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1 Introduction

The area of saline soils on the desiccated seafloor of the Aral Sea comprises about 42,000 km2 (which is about three quarters of the dry seafloor, Chap. 2, Fig. 2.7). Within the agricultural areas with irrigated lands, a major proportion is secondarily saline; this amounts to about 22,000 km2. In total, this means that the salt desert areas in Middle Asia have increased by more than 60,000 km2 within the last 50 years. Salt desertification is spreading within the whole area, not only in the Aralkum. But it is a very old problem of mankind (Jacobsen and Adams 1958). All arid countries face the salinity problem (Waisel 1972, 2001; Hammer 1986; Oldeman 1994; Breckle 1982, 1989, 2002a, b; Wichelns 1999), e.g. in Australia (Dregne 1986), California (Sheridan 1981; Law and Hornsby 1982; Rhoades 1990), India (Singh 2009), China (Yang et al. 2005) and Iran (Shiati 1991). However, the Aral Sea basin is one of the most striking examples of salt desertification (Geldyeva et al. 1998; Novikova et al. 1998). The forecast that the eastern basin of the Aral Sea will have disappeared by 2010 (Breckle and Agachanjanz 1994; Agachanjanz and Breckle 1994) and huge solonchak areas will spread out was totally right, as can be seen now. A huge salt swamp has been observed already in 2009 (chap. 2).

The coast of the Aral Sea and the dry seafloor of the former Aral Sea are an excellent model where the processes of salt desertification can be seen (Glazovskii and Orlovskii 1996; Breckle and Wucherer 2007). In general, soil salinity assessments are essential for mapping land degradation in drylands as well as for agricultural surveys, and remote sensing is a helpful tool (Metternicht and Zinck 2008).

The strategies of plants for regulating salt content and for coping with salt stress are a precondition for survival, whether they are halophytes or nonhalophytes. The adaptation of plants to NaCl has to cope with the general osmotic effects of the ions, but also with the specific ionic effects of Na+ and Cl– on the metabolic processes (Fig. 12.1). Halophytes have evolved during long-term evolution by selection of tolerant ecotypes in several plant taxa. In Central Asia, there is a biodiversity centre of halophytes (Wucherer et al. 2001). In arid sites with a continental climate, various types of salinity are known (chloride, sulphate, carbonate, magnesium and boron), more variable than along ocean coasts, depending on soil properties, climatic conditions and ecosystem processes. The presence of excessive ions in such ecosystems dominates over many other environmental factors. Only the supply of water is the other decisive factor in ecosystem development.

Fig. 12.1
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The interrelations of NaCl effects on various complexity levels in plants (modified from Breckle 2005)

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The invasion of the desiccated seafloor by halophytic species occurs under climatic conditions (chap. 4) which are rather variable from year to year (Breckle et al. 2001). The halophytic species, nevertheless, are on the other hand indicators of the degree of salinity at their site, and thus can be used to monitor salinity. A novel list of indicator values for salinity is presented (see below, Table 12.9). This can be used also for the necessary means of phytomelioration (Chaps. 15).–17

1.1 Halophyte Groups

Middle Asia and Central Asia are the evolutionary centres of many genera and species of the Chenopodiaceae. The Chenopodiaceae are characterized by their ability to accumulate inorganic ions, mainly sodium (Na+). Only a few other angiosperm families are similarly able to withstand high soil salinities, e.g. Zygophyllaceae, Frankeniaceae, Tamaricaceae, Plumbaginaceae and a few grasses. However, there are many more genera in various angiosperm families which have evolved some degree of salt tolerance. In drylands, salinity has been such an important ecological factor that mechanisms of salt tolerance have evolved several times.

Plants have developed various mechanisms to cope with salinity. Table 12.1 gives an overview of some of the strategies which can be found in halophytes and which are sometimes even combined. Often morphological structures are typical for distinct adaptation strategies. Especially halosucculence of stems or leaves, or both, is very common in halophytes strongly adapted to salinity. Thus, succulent halophytes are either leafless and stem-succulent or have fleshy and succulent leaves. In both cases this kind of succulence has two components: the basic one is a genetically controlled succulence, whereas the second is a modifying variable and can be induced by salts to a considerable degree. These types of halosucculence have to be distinguished from xerosucculence.

Table 12.1 Control mechanisms of halophytes to strive on saline sites (Breckle 1990, 2002a) and the main morphological strategy type
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There are leaf-succulent euhalophytes which are annuals, e.g. some Suaeda species, Halopeplis, Halimocnemis, Gamanthus, Girgensohnia, etc. Other leaf succulents are herbal perennials (e.g. Plantago, Aster, Suaeda), and others are shrubs (e.g., some members of the genera Salsola, Suaeda, Nitraria and Kochia). In some others, the succulence of the fruit or parts of the fruit became very pronounced (Gamanthus). Regarding the adaptations of the photosynthetic pathway which have evolved, it is obvious that succulence has altered the anatomical structure dramatically, as can be seen in the various types that are exhibited by Salsola and Suaeda (Shomer-Ilan et al. 1981).

The stem-succulent euhalophyte lack leaves or have only minor scalelike leaves. The young stems are succulent, the older ones in perennial species can become rather woody. Salicornia and some species of Anabasis, for example, are annual stem-succulent species. Perennial stem-succulent halophytes are also found in Anabasis, Kalidium, Aellenia, Ofaiston, Halostachys, Haloxylon, etc. and also in the woody subshrub Halocnemum strobilaceum (Fig. 12.2), which is one of the most salt-tolerant species.

Fig. 12.2
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Halocnemum strobilaceum, young shoots (photo: Breckle, May 2004)

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In contrast to halosucculents, most xerosucculents in general are very sensitive to salinity.

Many halophytes exhibit a rather rapid turnover of their leaves. The rosette leaves in Limonium vulgare are replaced during the vegetation period two or three times, and the leaves of Aster tripolium rather soon become yellow and new leaves replace them. This replacement is a mechanism of removal of large quantities of salt. Old leaves with high salt content are steadily replaced by younger leaves in many Juncus species. This is certainly one adaptation mechanism that enables the plant to get rid of excessive salts by shedding plant organs. A less specific adaptation is the rapid production of new leaves and dropping old leaves rich in salt. This can be observed in many pseudohalophytes. But the loss of leaves affects the supply of assimilates or hormones to the growing organs and thereby affects growth (Munns 1993; Munns et al. 1995).

But even more important in some halophytes is the existence of specific cell structures which can recrete (recretion in the sense of Frey-Wissling 1935, meaning elimination of substances not metabolically changed) inorganic ions, especially Na+. This is done by salt glands, which have evolved several times in the angiosperms, and by bladder hairs. Salt glands eliminate salt to the outside (e.g., Tamarix, Frankenia – see Fig. 12.3 – Glaux and Limonium as well as some grasses); Bladder hairs accumulate salts in their huge vacuole (Atriplex, see Fig. 12.4; to a less extent Halimione, Salsola, Chenopodium) (Black 1954; Berger-Landefeldt 1959; Schirmer and Breckle 1982; Breckle 1992). In both cases the salts are physiologically isolated from active tissues. Here also the turnover of salt is rather rapid by recreting salt with salt glands in the exocrinohalophytes or into big bladders in the endocrinohalophytes.

Fig. 12.3
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Frankenia hirsuta, in flower with many dry recreted salt crystals (photo: Breckle, May 2004)

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Fig. 12.4
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Atriplex pratovii. (a) Intact bladders from the lower side of leaves. (b) Crushed bladders from the upper side of leaves after wilting, forming a layer of salt crystals. North Aral Sea (photo: Breckle, a – May 2003; b – May 2004)

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Nonhalophytes exhibit almost none of these morphological adaptations. The dominant processes in the various morphological halophyte types are indicated in Table 12.1.

In general, it should be kept in mind that salt tolerance of a plant is not defined by the act of individual genes, by the individual regulation of each of them or by one specific metabolic process. Salt tolerance is a whole plant response (Hedenström and Breckle 1974; Breckle 1990, 1995; Munns 1993; Naik and Widholm 1993; Flowers and Yeo 1995; Ramani and Apte 1997), where many processes, such as efficient potassium pumping and accumulation, synthesis and transport of compatible solutes, plant signalling systems involved in tissue and in developmental regulation (Winicov and Bastola 1997), etc. are only some of many other important adaptations which are equilibrated in a harmonic way to fulfil those adaptive processes mentioned in Table 12.1.

It has to be stressed that salt tolerance has at least two quite differing aspects. One is the upper limit of salt that can be tolerated by an individual plant, which is necessary for survival. The other is the existence of a plant species that exerts successful reproduction, which is necessary for ecological success.

Salt tolerance of plants varies very much. It varies during different growth or development phases (Tobe et al. 2004, 2005), with ionic constitution of the soil solution (e.g. the presence of Ca and K as antagonists of Na), with microclimatic conditions (e.g. relative humidity), with life form and halophyte strategic type, with the plant organ affected by salinity and with the genetic variability of each species forming ecotypic varieties. Also, the effects of salinity on different growth stages and growth processes of plants have to be taken into account (Ungar 1996). Germination and seedling growth is normally more sensitive than growth of established adult plants.

For halophytes osmotic adaptation is accomplished not only by synthesis of organic compounds but also by absorbing inorganic ions, accumulated in the vacuole, counterbalanced by compatible solutes in the cytoplasm. As a rule, the osmotic potential of leaf cell sap normally differs by 0.5–1 MPa from that of the soil solution, enabling uptake of water.

1.2 Ion Pattern of Halophytes

For a long time, halophytes had been classified into chloride halophytes, sulphate halophytes and alkali halophytes, according to the main ions in cell sap or ash (Walter 1968). The alkali halophytes are those where a high proportion of organic acids (e.g., oxalate in Halogeton with up to 30% dry matter) are accumulated. It has long been known that halophytes are able to take up nutrients from the soil despite an excessive content of Na+ and Cl–. Most halophytes discriminate between Na+ and K+ and only few species are really sodiophilic (Moore et al. 1972). To demonstrate the characteristics in K+/Na+ discrimination, it is necessary to have the relevant soil samples from the rhizosphere of the respective plants. Then the accumulation factor for sodium in comparison with potassium can be calculated. It is easily seen that most species under a wide range of given cation ratios in the soil favour potassium uptake. The widespread Chenopodiaceae Salicornia europaea and Suaeda maritima can be termed sodiophilic, and so can Climacoptera aralensis and Suaeda acuminata (Tables 12.2, 12.3 and 12.4), whereas Petrosimonia triandra exhibits a rather balanced Na+/K+ ratio. In contrast, the grasses Puccinellia distans and Stipagrostis pennata and Eremosparton aphyllum very selectively accumulate potassium by a factor of 10–100 according to the soil Na+/K+ ratio; even in saline soils their Na+/K+ ratio is between 0.10 and 0.40 (Table 12.4). Slightly more sodium is accumulated in some Brassicaceae, e.g. in Malcolmia africana. All other Chenopodiaceae are more or less halophytic and exhibit rather high Na+/K+ ratios (Table 12.4), which is not really very different from the results from hot-water extracts and from acidic extracts (Tables 12.3 and 12.4). However, in the pseudohalophytes or nonhalophytes, the amount of nonvacuolar alkali ions (which are extracted additionally with the acidic extract) is considerably higher (Table 12.4). This is due to the calcium content, where by an acidic extraction up to 60 times higher amounts are analysed.

Table 12.2 Ion pattern of some common halophytic species of the Aralkum, analysed from hot-water extracts (upper figure) and from acidic extracts (lower figure in italics). Comparison of samples from Bayan (Ba) and from Karabulak (Ka); n number of samples, ion content (mmol kg–1 dry matter and standard deviation)
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Table 12.3 Ion pattern of some common halophytic species of the Aralkum, analysed from hot-water extracts. From Bayan (Ba) and from Karabulak (Ka). n number of samples, ion content (mmol kg–1 and standard deviation). For each species the halophyte type is indicated (second column; for an explanation of the abbreviations, see Table 12.1
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Table 12.4 Ion pattern of some common halophytic species of the Aralkum, analysed from acidic extracts. From Bayan (Ba) and from Karabulak (Ka). n number of samples, ion content (mmol kg–1 and standard deviation), in parentheses factor for increased content related to hot-water extracts, see Table 12.3
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It is obvious that leaf succulents and stem succulents, such as species from the genera Suaeda, Salicornia and Halocnemum, accumulate considerably more Na+ and Cl– (3,000–5,000 mmol/kg) in comparison with other species. The ionic contents (Na+ and Cl–) of Climacoptera species and of Ofaiston monandrum are lower (2,000–3,500 mmol/kg) in comparison with those of species from Salicornia and Suaeda. Even lower are the values from Petrosimonia triandra. On the other hand, the Na+ and Cl– accumulation of pseudohalophytes such as Euclidium syriacum and Stripagrostis pennata is very low.

The respective data on soil from the sites studied are given in Table 9.1 along the Karabulak gradient transect. All sites are rather alkaline. Salinity is also very variable between sites and between distribution along horizons. This depends on season, as salinity changes with evaporative demands in summer along the very long capillaries in loam and clay to the upper horizons and this may form a salt crust. However, lower horizons also often have a rather high salinity level, whereas middle horizons may store less saline water from winter snow or rains. This is shown by two examples of soil profiles (Fig. 12.5). In both soil profiles it is obvious that the sulphate salinity is as high as or even higher than the chloride salinity, but differs in the horizons.

Fig. 12.5
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Ion content in soil horizons of the Aralkum. Left: Soil profile Bayan Pr10 with main salt accumulation, mainly chloride in topsoil. Right: Soil profile Bayan Pr16 with salt, mainly sulphate accumulation in topsoil and in lower horizons

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It should be briefly mentioned that the various members of the Chenopodiaceae on the Aralkum cannot be put into one group of physiotypes (Reimann and Breckle 1993). Under natural conditions the sodium levels vary very much, as do the levels of other ions. There are many articles on the chemistry of halophytes and their internal ion composition (Albert 1982), as well as on the normally taxon-specific accumulation of compatible solutes (Popp 1985). The main characteristics of the physiotypes, e.g. Brassicaceae and Poaceae, are represented in the same ion pattern, as Albert (1982, 2005) extensively described.

It is always an open question to what extent the edaphic conditions influence the ionic pattern and content in plants (Mirazai and Breckle 1978). The Pontic–Irano– Turanian Suaeda acuminata (Fig. 12.6) is very common in Central Asia (Wucherer 1986). This species exhibits a wide ecological amplitude and thus can be found on very contrasting saline stands. Within the Karabulak transect on the northern coast of the Aral Sea, at seven localities Suaeda acuminata is present (Table 9.1).

Fig. 12.6
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Suaeda acuminata. Remnants from previous year, new seedlings and saplings. North Aral Sea (photo: Breckle, May 2003)

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It is obvious that the sodium and chloride contents of the aboveground plant organs of Suaeda acuminata on degraded coastal solonchaks and puffy coastal solonchaks are significantly lower. These soils contain significantly less salt in the topsoil. On these stands the sodium content is higher than the chloride content in comparison with the marshy solonchaks and crusty coastal solonchaks (Table 12.5). This example of Suaeda demonstrates that the ion content in halophytes growing on real solonchaks with high salinity is not distinctly influenced by the edaphic conditions.

Table 12.5 Main ions in the stems and leaves of Suaeda acuminata (mol kg–1 dry matter) from the Karabulak transect and soil characteristics (10–20 cm) of the site. EC electric conductivity of soil extracts (mS cm), DSF dry sea floor (from the 1970s, 1980s or 1990s)
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Balnokin et al. (1991) studied the sodium, chloride and proline contents in Salicornia europaea, Climacoptera aralensis und Petrosimonia triandra along the Bayan transect on the eastern coast of the Aral Sea (Table 12.6). The content of proline as one of the typical compatible solutes apparently exhibits no clear correlation to the storage of salt in the plant tissues.

Table 12.6 Ion content and proline content (mmol kg–1 fresh weight) in green tissues of halophytic plants from the Aralkum
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The selectivity against ions differs in the various species according to their natural occurrence. The halophytes Limonium gmelinii and Limonium ramosissimum are very potassiophilic, as can be seen by the strong change in ion pattern (Fig. 12.7, left side) between nutrient solution and leaf cell sap. Again, there is a major change in ion composition between leaf cell sap and the recreted fluid. The ion pattern changes in such a way that the cytoplasm of the leaf cells is kept relatively low in sodium, whereas the gland fluid is rich in sodium (Fig. 12.7, right side). Such behaviour is not recognizable in Limonium sinuatum, a plant which inhabits slightly saline stands. In that species selectivity in both cases of transport is low (Fig. 12.7). It was also shown that the activity of the salt glands of the halophilic Limonium species (Wiehe and Breckle 1989) and Aeluropus has a threshold value and these start to recrete NaCl only after a distinct salinity level in the leaf is reached (Pollak and Waisel 1979).

Fig. 12.7
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Ion conditions in three Limonium species in salinity cultures (Wiehe and Breckle 1989) with various nutrient solutions differing in Na+/K+ ratio

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There are many indications that also in the stem- and leaf-succulent halophytes, in the recretohalophytes and pseudohalophytes from the dry Aral Sea floor, different mechanisms and strategies for the adjustment and regulation of the salt concentration in the plant tissues are operating (Breckle 1995) and thus a differing salt tolerance in the various species leads to a specific pattern of species and halophyte types along salt gradients.

The sequence of species along the salt gradient in a rich halophytic area, as it is in the Central Asian deserts, reveals a typical sequence of the dominating halophyte types. Along the salt gradient (Breckle 1986), which can be derived from salinity measurements in a mosaic vegetation, it is obvious that the stem succulents and then the leaf succulents play the major role close to the saline lakes or basins, where salinity is high. The recreting halophytes (exocrinohalophytes, endocrinohalophytes) dominate in the middle part of the transect, where salinity is more variable as is water supply. This part of the transect is characterized by less water availability and often here a much higher proportion of C4 plants occurs. This is also the case on the less saline side, where the pseudohalophytes and finally on almost salt-free substrates the nonhalophytes predominate and other ecological factors, such as water availability, water supply and nitrogen source, govern the vegetation mosaic. However, on the desiccated seafloor of the Aral Sea an equilibrium of halophyte types has not yet been reached, the dynamics of changing ecological conditions from year to year is so drastic that only by chance a mixture of more or less adapted species is found, which in part resemble the sequence of the halophyte types discussed.

1.3 Ecological Salinity Indicator Values for Plants of the Aralkum Region

The ecological behaviour and adaptation to distinct natural site conditions is the result of the competitive ability of a species. This depends on the floristic pattern of the region and the competitors present. Normally under natural conditions with a fluctuating climate from year to year, a dynamic equilibrium can be observed, and if the main ecological conditions vary within a rather constant range, a set of species will form a rather constant community.

Under saline conditions, the salt content of the soil plays a major decisive role for which species can compete successfully (Adam 1990). By comparing many sites with different salinities, one can evaluate the distinct ecological optimum (not ecophysiological optimum without competition, which can be rather different: many plants grow better under low salinities but are pressed to higher salinity sites by competition, where they can grow, but not optimally). This ecological optimum can be used to grade the ecological salinity tolerance by an indicator value (S value, see Table 12.7). Such indicator values are used rather widely in various regions for various ecological parameters, e.g., pH, nitrogen supply, drought tolerance and heat tolerance (Ellenberg et al. 1991). For salt tolerance a scale from 0 to 9 can be used (Table 12.7), where 9 means the highest salt tolerance. In contrast to many other indicator values, the distribution of the S values over the whole scale is oblique since most species belong to the nonhalophyte group, which has an indicator value of S = 0 or S = 1.

Table 12.7 Definitions of the S value, the ecological salinity indicator value (see Breckle 1985; Ellenberg et al. 1991)
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By long-term observations and comparing many sites, one can define S values for many species. A few species are very variable in their adaptation to saline site conditions, and those species have no definite S value (S = X). For others, their typical site conditions are not known exactly (S = –), and will be revealed only in the future.

It should also be kept in mind that the S-value list is only valid for a distinct region; it depends on the whole given flora and the respective competitive plant communities.

The Aralkum flora is very rich in halophytes; thus, the percentage of species with high S values (above 4) is rather high in several plant families (see Table 12.8). Other plant families are represented in the Aralkum by a quite high number of species but still mainly prefer sites with low salinity (Polygonaceae, Brassiceae, Fabaceae).

Table 12.8 Number of species of halophytic strategy types and related salinity indicator values for the halophytic flora of the Aralkum
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All species of the Aralkum flora are listed in Table 12.9 with their ecological salinity indicator value (S value), their halophytic strategy type and their life form. It is easily recognizable that the halophytic type and the S value are rather strongly correlated.

Table 12.9 Species of vascular plants from the Aralkum, indicating their salinity indicator values, halophytic character and life form (Ch chamaephytes, G geophytes, B with bulbs, P parasitic, R with rhizomes, H hemicryptophytes, Hy hydrophytes, Ph-m microphanerophytes, Ph-n nanophanerophytes, T therophytes). Salinity tolerance is expressed as indicator value S (for definitions of S values, see Table 12.8; S = 0, nonhalophytes, are not indicated here, mainly are within the “not known” group; S = 9, extreme halophytes, only growing on strongly saline stands; X indifferent, – not known)
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1.4 Salinity

Over all the oceans, seawater is rather homogeneous in chemical composition, with a strong preponderance of NaCl. In deserts with their arid climate, salinity is caused not only by atmospheric input (cyclic salt; Teakle 1937; Breckle 1976, 1985), but also by leaching of the rocky material of the hydrotope within the endorheic basin, where the water runoff from the rare precipitation events is collected in the erosion basin, forming salt lakes, which may have accumulated also some other ions besides Na+ and Cl–, mainly SO 2–4 , HCO –3 , Li+, Mg2+, borate, etc. (Breckle 1975a, b, 1990). Thus, in some parts of the world, salinity is caused not only by chloride, but in temperate and cold arid continental regions it may be caused by sulphate or carbonate accumulation (Curtin et al. 1993). In the Aralkum the desiccated substrate of the seafloor is rich in chloride and sulphate. This can change within deeper horizon layers (Fig. 12.5). It can also be seen indirectly by the various water sources in the region (Table 12.10) with very variable ion content and salinities. The ratio between ions is not as similar as in open-ocean water, which is rather constant within narrow limits all over the world. If the Na+/Cl– ratio is distinctly higher than 1, the counterbalance is normally by sulphate (sulphate salinity); if the Mg2+/Na++K+ ratio is distinctly higher than 0.1, the water has a bitter taste. If there is a sufficient portion of potassium and alkali earth ions present, the salinity by sodium is not as severe as with pure NaCl salinity. Plants can adjust to such conditions and are able to absorb nutrients by discriminating ions. Thus, a typical halophytic community is a mixture, and is often composed of several species, where some of these species are not real halophytes. They occur only accidentally in such plant communities of oligohaline marshes, but have their optimal growth and performance in nonsaline vegetation. A typical spatial or temporal niche segregation enables nonhalophytes or pseudohalophytes to migrate and to invade halophytic stands.

Table 12.10 Ion content (g kg–1) of some water samples from the Aralkum
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2 Conclusions

Investigation of the adaptive mechanisms of the various halophyte types as well as succession processes is essential to obtain an adequate species composition for phytomelioration of the saline soils of the Aralkum. Ion pattern, halophytic strategy to cope with salinity and life form are very variable in halophytes; their distinction from less tolerant pseudohalophytes or nonhalophytes is only gradual. The salinization of the substrate on the dry seafloor varies to a great extent, causing a wide variety of saline soil types. Various solonchaks have developed: marshy solonchaks, crusty and puffy solonchaks, solonchaks slightly covered by sand, degraded coastal solonchaks, takyr solonchaks, etc. Studying natural halophytes is thus very important not only for all those regions where salinity has reached such a level that desalinization techniques are much too costly but also for quasi natural sites with their ecological dynamics. Understanding the adaptation of halophytes to saline sites and understanding their abilities to compete in saline communities is a good precondition for better use of halophytes. The applicability of the ecological salinity indicator value (S value) may be also worthwhile for adjacent agricultural areas with salinized fields and weeds for fast characterization of the sites.

Ecotypes and biogeography, germination and establishment, competition and nutrient availability under high salinity and alkalinity are subjects on the ecosystems level which have to be investigated further. Investigations of halophytic ecosystems, of the salinity process in agrarial systems and of plant strategies for salt regulation are urgently needed in the Aral Sea region, where salt desertification has become dominant.