The continual increase in world population, coupled with the expansion of salt affected lands into agricultural lands, places additional pressure on global agriculture to produce enough food to feed the growing population. Salt-tolerant plants, namely halophytes, provide a sensible alternative to increase productivity in saline lands where traditional crops such as wheat and canola are unproductive. Halophytes can also be used simultaneously for land rehabilitation. This review covers the physiology of halophytes that enable them to thrive in a salt-stressed environment as well as their uses in food production and phytoremediation of saline or contaminated lands.
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Global population is expected to increase by 2.6 billion over the next 40 years to 9.1 billion. In order to meet this growing demand for food and fiber, global agriculture is tasked to increase its productivity by more than 110 %. (FAO, 2005). Expanding cultivation into new areas is undesirable mainly due to the detrimental environmental impacts associated with it. The removal and disturbances of these previously uncultivated areas can have wide ranging and long-term consequences to the terrestrial and aquatic ecosystems via deforestation and eutrophication etc. (Tilman, 1999). As such, improving crop productivity per unit area of existing cultivated land is critical to feed the growing population. However, due to land degradations of cultivated areas worldwide, agriculture is gradually being pushed to marginal and salt-affected lands. Globally, these saline lands cover an area of 831 million hectares, and spans all continents including Africa, Asia, Australasia as well as the Americas (Rengasamy, 2006). In Western Australia alone, 6.5 million hectares of agricultural land are at risk of dryland salinity due to land degradation (ANRA, 2002), and traditional crops such as wheat and canola will then be unproductive to be farmed.
Halophytes are plants capable of surviving and being productive in a saline environment. As such, halophytes can be grown in saline areas in which traditional crops falter, as well as in regions increasingly affected by dryland salinity. Although halophytes constitute a small percentage of the known plant population, they play a number of useful roles in the environment. The first part of this review focuses on the physiology of halophytes that allow them to succeed in a saline environment, and the second part discusses the potential uses of halophytes in increasing global food production, either directly as a food source or through their phytoremediatary capabilities.
Halophytes are highly specialized and evolved plants capable of acquiring nutrients from a high salt environment in which glycophytes (salt-sensitive plants) are either unproductive or unable to survive. In this first part of the review, the physiology of halophytes, in particular ion compartmentation, production of organic solutes, salt glands and bladders, as well as leaf and shoot succulence is discussed.
Physiology of Halophytes
Intracellular cytosolic enzymes in both glycophytes and halophytes are equally sensitive to salt (Glenn and Brown, 1999). Under typical physiological conditions, high cytosolic K+/Na+ ratio is maintained (Tester and Davenport, 2003) to ensure normal cellular functions. The maintenance of this ratio in the plant’s cytosol is energy dependent, and is mediated by pathways for Na+ extrusion or by compartmentation of Na+ into the vacuole (Blumwald, 2000). Unlike glycophytes, halophytes have developed mechanisms to sequester excess Na+ into the vacuoles to avoid Na+ toxicity in the cytosol. The transport of Na+ into the vacuoles is mediated by cation/H+ antiporters driven by the electrochemical gradient of protons generated by the vacuolar H+ translocating enzymes such as H+-ATPase (Gaxiola et al., 2007). These transporters play an essential role in the sequestering Na+ ions into the vacuole or exclusion outside the cell of the halophytes, ultimately allowing them to tolerate much higher salt concentrations compared to the glycophytes.
Production of compatible solutes
Solute transport is a process regulated by environmental and endogenous signals. Environmental stresses such as salinity affects solute transport in plants and can cause changes in the partitioning of carbon and nitrogen. In addition to compartmentalizing extra salt in its vacuoles, halophytes can produce organic solutes. These osmotically active solutes are synthesized in order to maintain normal cellular functions in response to a drop in the osmotic potential within the plant (Glenn and Brown, 1999). Depending on the halophyte species, a variety of organic solutes ranging from proline, sucrose to pipecolatebetaine (Rhodes and Hanson, 1993) can be produced. Unlike inorganic solutes such as Na+, these compounds do not induce toxicity even at high concentrations (Ashraf and Foolad, 2007), and serves as a key adaptation to halophytes’ survival in a saline environment.
Salt glands and bladders
As an adaptation to saline environments, halophytes frequently have specialized structures designed for extruding salt from tissues. Salt glands and bladders play an important role in internal ion regulation by transporting ions away from the mesophyll cells to the leaf surfaces. Once deposited on the leaves, crystallization occurs and the salt crystals are washed or blown away.
Salt glands consist of several specialized cells and are located in the depressions of leaf epidermis. When grown in highly saline environments such as seawater, the excreted ions are typically Na+ and Cl-, and excretion increases with increased levels of salinity. Found in both halophytic monocotyledons and dicotyledons (Khan and Weber, 2006), these glands allow for massive amount of salt to be removed and are important organs for salt management.
Salt bladders are derived from modified epidermal hairs and typically have a stalk cell and a bladder cell. Stalk cells serve as ion transporters from mesophyll cells to the bladder cells. As salts accumulate in the bladder cells, expansion occurs until they burst. The bursting action allows salt to be discharged on leaf surfaces. By accumulating salt in the bladder cells, ion toxicity is prevented from building up in the mesophyll cells and this constitutes an important mechanism for the protection of young leaves. This specialized organ is a common feature on the salt tolerant halophytes in the family Chenopodiaceae, and includes the saltbushes (Atriplex sp.) (Khan and Weber, 2006).
Leaf and stem succulence
Highly vacuolated and large cells resulting in fleshy or thick leaves and stems are a common feature in halophytes. Despite the poor understanding of the anatomical response leading to succulence, Na+ ions are believed to be responsible (Khan and Weber, 2006). Succulence is not confined to halophytes alone. Non-halophytic plants, such as the cotton, increase succulence when grown at a high salt concentration. Despite its succulence, plant growth is still impaired by high levels of salt. In contrast, the Atriplex spp., in conjunction with its salt bladders, utilizes succulence as additional storage for excess salts, and thus reduces ionic toxicity on the mesophyll cells.
Naturally salt-tolerant species are used in agriculture, mainly to provide forage, medicine, and aromatics (Qadir et al., 2008). In Australia, Barrett-Lennard (2002) identified 26 salt-tolerant plant species of potential economic value to agriculture. Examples of these useful halophytes include the potential oil-seed crops Kosteletzkya virginica, Salvadora persica, Salicornia bigelovii, and Batis maritime. Useful fodder crops include Atriplex spp., Distichlis palmeri and biofuels (Flowers et al. 2010). In addition, growing halophytic biofuel crops on saline agricultural land would help to counter concerns that the biofuel industry reduces the amount of land available for food production (Qadir et al., 2008). This second part of the review explores the potential uses of halophytes in the context of Australia in increasing food production directly as a food source or through their phytoremediatary capabilities in abiotic stress management.
Halophytes grown on saline agricultural land helps improve site productivity by providing ground cover to prevent erosion as well as increase the organic contents in saline soils. Atriplex species are now widely used throughout the Meditteranean areas, including Australia, for the purpose of rehabilitating saline land and to increase forage productivity. Saltland pastures provide fresh feed for the entire year, including the summer months in Australia. Furthermore, many studies have been done on halophyte species that can be used for fodder, in particular Atriplex nummularia, A. halimus and A. lentiformis (Choukr-Allah, 1997). These three species are now well established in the Meditteranean basin. When used in conjunction with deep-rooted perennials such as Eucalyptus occidentalis, halophytes can help to restore the hydrologic balance on areas affected by dryland salinity. This can potentially allow vast areas to be reclaimed (Barrett, 2000) and subsequently used to plant traditional crops such as wheat and barley.
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Halophytes as food sources
Oilseed crops are grown for the oil contained in the seeds. Seeds of various halophyte species, such as Salicornia bigelovii, Haloxylon stocksii, and Halogeton glomeratus contain 70-80% of high quality and unsaturated edible oil (Ladeiro, 2012). A controversial species underutilized for its edible qualities is Diplotaxis tenuifolia (Rocket). Rocket is widely used in Europe where it is regarded as a delicacy. It is naturally adapted to Mediterranean-type climate, including saline and dry ecosystems. Rocket is able to compete strongly with other pasture plants and can reproduce via seeds and root fragments. Studies have shown that it is able to grow and reproduce at salinity levels of up to 300 mM NaCl, and can be grown at levels up to 100 mM NaCl without losing its nutritional values (Ladeiro, 1997). In Australia, however, rocket is regarded as an agricultural weed found mainly in poorer pastures in the Eyre Peninsular of South Australia and Victoria (DAFWA, 2007). Thus, if Rocket is to be used as a food source in Australia, proper containment strategies must be in placed to prevent it from spreading into unwanted areas.
Halophytes in abiotic stress management
Desalination of saline soil
As dryland salinity increasingly affects huge areas of cultivated land, numerous physical, chemical and biological methods have been developed for reclaiming these saline soils (Shahid, 2002). Biological methods include crop rotation, inputs of organic manure as well as the use of salt-tolerant crops (Shahid, 2002). The ability of plants to accumulate huge amounts of salt is highly dependent on the capacity of their aboveground biomass (Rabhi et al., 2010). This ability is especially important in the drier regions of Australia where rainfed systems are used and rainfall events are not reliable enough to reduce the salt concentration in the rhizosphere (Shahid, 2002). Halophytes are the most important group of plants used in soil desalination due to its salt accumulating and salt-tolerant characteristics. High salt resistance, high aboveground biomass, and high degrees of economic utility (fuel, fiber, and oil seeds etc.) (Rabhi et al., 2010) are key requirements to assess a plant’s usefulness in desalination. Sesuvium portulacastrum is a naturally occurring halophyte species in western Australia. Most importantly, it is able to accumulate huge quantities of Na+ within its aboveground organs. In addition, Sesuvium portulacastrum has been used in other parts of the world for desalination of salt-affected lands (Patil et al., 2012) and should be studied further in the context of Australia for similar purposes.
In cultivated soils, contamination by heavy metals (i.e. Zn, CU, Cd, Fe, As, etc.) is a serious environmental problem. Throughout evolutionary history, plants have developed various detoxification mechanisms in response to allelochemicals produced by competing organisms. Thus, a biological method of rehabilitating contaminated lands utilize plants to decontaminate affected sites and is termed phytoremediation. Phytoremediation exploits the natural ability of plants to absorb, accumulate, storage and degradation of both organic and / or inorganic compounds. In this regard, halophytes show the most success in terms of adaptations to a variety of abiotic stresses including heavy metal stress.
Mechanism of phytoremediation
Physical removal and bioconversion of compounds by plants are termed phytoextraction and phytotransformation or phytodegradation respectively. Phytoextraction utilizes the plant’s ability to take up a range of chemical compounds through the root system, translocate them through the vascular tissues and eventually compartmentalizing these compounds in different organs such as leaves and stems. For a compound to be readily available to a plant, soil conditions e.g. clay content and pH play a crucial role. Incorporation of soil amendments e.g. lime has been shown to increase the availability of lead (Pb) and uranium (U) by more than 100-fold (Chen et al., 1998). Using this approach, successful remediation of agricultural soils contaminated with selenium (Se) in the US had been recorded (Eapen et al., 2006). Similarly, the Australian saltbush (Atriplex nummularia var. De Koch) has been successfully used in rehabilitating mercury-contaminated sites, with studies showing undetectable levels of mercury just 72 hours after plant introduction (Khondaker and Caldwell, 2003). The compartmentation of metals into the aerial organs of the plant allows for easy harvesting and can be processed to reclaim economically important metals or disposed off as hazardous waste in landfills.
Phytochelatins (PCs) play a crucial role in phytodegradation and phytotransformation. PC production in plants is stimulated by the presence of heavy metals. PCs are metal-binding peptides and works by mobilizing heavy metal compounds in the cytosol and then sequestering PC – metal complexes in the vacuoles of plant cells. Upon absorption of heavy metal compounds, PCs and enzymes such as e.g. oxygenase, peroxidases and reductases etc. are produced in large quantities. Degradation of these heavy metal compounds occurs and the biodegraded constituents are then converted into inert forms stored in the lignin or released as exudates (Watanabe, 1997). In phytotransformation, the absorbed heavy metal compounds are biochemically bonded by PCs and enzymes to cell tissues in inert forms where they are eventually compartmentalized (Watanabe, 1997). In Australia, great success in the use of native Halosarcia pergranulata to revegetate old mining areas has been recorded.
Conclusion: Going into the future
Sustainable agriculture is continuously threatened by the decreasing availability of freshwater and arable land. Global agriculture is pressured further by the demand for more food by the growing population. In addition, saline agriculture will be of particular importance to Mediterranean countries, including Australia, due to the widespread increase in soil degradation and unfavourable climatic conditions. With these issues, saline agriculture involving the use of halophytes plays a crucial emerging role.
Halophytes have demonstrated their importance with is wide range of uses ranging from food production to phytoremediation of stressed environment. By growing and developing agriculture on marginal saline lands, halophytes can help augment the global sources of food, forage, medicine and plant-based chemicals for the growing population. By understanding the stress mechanisms in halophytes, the knowledge can be used in extracting valuable genes for transgenic manipulation in traditional crops.
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