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Salinity reduces plant growth and yield by two mechanisms, osmotic stress and ion cytotoxicity (Mumms and Tester, 2008). Mumms et al (1995) proposed a two phase model of salt injury where growth is initially reduced by osmotic stress and then by Na+ toxicity. According to this biphasic model, growth is first reduced by the decrease in soil osmotic potential (Ïˆo), caused by salt outside the plant rather than within it. The induced osmotic stress is controlled by inhibitory signals from the roots and, genotypes differing in salt resistance, respond identically in this first phase. Ionic stress develops over time and is due to a combination of ion accumulation in the shoot and to an inability to tolerate ions that have accumulated. The adaptation responses to ionic stress by plants are of two distinct types: Na+ exclusion and tissue tolerance, and is the result of different abilities by plants to exclude or to sequester toxic ions into vacuoles. In this second phase of growth reduction, genotypes varying in salt resistance may respond differently. Secondary stresses induced by salinity, such as nutritional imbalances and oxidative stress, are also responsible of reduced plant growth.
It is difficult to separate the osmotic effect from specific ion effects that overlap during the development of salinity stress, thereby some uncertainty exists regarding the relative importance of both mechanisms. Rengasamy (2010) conducted a pot experiment on wheat growth where the plants were exposed to NaCl or to a Hoagland nutrient solution at different salinity levels. The results evidenced that the osmotic effect is continuous but, at low level of salinity, the ionic effect may be significant in reducing growth and, the application of nutrients (Hoagland solution), alleviates the salinity stress on plants. However, above a threshold value of soil solution salinity, the osmotic effect becomes the dominant mechanism, limiting the growth. Although the term salinity implies high concentration of salts in soil, NaCl contributes the most part in soil salinity and this explains why all plants have evolved some mechanisms to regulate NaCl accumulation or exclusion. Moreover the specific-ion toxicity of NaCl is not only the result of an excessive Na uptake, but a combined contribution of both Na+ and Cl- as well. In fact Cl- concentrations may be higher than those of Na+ ions, cations that can be adsorbed by soil particles. Generally, anions like Cl- are repelled from soil surface and retained in soil solution where they can accumulate also at large amount, controlling the overall salt concentration of the soil solution. For most species Na+ appears -to reach a toxic concentration before Cl- does, however for some crops, such as soybean, citrus and grapevine , Cl- is considered to be the more toxic ion (Storey and Walker, 1999).
Hydroponic versus soil systems
The majority of works regarding salt effects and developing selection criteria for improved salt tolerance in plants has been done using solution culture, assuming that responses in hydroponics mimic those in soil. In a recent study on barley, Tavakkoly et al., (2010) show that the effects of salinity on plants differed between the hydroponic and soil systems. The salt concentration in the rhizosphere may increase, as a result of decreasing water content in the vicinity of the roots, due to the high transpiration demand and low hydraulic conductivity of soil. This does not occur in solution culture, where no ion gradients will build up and neither depletion nor salt accumulation in the rhizophere will occur. In soil sudden changes of salt concentration are unlikely because of the soil buffering capacity, associated with the cation exchange soil complex (Vetterlein et al. 2004). Thus, plants in soil have more time to adapt to the increase of salinity than plants in hydroponic system. This is of a particular importance for cellular homeostasis adjustments that require ion uptakes and compatible solute accumulation. A result consistent with the two systems is the more significant negative effect of osmotic stress on plant growth, in comparison with the specific ion effects (Tavakkoli et al., 2010). This agrees with assumptions by Munns (2005) and Rengasamy (2010) which indicated that the biggest reduction in growth is caused by the osmotic stress and a relatively smaller effect is due to the genetic differences in ion exclusion.
Soil constraints on root growth in saline environment
The evaluation of the average soil salinity and water content of a specific soil layer cannot be considered comprehensive to calculate the effective soil solution salinity roots are exposed to. In fact it does not consider aspects concerning interactions between roots and soil, at the root/soil interface. Driven by transpiration of the shoot, saline soil solution moves from the bulk soil to the root surface, where water uptake occurs, but most ions are excluded. Consequently, rhizospheric soil can be up to 15 times more saline than the bulk soil and this gradient is also more expressed under conditions of higher ET demand. The osmotic water potentials of the soil solution contacting the root surface are significantly lower than the bulk soil and this gradient initiates a flow of soil solution directed to the root surface (mass flow). Then, the increase in soil salinity, as result of evaporation, occurs at the soil interface, while the site of separation of salts from the soil water, due to root water uptake, takes place at the soil-root interface.
In many saline soils a deterioration of the structure leads some physical constraints that in the root zone appear principally in the form of compaction and crusting. Low porosity restricts rates of water and nutrient uptake by roots as well as gas exchange, whereas high soil strength directly inhibits root elongation and expansion. Soil oxygen movement to roots is critical to maintain adequate respiration for plant growth. Under anoxic conditions some bacteria shift metabolic pathways so as to utilize alternative terminal electron acceptors and produce some substances , such as hydrogen sulphyde, that are toxic for plants. Roots need nutrition, water aeration and low mechanical strength to grow and function in the soil environment. The study of interactions between root properties (morphology and activity) and soil conditions are relevant to assess the water supply of plants and the salt tolerance of plants. If root growth and physiological processes in the root are affected, adverse leaf water status and top growth can occur through both hydraulic and biochemical signals.
Na+ uptake and accumulation in roots
In most plants, roots should exclude 98% of the salt in soil solution, allowing only 2% to be transported to the shoot. Then roots filter out most of the salt in the soil while taking up water and play a fundamental role in protecting the plants from excessive uptake of salts. Furthermore roots have a remarkable ability to control their Na+ and Cl- concentration, that is rarely much higher than in external solution. (Munns, 2005)
Unidirectional influx and efflux provide the two main components of the currently accepted model of Na+ uptake in plants. Na+ ions passively enter the cell, down the ion's electrochemical potential gradient, and exit the cell via a secondarily active proton-driven sodium with a probable Na+:H+ stechiometry of 1:1 (SOS1; Shi et al. 2002). This process consumes significant cellular energy. Recently Malagoli et al (2008) showed that the energy predicted to drive active Na+ efflux in rice roots was much greater than the measured one. This discrepancy may indicate the involvement of more Na+-specific transport systems, and, interestingly, a sodium-potassium-chloride transporter has recently been discovered in A. thaliana (Colmenero-Flores et al., 2007). Then, it was suggested a possible mechanism in which active Na+ efflux is energized differently from current models, possibly via its coupling to passive fluxes of ions other than protons.
The compartmentation in root vacuoles of remaining Na+ is achieved by tonoplast Na+/H+ antiporters. A passive leakage of Na+ back to the cytosol (possibly via tonoplast nonselective cation channels) requires a constant resequestration of Na+ into vacuoles (Apse et al., 1999, 2007). This mechanism allows plants to minimize or delay the toxic effects of high concentrations of salts, so genotypes with a poor ability to sequester salts have a greater rate of leaf death. Therefore, an efficient sequestration system may improve tissue tolerance by plants, perhaps by reducing cytosolic Na+ concentrations.
As the water moves from the soil across the root cortex ions are transported by this stream towards the stele. Some X-ray microanalysis on roots of wheat plants, showed that the root cortex is the main barrier to Na+ transport into the stele, rather than the endodermis (Lauchli et al., 2005) and the highest concentration of Na+ was in the cell layer of pericycle. Similar results show substantial sequestering of large amount of Na+ and Cl- in vacuoles of pericycle cells in grapevine roots, grown at relatively low salinity (25 mM NaCl), suggesting an important role of pericycle in the radial transport of Na+ and resulting xylem loading (Storey et al., 2003).
How salinity is sensed in roots
The perception of salinity is achieved by both ionic and osmotic stress signals in plants. The responses of root cells are finalized to maintain their own correct functionality, despite of the condition of elevated Na+ concentration. Long distance signals to shoots are activated in the form of hormones or their precursors, in fact the reduction of leaf growth under salinity is independent of carbohydrate supply and water status (Turkan and Demiral 2009). Abscissic acid (ABA) plays a central role in root-to-shoot and cellular signaling, but gibberellins are also involved. ABA can inhibit leaf elongation by lowering the content of active GA, as observed in barley leaves (Munns et al. 2006) .
Root growth is usually less affected by salinity than leaf growth. Root elongation rates recovers remarkably well after exposure to NaCl or other osmotica and, unlike leaves, the recovery takes place despite turgor. Changes in wall properties must occur, but the mechanism is unknown. With time, reduced initiation of new lateral or seminal roots is evident.
Signals within root cells are likely independent from ABA. Plants respond directly and specifically to addition of Na+ within seconds. Then, a plasma membrane protein must be the sensor, but this is still obscure. The first recorded response in roots is an increase in [Ca+2]cyt from an influx across the plasma membrane and also from the tonoplast. This perturbation in Ca+2 level activates salt stress signaling, sensed by a protein (SOS3) that interacts with a protein kinase, identified as SOS2. The complex SOS3/SOS2, enabling the phosphorylation, activates the membrane bound Na+/H+ antiporter, SOS1, that is responsible of Na+ efflux.
The discovery of the SOS (Salt-Overlay-Sensitive) pathway in Arabidopsis clarified how Na+ (ionic stress) is sensed and the relationship between ion homeostasis and salinity tolerance. However, Arabidopsis is a glycophite species, sensitive to moderate levels of NaCl, and the adaptive responses to Na+ in this plant should be extrapolated with caution. In fact, if Arabidopsis remains a useful model to study and discover plant Na+ transport processes, the identification of signaling pathways in salt tolerant species is more relevant to define adaptive rather than dysfunctional responses to salinity. The relationship between Na+ tolerance and Na+ accumulation is different in Arabidopsis and cereals (Tester and Davenport 2003). More work is necessary for the identification of the different mechanisms that are fundamental to specific aspects of salinity tolerance, and also the evaluation of the time of exposure and the severity of salt treatment are important, because they determine the physiological and molecular changes that are detected.
Root form and function in saline environment
Root system is the main interface between plants and their environment, and shows a high degree of plasticity in its development in response to local heterogeneity of the soil. On the level of the individual root and the entire root system, various morphological parameters such as length, section, surface area, root hairs are used as potential indicator of root plasticity. Moreover, responses of biomass allocation patterns and structural traits such as specific root length, root tissue density and root diameter distribution, are associated with acquisition capacities for below-ground resources and respond to stresses and environmental changes. Therefore, some morphological modifications, at the individual root level, can affect the structural and physiological characteristics of the entire root system and this can change water uptake and nutrient supply by plants.
Rice is considered a moderately salt-sensitive crop, although a large variability exists among cultivars as well as between developmental stages (Bahaji et al., 2002). A delay in the emergence of primary, adventitious and lateral roots and a subsequent inhibition of root development, in terms of number and length, were common responses to osmotic and saline treatments. However some specific NaCl responses were detected and concerned lateral root development. In particular, lateral roots were thicker as well as more densely arranged and more irregular spaced than those of control plants. Furthermore some bifurcations were occasionally noticed in primary and adventitious roots of NaCl stressed rice seedlings. In rice, under salinity, silicon can accumulate to high levels and it reduces Na+ loading to xylem in plants. X-ray microanalysis of root transverse sections showed there was the greatest silicon deposition in the endodermis. Silicon deposition restricted the movement of water and ions through the apoplast so the Na+ uptake was reduced by blocking the influx through the apoplastic pathway. It has been reported a positive role for silicon in reduction of salt stress in many crop grasses, including wheat, maize and barley (Munns 2002, 2006, Flowers 2004).
Wang et al. (2009) showed that high salt exposure suppressed lateral root initiation and organogenesis in Arabidopsis thaliana, resulting in the abortion of lateral root development but, on the other hand, salt stress markedly promoted lateral root elongation. The lateral root shaping is considered a prime example of developmental plasticity because both, number and placement of lateral roots, are highly responsive to external cues. This indicates that there must be a signal transduction pathway that interprets complex environmental conditions and makes the "decision" to form a lateral root at a particular time and place. Auxin plays a key role in shaping plant architecture and it mediates responses to a broad range of external signals. Histochemical staining, physiological experiments using transport inhibitors and genetic analysis revealed that the quantity of auxin and its patterning in roots were both greatly altered by exposure to high concentrations of salt stress and auxin transport pathway is important for adaptive root system development under salt stress ( Malamy 2005).
Root hairs can make up 70-80% of the root surface area. They play an important role in nutrient uptake and, root hair number and density, generally increase as a consequence of a nutrient stress (Glory and Jones 2000). Wang et al. (2008) showed that in Arabidopsis thaliana root hair number and density decreased significantly under salinity, in a dose-dependent manner, and they reported a physiological mechanism for root hair development in response to salt stress. They hypothesize that salt stress may affect cell-fate specification and the reduction in root hair number is likely caused by a decrease in the epidermal cells differentiating into trichoblasts. The inhibition was sensitive to ions but not to osmotic stress, and was considered an adaptive mechanism to avoid excessive ion uptake, by reducing the absorptive area when ion disequilibrium occurs in roots. Furthermore, as the high sensitivity of root hairs towards salt, they suggest a possible role of root hair alteration as an early indicator of salt stress and plant response.
Salinity stress is also responsible of a thickening of roots. Some studies were carried out on growth and changes in structure of root cells in Kikuyu seedlings grown in Hoagland nutrient solution with different salt concentrations ( Panuccio et al 2002, 2003, Muscolo et al. 2003). The cross sections of the primary structure of Kikuyu grass roots exposed to 50 and 100 mM NaCl did not show significant changes in the cortex growth and stele development; in contrast 200 mM NaCl caused a significant reduction in the relative volume of the endodermis around the central cylinder, a thickness of the Casparian band and an increase in the number and diameter of root metaxylem vessels. These anatomical modifications may increase the mechanical resistance and decrease the root permeability to avoid the toxic effects of ions in excess.
Lens culinaris has always been considered a salt sensitive species, but the microsperma landrace "Ustica" is a genotype that behaves like a salt tolerant one because of its adaptation to the particular environment of the homonymous little island (North of Sicily). Some studies have been conducted to evaluate salt effects and plant responses in Ustica seedlings, grown for 20 days in microcosms, using agrilite as solid substrate and in presence of different salt concentration (0, 50, 100, 200 mM NaCl) (unpublished data). Various morphological root parameters such as length (cm), diameter (mm) and surface area (cm2) were tested, by using an image analysis system. The results were also compared with those of a commercial cultivar (Eston), as they are valuable parameters when describing and comparing root systems. In both cultivars, the length of lateral roots was more affected than that of primary roots, but to different extent (Fig.1). The parameter "root length" is considered more important than the "root weight" to indicate the root functionality, because it expresses the potential for solute and water uptake (Ryser 2006). In Eston seedlings, exposed to 100 mM NaCl, no lateral roots were expressed; differently, seedling of Ustica showed an inhibition of the lateral root length, even though the number was not significantly influenced. Generally, a water supply reduction in plants, brings to a lower lateral root production (Fig.1). The Specific root length (SRL) values were higher in Ustica than in Eston seedlings (Tab.1). SRL is the length-to-mass ratio, it is believed to characterize economic aspect of the root system and is frequently used as indicator of root fineness. Then, higher SRL results from longer and thinner roots per unit construction cost (root mass) and this root apparatus is more effective in water and nutrient uptake (Fitter 1991) . SRL is a complex parameter that includes variations in root diameter and root tissue density, which respond to environmental conditions differently (Ryser 2006) . In the Ustica variety, a salinity increase leads to an increase in SRL (Tab.1), due to thicker lateral roots and the root diameter distribution was shifted towards larger diameter classes (Fig.2). Root diameter distribution is usually expressed as the mean diameter but sometimes it does not necessarily characterize a response of root system structure adequately. In fact, fine and coarse roots show different responses, indicating that root diameter classes should be considered as functionally distinct and regarded separately to fully understand stress responses of root systems. It is known that roots with a smaller root diameter can contact a larger soil volume per unit root surface area, however the maintenance carbon cost of producing finer roots may be higher as these will have be replaced more frequently (Fitter 1991). In Ustica plants, coarse roots, for both principal and lateral, prevailed under high salinity conditions. This result can be explained by considering that, under salinity, the construction costs per root length should be minimized because of the onset of growth limiting conditions, and the root development resulted further inhibited to counter water stress and ion toxicity due to the salt around the root. Apart from the effects on root biomass production, contrasting root morphological responses of ecotypes to salt treatments might be partially responsible for dissimilar abilities to tolerate salinity. Structural and morphological differences in roots certainly play an essential role for nutrient and water uptake by plants from saline soil and the study of these parameters can help to determine different mechanisms underlying salt toxicity and the way plants can cope with saline conditions . Some modifications of root morphology should not be considered a simple growth stopping, but rather an induced reorientation of growth which is related to stress avoidance. These information could be considered an important tool in studies that involve salt tolerance improvements in plants.