ISIS Report 24/02/09
Saline Agriculture to Feed and Fuel the World
Shortage
of fresh water poses a much bigger threat to world food supply than the shortage
of fossil fuels; cultivating salt-tolerant crops could solve both problems
Dr. Mae-Wan Ho
and Prof. Joe
Cummins
A fully referenced and illustrated version of this article is posted on ISIS
members’ website. Details here
An electronic version of the full report can be downloaded from the ISIS online
store. Download Now
Fresh water in short supply and salinization widespread
People now
use about half of the global supply of fresh water, and good fresh water is
becoming an expensive resource. About 1 percent of water on earth is fresh
while another 1 percent is brackish (water that has more salt than fresh water,
but not as much as seawater), while 98 percent is sea water. Agriculture not
only has to compete for limited fresh water resources with home and industrial
use; it is being threatened by the spread of soil salinization.
Irrigation of food and feed crops contributes to salinization. High rates
of evaporation and transpiration lead to salt accumulation in the root zones
as salts are drawn from the deep layers of the soil. Global warming also accelerates
salinization as sea level rises and floods coastal regions. Soil salinization
is irreversible in arid regions because water is not available to leach the
accumulated salts out of the soil. As salinity increases, crop yields decline,
because most existing crop plants are not salt-tolerant.
Saline agriculture to the rescue
To cope with
the shortage of fresh water and increasing salinization of agricultural land,
there has been renewed interest in saline agriculture: cultivating crops that
are salt-tolerant, so they can grow in brackish water and sea water [1].
Two prominent advocates of saline agriculture are NASA scientists Robert Hendricks (Glenn
Research Center,
Cleveland, Ohio)
and Dennis Bushnell (Langley Research
Center, Hampton,
Virginia). They want to see halophytes (salt-tolerant
plants) being used for food, feed, and fuel [2].
They point out that halophytes could be grown in coastal areas, marshes, inland
lakes, desert regions with subterranean brackish aquifers, and directly in
oceans or seas. Cultivating halophytes
would not compete for land that should be cultivating food [3] (see Biofuels: Biodevastation,
Hunger & False Carbon Credits, SiS 33); it would provide more food and feed; and as added
bonus, halophytes provide shoreline erosion protection and feeding areas for
birds, fish and animals.
Some
halophytes may even reclaim the land for freshwater plants. They can leach
soil salt through enhanced percolation and, to some extent, through storing
salt in their leaves that are harvested and removed from the fields.
By selecting and growing both micro and macro halophytes, we could get proteins,
oils, and biomass to provide food, food, and fuel needs.
The oceans are also vast reservoirs of nutrients (nearly 80 percent of required
plant nutrients) that could be recycled back to the land for greater sustainability
in the grand circular eco-economy of nature (see The Rainbow and the Worm, The Physics
of Organisms [4] for more).
Visions
of large-scale industry based on halophytes go back to the 1990s [5, 6], when
it was already seen to provide sustainable fuel-food supply while increasing
the sequestration of carbon dioxide from the atmosphere.
Halophytes under development for food feed and fuel
Bushnell [7]
points out that there are some 10 000 halophytic plant species, of which 250
are potential staple crops. Vast land areas worldwide are salt affected and
major regions overlay saline aquifers. A number of halophytes are now under
development [2].The glasswort (Salicornia bigelovli) is a leafless
annual salt-marsh plant with green jointed and succulent stems indigenous
to the Arabian Sea coasts of Pakistan and India on the margin of salt lakes
and Sri Lanka [8]. It produces seeds that are 30 percent oil and 35 percent
protein; the oil is similar in fatty acid composition to safflower oil, and
hence suitable for edible oil production. Its yield is also superior to soybeans
and other oil seeds [2]. The seawater foundation has several
hundred hectares under development (www.seawaterfoundation.org).
S. bigelovii, farm2.static.flickr.com
The seashore mallow (Kosteletzkya
virginica), a perennial, is one of the many salt-tolerant plants that grow
wild on the coastal marshlands or inland brackish lakes, and serves as a source
of both feed and
fuel [9]. The oil content of the seed is 18 percent,
similar to soybean with a fatty acid composition more like cotton seed; but
unlike them both, it is a perennial, saving a lot of labour in resowing and
sequestering more carbon in the deep roots (See [10] Ending 10 000 Years
of Conflict between Agriculture and Nature, SiS 39, for the advantages
of perennial crops which are being bred in the Land Institute, Kansas, in
the USA to replace the annuals we now grow.)
K. virginica, farm2.static.flickr.com
Distichlis spicata, another
perennial, is one of the halophyte grasses used in response to saline-affected
lands, and is most suited to the high temperatures and high-radiation regimes
in the summer months of southern Australia.
In an extensive soil sampling survey conducted sites in Western
Australia where D. spicata had been growing for 8 years, a marked improvement
in the soil was found compared to control soil, where no grass had been growing.
There was a 12-fold increase in water percolation plus increases in carbon
and nitrogen content [11]. Australia
had an estimated 5.7 million hectares of saline-affected land in 2000, and
projected to reach 17 million hectares by 2050. A test carried out there in
2002 [12] confirmed that several NyPa Distichlis cultivars grow well
in sea water, with green matter yields up to 25 tonnes/ha and tolerating 1.5
times ocean salt conditions.
D. spicata, farm2.static.flickr.com
John Gallagher who heads the Halophyte
Biotechnology Center
at the University of Delaware
has been developing halophytes cultivated in seawater for a long time [13],
producing
hay, protein rich grain, and a spinach-like vegetable.
Algae halophyte for biodiesel
There is a
great deal of activity directed at producing biofuels from algae, the potential
of which
we reported earlier [14] (see Green
Algae for Carbon Capture & Biodiesel, SiS 30). The hope is to find halophytic
algae that produce more than 30 percent its biomass in oil, and cultivation
methods that make it commercially feasible [15]. Many companies have invested
in research and development efforts to bring the cost of culture down and
the production up to the goal of 50g/m2/day of dry biomass set
by the US Department of Energy. Currently, an Israeli company Seambiotic
maintains a 1 000 m2 site that can produce approximately 23g/m2/day,
according to its scientific advisor and algal growth expert Ami Ben-Amotz.
This translates to more than 5 600 gallons/ha/year of algal oil, compared
to palm oil yield at 1 187 gal/ha/y, Brazil
ethanol at 1 604 gal/ha/y, and soy oil at 150 gal/ha/y.
Hendricks
and Bushnell [9] estimate that the theoretical biomass conversion efficiency
is 22 percent of the photosynthetic active radiation (400 to 700 nm), or 10
percent of total solar radiation, and is equivalent to 100 g dry biomass per
day. In the case of algal oil, it would produce 24 500 gal/ha/y. As some 43
to 44 percent of the Earth landmass is arid or semi-arid, there is considerable
potential for developing a multiplicity of seawater irrigated halophyte cultivation
and algal aquaculture. An area the size of the Sahara
desert (13.6 percent of the world’s arid and semi-arid area) would be sufficient
to produce 16 times the energy used by the world in a year (2004). On the
current status of the art, algal aquaculture would produce 27.6 percent of
the energy used in 2004.
Algae ponds, electricitybook.com
Livestock that can thrive on halophytes
There is already
research indicating that various livestock can thrive on halophytes or a combination
of halophytes and conventional feed.
Sheep fed with
halophyte forage was compared with sheep fed Bermuda grass forage or Bermuda
grass mixed with salt to simulate the salt content of the halophyte. Halophyte-fed
lambs gained weight at the same rate as control while the salt amended control
gained significantly less. The halophyte diet appears to have contained balanced
nutrients, which render their high salt level less detrimental than adding
the same salt levels to Bermuda grass hay [16].
Cattle fed a halophytic grass gained weight equally to maize fodder fed controls
[17]. An extensive review listed numerous halophytes including grasses and
legumes that provide suitable forage for animals. The review indicated that
grazing halophyte alone can result in salt overload for some animals so they
stop feeding and begin to lose weight. A mixed ration of halophyte with conventional
hay or maize is therefore advisable. The most salt tolerant farm animal is
the camel, followed by sheep, then cattle, followed by horses, and the least
tolerant are pigs and chickens [18]. Camels appear to be a promising source
of meat in areas where halophytes irrigated with sea water can pasture large
camel herds. Camels tolerate drinking water containing up to 2 percent sodium
chloride while sea water contains in the range of 3.5 percent sodium chloride.
Camels thrive while consuming brackish water and halophytes [19].
Domesticating wild halophytes are the way forward
In view of
so many existing naturally salt-tolerant plants, researchers Jelte Rosema
at the Free University, Amsterdam in the Netherlands, and Tim Flowers at the
University of Sussex Brighton, in the UK think that the best way ahead is
to domesticate wild plants and cross-breed them to produce higher yields [1,
20}. Plants such as sea kale and the asparagus-like samphire, which grow along
the coast all over the world have been eaten for thousands of years. Sea kale
is now farmed in the Netherlands. Spinach and
beetroot are closely related to samphire, and crops such as sugar beet can
grow well in salty conditions. .
Genetic modification experiments have been conducted for more than 30 years
to try to make crops such as wheat or rice salt tolerant. But Rozema and Flowers
say that the genetic manipulations necessary to achieve that for commercial
growing may be too complex at present..
Rana Munns’s research team at the Australian CSIRO (Commonwealth Scientific
and Industrial Research Organisation) in Canberra
had succeeded in breeding a new variety of salt-tolerant durum (pasta) wheat
by crossing with an ancient Persian variety [21]. Modern durum wheat is not
salt tolerant, but wheat originated from around the Mediterranean
which is a heavily salt-affected area. So the researchers went back to the
original wheat varieties to find some that were salt tolerant and crossed
them into the current wheat. They knew that bread wheat tolerates salty soil,
because its roots are good at excluding the salt and letting in the other
nutrients, so they looked for salt in the leaves and selected for those that
had hardly any salt in them. They found an ancient variety from what is today
Iran,
which they crossed with the modern durum wheat to get a new salt-tolerant
variety. The ability to exclude sodium was associated with two genes Nax1
and Nax2 [22].
Identifying genes involved in salt tolerance
Substantial
effort has been dedicated to identifying genes and genetic networks involved
in salt tolerance, so that crop plants could be enhanced in salt tolerance
by conventional selection and breeding. Another approach is to introduce transgenes
into the crop plants to enhance salt tolerance, or influence expression of
the salt tolerance genes. The naturally highly tolerant crops include beetroot,
barley and rye. Moderately tolerant crops include spinach, rice, tomato,
olive, wheat, cabbage and oats [23].
Identifying
the genes for salt tolerance in halophytes facilitates the improvement of
those crops but also provides a source of genes
for improving the salt tolerance of conventional crops. Transcript profiling
of salt tolerant red fescue grass (Festuca rubra ssp. Litoralis) revealed
a complex regulatory network controlling salt stress response. The salt regulated
transcripts included those involved in regulating gene transcription and signal
transduction found in the cells of the root epidermis, cortex, endodermis
and the vascular tissues; while other tissue cells had less active salt transcript
activity. The gene transcription results showed coordinated control of ion
homeostasis and water status at high salinity [24]. Heat stress was found
to alter the expression of salt stress induced genes in the halophyte smooth
cord grass [25].
Small
proteins that regulate salt stress response in Arabidopsis were identified.
Over- expressing one of those genes results in salt tolerance in
the plant. Salt directly affects the small protein’s
signalling by inducing its degradation [26]. Proteomic analysis on grapevine
revealed that 48 out of 800 proteins were altered after exposure to high salt,
including 32 that were up regulated, 9 down regulated, and 2 newly expressed.
The salt stress response suggests that salt spreads systematically throughout
the plant [27]. A gene transcription map was used to identify a set of genes
related to salt tolerance in salt-sensitive indica rice seedling compared
with a natural salt-tolerant relative. Over one thousand salt regulated genes
were identified and several mapped to a QTL (quantitative locus) for salt-tolerance
on chromosome 1. Selected members of the genes are considered candidate transgenes
for crop improvement [28].
Small
regulatory RNA response to salt stress was studied in maize roots. Micro array
analysis identified 98 regulatory RNAs that were altered in activity following
exposure to salt, along with 18 regulatory RNA molecules that were only active
in salt tolerant maize [29].
The
results of these studies do confirm the complexity of salt tolerance, which
is why transgenesis has so far failed in produce salt tolerant crop plants
beyond the greenhouse stage. On the other hand, these results will help considerably
in enhancing the salt tolerance of crops by marker assisted conventional selective
breeding.
Transgenic salt tolerant crops
There have
been a number of attempts to create salt tolerant crops by introducing and
over-expressing certain ‘major’ genes involved in salt tolerance.
Transgenic salt tolerant tomato plants were created by over-expressing
a gene taken from Arabidopsis, encoding the vacuolar Na+/H+
antiporter protein. The transgenic tomato accumulated salt in the leaves but
not in fruit. The transgenic protein transports sodium ions from the cytoplasm
into membrane-bound vacuoles within the plant cell, thereby isolating them
from the cell cytoplasm. Tomatoes that are normally somewhat resistant to
salt become sufficiently resistant to survive exposure to 1.2 percent sodium
chloride that kills the non-transgenic controls [30].
Transgenic
salt tolerant sugar beet expressing the same Arabidopsis vacuolar sodium/hydrogen
antiporter gene used in the salt tolerant tomato accumulated more soluble
sugar but less salt in the storage roots than did unmodified beets [31]. The
same gene over-expressed in trangenic tall fescue (a perennial grass) enabled
the grass to survive 1.2 percent sodium chloride [32]; while transgenic maize
with the antiporter gene survived 0.8-1.0 percent salt solutions [33].
Mn superoxide dismutase (SOD) is a critical enzyme eliminating
reactive oxygen species in plants under environmental stresses. Transgenic
Arabidopsis over-expressing it (more than 2 fold) tolerated 150 mM
(~0.9 percent NaCl). Other antioxidative enzymes such as Cu/Zn-SOD Fe-SOD,
catalase and peroxidase in transgenic plants treated with NaCl were also markedly
higher than those of wild type plants, and contents of malondialdehyde were
lower than those of wild type plants, which shows that Mn SOD plays a key
role in protecting the plant against reactive oxygen species in stressful
conditions [34].
Over-expression
of an NAC transcription factor from rice enhanced both drought resistance
and salt tolerance [35]. The NAC transcription family is large and diverse; it includes those regulating
embryonic, floral and vegetative development, lateral root formation
and auxin signalling, defence, and abiotic stress.
Conclusion
Salt tolerance
is clearly a very complex character, linked to stress and other developmental
responses. Not only is it difficult to genetic engineer successfully in crop
plants. Apart from the usual hazards inherent to genetic modification [36]
(GM
is Dangerous and Futile, SiS 40), the
very complexity of salt tolerance increases the possibilities for unexpected,
unintended effects.
On the other hand, as we have shown, there is much scope for domesticating
a range of existing halophytes that already perform well in salt-affected
environments, and for improving salt tolerant crops by conventional marker-assisted
breeding. That is by far the best way forward
| 
