Biosalinity Awareness Project

...understanding the impact of salinization and implications for future agriculture

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Halophytes ~ What, Why & How?
The prefix ‘halo’ and root ‘phyto’ are translated as salt and plant respectively.  Thus halophytes are often described as salt-tolerant, salt-loving, or saltwater plants whereas practically all of our domesticated crops are considered glycophytes (‘glyco’ or sweet) having been selected and bred from sweet or freshwater ancestors.  At the extremes, it is not difficult to distinguish between the two.  Yet at closer proximities in salt-tolerance, the distinction can be arbitrary or blurred with no fixed parameters for their classification. 

What are Halophytes? Some Distinguishing Features

Why Halophytes? Economic & Environmental Benefits

How? Domestication & Improving Salt-Tolerance
 

What Are Halophytes? Some Distinguishing Features

Halophytes are generally defined as rooted seed-bearing plants (i.e. grasses, succulents, herbs, shrubs and trees) that grow in a wide variety of saline habitats from coastal sand dunes, salt marshes and mudflats to inland deserts, salt flats and steppes.  These highly adaptable plants, which can accrue relatively large amounts of salt, are found in every climatic zone where there is vegetation, from the tropics to tundra.  A slightly more refined classification of these salt-tolerant plants differentiates between hydro-halophytes that grow in brackish/saline wetlands, and xero-halophytes that are particularly well-suited to deserts and other low-moisture environments. 

Distinguishing Features

Most halophytes are deep-rooting perennials that achieve their optimum growth and yield potential at thresholds between 6-20 dS/m (EC), levels at which virtually all of our modern crops would perish.  Some of the more prolific thrive in the coastal regions and arid inland deserts with concentrations of 45 dS/m (seawater) and above.  With their vigorous growth and root development, these opportunistic plants are often able to take advantage of less saline moisture within the soil profile and adapt to seasonal variability in salinity by altering germination, growth, and reproduction cycles to best suit their survival needs. 

In general, halophytes produce salt-free seeds which require freshwater and temperatures greater than 55°F (13°C) in order to germinate properly; there are exceptions among the extreme which are able to germinate at seawater concentrations.  As they grow into seedlings and mature, halophytes begin to develop and exhibit the salt-tolerance mechanisms for which they are known.  Distinct life-cycle variations in salt-tolerance have been observed in certain halophytes  including increased sensitivity when a plant is producing seed or forming buds.

Once established, halophytic perennials are better able to retain moisture in the root zone than shallow-rooting annual crops.  Although well-adapted to sandy well-drained soils, persistent root penetration also enables them to perform in denser clay mediums.  Some hydro-halophytes that grow best in brackish or slightly saline waters can shut down completely when exposed to seawater intrusions, staying dormant for extended periods of time before resuming growth as salinity declines.

Native Habitats

Halophytic grasses predominate in the salt marshes, sand dunes, and intertidal zones of the temperate coasts while annual herbs and succulents tend to colonize the shifting mudflats.  Reeds, rushes, and sedges are typically found at the back of salt marshes (subject to freshwater influxes); the more stable, higher ground supports a variety of salt-tolerant herbs, shrubs, and trees.  A number of these species have adapted to saline habitats further inland and distinct climatic zones extending north and south.

Mangrove shrubs and trees populate the inundated portions of the tropical coasts (known as a mangle).  These habitats are often dominated by a few species of mangroves and understory plants which take their place in the mangle according to their salt-tolerance and competitive advantage.  Humid coastlines tend to foster greater diversity while the more arid shores are usually limited to one specie with a small number of herbs and succulents making up the undergrowth.  Halophytic grasses, shrubs, and trees (palms) are found on saline soils further inland.

In spite of their limited diversity, halophytic flora in the coastal zones play an important role in protecting habitats and maintaining ecological stability: preventing erosion and seawater incursions into freshwater habitats as well as providing food and shelter for a far greater number of animal species, both aquatic and terrestrial.  In general, hydro-halophytic communities tend to generate more biomass than those of xero-halophytes due to differences in air and soil moisture. 

The arid and semi-arid regions (roughly half the world’s land surface) are home to a number of shrubby plants and trees as well as corresponding varieties of the coastal grasses, herbs, and succulents.  Able to withstand salinity, drought, heat/cold, and other harsh environmental conditions in both temperate and tropical climates, many of these desert halophytes retain relatively high concentrations of salt, tannins, and saponins in their leaves. 

PHYSIOLOGICAL RESPONSES TO SALINITY

For glycophytes, salinity is an abnormal growth condition and thus perceived as a stress, whereas for halophytes it seems more appropriate to describe salinity as normal and more of a constraint.  Over time, halophytes have evolved to manage and adapt to these constraints; employing a number of physiological mechanisms (biochemical and morphological) that enable them to tolerate elevated concentrations of sodium and chloride in soil moisture.  A long history of adaptation to environmental salinity has conferred to halophytes a hardy and robust constitution with a significant degree of resistance to heat/cold, drought, disease, and pests that plague many of our freshwater crops.

In the last few decades, there has been a growing body of knowledge, scientific research, and anecdotal evidence on plant response to salinization.  A better understanding of how halophytes tolerate elevated salt concentrations has shed some light on the specific mechanisms employed to cope with osmotic stress, the most attributable cause of growth/yield decline.  Perhaps the most distinguishing feature of a halophyte is known as osmoregulation, referring to its enhanced ability to regulate osmotic imbalances that result from increased salinity.  Whether they demand or merely tolerate elevated levels of salt in their environment, halophytes compensate for the high osmotic (low water) potential in the soil moisture by increasing the osmotic potential within their cells and tissue.  The buildup of salts in the plant’s cytoplasm (cell sap) generally increases until it is equal to the concentrations found in the soil, thereby sustaining adequate water intake and cell turgor necessary for continued growth and survival.  By maintaining high internal salt levels, they are able to avoid many of the associated ill-effects of dehydration: i.e. nutrient deficiencies and specific ion toxicity.

Halophytes depend on a combination of interrelated processes, often described as metabolic peculiarities both within their cells, and at higher levels (in whole plant adjustments) which enable them to manage osmotic imbalances.  They depend on these unique regulatory mechanisms, integrated within an overall survival strategy, to maintain the uptake, transport, and excretion of water and salts (nutrients).  Osmoregulation is the basic underpinning of salt-tolerance in halophytes whereby the cytoplasm continues its healthy function in spite of increased salt accumulation.

Exclusion and Excretion

Virtually all halophytes (and most glycophytes) are able to exclude sodium and chloride ions from their soluble nutrient uptake contributing to higher levels of soil salinity.  Salt exclusion by the plant’s roots is often described in terms of elemental substitution or the preferential ion selection of potassium over sodium.  In addition, certain halophytes are known to have feeder roots with an outer protective layer and a waxy inner membrane that effectively filter out salts while allowing water to pass through.

The storage or compartmentation of excess salts within certain organs of the plant is another exclusionary mechanism (known as intraplant allocation) that predominates at the root level and contributes to overall plant salt-tolerance.  As a result of stomatal closure and reduced rates of transpiration, many halophytes are able to confine excess salts within their extensive root systems and the lower parts of the shoot in order to restrict its translocation to the rest of the plant.  Both external and internal exclusionary processes help facilitate healthy leaf development and overall biomass growth while minimizing whole-plant disturbances caused by increased salinity.

Excretion is perhaps the most readily observable self-regulating behavior.  This adjustment is often characterized by the secretion of salty sap through epidermal pores, glands, and bladders located on the plant’s roots, shoots, and leaves.  Intercellular transport mechanisms (pumps) move excess salt ions from surface cells to the outside of the leaf or stem leaving visible crystal deposits once the water has evaporated.  The more highly-evolved halophytic grasses, shrubs, and trees employ this device regularly in order to desalinate internal fluids by excreting sodium and chloride ions at critical periods in their development.

Succulence and Abscission

The ability to retain water, and thus dilute internal salt concentrations, causes leaves and stems to become distended or succulent.  This physiological adjustment in the plant’s water potential is somewhat similar to that of xerophytes when coping with water stress under arid conditions.  Succulents and other halophytes have the capacity to sequester sodium ions in vacuoles (pockets) within the cell by active transport mechanisms and intracellular pumps that help maintain constant levels of salt within the cytoplasm.  This inhibits ion toxicity and helps maintain cell turgor (rigidity) while slight accumulations of water, potassium, and manufactured organic proteins (i.e. proline, mannitol, sucrose and glycine betaine) keep the cell sap from dehydrating and allow for the proper function of essential metabolic processes.  Increased protein production, which requires additional carbon synthesis, is a direct response to the increased salt content and changing osmotic requirements of the cell. 

Another efficient desalination mechanism, the abscission or die-off of older salt-laden leaves and stems (and the subsequent regrowth of new salt-free ones) has a similar detoxifying effect, ensuring plant survival through continuous cycles of salt purges.  Similarly, certain shrubby halophytes have bladder (water storage) hairs on their leaves that become engorged with salts, die, and fall off to make way for new growth. 

Why Halophytes? Economic & Environmental Benefits

Halophytes represent an underexploited and potentially valuable source for many of our basic necessities, increasingly so as salinization and the cost of good quality irrigation water continues to rise.  The availability and expense of freshwater is often the most limiting constraint on the expansion of fertile lands for conventional farming while excessive soil salinity has led to the large-scale abandonment of formerly productive lands.

The greatest potential economic value of halophytes resides in their use of abundant ‘poor quality’ resources (i.e. saline soils and water) to sustain or extend the cultivable life of irrigated farms and bring vast tracts of marginal or non-arable lands under commercial production.  Profitable halophyte utilization has been demonstrated in new coastal agro-ecosystems, sustainable wetland and desert cultivation, and renewed plantings on degraded soils; given the existing infrastructure on previously farmed lands, salt-tolerant crops (i.e. leguminous trees, shrubs, and ground cover) can be planted for immediate benefit.  As the life of the soil gradually returns, mixed cropping systems with salt-tolerant glycophytes can then be introduced, all the while maintaining vegetative cover and conferring continuous benefits.  Over time, improved soil quality will giver farmers more flexibility in dealing with salinity constraints.

COMMERCIAL PRODUCTION

Although many halophytes have long been wild-harvested by indigenous populations inhabiting the coasts, delta marshlands, and arid deserts of the world, serious efforts to domesticate halophytes for commercial production did not begin until the 1960’s.  Thus far, botanists have identified and categorized over 2,000 halophytic plant species from more than 550 genera in over 100 families.  Some of these have the potential for crop production on a commercial-scale, yielding a sustainable supply of renewable resources including food, fodder and forage, fiber and fuel, green manure and other farm inputs, timber and construction materials as well as raw materials for pharmaceutical, industrial, and household products. 

Various studies indicate that more than fifty salt-tolerant species show promise as future sources of nutritious grain and oil while hundreds more bear edible or economically useful roots, trunks, bark, stems, leaves, flowers, fruit, and seeds.  The sustainable cultivation of halophytes and other salt-tolerant crops on appropriate lands can serve commercial purposes without the degradation associated with large-scale annual monocultures and modern industrial agriculture in general.  However, at the present time, much of the speculation about their commercial potential is based primarily on limited field experimentation and analysis, examples of indigenous usage and recent consumer acceptance, and anecdotal extrapolations from various parts of the world. 

Refer to the Salt-Tolerant Plants page for a listing and description of prospective species identified as high-priority or already being cultivated.  The Current Research page contains an extensive listing of organizations and projects promoting halophyte utilization in the field.

Grains & Oilseeds

An important feature of halophytes is that they do not generally accumulate salts in their seed, enhancing their potential for immediate use without additional treatment.  The relatively small size of their seeds, perceived as a disadvantage in harvesting and processing, can often be compensated for by relatively high yields per hectare.  Although they are not generally considered food staples or particularly salt-tolerant, teff and amaranth are examples of extremely small grains that have met commercial expectations while providing superior nutritive qualities.  Similarly, a number of salt-tolerant cereal grasses (Chenopodium, Distichlis, Pennisetium, Sporobolus, Uniola, Zizania, and Zostera) produce large amounts of small high-protein seeds that contain a good balance of amino acids and essential fatty acids, essential vitamins and minerals, and important starches/carbohydrates.

Some of these seed-bearing halophytes (Acacia, Allenrolfea, Argania, Batis, Cocos, Kosteletzkya, and Salicornia) can be cultivated for multiple consumer products as well as conservation purposes.  Like the multipurpose coconut, the coastal salt marsh succulent Salicornia bigelovii is extremely salt-tolerant and one of the most versatile halophytes currently under commercial production.  It is now being harvested as a green vegetable (sea asparagus) for specialty markets while the seed is pressed for its high quality edible oil; the residual meal provides superior feed for livestock and farmed shrimp/fish.  In addition, Salicornia stem and straw can be utilized as cut hay in mixed feeding regimes or manufactured into pressed board for construction purposes.

Vegetables, Fruits & Nuts

Conventional crops harvested for these purposes tend to be the most salt-sensitive.  At the present time, certain halophytes (Aster, Basella, Cakile, Capparis, Ceratonia, Crambe, Crithmum, Eleocharis, Lycium, Manilkara, Opuntia, Phoenix, Pithecellobium, Pouteria, Santalum, Sesuvium, Tetragonia, Trapa, and Ziziphus) are being cultivated for their vegetables, fruits, and nuts.  Many of these are grown for local/regional consumption or specifically for ethnic and specialty markets in the developed countries.  For other potential crops, selection and breeding will undoubtedly improve upon desirable traits such as size and quality.  In this regard, new grafting techniques with salt-tolerant rootstock have already proven successful for certain fruit trees.  Ultimately, the commercial success of halophytic produce in the near future will depend largely on consumer acceptance of new tastes and textures in their diet.

Fodder, Forage & Green Manure

In the past, halophytic grasses, shrubs and trees, containing digestible protein levels comparable to conventional livestock feed, were planted as forage or harvested for fodder.  In spite of their recent decline, forage and fodder still account for the bulk of commercial halophyte cultivation around the world: these include grasses (Distichlis, Hedysarum, Kochia, Paspalum, Puccinellia, Spartina, Sporobolus, and Thinopyrum), shrubs (Atriplex, Salsola, and Suaeda), and trees (Acacia, Cassia, Luecaena, and Prosopis).  Due to the relatively high salt content in their tissue (between 10-50% of their dry weight), the potential is greatest when interplanted with native forage or used in mixed feeding regimes as a dry season browse and fodder supplement.  Many of these salt-tolerant plants have also demonstrated promise in the extraction and production of leaf protein concentrates that are being increasingly used in animal feeds.  Certain nitrogen-fixing halophytes (Albizia, Cassia, Cyamopsis, Luecaena, Pongamia, Sapium, Sesbania, and Trifolium) have been effectively utilized as cover crops, green manure, mulch and compost.

Agro-Forestry & Conservation

Halophytic polycultures for fuel, timber, and conservation offer an extremely valuable asset in the creation of sustainable agro-forestry schemes, the stabilization and rehabilitation of degraded environments, and the deceleration of deforestation and desertification.  Salt- and drought-tolerant shrubs and trees (Acacia, Causarina, Eucalyptus, Melaleuca, Prosopis, and Tamarix) can be planted for conservation and harvested for much-needed cooking/heating fuel and timber.  The replanting of mangrove forests (Avicennia and Rhizophora) on the tropical coasts represents another immediate low-cost opportunity for both environmental restoration and sustainable commercial production.  Mangroves have traditionally been used for fuelwood, charcoal, and building materials, however, over the last 100 years, this vital natural resource has been reduced by half.

Medicinal, Industrial & Domestic Uses

Some halophytes (Cyamopsis, Grindelia, Parthenium, and Simmondsia) have recently experienced increased demand, and are now being harvested commercially for their gums, oils, and resins.  These plant by-products are commonly used in household goods production, food processing, and heavy industrial applications.  Others (Azadirachta, Balanites, Calophyllum, Catharanthus, Hippophae, and Melaleuca) are well-known for their bioactive derivatives and have long been considered essential ingredients for pharmaceuticals, agricultural pesticides, traditional medicines, and natural cosmetics.  Historically, a number of halophytes (Hibiscus, Juncus, Phragmites, Scirpus, Typha, and Urochondra) have been cultivated and/or wild-harvested for paper-pulp, fiber, and other raw materials used in cottage industries and large-scale processing.  In addition, a few (Beta, Leptochloa, and Nypa) are now being considered as sources of renewable phytofuels such as ethanol and biogas.

Ornamentals

A diverse group of halophytes, from grasses to trees, can be efficiently utilized for landscaping and ornamental purposes under saline conditions; salt-tolerant lawn and turf grasses, cut flowers, and landscape plants tend to increase the availability of freshwater for more essential applications.  Although some of these decorative plants (Acrostichum, Catharanthus, Causarina, Conocarpus, Eucalyptus, Hibiscus, Mairreana, Melalueca, and Thespesia) have been identified under their primary economic uses, most are not included in this website.  For an listing of ornamentals, see the University of Osnabrück Halophyte Listing.

ENVIRONMENTAL Conservation

Halophytes are especially well-suited for using brackish/saline water often requiring little or no freshwater in order to rehabilitate degraded vegetative habitats.  For many, the application of both fresh and saline waters in mixed or alternating irrigation programs can provide appreciable cost reductions and resource savings.  Integrated resource management schemes and the multiple use of drainage water for increasing salt-tolerant crops can significantly reduce on-farm consumption and replenish freshwater reservoirs.  With proper management and waste disposal, these schemes can also prevent the further salinization of aquifers and groundwater of surrounding lands and habitats.  The emergent use of salt-tolerant grasses for non-essential commercial purposes (i.e. recreational turfs, ornamentals, and landscaping) highlights the ability of halophytes to take advantage of more abundant saline water resources.

Most halophytes are perennials of varying life spans, where even the short-lived grasses and herbs tend to have extensive and highly opportunistic root systems.  They are often able to exploit increasingly scarce resources, reduce surface evaporation, and control soil erosion that results from natural and human disturbances.  Over time, deep persistent root development and mineralized leaf litter improve soil texture, composition, and fertility by augmenting its organic matter (humus) and nurturing healthy biological processes that were lost to salinity.  Furthermore, the depth and structure of perennial root systems may also increase permeability and the rate at which salts are leached from soils, reducing surface deposits and scalding.

Under waterlogged conditions, halophytes have demonstrated the ability to reduce saline water tables and, to a certain extent, reclaim affected lands.  These deep-rooting trees and shrubs, with their continuous demand for water, help manage salinity and moisture in the upper soil layers, and tend to drive salts below the root zone of most other plants.  In addition, salt-tolerant vegetative bands around agricultural fields can be strategically planted to filter drainage waste (chemical residues) and intercept lateral flows of saline groundwater.  Some of the most promising options for revegetating salt-affected lands include alternate (fallow) and inter-cropping systems, agro-aquaculture, and agro-forestry plantations as well as dedicated conservation schemes.

In the temperate regions, salt marsh grasses and other halophytes help stabilize coastal soils and wetland ecosystems while in the tropics, mangroves are essential for the protection and preservation of fragile marine habitats.  The planting of xero-halophytic grasses, shrubs, and trees in certain arid regions has been proven effective in stabilizing shifting sand dunes, revegetating degraded soils, and increasing agricultural productivity while, at the same time, ensuring a sustainable supply of fuelwood and forage.  On some rainfed lands, the farming of extreme halophytes may actually result in the net removal of salts from topsoils.

Phytoremediation

Phytoremediation is the cultivation of plants for the purpose of reducing soil and water contamination (by organic and inorganic pollutants) that results from the improper disposal of aquaculture, agriculture, and industrial effluents.  On salt-affected soils, phytoremediation is often the only effective and economical method of removing or reducing contaminates, particularly when covering large areas where physical/chemical treatments and leaching are too expensive or unfeasible.  The revegetation of affected lands with water-hungry plants can also assist in mitigating the further spread of pollutants associated with leaching and erosion.

Some glycophytes have the capacity to reduce inorganic pollutants from non-saline soils and wetlands contaminated by nuclear, mining, and other industries.  Likewise, certain halophytes are able to accumulate and transform toxic levels of heavy metals (i.e. lead, cadmium, and selenium) into benign organic compounds.  Research in the San Joaquin Valley suggests that Salicornia is uniquely able to convert high concentrations of inorganic selenate into organic selenium through a process known as volatilization (vaporization).  In this particular case, Salicornia cultivation may also confer economic benefits as the plants can be harvested for selenium-rich animal feed.  A number of halophytic grasses have been proven to be effective in revegetating brine-contaminated soils that typically result from gas and oil mining.

Inorganic elements predominate in the soluble nutrient uptake of plants, oftentimes in the same proportions found in the soil moisture; certain organic solutes are absorbed by the roots, though in relatively small proportions.  In turn, plant roots exude vital nutrients (i.e. proteins and carbohydrates) that encourage the growth and development of soil organisms which help metabolize and reduce the level of organic contaminates.  Thus, well-known techniques (i.e. the application of green manure, compost, and mulch) that support this symbiotic relationship are essential when attempting to break down organic toxins and cleanse the soil. 

Carbon Sequestration

All plants extract carbon dioxide from the atmosphere for photosynthesis and biomass production.  In general, halophyte biomass yields are comparable to those of glycophytes yet the associated costs of cultivation are often far less particularly in areas where there is an over abundance of saline resources.  Halophytic agro-forestry plantations may represent an cost-effective option for sequestering carbon and reducing their elevated levels in the biosphere.  Under the auspices of the UN Conference for the Environment held in 1992, a jointly implemented Salicornia plantation for carbon sequestration has been initiated in Soñora, Mexico.  The first phase (only 30 hectares) to develop carbon accounting procedures and other methodologies is envisioned to expand to 50,000 hectares in this two-part project.

Schemes to colonize inland deserts and other suitable wastelands with appropriate halophytes could be spaced out geographically or situated on elevated plateaus for maximum benefit.  One approach to phyto-sequestration is to plant carbon-hungry shrubs and trees on marginal lands in order to bolster fragile transitional zones and encourage the vegetative perimeter to march back into the desert.  While trying to determine if indeed halophytes can be effectively utilized as carbon sinks, their potential for meeting our more immediate needs for crop alternatives and environmental conservation could be adequately assessed.  Once halophytes become commercially viable, the costs of testing the sequestration hypothesis could be reduced further. 

How? Domestication & Improving Salt-Tolerance

Research on halophytes and the initial interest in biosaline agriculture began in the early 1900’s however it was not until the 1950’s that Hugo Boyko, one of the modern pioneers of saltwater agriculture, began to apply these methods in the field and push the limits of salt-tolerance in conventional crops.  His insights into growing economically useful plants under saline conditions provided an important starting point for subsequent research and the development of the biosaline production concept.  In the early 1970’s, researchers at the University of Arizona and Ben Gurion University of the Negev initiated groundbreaking work on halophyte domestication and the feasibility of large-scale biosaline applications.  Serious efforts to collect and evaluate wild halophytic germplasm in simulated field experiments were undertaken for the first time, recognizing the commercial viability of halophytes and other salt-tolerant crops.

Given the accelerating rate of salinization and prohibitive cost of most reclamation techniques, two distinct options have emerged for improving crop salt-tolerance.  One is the domestication of halophytes and other wild species that grow ‘normally’ on salt-affected lands and do not require large amounts of freshwater.  The other is to improve the salt-tolerance of existing commercial crops allowing farmers to cultivate affected lands utilizing brackish or saline water for irrigation.  Halophyte domestication and improved crop salt-tolerance have yet to be given a high priority due to the fact that salinity is still perceived as a localized constraint rather than a regional or global stress on agricultural production.  In the more insulated economies of the developing world, biosalinity is beginning to seriously threaten their ability to meet the needs of rapidly expanding populations dependent upon local production. 

DOMESTICATION OF HALOPHYTES

As stated previously, the primary impetus for domesticating halophytes (shifting from wild harvesting to cultivation) is the (1) escalating salinization, degradation, and contamination of our soils and water that has led to the present state of biosalinity, (2) difficulty and cost of reversing these trends, and (3) promise of valuable germplasm resources for the development of future salt-tolerant crops.

Domestication refers to the process of plant selection and manipulation that has prevailed for the last 10,000 years, making wild species increasingly adaptable to diverse habitats and more useful to humans.  Our first agricultural crops were fast-growing, nutritious, and sufficiently palatable with energy- and cost-efficient yields that could be harvested within months.  Many of these early cultivars (i.e. wheat and barley) were self-pollinating and did not require much human management or intervention.  In addition to their desirable food qualities, plant selection was also determined by the ease of seed collection, non-shattering qualities, and their propensity to germinate (thin moisture-sensitive seed coats). 

As there have been no major plant domestications in the last 4,000 years, many historians conclude that our ancestors were extremely thorough in their screening, selection, and breeding of edible wild species.  Of the approximately 250,000 plant species, only a few thousand are considered edible by humans, many of which provide only minor supplements to our modern diet.  The dozen or so staples that comprise more than 95% of our nourishment are cereals (wheat, corn, rice, barley, and sorghum), pulses (soybeans, peas), tubers (potato, manioc and sweet potato), sugars (cane and beet), and banana varieties. 

Another factor contributing to effective plant domestication is their removal from competitive environments and cultivation in open fields, giving plants the opportunity to shift the energy needed for aggressive root development and overall survival to above ground biomass (seed and leaf) production.  As an alternative to improving the salt-tolerance of plants that already have desirable commercial traits, the domestication of halophytes is focused on improving the agronomic characteristics of wild salt-tolerant species through selection and breeding. 

By far, the most critical feature of successful halophyte domestication is the infusion of private and public capital for germplasm collection, breeding programs, and biosaline applications in the field.

Germplasm Collection

The collection and preservation of seed and other hereditary materials (germplasm) is perhaps the most immediate priority as it may be some time before halophytes can be bred for commercial and environmental purposes.  The availability of seed and germplasm for research, breeding, and experimentation must be secured by both public and private institutions as an extended gene pool will prove invaluable for future domestication. 

Many of the same techniques (i.e. collection, preservation, and distribution) employed by commercial seed banks and public germplasm collections can also be used for most halophytes.  Private collections are often limited in order to suit the specific requirements of associated researchers while larger public facilities could be broader in scope and serve more diverse needs.  Due to the myriad of factors influencing plant response to salinity, attention should be given to the identification and collection of genotype or population variations among the most promising species.  Germplasm collection and preservation must be given precedence until the economic value of halophytes is fully recognized, markets are established, and commercial seed companies begin to take over this function.

Breeding Programs

Past and present utilization often determines the relevant properties to be screened: i.e. salt-tolerance, nutritional value, palatability, and digestibility.  Screening for the size and quality of yields at different levels of salinity is a crucial step in creating new cash crops.  Other considerations include their adaptability to diverse habitats and current agricultural infrastructure/technology including production, processing, and distribution.

Open field trials (or as close a simulation as possible) during normal growing seasons should be the highest priority once halophytes with desirable traits have been identified.  The aim of breeding cultivars with commercial value and/or other economic incentives linked to conservation involves the continuous improvement of crop characteristics and shifting halophyte programs from the greenhouse into the field.

Commercial Viability

The prospect for adopting halophytes in commercial agricultural production depends on a number of economic factors including the cost of saline resources (soil and water) and other inputs, plant yield assessments, harvesting, processing, and marketing requirements, consumer and end-user acceptance as well as the appropriate valuation of associated environmental costs and benefits.  The current determination of profit is, at best, an imprecise tool for analysis given the skewed values ascribed to agricultural production, and the cost of inputs and farming technologies.  Government-supported price and subsidy policies linked to the production of conventional crops artificially reduce these costs and aggressively support farmers participating in modern industrial agriculture. 

For halophytic crops, the cost of more abundant saline resources (i.e. land and water) is often significantly lower than those needed for freshwater cultivation; the design, planting, and management of salt-tolerant crops on previously-irrigated farms may further reduce costs by taking advantage of existing on- and off-farm infrastructure (i.e. irrigation, drainage, and mechanization).  Actual yields per hectare and the cost of harvesting, processing, and marketing halophytic produce are also important considerations in evaluating their commercial feasibility.  However, until the environmental costs (of chemical monocultures) and benefits (of halophyte cultivation) are properly monetized, traditional cost/benefit analysis will be unable to accurately reflect the economic viability of halophytes. 

IMPROVING CROP SALT-TOLERANCE

The complexity of determining the genetic traits and physiological mechanisms (both biochemical and morphological) that confer salt-tolerance is another important limitation on the development of more salt-tolerant crops.  A lack of specific knowledge about sodium transport processes at both the cellular and whole-plant level has hindered efforts to improve salt-tolerance by either conventional or biotechnological methods. 

It is generally acknowledged that salt-tolerance is a quantitative trait determined by an orchestrated process of sub-traits involving multiple genes, perhaps hundreds or thousands, that are linked to the plant’s ability to minimize salt accumulations and their deleterious impact on growth.  Multiple gene transfers, which occur naturally in conventional breeding, are now being proposed in transgenic approaches to reduce salt-stress in commercial crops.  Refer to the University Research section of the Current Research page for summaries and links to recent developments.

Screening, Selection, and Breeding

Natural selection is driven by either environmental adaptation (hybrids) or, less commonly, sexual preferences (mutants), both of which have the potential to create new genetic or population profiles.  As most offspring do not produce viable seeds, new plant species and genotypes tend to evolve from a few fertile hybrids.  Researchers at Indiana University, Bloomington have recently concluded that hybridization may be more important than mutation or engineering in causing rapid, widespread genetic transformation.  They have shown that a small percentage of fertile sunflower hybrids can produce enough variation to lead to new stress-tolerant species while, at the same time, retaining a wide range of desirable traits from their parents.

Conventional breeding techniques (i.e. screening, recurrent selection, and interspecific hybridization) that exploit existing genetic variability have historically demonstrated the greatest potential for designing new cultivars and increasing salt-tolerance among existing crops.  These ancient methods for improving crop characteristics through the deliberate reproduction of superior plants still holds tremendous promise for domesticating wild species and developing economically useful crops with higher salinity thresholds. 

Given the wide variability of salt-tolerance within certain species and genotypes, simple screening procedures for yield and quality under increasingly saline conditions must be considered as the first step in improving resistance to salt-stress, particularly among important staple crops with maximum salinity thresholds between 5-8 dS/m.  Recurrent selection (the recombination of individual plants) can then be utilized for the creation of salt-tolerant populations that did not previously exist.  Although it may take many generations of crosses before these desirable traits are reinforced and the undesirable ones eliminated, accelerated breeding programs can lead to the formation of new varieties within a few years (in some cases). 

More advanced breeding programs involve controlled interspecific crosses (between species of the same genus) among selected parentage that demonstrate salt-tolerance as well as desirable crops traits.  These hybridization techniques have already had modest successes in improving salt-tolerance within certain cross-pollinating species, such as rice, tomato, wheat, and barley.  Researchers at the University of California, Davis produced a number of commercial tomato hybrids in the 1970’s by crossing domesticated varieties (Lycopersicon esculentum) with inedible wild species (L. peruvianum, L. penellii and L. cheesmanii) from South America.  In some cases, improved salt-tolerance and other characteristics (i.e. higher sugar content) resulted in increased economic benefits for California tomato producers. 

In West Africa, a number of native rice varieties (Oryza glabra), with increased resistance to salt and other abiotic stresses, were selected for their potential compatibility with the widely cultivated Oryza sativa.  As a result, the first true-breeding interspecific lines, known as NERICA, were made available to farmers in 1994.  Although salt-tolerance thresholds may increase marginally, the impact on productivity can be significant, particularly in the developing countries.  By just pushing the envelope of salt-tolerance, improved crop varieties can extend the arable life of salt-affected soils currently under cultivation as well as restore marginal lands to agricultural production.  Wild varieties of barley, wheat, rye, rice, cotton, millet, beets, dates and certain legumes have demonstrated their ability to sustain normal yields when subject to slight increases in salinity.  Thus, inter-specific crosses involving domesticated crops and their wild salt-tolerant relatives have the potential to create increased genetic variability that may prove extremely useful in future biosaline production.    

Far less common are programs that focus on crosses among species of different genera as in the creation of triticale, a commercially-viable offspring of selected wheat and rye parentage.  Since the 1960’s, the few breeding programs concerned with intergeneric crosses between halophytes and glycophytes have been relatively unsuccessful in producing any new crop varieties.  To date, selection and breeding have turned out only a few hybrids or cultivars with both improved salt-tolerance and desirable crop characteristics.  As a result, some researchers have suggested that breeding for salt-tolerance should be abandoned and that crops should be selected strictly on the basis of yield.  This is due, in part, to the associated losses in yield potential when breeding for salt-tolerance as well as the lack of uniformity in salt-affected fields which can be adequately compensated for with higher yields.

Genetic Engineering

Salt-tolerance is a multi-genic trait where responses to salt-stress include biochemical reactions at the cellular level as well as whole-plant morphological adjustments.  Although many researchers claim to have genetically engineered improved salt-tolerance in certain commercial crops, only a handful have been registered or patented and made available to producers.  It is still not yet understood how individual genes, and orchestrated combinations thereof, behave within the whole-plant’s physiological response to salinity.  Until now, only a small number of genes have been identified and isolated, and their role in conferring salt-tolerant traits to salt-sensitive crops is still unclear.  For many researchers, it is difficult to see how one could 'surgically' introduce these genetic traits into plants without the associated biochemical and morphological mechanisms to implement them. 

In the future, genetic engineering may offer incremental techniques (pyramiding) for making commercial crops more salt-tolerant.  Current transgenic approaches typically involve the transfer or introduction of genetic traits in order to enhance a plant’s capacity for excluding and/or tolerating excess salts.  However, advances in the design of transgenic crops, initially dependent upon the identification of salt-tolerant genes, face a number of further obstacles such as the extremely small size of genes, crude methods to isolate, remove and transfer them, and our limited ability to regenerate new plants (in vitro) from single cells.  As mentioned, the creation of salt-tolerant transgenic plants often involves certain trade-offs, such as lower productivity and yield potential.

One approach to genetically engineering improved salt-tolerance involves increasing the levels of protective osmolytes (i.e. proline, trehalose, mannitol, and glycine betaine), active solutes within the cytoplasm that mitigate the effects of abiotic stresses (i.e. salt, water, and heat/cold).  Another transgenic approach focuses on the manipulation or 'overexpression' of genes that regulate intracellular transfers and salt accumulation within the cell sap.  A recent development at the Universities of California and Toronto, that has attracted considerable attention, involves the insertion of genes from cress weed (Arabidopsis thaliana) into rapeseed and tomato plants, enabling them to accommodate higher salt concentrations (vacuole sequestration) in their cells and tissue while maintaining normal leaf, seed, and fruit production.  In the case of tomatoes, researchers assert that a single genetic manipulation was sufficient to induce higher salt-tolerance without compromising growth and quality due to their relatively simple genome.

Within the past decade, progress with molecular markers and quantitative trait loci (QTL) has allowed scientists to map and tag genetic traits associated with salt-tolerance at the cellular level.  Traditional breeding and biotechnology programs are now being accelerated by the use of molecular markers that indicate the presence of salt-tolerant traits without the necessity of laborious and time-consuming screening procedures.  Ultimately, these traits must prove to be inheritable and sustainable over many generations. 


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