Introduction

Halophytes, plants that survive to reproduce in environments where the salt concentration is around 200 mM NaCl or more, constitute about 1% of the world flora (Flowers and Colmer 2008). Halophytes are receiving substantial attention from botanists and agriculturists because soil salinity is increasing in many parts of the world. These increases largely result from low quality irrigation water (water with high salt content), high evaporation-to-rainfall ratios, and, in coastal lowlands, from rising sea levels associated with global warming. As fertile soils become salinized, the yield of conventional crops decreases (Munns 2005). Between 1993 and 1995 in the Sacramento Valley of California, for example, soil salinity increased by 1 ds·m−1, and this reduced yields by an estimated 10% (see Rozema and Flowers 2008). Yield reductions are a serious problem because the world population is rapidly increasing. Social scientists expect that the world population will reach 9.3 billion by 2050 (United Nations Population information Network www.un.org/popin/data.html).

Although salinization in agricultural soils can be avoided or reduced if crops are irrigated with fresh water (water with low salt content), fresh water makes up only 1% of the water on earth. There is an equivalent supply of brackish water (1%) and a vast quantity of seawater (98%). Agriculture must compete with domestic and industrial uses for this fresh water, and high quality water is rapidly becoming a limited and expensive resource. It is time that we therefore must find ways to grow crops without applying large volumes of fresh water (Rozema and Flowers 2008).

One way to deal with the problem of saline soils is to develop halophytic crops, i.e., crops that are adapted to a saline environment. An important step in developing halophytic crops is to document and understand those halophytes that are already growing in our natural and agricultural ecosystems. The current paper describes a series of surveys conducted from 1995 to 2004 whose objective was to study the distribution, economic potential, and types of halophytic plant species growing in China.

Materials and methods

Salinized soils are located in eight regions in China (Fig. 1) (Soil Institute of Nanjing, Chinese Academy of Science 1978; Wang and Li 1993). From the middle of June to early August and from early September to late October in every year from 1995 to 2004, samples of halophytic plants and salinized soils were collected from each of these regions. The halophytes were identified to species with the aid of the Chinese Plant Flora (Editorial board of the Chinese Academy of Science 1977, 1979, 1985, 1987, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999) and the Xinjiang plant Flora (Editorial board of Xinjiang plants 1992, 1994, 1996, 1999) (Wu 1980; Zhao et al. 2002). The soil samples were analyzed by conventional methods (Soil Institute of Nanjing, China Academay of Science 1978). The salt concentrations (NaCl, NaCl and Na2SO4, or NaCl and Na2CO3) of the salinized soils ranged from 0.98 to 2.4% in regions 2 through 8. The salt concentrations (NaCl) ranged from 3.0 to 3.6% in the southern part of region 1, and from 0.8 to 4.2% in the northern part of region 1.

Fig. 1
figure 1

The classification of the salinized soil region in China: 1. Coastal humid—semihumid salinized region; 2. Semimoist—semiarid grassland and meadow salinized region in northeastern China; 3. Semihumid and semiarid cultivated meadows salinized region on the Huang Huai Hai Plain; 4. Plateau drought–fog–desert grassland salinized region on inner Mongolia; 5. Semi-drought-semifog desert salinized region in the middle and upper reaches of the Yellow River; 6. Arid, fog-desert salinized region in Gansu, Xinjiang, and inner Mongolia; 7. The extremely arid-fog–desert salinized region in Qinghai and Xinjiang; 8. High, cold–fog–desert salinized region in Tibet (Wang and Li 1993)

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Results

Species of halophytes in China

Based on surveys from 1995 to 2004, China contains 587 species of halophytes (Table 1) (Li 1988, 1990; Zhao and Li 1999; Zhao et al. 2002; Xi et al. 2006); these species occur in 242 genera and 71 families and represent 10% of the angiosperm species in China. The families with the most species are the Chenopodiaceae (120 species in 29 genera) (recently some taxonomists suggest the Chenopodiaceae may be merged into the Amaranthaceae: http://www.mobot.org/MOBOT/research/APweb/), the Asteraceae (82 species in 26 genera), the Poaceae (25 genera, 55 species), and the Fabaceae (51 species in 24 genera) (some taxonomists divied the Fabaceae into Minosaceae, Caesalpiniaceae and Papilionaceae: http://www.mobot.org/MOBOT/research/APweb/); these four families (Chenopodiaceae, Asteraceae, Poaceae and Fabaceae) contain 52.3% of the Chinese halophytes. The species of Chinese halophytes reported in Table 1 represent 37% of the world halophytes reported by Aronson (1989) and 22% of world halophytes reported by Menzel and Lieth (1999).

Table 1 The taxonomy, life form, and distribution of halophytes in China
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Physiological and ecological types of Chinese halophytes

Physiological types

The classification of Chinese halophytes into three physiological types is based on salt uptake and storage, as described by Breckle (1995). These three types are salt-secreting halophytes (exorecretohalophytes and endorecretohalophytes), euhalophytes (i.e., salt-diluting halophytes), and pseudohalophytes (i.e., salt-excluding halophytes). The exorecretohalophytes secrete salt from epidermal salt glands of leaves and stems; in China, these include species of Limonium, Tamarix, Spartina, Avicennia, and Frankenia. The endorecretohalophytes discharge salt from epidermal bladders on the leaves; in China, these include species of Atriplex, Chenopodium, Halimione and Mesembryanthemum.

The euhalophytes have succulent leaves or stems. In China, the genera with succulent leaves include species of Suaeda, Salsola, Petoroimonia, and Reaumria. Those genera with succulent stems include species of Halostachys, Halocnemum, Ralidium, Kalidium and Salicornia.

The pseudohalophytes or salt-excluding halophytes include species of Phragmites, Artemisia and Juncus.

Ecological types

Chinese halophytes can be divided into three ecological types: mesohalophytes, zerohalophytes, and hydrohalophytes. Mesohalophytes normally grow in salt meadows, salt shrublands, and other semiarid habitats with moderate soil water content; mesohalophytes include species of Atriplex, Limonlum, Aeluropus, Apocynum, and Suaeda. Xerohalophytes grow mainly in salt deserts, i.e., in the deserts of Xinjiang, Gansu, Inner Mongolia, and inland China. Examples of zerohalophytes are Anabasis aphylla, as well as the species of Haloxylon , Tamarix, Kalidium, Halocnemum, Halostachys, Suaeda, Glycyrrhiza, and Zygophyyllum. Hydrohalophytes are divided into emergent halophytes and submerged halophytes. Emergent halophytes grow in salt marsh and coastal marsh habitats, such as mangroves; emergent halophytes include the species of Bruguiera, Kandelia, Rhizophora, Acanthus, Lumnitzera, Nypa, Sonneratia, Acrostichum, Hibiscus, Triglochin, and Salicornia. As the name indicates, submerged halophytes grow beneath the surface of sea water, and include Ruppia rostellata, Zostera merina, Phyllospadix japonica, Cymodocae spp., Halodule spp., and Halophila spp. (Hou 1982; Ramadan 1998; Zhao and Li 1999).

Distribution of Chinese halophytes

Because halophyte vegetation is azonal (i.e. intrazonal vegetation), the distribution of Chinese halophytes is closely related to the distribution of Chinese salinized soils: where there are saline soils, there are halophytes. Chinese salinized soils can be divied into eight subregions based on the similarities of soil, biology, and geographical location (Ramadan 1998; Wang and Li 1993; Wu 1980) (Fig. 1; Table 2).

Table 2 The salinized regions, climate trends, soil types and common halophytes in China
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Discussion

The results of this survey demonstrate that although the halophyte vegetation in China is a azonal community, the large-scale distribution is also restricted by climate, e.g., the 38 families halophytes of Xinjiang may be divided into 6 types based on their geographic distribution (Table 3). The families of world large-scale distribution are 50% of 38 families from analysis of their geographic components, The temperate zone component (include tropic zone distribution, tropic zone, temperate zone distribution and temperate zone distribution) are 42.1%, and the tropic zone, subtropic zone distribution and subtropic zone, temperate zone distribution are 7.9%. These results show that the distribution of Xinjiang halophytes and the climate zone of Xinjiang are identical, despite halophyte vegetation is an azonal vegetation, their large-scale distribution is first restricted by the area climate zone of Xingjiang. Again, higher world large-scale distribution component illustrate that the narrow ecology adapatic range families are easily suffered from the influce of Xinjiang arid climate, only the world large-scale distrinutive families of more wide adapatic range may be developed better in the adverse circumstances.

Table 3 Analysis of geographic distribution of 38 families of halophytes in Xinjiang
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Among the 587 halophyes in China, many have economic potential. Some can provide the starch, sugar, protein, or oil materials, and these include Acacia spp., Suaeda spp., Salicornia spp., Glycyrrhiza uralensis, Asparagus persicus, Pandanus tectorius. Some of these halophytes can serve as fodder, and these include Salsola iberica, Aeluropus sinenesis, Phacelurus latifolius, Achnatherum splendens, Atriplex spp., Kochia spp., Halostachys caspica, and Alhagi sparsifolia, e.g., Atriplex spp. produced 12.6 to 20.9 t/ha of biomass containing 9.9% to 19.5% of protein when growing in full-strength seawater (Dong 1994; Zhao and Feng 2001). Otherwise, more than 100 species of Chinese halophytes can be used as Chinese medicines. Those halophytes include Lycium chinense, Glycyrrhiza uralensis, Apocynum venetum, and Nitria tangutorum. Some of these medicinal halophytes have been cultivated in the saline soils of northwesten China, and the plants have been marketed domestically (China Medicinal Materials Co. 1994; Zhao and Feng 2001). Except above economic materials, some halophytes also serve as fiber, tannin materials,(fibers such as Ashmatherum splendens, Apocynum venetum, Poacynum pictum, Tamarix chinensis, Sesbania canabinia, Phragmites australis, Juncus effusus; tannins such as Kandelia candel, Ceriops tagol, Bruguiera gynmorrhiza, Rhizophora styosa, Rhizophora apiculata, Xylocarpus grantum, and Lumnitzera racemosa).

Some halophytes, such as Tamarix spp., Alhagi sparsifolia and Elaeagnus omgustifolia can also serve as wind breaks and stablize sand. As well as can survive air pollution, such as Phragmites australia (absorb NH3), Tamarix spp. (tolerate SO2, Cl2, and HF) (Zhao and Feng 2001).

The great nature world gifted many economic potential halophytes to us. We ought to better pretect, improve and use them by mordern agrobiotechnology: (1) some bigger salt tolerant and bigger economic potential halophytes may be grown in the coastal area irrigated by sea water; (2) we may irrigated them with brackish water, or fresh water plus brackish water in the dry salinized lands. In other words, we have to deal with the changes of future climate and the increace of population in the world. So we must develop saline agriculture. Again, we also take the transgenic technology of transformation of conventional crops with different halophytes genes to increase their salt tolerance. Although it is not getting success now, we believe that this technic is bound to gain a success in near future.