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The most commonly described symptom of chlorine deficiency is wilting of leaves, especially at the margins. As the deficiency becomes more severe, the leaves may exhibit curling, shriveling and necrosis. Roots of chlorine-deficient plants become stubby with club tips. In chlorine-deficient wheat, the symptoms are expressed as chlorotic or necrotic lesions on leaf tissue (Engel, Bruebaker and Emborg 2001). In coconut palm, the symptoms are exhibited as wilting and premature senescence of leaves, frond fracture, and stem cracking and bleeding (Marschner 1995). Coconut palm is of great economic importance in the tropics and subtropics and drought is one of the main environmental factors that limit coconut productivity. In this plant, chloride is an importance factor in the mechanisms governing stomatal opening and closure and is also important for stomatal regulation, particularly during the dry season. Moreover, its high concentration in coconut leaf tissues means that it acts as an osmoticum in maintaining tissue turgor during drought (Braconnier and Bonneau 1998). Differences in the gas exchanges in coconut during the dry and the rainy seasons confirmed the important role of chloride in this palm. In the dry season, chlorine deficiency has a depressive effect on gas exchanges right from the morning, which worsens as the day wears on. This results in reduction of stomatal conductance and net photosynthesis. Under moderate drought, coconut palms not suffering from a chlorine deficiency respond to higher evaporative demand by increasing their stomatal conductance and transpiration, and by maintaining a reasonable level of net photosynthesis. Under the same conditions, deficient palms react by reducing their stomatal conductance and net photosynthesis, hence expressing a state of stress. The chloride therefore enables coconut palms to withstand the dry season, by maintaining a relatively high level of leaf gas exchanges (Braconnier and Bonneau 1998).
Resistance against pathogens
Addition of chlorine has been reported to reduce the severity of at least 15 different foliar and root diseases on 11 different crops (Heckman 2007). Several possible mechanisms may explain the effects of chlorine nutrition on disease suppression and host resistance. In acid soils, chloride inhibits nitrification (Rosenberg, Christensen and Jackson 1986). Keeping N in the ammonium form can lower rhizosphere pH and influence microbial populations and nutrient availability in the rhizosphere (Heckman and Strick 1996). Competition between chloride and nitrate for uptake also tends to reduce nitrate concentrations in plant tissues. When plants take up more ammonium and less nitrate, it usually causes rhizosphere acidification, which in turn, may enhance Mn availability (Thompson, Clarke and Heckman 1995). Chlorine can also enhance Mn availability by promoting Mn-reducing microorganisms in soil. Factors which increase Mn availability have been associated with improved host resistance to diseases in grain crops (Huber 1989). Higher concentrations of chlorine in plant tissues can also enhance water retention and turgor when roots have been attacked by pathogens. The amount of organic acids, such as malate, in plant tissues and exuded from roots, decreases with chlorine supply. This action deprives pathogens of an organic substrate (Goos, Johnson and Holmes 1987).
Effect of nickel deficiency on plants stress responses
Nickel is the latest element to be classified as essential for plant growth, however, its agricultural and biological significance is poorly understood. This is mainly because of the low levels of Ni needed by plants in relation to the relative abundance of Ni in soil (Marschner 1995). The knowledge of Ni uptake by plants is indeed very limited, and apart from the observation that Ni is quite mobile as compared to other heavy metals, little is known about the uptake mechanism and translocation under Ni-limiting conditions (Brown 2007). There are several enzyme systems (NiFe-hydrogenase, carbon monoxide dehydrogenase, acetyl-CoA decarbonylase synthase, methyl-coenzyme M reductase, superoxide dismutase, Ni-dependent glyoxylase, acireductone dioxygenase, and methyleneurease) in bacteria and lower plants (Mulrooney and Hausinger 2003) that are activated by Ni, however, the activation of urease appears, to date, to be the only enzymatic function of Ni in higher plants (Gerendás et al. 1999). Urease contains two Ni ions at the active site (Ciurli 2001).
The metabolic effects of Ni deficiency have been reported in cereals (Brown, Welch and Madison 1990), legumes (Gerendás and Sattelmacher 1997) and perennial species (Bai, Reilly and Wood 2006). These include reduced urease activity, induced metabolic N deficiency, disruption of N metabolism via ureide catabolism, amino acid metabolism, and ornithine cycle intermediates. Disruption of ureide catabolism in Ni-deficient leaves resulted in accumulation of xanthine, allantoic acid, ureidoglycolate, and citrulline, but total ureides, urea concentration, and urease activity were reduced. Disruption of amino acid metabolism in Ni-deficient leaves resulted in accumulation of glycine, valine, isoleucine, tyrosine, tryptophan, arginine, and total free amino acids, and lower concentrations of histidine and glutamic acid. Nickel deficiency also disrupts the citric acid cycle, the second stage of respiration, where Ni-deficient leaves contained very low levels of citrate compared to Ni sufficient leaves. Disruption of carbon metabolism was also via accumulation of lactic and oxalic acids (Bai, Reilly and Wood 2006).
According to these results, Ni deficiency substantially disrupts several metabolic pathways and results in distinct biochemical-based symptoms of Ni deficiency even before the development of morphological symptoms associated with disruption of vegetative growth processes. The magnitude of metabolic disruption exhibited in Ni-deficient plants is evidence of the existence of unidentified physiological roles for Ni in plants. This finding in combination with the diverse known functions of Ni in bacteria suggests that Ni may indeed play a role in many, yet undiscovered processes in higher plants (Brown 2007).
Improvement of our knowledge of the biochemical role of Ni in plants may bring new insights into how Ni nutrition affects plants stress responses. Genetics and molecular biology approaches may be useful in identification of the roles of Ni in the biochemical processes particularly under stressful conditions similar with the studies on Mo and its effect on the plants stress response via ABA metabolism.
Effect of beneficial elements on plants stress responses
Mineral elements which either stimulate growth but are not essential or which are essential only for certain plant species or under specific conditions, are defined as beneficial elements. This definition applies to sodium (Na), silicon (Si), selenium (Se), cobalt (Co) and aluminum (Al) (Marschner 1995). The main physiological functions of beneficial elements are presented in Table 5.
For elements defined as beneficial, instead of application of the word "deficiency", it seems to be more practical to focus on the plant responses in the presence of these elements. In this section we will summarize evidences showed plants response to supplementation with beneficial elements when grown under various stressful environmental conditions.
Effects of sodium supplementation on plants stress responses
Sodium has been studied more for its negative effect at excess levels (salt stress) than as a beneficial or essential element. Sodium is essential only for some C4 species, but is undoubtedly beneficial to the growth of euhalophytes. It may stimulate the growth of some species with an evolutionary history in saline environments, and even of apparently totally glycophytic species under certain conditions.
Although Na has not been shown to be an "essential nutrient" for most plants, there is a high degree of Na utilization in many plants and some utilization in most if not all plants. The criteria described by Arnon and Stout (1939) that must be met for an element to be considered as an "essential nutrient" for plants are based exclusively on ecological considerations for survival and reproduction; high yield or biomass production may or may not be an important aspect, and may not even be associated with nutrient essentiality. For example, some mineral elements such as Na, Se and Si may promote increased biomass production, but may not be required for the species to survive. To overcome some of the limitations and difficulties associated with a strict definition of "essentiality," the term "functional or metabolism nutrient," has been suggested (Nicholas 1961) which is defined as "any mineral element that functions in plant metabolism irrespective of whether or not its action is specific." Recently (Subbarao et al. 2003), this term has been defined as "an element that is essential for maximal biomass production or can reduce the critical level of an essential element by partially replacing it in an essential metabolic process." This section deals with this issue and present evidence to support the notion that Na should be considered as a "functional nutrient," based on the above definition.
Because of the chemical similarity between K and Na, it is generally assumed that K and Na compete for common absorption sites in the root. Sodium, even in 20-fold excess, fails to compete significantly with K under mechanism I, while Mechanism II does not discriminate K from Na and thus Na can competitively inhibit the absorption of K. Recently Kin channels (inward rectifying K channels) have been reported in different root cells, including cortical, root hair, stelar and xylem parenchyma cells, that can sense K concentrations (Blumwald, Aharon and Apse 2000). Although it is widely believed that mechanism I does not have much affinity to transport Na in the presence of adequate K, for some crops such as beets this mechanism may be transporting Na independent of the external concentration. Several Atriplex species take up Na in preference to K. In these species, Na competes with K during uptake, but K does not compete with Na. Thus specific mechanisms of Na transport at low concentrations and in the presence of K are open to further investigation (Subbarao et al. 2003).
Effects in C4 species
In some C4 species such as Atriplex vesicaria (Chenopodiaceae), Amaranthus tricolor (Amaranthaceae) and Panicum miliaceum (Poaceae), Na is required for the function of CO2 concentration mechanism, plays a critical role in the regeneration of phosphoenolpyruvate (PEP) in mesophyll chloroplasts, has a role in Chl synthesis, pyruvate uptake into chloroplasts via Na+/pyruvate co-transport system and in nitrate assimilation (Marschner 1995). Sodium deficiency impairs conversion of pyruvate to PEP in the mesophyll chloroplasts, leads to a reduction in PSII activity and ultrastructural changes in mesophyll but not bundle sheath chloroplasts, and reduction of nitrate-reductase activity (Marschner 1995). In sorghum species (Sorghum L.), there is a specific effect of higher concentrations of Na on the kinase that regulates the activity of PEP carboxylase, the primary carbon-fixing enzyme in C4 and crassulacean acid metabolism (CAM) plants (Monreal et al. 2003).
In natrophilic species such as sugar beet when the availability of water in the substrate is high, Na decreases the total dry mass per unit water consumption i.e. water use efficiency. If, however, the availability of water in the substrate is low, water use efficiency remained unchanged in plants supplied with Na but increases sharply in plants receiving a K supply only. Improvement of water balance of plants when the water supply is limited is obviously occurs via stomatal regulation. With a sudden decrease in the availability of water in the substrate (drought stress) the stomata of plants supplied with Na close more rapidly than plants supplied with K only and, after stress release, exhibit a substantial delay in opening. As a consequence, in plants supplied with Na the relative leaf water content is maintained at a higher level even at low substrate water availability (drought periods, saline soils) (Marschner 1995).
An improved osmotic adjustment is a major factor in growth stimulation of halophytes by high Na supply. Growth responses of halophytes to Na under saline conditions reflect the need for an osmoticum during osmotic adjustment to salinity stress. Many halophytes osmotically compensate for high external osmotic potential by accumulating Na salts, often NaCl from the environment. Growth stimulation by Na is particularly apparent in the Chenopodiaceae and among non-chenopods, some cultivars of tomato adapted to saline soils has been reported to respond positively to additional Na (Hajiboland et al., 2010). In the presence of Na, cell expansion in natrophilic species is maintained and water balance is even improved. In these species, not only can Na replace K in its contribution to the solute potential in the vacuoles and in the generation of turgor and cell expansion, it may surpass K in this respect since it accumulates preferentially in the vacuoles. The superiority of Na can be demonstrated by the expansion of sugar beet leaf segments in vitro as well as in intact sugar beet plants, where leaf area, thickness and succulence are distinctly greater when a high proportion of K is replaced by Na (Marschner 1995). In sugar beet, mild salinity (5.5 dS m-1) caused significant improvement in the yield and sugar content of storage roots (Hajiboland, Joudmand and Fotouhi 2009). Sugar beet cultivars differ in the response to low (50 mM) salinity (Hajiboland and Joudmand, 2009). In the cultivar with positive response to low salinity (IC), in addition of higher dry matter production and broader leaves, membrane integrity was even improved under low salinity (Table 6). Fractionation of Na in the leaves showed that proportional Na in cell sap (mainly vacuole) was higher in IC and in contrast, the proportional Na in residual fraction (comprised mainly from cell wall) was lower in this cultivar (Table 6). Allocation of more Na to the cell sap may result in facilitating control of water balance of leaf cells and causes an improvement of cell expansion and production of broader leaves (Hajiboland and Joudmand, 2009). Finally, salinity stress is known to induce CAM photosynthesis in the facultative CAM species, such as Mesembryanthemum crystallinum L., (Aizoaceae) and Sedum album (Crassulaceae) (Cushman and Bohnert 2002). CAM is a metabolic adaptation of photosynthetic CO2 fixation that improves water use efficiency by shifting net CO2 uptake to the night, thereby reducing transpirational water loss.
The presence of Na in the environment and its uptake by plants can reduce the amount of K required to meet the plants basic metabolic requirements. K functions in plants can be summarized as both biophysical (non-K-specific role as an osmoticum in the vacuole) and biochemical (specific and non-specific roles in the cytoplasm). The need of monovalent cations in some plant species can also be filled by Na, thus reducing the required critical level of tissue K. In natrophilic species such as sugar beet with a high ability for substitution of K by Na, in old leaves nearly all the K can be replaced by Na that made K available for specific functions in meristematic and expanding tissues. Sodium alleviates K-deficiency symptoms and decreases the critical foliar K concentration at which K-deficiency symptoms appeared (Subbarao et al. 2000).
Effects of silicone supplementation on plants stress responses
Silicon is the second most abundant element both on the surface of the Earth's crust and in the soils. Although Si has not been considered as an essential element for higher plants, it has been proved to be beneficial for the healthy growth and development of many plant species, particularly graminaceous plants such as rice and sugarcane and some cyperaceous plants (Marschner 1995). The beneficial effects of Si are particularly distinct in plants exposed to abiotic and biotic stresses. Epstein and Bloom (2005) have recently established a new definition for essential elements in higher plants. According to these authors, an element is essential that fulfils either one or both of the following criteria: (i) the element is part of a molecule which is an intrinsic component of the structure or metabolism of the plant, and (ii) the plant can be so severely deficient in the element that it exhibits abnormalities in growth, development, or reproduction, i.e. 'performance', compared to plants with lower deficiency. Accordingly, Si will be an essential element for higher plants, which is to be generally accepted in the near future.
Over last two decades, extensive studies have been performed aiming at understanding of the possible mechanism(s) for Si-enhanced tolerance of higher plants to both abiotic and biotic stresses (Liang et al. 2007). More recently, rapid progress has been also made in Si uptake and transport in higher plants. The uptake of Si was found to be the result of two different transport mechanisms. A low affinity transporter (Lsi1) found on the lateral roots of rice plants is responsible for the uptake of silicic acid from the external solution to the root cortical cells (Ma and Yamaji 2006). The transporter has been localized on the distal cells of exodermis and endodermis. A second transporter has also been identified in rice which is responsible for xylem loading of Si (Mitani and Ma 2005).
In this section we review current knowledge on the roles of Si in conferring tolerance to plants against abiotic stresses. Because of a well-documented role of Si in the plants resistance against biotic stress factors such as pathogens, we will give also a brief overview on this effect of Si.
Optimization of silicon nutriton results in increased mass and volume of roots, giving increased total and adsorbing surfaces (Kudinova 1975). These plants could more efficiently extract water from drying substrate than plants without Si supplementation. Experiment with citrus (Citrus spp.) has demonstrated that with increasing monosilicic acid concentration in irrigation water, the weight of roots increased more than that of shoots (Matichenkov, Calvert and Snyder 1999b). The same effect was observed for bahia grass (Paspalum notatum Flügge) (Matichenkov, Calvert and Snyder 2000). Greater root/shoot mass ratio provides greater water absorption surface and lower transpiration area leading to a considerable increase in plants tolerance to drought. Silicon deposits in cell walls of xylem vessels prevent compression of the vessels under conditions of high transpiration caused by drought or heat stress. In addition, the silicon-cellulose membrane in epidermal tissue also protects plants against excessive loss of water by cuticular transpiration. This action occurs owing to a reduction in the diameter of stomatal pores and, consequently, a reduction in leaf transpiration (Snyder, Matichenkov and Datnoff 2007). In rice plants, Si can alleviate water stress by decreasing transpiration. Rice plants have a thin cuticle and the formation of a cuticle-Si double layer significantly decreases cuticular transpiration. Since water stress causes stomata closure and reduction of photosynthetic rate, Si stimulates the growth and photosynthesis of rice more clearly under water-stresses than non-stressed conditions (Ma, et al. 2001). Furthermore, deposition of Si in rice increases the thickness of the culm wall and the size of the vascular bundle preventing lodging. Sterility is related to many factors including excess water loss from the hull. Transpiration from the panicles occurs only from the cuticle of the hull because the hull has no stomata. Silicon deposition on the hull decreases the transpiration from panicles by about 30% at either milky or maturity stage, preventing excess water loss. This is the reason why Si application significantly increases the percentage of ripened grain (Ma, et al. 2001).