permaculture

["Lawrence F. London, Jr." <lflj@intrex.net>]

http://lists.ifas.ufl.edu/cgi-bin/wa.exe?A2=ind0902&L=sanet-mg&T=0&O=D&P=119674
SANET-MG Archives -- February 2009 (#171)
Subject:         Saline Agriculture to Feed and Fuel the World
From:   jcummins <[log in to unmask]>
Reply-To:       Sustainable Agriculture Network Discussion Group <[log in to 
unmask]>, jcummins <[log in to unmask]>
Date:   Wed, 25 Feb 2009 20:55:18 -0500

http://www.i-sis.org.uk/SalineAgriculture.php
ISIS Press Release 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.

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.