Salt tolerance mechanisms in quinoa (Chenopodium quinoa Willd.)

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Salt tolerance mechanisms in quinoa (Chenopodium quinoa Willd.) Verena Isabelle Adolf a , Sven-Erik Jacobsen a,∗ , Sergey Shabala b a b

Faculty of Life Sciences, University of Copenhagen, Højbakkegaard Allé 13, DK-2630 Taastrup, Denmark School of Agricultural Science, University of Tasmania, Private Bag 54, Hobart, Tas 7001, Australia

a r t i c l e

i n f o

Keywords: Soil salinity NaCl Halophyte Tolerance mechanisms Oxidative stress Osmotic adjustment

a b s t r a c t In the face of diminishing fresh water resources and increasing soil salinisation it is relevant to evaluate the potential of halophytic plant species to be cultivated in arid and semi-arid regions, where the productivity of most crop plants is markedly affected. Quinoa is a facultative halophytic plant species with the most tolerant varieties being able to cope with salinity levels as high as those present in sea water. This characteristic has aroused the interest in the species, and a number of studies have been performed with the aim of elucidating the mechanisms used by quinoa in order to cope with high salt levels in the soil at various stages of plant development. In quinoa key traits seem to be an efficient control of Na+ sequestration in leaf vacuoles, xylem Na+ loading, higher ROS tolerance, better K+ retention, and an efficient control over stomatal development and aperture. The purpose of this review is to give an overview on the existing knowledge of the salt tolerance of quinoa, to discuss the potential of quinoa for cultivation in salt-affected regions and as a basis for further research in the field of plant salt tolerance. © 2012 Published by Elsevier B.V.

1. Introduction Yield loss due to saline soils is a common problem all over the world as most crop plants are glycophytes and, hence, sensitive to salinity. 97.5% of the world’s water is saline, and large land areas are naturally saline. Human activities exacerbate the problem in many affected regions (Munns and Tester, 2008). Therefore, new approaches are necessary to cope with these problems. One option is the use of halophytic crop species, which can tolerate high levels of soil salinity (Koyro et al., 2008). The seed crop quinoa belongs to the Amaranthaceae, a plant family that comprises by far the highest proportion (44%) of halophytic plant species (Flowers et al., 1986). Quinoa originates from the Andean region of South America, where soil quality is poor and climatic conditions are harsh. The plants had to adapt accordingly in order to withstand frequent drought (Garcia et al., 2003, 2007; Jacobsen et al., 2009), frost (Jacobsen et al., 2005, 2007), hail and wind at an elevation of 3500–3900 m above sea level (Jacobsen et al., 2003). Quinoa is a facultative halophytic plant species with varieties being able to cope with salinity levels as high as those present in sea water (electrical conductivity (EC) 40 dS/m

Abbreviations: ABA, abscisic acid; NSCC, non-selective cation channels; HKT, high-affinity K+ transporter; NHX, tonoplast Na+ /H+ exchanger; KOR, outwardrectifying K+ channel; SKOR, shaker-like depolarisation-activated outwardrectifying K+ channel; NORC, non-selective outward rectifying cation channel. ∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S.-E. Jacobsen).

to 400 mM NaCl) (Jacobsen et al., 2001, 2003; Koyro and Eisa, 2008; Hariadi et al., 2011). Quinoa has been cultivated in the Andes for at least 7000 years (Pearsall, 1992) and constitutes an important seed crop in this region. The interest in this seed crop is increasing all over the world (Jacobsen, 2003, 2011), not only because of its stress tolerance, but also due to its exceptional nutritional quality (Repo-Carrasco et al., 2003; Vega-Gálvez et al., 2010; Stikic et al., 2012). The seed has an outstanding composition of essential amino acids, is rich in vitamins (A, B2 , E) and the minerals calcium, magnesium, iron, copper, zinc and lithium, and it represents a valuable source of carbohydrates and essential fatty acids for human nutrition (Koziol, 1992; Ranhotra et al., 1993; Repo-Carrasco et al., 2003; Llorente, 2008).

2. Establishment and productivity of quinoa under salinity 2.1. Germination and seedling establishment under salinity Seedling establishment is a critical process in a plants’ life, especially in the presence of adverse environmental factors (Bohnert et al., 1995). When compared with glycophytes, halophytes can cope with high salt levels during germination (Malcolm et al., 2003; Debez et al., 2004). However, it has been shown in several studies that even halophytes are relatively sensitive to salinity during the stages of germination and seedling emergence (Ungar, 1996; Khan and Abdullah, 2003; Debez et al., 2004; Tobe et al., 2000; Malcolm et al., 2003) and quinoa’s most

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Please cite this article in press as: Adolf, V.I., et al., Salt tolerance mechanisms in quinoa (Chenopodium quinoa Willd.). Environ. Exp. Bot. (2012), http://dx.doi.org/10.1016/j.envexpbot.2012.07.004

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After 4 d

After 7 d

Recorded hourly for 14 h

150 and 300 mM

Titicaca (5206)

Four genotypes (PRJ, PRP, UDEC9, BO78) Ruiz-Carrasco et al. (2011)

Ruffino et al. (2010)

Hariadi et al. (2011)

Denmark (selected from material originating from a cross between southern Chilean and Peruvian lines) Chile

0, 100, 200, 300, 400, 500 mM

No effect on germination %, but 2 h delay Sign. effect only at >400 mM 400 mM: still >90% 500 mM: 24% Only for BO78 inhibition of germination at 300 mM 250 mM

25 dS/m Peru

Out of 182 accs. tested: 15 most tolerant Sajama Gómez-Pando et al. (2010)

0, 200, 400, 800, 1200 mM Altiplano of northern Chile Temuco in southern Chile Amarilla and Roja Hueque and Pucura Delatorre-Herrera and Pinto (2009)

Bolivia

After 10 d

After 22 h

After 14 h After 7 d Recorded after 5–10 h 400 mM 57 dS/m 100 and 200 mM Sajama Kancolla Robura and Sajama Prado et al. (2000) Jacobsen et al. (2003) Schabes and Sigstad (2005)

Bolivia Peru Bolivia

Time Germination effect

No decrease in germination % at 0, 100 and 200 mM NaCl treatment, but delay in germination. At 300–500 mM NaCl, delay and reduction in germination percentage. (Effect is temperature dependent). 14% 75% Robura more sensitive to salt during germination than Sajama. LD50 (NaCl conc. causing 50% reduction): Amarilla: 400; Roja: 320; Hueque: 200; Pucura: 360 mM 60% 0, 100, 200, 300, 400, 500 mM

NaCl conc. Origin

González and Prado (1992)

Bolivia

Accession/Cv.

Sajama

Authors

critical period is the time of establishment (Jacobsen et al., 1994, 1999). Gómez-Pando et al. (2010) tested the germination percentage of 182 selected from about 2500 quinoa accessions under saline conditions and observed significant differences in germination rate between the accessions. The fifteen most tolerant accessions used in the study showed a germination percentage of 60 in 25 dS/m saline water. Previously, it was observed that seeds of the Peruvian quinoa cultivar Kancolla had a germination percentage of 75 at a salt concentration as high as 57 dS/m measured after 7 days (Jacobsen et al., 2003). Ruiz-Carrasco et al. (2011) tested the germination of four Chilean quinoa genotypes treated with 0, 150 or 300 mM NaCl (∼0, 15, 30 dS/m), respectively. Only at the highest salinity level and in one of the accessions they found a significant reduction in germination rate. Hariadi et al. (2011) observed a significant inhibitory effect of salinity on seed germination only for concentrations higher than 400 mM NaCl. In a comparison between the Bolivian cultivars Robura and Sajama, Robura was found to be more sensitive to salinity during germination with a tolerance limit of 100 mM for NaCl (Schabes and Sigstad, 2005). Prado et al. (2000) observed a reduction of germination percentage to 14% after 14 h at 400 mM NaCl for the cultivar Sajama. The same cultivar was studied by Ruffino et al. (2010) under 250 mM NaCl and they could not detect any significant effect on germination percentage after 14 h, however a 2 h delay in germination occurred under salinity. Similarly, González and Prado (1992) found that germination percentages did not differ significantly between 0 and 200 mM NaCl treated seeds. However, delay time in germination increased with increasing salinity. At 300–500 mM salinity, germination percentage was also reduced. Delatorre-Herrera and Pinto (2009) tested germination in four Chilean quinoa genotypes and found that at 400 mM NaCl, which was the concentration that caused a 50% reduction in germination in the most salt tolerant genotype, germination rate was lower for genotypes from non-saline areas, which germinated after 22 h compared to 10 h for those originating from a saline area. These findings suggest that salinity primarily delays germination commencement before affecting the germination percentage. Table 1 provides an overview on the studies on quinoa germination and facilitates comparisons of salt concentration, cultivar and time duration of germination. Establishment and growth of seedlings, germinated and grown at 150 and 300 mM NaCl, showed that seedling biomass and root/shoot length ratio were differentially affected in seedlings of four different genotypes at 300 mM NaCl treatment, whilst there was no significant effect in any of these genotypes at 150 mM NaCl (Ruiz-Carrasco et al., 2011). It may be concluded that the capability of germination and seedling establishment under saline conditions is dependent on cultivar and probably also on the substrate in which the seeds germinate. However, quinoa proves in general a great capability of dealing with salinity in this early and critical phase of development. The high tolerance of quinoa to salt stress at germination has been discussed to be the result of a significant gradient in the distribution of possibly toxic (Na+ and Cl− ) ions and essential ions such as K+ , Mg2+ , Ca2+ , PO4 3− and SO4 2− , across the seed coat and additionally a change in the allocation of elements in the embryo. These observations have been made in seeds harvested from salt treated plants and it has been concluded that a highly protected seed interior may be the reason for the high tolerance of quinoa seeds to salinity (Koyro and Eisa, 2008). Hariadi et al. (2011) suggested that seed viability was dependent on its ability to exclude toxic Na+ from the developing embryo in order to avoid ion toxicity. In summary, it seems that the high salt tolerance of quinoa seeds may be ascribed to structural and physiological protective features of its seeds, which are worth to be studied in more detail.

After 30 h

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Table 1 Overview on the studies on quinoa germination performance. In the table, the units used in the original source are presented. 1 dS/m–10 mM NaCl.

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2.2. Osmotic and ionic effects on germination Salt stress consists of two components, an osmotic effect and specific ion-toxicity (Munns et al., 1995; Munns, 2002). Data and interpretations regarding the degree of these effects on quinoa germination are contradictory. Prado et al. (2000) observed that many of the seeds that had not germinated under salinity, germinated after washing with distilled water, which is consistent with González and Prado (1992), but in conflict with data provided by Hariadi et al. (2011). Based on the comparison with the effect of isotonic mannitol and PEG they concluded that specific ion toxicity was the major reason for the decline in seed germination. Delatorre-Herrera and Pinto (2009) separated the osmotic and ionic components of salinity in order to analyse their proportional effects on quinoa germination. They calculated the ionic effect as the difference between the total and the osmotic effect of salinity. The total effect was the observed effect of the NaCl treatment on germination when compared to treatment with pure water, and the osmotic effect was the difference between germination percentage in distilled water and an isotonic polyethylenglycol solution. Therewith, they showed that the osmotic and ionic components of salinity had a different effect on germination in different quinoa genotypes. An explanation for these conflicting findings might be the differential sensitivity of quinoa genotypes to the osmotic and ionic stress component of salinity. Further explanations may be that iontoxicity is reversible until it reaches a certain degree, which allows recovery when washing seeds with distilled water. In addition, the ability to recover and the threshold of salinity for recovery may be cultivar dependent. Further studies are needed to develop a standardized method for separating the two factors, osmotic and ion-toxicity, and which allow a comparison of independent trials. The knowledge of a genotype-specific proportional contribution of the two factors on germination may help to customize genotypes to particular field conditions. High osmotolerance during germination may favour cultivation on drought affected and slightly saline fields, whilst a high tolerance to ion-toxicity would be advantageous under highly salt affected conditions. 2.3. Physiological processes in quinoa seedlings under salinity In addition to affecting seed germination, salinity also influences numerous biological processes during seedling establishment in quinoa. These include ionic and osmotic homeostasis, chlorophyll and carotenoid synthesis, photosynthesis, carbon partitioning, lipid and protein synthesis, and thus whole plant metabolism and growth (Prado et al., 2000; Ruffino et al., 2010). Prado et al. (2000) observed changes in glucose, fructose and sucrose content between salt-treated and non-treated seedlings. An initial decrease in the ratio (total soluble sugar)/(dry weight) in cotyledons under saline conditions, and a subsequent increase, was explained by an initially low synthetic activity followed by an increase in carbohydrate metabolism. An increase in sucrose content in the embryonic axis was assumed to be associated with an active osmotic adjustment, low invertase activity, or a general decrease in total metabolic activity under salinity (Prado et al., 2000). Total soluble sugars such as sucrose and glucose were higher in the cotyledons of salt treated than in control plants (Ruffino et al., 2010). This was suggested to be the result of perispermic starch hydrolysis or conversion of sugars for osmotic adjustment. Fructose content decreased and was thus not considered an osmotic agent. Similar responses were observed by Rosa et al. (2009) when analysing sucrose–starch partitioning and related enzymes in saltstressed and salt-acclimated seedlings under low temperature. They reported higher activities of sucrose–phosphate synthase (SPS; EC 2.3.1.14) and soluble acid invertase (SAI; EC 3.2.1.26) in

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salt-stressed plants and an increase in soluble sugars and proline under salinity, which they also argued to be essential for the maintenance of osmotic balance under saline conditions. The response of seedling carbohydrate metabolism to salinity seems to be pronounced, and the enhanced production of soluble sugars may allow the plantlets to adjust osmotically to their saline environment. An efficient osmotic adjustment under saline conditions belongs to the most important factors of plant salt tolerance (see also Section 4.1) and seems to be a main focus of metabolism at the early seedling stage. 2.4. Biomass, yield and seed quality under saline conditions Of the almost 2500 quinoa accessions (Gómez-Pando et al., 2010), more than 200 have been tested under saline conditions, and shown to differ in their response to salinity. Differences in tolerance can be observed already at germination, and later during the growth cycle. Salt tolerance at germination is however not necessarily correlated with the degree of tolerance at later developmental stages, as demonstrated by Adolf et al. (2012) and other still unpublished data. For the variety Andean hybrid, Wilson et al. (2002) could not detect any significant reduction in plant height, leaf area or plant fresh weight, until a threshold of 11 dS/m was reached in a mixed salt treatment. They observed a tendency for an increase in both leaf area and dry weight of plants grown at 11 dS/m compared to those grown at control level (3 dS/m). In a semi-dwarf “moderately tolerant” wheat variety the threshold was already reached at 7 dS/m (Wilson et al., 2002). For quinoa, optimal plant growth has been seen at NaCl concentrations of 100–200 mM (∼10–20 dS/m) (Hariadi et al., 2011). This is in accordance with Jacobsen et al. (2003) who observed that quinoa biomass production, seed yield and harvest index were higher under moderately saline conditions (10–20 dS/m) than under non-saline conditions. In two tested cultivars, seed yield was found to be highest at an electrical conductivity (EC) of 15 dS/m. An EC higher than 15 dS/m led to a decrease in yield in both tested cultivars (Jacobsen et al., 2001, 2003). There did not seem to be much difference in growth responses whether plants were grown in NaCl containing soil from the beginning, or NaCl was added gradually with irrigation water (Hariadi et al., 2011). The most sensitive characteristics with regard to salinity have been shown to be stomatal conductance, inflorescence size and plant height, and it has been suggested that screening for plant height may be a means of increasing seed yield under saline conditions (Jacobsen et al., 2001, 2003). However, a recent study showed height not to be a good predictor of biomass production under salinity and biomass is usually correlated with yield production (Adolf et al., 2012). Gómez-Pando et al. (2010) studied 15 Peruvian accessions in more detail. Whilst some accessions showed a reduction in height under saline conditions, others did not or showed even an increase. The same was observed for leaf and root dry weight, and yield under saline conditions. They calculated the Euclidean distance to the expert criteria as an integrated indicator for evaluating the best-suited accessions for saline soils. Low plant height, short duration of life cycle, maximum root and leaf dry mass, and maximum seed yield were regarded as desirable agricultural traits. We have recently conducted an experiment in greenhouse, comparing 14 quinoa varieties with respect to biomass production. Two varieties belonging to the Real type (Pandela rosada and Utusaya), which is adapted to the extremely harsh climatic conditions of the southern altiplano of Bolivia, and a cultivar from the southern Andes of Peru (Amarilla de Maranganí) were the least affected varieties with respect to relative biomass production and height compared to the other varieties included in the study (Adolf et al., 2012) (Fig. 1). However, the varieties least affected in height and biomass

Please cite this article in press as: Adolf, V.I., et al., Salt tolerance mechanisms in quinoa (Chenopodium quinoa Willd.). Environ. Exp. Bot. (2012), http://dx.doi.org/10.1016/j.envexpbot.2012.07.004

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KVL37

Huariponcho

Sayaña

Huallata

3706

5206

Pasankalla rosada

KVL52

Kamiri

Chullpi roja

Misa quinua

Amarilla de Maranganí

Utusaya

Pandela rosada 0 -10 -20 -30 -40

% BM reduction -50

% Height reduction

-60 Fig. 1. Percent fresh weight biomass (BM) and height reduction in 14 quinoa varieties (based on data in Adolf et al., 2012).

production had also lowest height and biomass under control conditions. The reason for this is probably that these varieties are adapted to extreme conditions and not selected for high biomass production. An aim in quinoa breeding is the development of varieties adapted to diverse agro-climatic regions, and at the same time having high seed yield and quality components (Bertero et al., 2004). There is only scarce information available about seed yield and quality of quinoa grown under highly saline conditions. The Peruvian quinoa cultivar Hualhuas (Koyro and Eisa, 2008) and Titicaca, a cultivar bred in Denmark from material originating from southern Chile (Hariadi et al., 2011) could complete their life cycle and produce seeds even at NaCl levels of 500 mM (∼50 dS/m). However, yield, number of seeds, size of seeds and C/N-ratio in the seeds were decreased under high salinity. The lowered C/N ratio at high salinity levels (>300 mM) was mainly the result of an increase in protein content, accompanied by a decrease in total carbohydrates. Salinity induced several changes in the seeds harvested from salt treated plants, such as a reduction of the volume of the perisperm, the amplitude of the leaf lamina (cotyledons) and leaf surface, suggesting a reduced storage capacity for starch, proteins and lipids, respectively, and ion composition (see also Section 4.1). These changes did however not cause visible damage to the seed nor did they affect seed viability (Koyro and Eisa, 2008). In summary, the effect of salinity on growth morphology differs between ecotypes and cultivars. In general, quinoa’s productivity was seen to be increased at salinity levels between 10 and 20 dS/m. At higher levels, decreases in growth and yield were observed. However, even at 500 mM NaCl some varieties can complete their life cycle. Salinity influences seed structure, seed components and localisation of nutrient reserves in quinoa. The effect of salinity on seed quality is an interesting and important aspect to be considered for further research. 3. The influence of temperature on quinoa’s response to salinity Germination and plant growth are dependent on several environmental factors and their combinations. Temperature, for example, has a considerable effect on germination, which has also been shown in quinoa. Base temperature, the temperature below which germination rate was equal to zero, has been estimated to 3 ◦ C and optimum temperature to 30–35 ◦ C for quinoa germination (Jacobsen and Bach, 1998). Chilo et al. (2009) found that decreasing

temperature and increasing salinity reduced the rate and percentage of germination and subsequent seedling growth. This is in accordance with the findings of González and Prado (1992), who showed that the detrimental effect of salinity was generally less severe at higher temperatures. Furthermore, they found that the ability of salt-treated seeds to recover after transfer to pure water was also temperature dependent, with a higher recovery rate at 22 ◦ C than at 0 ◦ C. Rosa et al. (2009) demonstrated that at the cotyledon stage of quinoa seedlings, growth of cotyledons and shoot–root axis were affected by low temperature (5 ◦ C), compared to seedlings grown at 25/20 ◦ C, but there was no difference between salt-treated (salt acclimated and salt stressed) plants and low temperature control seedlings. Additionally, low temperature modified sucrose–starch partitioning in salt-stressed and salt-acclimated cotyledons, leading to decreased growth. Also here, no differences between low-temperature control and salt-treated seedlings were observed. It was argued that low temperature may cause a decrease in carbon flux from leaves, maybe due to a low sink demand and a restriction of phloem transport by cold itself (Rosa et al., 2009). Further investigations are needed to evaluate the influence of the combined effect of temperature and salinity on quinoa germination and also on later plant growth stages. This knowledge may allow the assessment of a variety’s capability to perform under a certain combination of these two environmental factors and may therefore be of particular value when considering the cultivation of quinoa in salt affected regions in high temperature climates. 4. Mechanisms involved in quinoa’s salt tolerance It has been suggested that quinoa may exert several unique as yet unknown mechanisms that allow it to grow and reproduce in highly saline soils and that it is therefore an interesting model crop for the identification of mechanisms involved in specific ion transport processes under salinity (Wilson et al., 2002). It has been claimed that halophytic plants are not really different from glycophytes regarding their physiology and anatomy (Flowers and Colmer, 2008; Shabala and Mackay, 2011), but that halophytes may make a more efficient use of common salt tolerance mechanisms (Bohnert et al., 1995). However, a typical feature of halophytes is the presence of salt glands or salt bladders, specialized epidermal cells that sequester and excrete excess salt from metabolically active tissues (Liphschitz and Waisel, 1982). So, what makes quinoa so remarkably salt tolerant? 4.1. Osmotic adjustment To maintain growth under salinity, plants need to adjust to the increased external osmolality, which can be achieved by accumulating a variety of molecules in the cytoplasm. Two major avenues are available for plants, which are the de novo synthesis of compatible solutes (organic osmolytes), and the accumulation of inorganic ions (Flowers, 2004; Shabala and Shabala, 2011). Osmolyte biosynthesis is associated with energetic costs resulting in yield penalties (Shabala and Shabala, 2011). Therefore, most halophytic plants maintain turgor by sequestering Na+ and Cl− (free osmolytes) in shoot cell vacuoles, and use compatible solutes only for osmotic adjustment in the cytosol (Flowers and Colmer, 2008; Shabala and Mackay, 2011). In quinoa plants it was shown that up to 95% of osmotic adjustment in old leaves and 80–85% of osmotic adjustment in young leaves was achieved by means of accumulation of inorganic ions (Na+ , K+ and Cl− ) in plants treated with NaCl concentrations within the range of 0–500 mM (∼0–50 dS/m) (Hariadi et al., 2011). Whilst the energetic benefit of this strategy is obvious, a

Please cite this article in press as: Adolf, V.I., et al., Salt tolerance mechanisms in quinoa (Chenopodium quinoa Willd.). Environ. Exp. Bot. (2012), http://dx.doi.org/10.1016/j.envexpbot.2012.07.004

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high concentration of accumulated Na+ in the cytosol is equally detrimental for both glycophytes and halophytes, and, hence, must be avoided (Flowers and Colmer, 2008). This implies an efficient Na+ sequestration in leaf vacuoles. Young leaves, which have a low degree of vacuolation (Kim, 2006) had consistently higher K+ and lower Na+ concentrations, as compared with old leaves, at different salinity levels (Hariadi et al., 2011). An increase in the vacuolar Na+ content, required for the maintenance of the cell turgor, must be accompanied by a concurrent increase in cytosolic osmolality. This is achieved by either increasing cytosolic K+ , or by accumulating organic osmolytes, compatible solutes, in this compartment. Indeed, increases in soluble sugars, proline and glycinebetain have been reported in quinoa (Jacobsen et al., 2007; Ruffino et al., 2010). Osmotic adjustment plays also a pivotal role at germination and seedling establishment. In the face of a reduced osmotic potential in the surrounding soil, an adequate water content must be maintained in the seed. Koyro and Eisa (2008) suggested that a reduced matric potential in the seed interior may counteract desiccation. Based on their studies on seeds descendent from salt treated plants they assumed that increased protein levels in pre-adjusted seeds lead to a lower matric potential. Furthermore, they showed that salinity caused an acceleration of germination in pre-adjusted seeds, which they argued to be the result of an accumulation of Na− and Cl− in the seed pericarp and a synthesis of organic solutes in the seed interior, which lowers the water potential in the walls of the plant ovary and thereby facilitates water uptake. 4.2. Osmoprotection Both osmotically-induced stomatal closure and accumulation of toxic levels of Na+ in the cell’s cytosol under saline conditions reduce a plant’s capacity to fully utilise light absorbed by the photosynthetic pigments and leads to the formation of various reactive oxygen species (ROS) (Shabala et al., 1998; Sudhir and Murthy, 2004; Tavakkoli et al., 2010). These ROS should be scavenged to avoid detrimental effects such as lipid peroxidation, DNA damage, protein denaturation, carbohydrate oxidation, pigment breakdown, and impairment of enzymatic activity (Noctor and Foyer, 1998). Two major antioxidant systems, enzymatic and non-enzymatic, are known to operate in plants (Scandalios, 1993). Among non-enzymatic antioxidants, ROS scavenging ability was reported for compatible solutes such as mannitol, myo-inositol and proline (Smirnoff and Cumbes, 1989; Bohnert et al., 1995; Noctor and Foyer, 1998; Szabados and Savouré, 2010) all of which are present in quinoa tissues (Aguilar et al., 2003; Ruffino et al., 2010). Ruiz-Carrasco et al. (2011) showed that 300 mM NaCl (∼30 dS/m) induced an accumulation of proline, which in the least salt tolerant genotypes was moderately increased, and 3–5-fold enhanced in the most salt tolerant genotypes. Furthermore, the ratio of the polyamines (spermidine + spermine)/putrescine was higher in the more tolerant genotypes under salinity. It was discussed that spermidine and spermine may play a more pivotal role under stress, which would explain the maintenance of higher levels of these compounds compared to the decline in putrescine. The extent of UV-B irradiation (medium wave 315–280 nm – ultraviolet radiation) damage to the photosynthetic machinery was much less in young leaves when compared with old leaves, which was attributed to the difference in the size of the organic osmolyte pool (1.5-fold difference under control conditions; 6-fold difference in plants grown at 400 mM NaCl (Shabala et al., 2012). Interestingly, exogenous application of glycinebetaine, previously shown to be non-effective in ROS scavenging in vitro (Smirnoff and Cumbes, 1989), substantially reduced detrimental effects of oxidative stress (UV light) onto quinoa leaf photochemistry (Shabala et al., 2012). This effect was attributed to the chemical chaperon action protecting PSII against oxidative stress (Halliwell and Gutteridge, 1990). In

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addition, proteins such as dehydrins have an effect in plant stress tolerance, for instance in salinity tolerance, where they have an osmoprotective function (Svensson et al., 2002; Saavedra et al., 2006; Brini et al., 2007). Dehydrins in the root, stems and leaves have been reported as an adaptation to water deficit produced by salinity (Rorat, 2006). In quinoa, at least four dehydrins were detected in seeds harvested from plants grown under salinity. One of them, a 30 kDa dehydrin was shown to accumulate in seeds of plants grown under high salt stress. Due to its nucleus-localisation and chromatin association in embryo tissue, it was considered as a protective agent for DNA in cells that are exposed to dehydration (Burrieza et al., 2011). It was further suggested that this ability of seeds to adjust to salt stress could be a source for the artificial selection and establishment of new cultivars with better stress tolerance. The balance between the accumulation of organic and inorganic osmolytes in quinoa may reflect not only the energetic cost of osmotic adjustment but also the protective role of organic osmolytes. These requirements may vary remarkably with plant age and the physiological characteristics of a specific tissue. The role of an enzymatic protection of quinoa metabolism under salinity is still unknown and worthwhile to become focus of future studies. 4.3. Sodium exclusion and xylem loading Na+ exclusion has always been considered as a highly desirable trait in glycophytes (Munns and Tester, 2008). In Arabidopsis, this exclusion is mediated by a Na+ /H+ exchanger located at the plasma membrane of epidermal root cells (Blumwald et al., 2000) and encoded by the salt overly sensitive (SOS1) gene (Hasegawa et al., 2000; Shi et al., 2000; Qiu et al., 2002). Functional SOS1 analogues have also been reported for a range of other species both in molecular (Mullan et al., 2007; Martínez-Atíenza et al., 2007) and physiological (Cuin et al., 2011) experiments. An accumulation of Na+ in the cytosol has been found to be anticipated by the tonoplast Na+ /H+ exchanger NHX, which translocates Na+ into the vacuole (Apse et al., 1999; Apse and Blumwald, 2007; Shabala and Mackay, 2011). Maughan et al. (2009) have cloned and characterized two SOS1 gene homologs (cqSOS1A and cqSOS1B) in quinoa and found a high level of similarity of these gene sequences to SOS1 homologs of other species. The expression of cqSOS1 upon application of NaCl was investigated in a cultivar originating from the salares of the Bolivian Altiplano. Gene expression analyses of cqSOS1A and cqSOS1B showed a 3–4 times stronger expression in root compared to leaf tissue in the absence of salinity. However, saline treatment (450 mM NaCl, ∼45 dS/m) caused an up-regulation of both genes in leaf, but not in root tissue (Maughan et al., 2009). With gene expression analyses of cqSOS1 and cqNHX1, Ruiz-Carrasco et al. (2011) confirmed the different responses of sodium antiporters to NaCl in shoots and roots. Furthermore, the genes were differentially regulated in different genotypes. The high homology of quinoa SOS1 sequences to other species and the differential expression of SOS1 and NHX1 under saline conditions suggest that also in quinoa cytosolic Na+ concentration and ion-homeostasis are regulated by a well coordinated activity of SOS1 and NHX Na+ /H+ exchangers. However, it was recently shown in Arabidopsis that NHX1 and NHX2 proteins played a comparatively greater role in K+ homeostasis than in Na+ compartmentation, and it has been concluded that in Arabidopsis ion transporters other than NHX1 and NHX2 regulate the translocation of Na+ into the vacuole (Bassil et al., 2011; Barragán et al., 2012). The question of how Na+ is sequestered into the vacuole remains to be studied in other plant species. Due to quinoa’s extraordinary salt tolerance it is an interesting candidate plant for detailed studies of this topic.

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The absence of an upregulation of SOS1 in roots under salinity questions whether Na+ exclusion is an important trait for salinity tolerance in quinoa and supports the idea that rapid Na+ loading into the xylem during the initial stages of salinity stress is beneficial for shoot osmotic adjustment (Shabala et al., 2010). Recent studies on the relatively salt tolerant crop barley showed that transgenic plants over-expressing HvHKT2;1 transporters had higher Na+ concentrations in the xylem, enhanced translocation of Na+ to the leaves, and superior salt tolerance compared with the wild type. It was suggested that one of the factors that limit salt tolerance in barley is the capacity to translocate Na+ to the shoot rather than accumulating and compartmentalizing it in leaf tissue (Mian et al., 2011). This provides another evidence for a secondary role of Na+ exclusion. SOS1 transporters are not only involved in Na+ extrusion from the roots, but also regulate Na+ loading into the xylem and Na+ retrieval from the xylem and thereby influence long-distance Na+ transport (Shi et al., 2002, 2003). This is also consistent with the arguments by Shabala and Mackay (2011) who suggest that the xylem Na+ loading in halophytic plants is an active process and requires an enhanced operation of SOS1 Na+ /H+ antiporters at the xylem parenchyma interface. Pharmacological experiments indicated a possible involvement of the high-affinity K+ transport protein HKT in sodium uptake in Suaeda maritima, a member of the Chenopodiaceae family (Wang et al., 2007). Furthermore, experiments on glycophytes have suggested that non-selective cation channels (NSCCs) play a dominant role in plant Na+ uptake through the plasma membrane (Tyerman and Skerrett, 1999; Demidchik et al., 2002). Wang et al. (2007) could however not confirm these findings in their studies on Suaeda. It remains to be studied which of the above transport systems mediate Na+ uptake in quinoa. 4.4. Potassium retention Cellular potassium retention and cytosolic potassium homeostasis are essential for salinity tolerance (Shabala and Cuin, 2007), however, massive K+ leak from the cytosol often occurs in both root and leaf tissues under saline conditions (Shabala, 2000; Shabala et al., 2005, 2006). The resulting depletion in the cytosolic K+ pool may activate enzymes involved in protein catabolism, and trigger programmed cell death in plant tissues (Shabala, 2009; Demidchik

et al., 2010). In this context, the remarkable salinity tolerance in quinoa may be attributed to its highly efficient K+ retention. It was reported that quinoa maintains relatively high K+ concentrations in the cotyledons during the seedling stage (Ruffino et al., 2010), and even increased K+ concentrations in xylem and leaf sap were observed under salinity in quinoa at a later developmental stage (Adolf et al., 2012). Na+ is used as a cheap osmoticum, so in order to maintain an acceptable K+ /Na+ ratio in leaves, the increased Na+ uptake needs to be accompanied by an increased transport of K+ to the shoot (Cuin et al., 2009). Studies in other species have shown that K+ loading into the xylem is a passive process, which is mediated by either NORC or SKOR outwardly-rectifying channels at the xylem parenchyma (Wegner and de Boer, 1997; Shabala et al., 2010). It remains to be shown if the same transporters mediate this process in quinoa. Also important is K+ retention in roots. Pharmacological analysis of NaCl-induced K+ fluxes from quinoa roots revealed that the latter could be suppressed by TEA+ , a known blocker of potassium-selective voltage-gated channels (Shabala, unpublished data). Thus, it is plausible to suggest that potassium retention in quinoa roots is achieved by preventing K+ efflux through depolarisation-activated K+ channels (KOR). Among other things, KOR permeability is regulated by the plasma membrane H+ ATPase activity, and the observed dose-dependent increase in net H+ extrusion from quinoa roots under saline conditions (Hariadi et al., 2011) is in full agreement with this suggestion. Furthermore, intracellular cytosolic K+ homeostasis was shown to be dependent on a well coordinated function of import and export of K+ at the plasmamembrane and tonoplast (Leigh, 2001; Barragán et al., 2012). K+ compartmentation in the vacuole was found to be mediated by tonoplast-localized NHX proteins in Arabidosis, which were also shown to be involved in cell turgor regulation, amongst others in stomatal guard cells, thereby regulating stomatal function and in turn water relations (Barragán et al., 2012). There is only scarce information on ion uptake and transport in quinoa. However, due to the sequence homology of some quinoa transporters to other species, and their differential expression under salinity, it is assumed that ion transport in quinoa is not operated by unique quinoa-specific transporters, but through a well-coordinated regulation of transporters that are commonly present in plants. Fig. 2 gives an overview on cation transporters known to be involved in ion homeostasis in plants, and marks the

Fig. 2. Summary diagram depicting key transporters and channels mediating Na+ and K+ homeostasis in quinoa and other plant roots.

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state of knowledge in quinoa. The functional characterisation of such cation channels and transporters in quinoa is a challenge for future research. Similar to other species, a significant part of Na+ uptake in quinoa roots is mediated by non-selective cation channels (NSCC) (Shabala, unpublished data). Also the dual affinity HKT symporter has been shown to be involved in Na+ uptake in other halophytes (Wang et al., 2007), and may have a similar function in quinoa. Low Na+ concentration in the cytosol and a balanced Na+ uptake in root and shoot tissue is maintained by an orchestrated activity of SOS1 at the plasma membrane and Na+ /H+ exchanger NHX at the tonoplast. Thermodynamically active Na+ xylem loading is required in most cases (Shabala and Mackay, 2011), implicating involvement of SOS1 located at xylem parenchyma boundary interface. NORC (nonselective outward rectifying cation channel) involvement becomes essential for cases when Na+ concentration gradients at both sides of parenchyma cell plasma membrane favours passive Na+ loading. Voltage-gated outward-rectifying (KOR) K+ channels mediate K+ efflux from quinoa roots. These are under the strict control by the plasma membrane H+ -ATPase, maintaining membrane negativity under saline conditions. To achieve rapid osmotic adjustment in the shoot, both Na+ and K+ are rapidly loaded in the xylem upon onset of salinity, with a statistically significant increase in shoot Na+ measured as soon as 6 h upon NaCl treatment (Mackay and Shabala, unpublished data). 4.5. Gas exchange, stomatal control and water use efficiency A reduction in photosynthetic activity under saline conditions is usually caused by a decreased stomatal conductance, which reduces transpiration rate but also CO2 uptake (Iyengar and Reddy, 1996). Gas exchange and transpiration have been shown to decrease in quinoa under salinity (Bosque Sanchez et al., 2003). Analysing the response of plants to different salinity levels and the combined effect of salinity and drought stress, Razzaghi et al. (2011a) found that an increase in salinity levels decreased the soil water potential and consequently leaf water potential and stomatal conductance in both fully irrigated plants and plants under progressive drought treatment. Stomatal conductance was significantly affected when leaf water potential dropped below a threshold level of −1.75 MPa. They calculated the critical values of relative available soil water as a threshold for a severe decrease of leaf water potential and thus plants damage. Furthermore, Razzaghi et al. (2011a) observed an increase in shoot and root abscisic acid (ABA) at decreasing soil water content, where ABA was already produced at mild stress conditions, suggesting that it was produced as a signal to regulate stomatal conductance. This finding confirms the results of Jacobsen et al. (2009), who found that hydraulic signalling does play a role as an early signal of drought. Proportional increases in ABA under salinity, and decreases in leaf and soil water potential, have been interpreted as a result of osmotic stress caused by water deficit rather than a specific response to ion-toxicity (Zhang et al., 2006). The comparison of stomatal conductance and photosynthetic CO2 assimilation in the two contrasting varieties Utusaya and Titicaca under salinity showed Utusaya to be less affected. Utusaya originates from the salar region of Bolivia and is expected to be highly tolerant to saline conditions, and Titicaca is a variety selected in Denmark from material originated in southern Chile. Utusaya maintained a relatively high stomatal conductance, with only 25% reduction in net CO2 assimilation compared to a 67% reduction in salt-treated Titicaca plants (Adolf et al., 2012). It may be assumed that Utusaya has mechanisms for an effective osmoregulation under saline conditions and that this variety has a relatively minor need to reduce water loss by transpiration. However, the stomatal conductance and therefore photosynthesis rate in this

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Fig. 3. Micrograph of the salt bladders on the abaxial and adaxial surface of a young leaf of quinoa.

variety were generally low under non-saline conditions, a classical trade-off between tolerance and productivity. This should be taken into consideration when selecting varieties for cultivation under different conditions and for breeding. Maximum photochemical efficiency of PSII, measured as Fv/Fm (ratio between variable and maximum chlorophyll fluorescence (Maxwell and Johnson, 2000)) was not affected by salinity, which suggests that PSII is not the main target of salinity stress (Hariadi et al., 2011; Adolf et al., 2012). Our recent studies have highlighted an interesting and previously not explored mechanism of controlling transpiration and water use efficiency (WUE) in quinoa by reducing stomatal density under saline conditions (Shabala et al., 2012). The observed salinityinduced reduction in stomatal density in quinoa may represent a fundamental mechanism by which quinoa plants may optimise WUE under saline conditions. Similar observations were confirmed in an independent study (Orsini et al., 2011). As shown by Boyer et al. (1997) a substantial amount of water (up to 28%) evaporating from the leaf surface may bypass stomata and be released through the cuticle. This cuticular transpiration is usually concentrated in the area surrounding stomata, where there are more and larger cuticular pores (Marchner, 1995). Whilst stomatal conductance is a dynamic process that can be rapidly regulated by ion fluxes in and out of guard cells (Blatt, 2000), cuticular transpiration relies almost exclusively on a passive, hydraulic permeability of the leaf surface and, hence, cannot rapidly adjust to changing conditions. Thus, even at much more modest rates of cuticular transpiration, reducing the density of stomata, and the more numerous cuticular pores associated with them, would be of benefit to osmotically stressed plants (Shabala et al., 2012). Water loss by transpiration is minimized by the production of ABA as an early signal for stomatal closure under mild salt stress, and by lowering cuticular and stomatal transpiration through the reduction of stomatal density. At high soil salinity, the osmotic effect caused by the salt ions needs to be compensated by irrigating at higher available soil water (Razzaghi et al., 2011a). 4.6. Salt bladders One of the distinct anatomical features of quinoa is the presence of salt bladders on both abaxial and adaxial leaf surfaces (Fig. 3). Epidermal Bladder Cells (EBC) are thought to be storage sites for excess Na+ , Cl− and K+ (Adams et al., 1998; Agarie et al., 2007). However, excretion through sequestration into EBCs was relatively modest (<20%) in quinoa compared with other halophytes (Orsini et al., 2011). EBC may also be used for the storage of water and

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Fig. 4. Summary of the knowledge on salt tolerance features in quinoa.

various metabolic compounds such as malate, flavonoids, cysteine, pinitol, inositol and calcium oxalate crystals (Adams et al., 1992; Agarie et al., 2007; Jou et al., 2007). Salt bladders may play an important role as a secondary epidermis to reduce water loss and prevent excessive UV-damage. EBC density was highest in young leaves, which was interpreted as a protective factor against UV-damage to leaf PSII (Shabala et al., 2012). However, the physical removal of EBC by gentle brushing did not result in significant changes in maximal photochemical efficiency (measured as chlorophyll fluorescence Fv/Fm ratio) in UV-exposed leaves (Shabala et al., 2012). Thus, the protective role of EBC is rather attributed to the accumulation of organic osmolytes with ROS-scavenging or chaperon ability than to the physical shielding of the leaf lamina against light. It should be noted that some highly salt tolerant genotypes had distinctly coloured (reddish) salt bladders on actively growing shoots. It can be speculated that this colouration is caused by an accumulation of betalaine pigments, as reported for Mesembryanthemum crystallinum (Adams et al., 1998). Given the high cost and a significant amount of time required to produce these pigments on demand, having them stored in EBC and available on request (Gershenzon, 1994), may give some genotypes a competitive advantage under stress conditions. As previously discussed in this review, quinoa shows an outstanding salt tolerance at all growth stages. Quinoa shares many common features of salt tolerance mechanisms with other plant species, especially halophytic species. However, quinoa seems to possess an efficient combination of morphological and metabolic characteristics and probably some unique as yet unidentified

features. Fig. 4 gives an overview on the present knowledge of salt tolerance in quinoa.

5. Breeding for stress tolerance 5.1. The importance of cross-tolerance Plant’s responses to salinity depend on salt type, level of salinity, plant species, genotype and growth stage (Läuchli and Grattan, 2007; Koyro and Eisa, 2008). Furthermore, when breeding for stress tolerance in plants, not only one stress factor is of importance, but a complex of environmental conditions including a number of cooccuring stresses need to be taken into consideration (Trognitz, 2003; Jacobsen et al., 2003). Different stress-signalling pathways cross-talk with each other, which leads to cross-tolerance meaning that a plant that is tolerant to one stress can also tolerate other stresses (Tuteja, 2007). This is of great importance when considering that, for example, in many areas heat, drought and salinity occur simultaneously. Cross-talk in signalling pathways makes the response of plants to different co-occurring stresses more efficient. Therefore, thorough investigations need to be carried out under different conditions and growth stages in order to be able to select species and cultivars of new food crops, such as quinoa, for cultivation in regions affected by a certain combination of abiotic stresses. The presence of five categories of quinoa, adapted to valley, altiplano, salt desert, sea level and tropics, as defined by Tapia (1997) is a valuable basis for the selection of material

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and breeding for different environmental and geographical conditions. 5.2. Salt tolerance is a complex trait Salinity tolerance is a complex multigenic trait showing heterosis, dominance and additive effects (Flowers, 2004). Salt tolerance is also multifaceted physiologically and is determined by a number of sub-traits specific for each tissue (Shabala and Cuin, 2007). To add to the complexity, fewer than 25% of the salt-regulated genes are salt stress-specific (Ma et al., 2006). Nonetheless, salinity tolerance in crops can be improved by pyramiding key physiological traits conferring the most essential characteristics (Shabala and Mackay, 2011). In quinoa key traits appear to be an efficient control of xylem Na+ loading and sequestration in leaf vacuoles, higher ROS tolerance, better K+ retention, and an efficient control over stomatal development and aperture. Quinoa may be used as a valuable donor of salt tolerance genes to other crops. The development of a genetic linkage map of quinoa was a first step towards the genetic characterisation and marker assisted selection of favourable agronomic traits (Maughan et al., 2004) and thus provides a basis for fine mapping of QTLs (quantitative trait loci) for salt tolerance in quinoa. The large genetic variability in salinity tolerance in quinoa is a great source for the selection and breeding for higher tolerance (Gómez-Pando et al., 2010; Ruiz-Carrasco et al., 2011; Adolf et al., 2012). The presence of cultivars being adapted to different altitudes and a wide range of climatic conditions allows selection, adaptation and breeding of cultivars for the use under different environmental constraints (Christiansen et al., 2010). This provides also the possibility of creating a double haploid population required for marker assisted selection. In quinoa, material should be selected among different ecotypes, local selections and genotypes for breeding of new cultivars. 6. The potential of quinoa in a salt-affected world Considering the problem of shrinking fresh water resources and increasing losses of arable land due to soil salinisation, new options and solutions need to be elaborated in order to sustain food supply for a rapidly growing world population. Research focuses more and more on the identification and selection of plant species that are naturally tolerant to drought and salinity. One of the major aims is to evaluate the potential of halophytic species for a broad and economic use in arid and semi-arid regions and to study their applicability for human and animal nutrition (Koyro et al., 2008). Quinoa originates from the Andes of South America, where it is grown in a large range of climatic conditions, which implies a high biodiversity within the species (Jacobsen and Mujica, 2002; Maughan et al., 2004). In the Andean highlands, agricultural production takes place under very unfavourable conditions, including drought, soil salinity, frost, wind and hail. Quinoa is adapted to these extreme conditions and represents an important crop for the Andean farmers (Jacobsen et al., 2003). The high tolerance to several abiotic stresses together with the large genetic variability, suggest a potential for use under different agro-climatic conditions in other parts of the world. Maas and Hoffmann (1977) evaluated the relative salt tolerance of a number of agricultural crops based on two parameters: (a) the maximal allowable salinity level without yield reduction, (b) the percent yield decrease per unit salinity increase beyond this threshold. They showed soil salinity levels as electrical conductivity (ECe, dS/m) and gave a qualitative tolerance rating of crops being sensitive (S) (EC: 0–8 dS/m), moderately sensitive (MS) (EC: 8–16 dS/m), moderately tolerant (MT) (EC: 16–24 dS/m), or tolerant

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(T) (EC: 24–32 dS/m). They also defined the salinity level unsuitable for crop production. Qadir et al. (2007) summarized the yield potentials of different crops as a function of average root zone salinity calculated from data reported by Maas and Grattan (1999). They showed the EC50 values (the soil EC causing 50% yield reduction) for several crops. The values fit well with the classification by Maas and Hoffmann (1977). Among the tolerant crops were triticale (Triticale hexaploide Lart.) and barley (Hordeum vulgare); wheat (Triticum aestivum) and sorghum (Sorghum bicolour) were classified as medium tolerant, whereas rice (Oryza sativa L.), potato (Solanum tuberosum L.) and maize (Zea mays L.) belong to the category medium sensitive. Common bean (Phaseolus vulgaris), carrot (Daucus carota) and several berry fruits were classified as salt sensitive by Maas and Hoffmann (1977). Razzaghi et al. (2011b) determined the EC50 for quinoa to 24 dS/m and confirmed hereby the classification of quinoa as a highly salt tolerant crop. These data and classifications are only a basis or guideline to relative tolerances among crops (Maas and Grattan, 1999). Absolute salt tolerance of a crop depends on a number of environmental conditions, including soil type and condition, climate and agricultural practices. In addition, variety-specific differences should not be underestimated. Furthermore, the slope of yield decrease beyond the threshold line is not the same for all crops, which gives an even greater variation in salinity tolerance ratings. Nevertheless, we may emphasize that quinoa can be characterized as an extremely salt tolerant crop. The increase in biomass production at moderate salinity in quinoa is another aspect that should be considered, as quinoa prefers moderate salinity levels where it grows even better than under non-saline conditions There is increasing interest in the possibility of using so-called “cash crop halophytes” to be irrigated with saline water of up to seawater salinity (Lieth, 1999; Koyro et al., 2008). Moreover, phytoremediation of saline soils may be important in the future. The preconditions for an applicable candidate for soil desalination are a high salt tolerance, a high biomass production, a considerable capacity for salt uptake, shoot sodium storage, and a high degree of economic utilisation (such as fodder, fuel, fibre, essential oil, and oil seeds) (Jacobsen et al., 2012). There might be a potential of quinoa for such purposes.

7. Quinoa cultivation and consumption in South America and other parts of the world 7.1. Small and large scale experience in quinoa cultivation In the Andes of South America, quinoa was produced for home consumption for thousands of years. During the last 30 years, there has been an increasing interest in the crop worldwide. Peru and Bolivia are the largest producers of quinoa. Total area and production of quinoa in Bolivia have increased from 10 000 ha and 5000 t twenty years ago to presently 50 000 ha and a production of 25 000 t. Export of quinoa has increased since 2001, and 90% of the total quinoa produced in Bolivia is now exported (Jacobsen, 2011). Yields in South America are less than 500 kg/ha in Bolivia, and 1000 kg/ha in Peru (Jacobsen, 2011). Potential yield is much higher, under experimental conditions, for instance in Bolivia 3 t/ha. In the United States, quinoa is grown to a small extent in the Colorado Rockies. Quinoa has been tested in a number of countries around the world and quite large differences were observed with respect to growth period and yield. The ability of different cultivars to mature was seen to depend on a mixture of environmental conditions (Jacobsen, 2003; Jacobsen et al., 2010). Successful trials have been made in Africa, Asia and Europe, and commercial production of quinoa outside the Andes is about to start, but still on a low scale.

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7.2. Quinoa consumption in South America In South America, quinoa is consumed as whole seed and in soups. The seeds are also ground to flour. The leaves of the quinoa plant can be eaten as a cooked green vegetable and quinoa may be fermented to produce beer. Quinoa consumption in South America is approximately 2 kg/person per year in Bolivia, which is low because of the large amount dedicated for export. In Peru the consumption is around 20 kg/person per year (Jacobsen, 2011).

7.3. Quinoa in the Western market and the perspective of quinoa products The interest in quinoa is increasing in the Western markets. One reason for the increasing interest in quinoa owes to its extraordinary nutritional characteristics. Its seeds are considered as a nutritionally complete food with a high protein content (12–18%) and an outstanding composition of essential amino acids. The seeds are rich in vitamins (A, B2 , E) and several minerals including Ca2+ , Mg2+ , Fe3+ , Cu2+ and Zn2+ . Moreover, they provide a valuable source of carbohydrates and essential fatty acids (Repo-Carrasco et al., 2003; Stikic et al., 2012; Koziol, 1992). The high nutritional quality makes it not only beneficial for vegetarians and vegans, but also for health-conscious people. Furthermore, quinoa is gluten free, which makes it favourable for people suffering from celiac disease, an autoimmune disease of the small bowel triggered by exposure to the protein gluten, which is present in wheat and other cereal species within the Triticaceae (Di Sabatino and Corazza, 2009). Important international markets include Europe, United States and Japan. Imports of quinoa by the European Union have increased rapidly during recent years. Particularly in France, The Netherlands and Germany, and more recently in UK and Scandinavia quinoa consumption has increased. Bolivia provides 94% of the EU quinoa import (CBI Market Information Database). On the Western markets, quinoa is sold as seeds, which are easy to cook and a tasty addition to many dishes. Furthermore, flour, pasta, muesli flakes, biscuits, energy bars and even beer are available. Moreover, quinoa has also been noticed by the NASA as a potential nutritious crop for long-term human space missions (Schlick and Bubenheim, 1993). The naturally high tolerance to abiotic stresses together with the excellent nutritional quality of its seeds are the reasons why the FAO has nominated quinoa as one of the crops that might contribute to global food security during the next century (FAO, 1998).

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Please cite this article in press as: Adolf, V.I., et al., Salt tolerance mechanisms in quinoa (Chenopodium quinoa Willd.). Environ. Exp. Bot. (2012), http://dx.doi.org/10.1016/j.envexpbot.2012.07.004