relation to the better understanding of how some plants could tolerate, escape, acclimate and adaptate to these severe abiotic stresses. This knowledge could have the support of studies involving since molecular (genomics and proteomics tools) aspects as ecotype researches (isotopes, imagging) and this combination will allow the faster comprehension of plant perfomance in different environments, as well the brinding of genotype and phenotype gap.
Acknowledgments
The authors would like to thank National Institute of Science and Technology Salinity (CNPq/MCT/Brazil) for the fellowships and financial support.References
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