Overview
The main objective of the Action is to collate existing knowledge of halophytes from gene function to ecosystems that will impact on conservation and management of saline environments and agricultural productions.
The growing human population presents a huge challenge to world agriculture. As more than 40% of the Earth is arid or semi-arid and most of the planet's water is saline, we advocate the sustainable use of these under-exploited resources for human benefit. Halophytes have evolved in saline habitats and are an untapped source of food, fibre and bioenergy. Deepening our understanding of halophytes and saline ecosystems will help combat salinisation, soil erosion, loss of biodiversity and bioproductivity.
Our goal is to create an interdisciplinary group of scientists to bridge gaps between disciplines by jointly exploring the biodiversity of halophytes, re-evaluating their uses as crops, including bioenergy, as sources of salt-resistance genes and for use in the restoration and rehabilitation of salinized or contaminated land. The Action will tackle the problems of salt-affected agricultural land and support the timely development of a saline agriculture using brackish water as a replacement or a supplement for diminishing freshwater.
What are Halophytes?
Salt plants or halophytes (from ancient Greek ἅλς hals,
"salt" and φυτόν phytón, "plant") form an ecologically
distinguishable group among the higher plants, which are adapted
to increased contents of easily soluble salts at their location
and can reproduce under these conditions.
Salt plants colonise salt-rich sites largely independent of
their width in dry to flooded habitats, often close to the sea
and on salt lakes. The mechanisms with which plants can adapt to
extreme environmental conditions and remain photosynthetically
active are very diverse. Some salt plants (the obligatory
halophytes) are promoted in their growth by moderate salt
contents, other salt plants do not need salt for their life
activity. They thrive much better on salt-free soils, but here
they are inferior to the competition of other plants.
Definition
There are several definitions of the term salt plant or
halophyte. One criterion, for example, is that the plants can
grow on salt soil with more than 0.5 percent salt in soil dry
weight. This value is easy to determine, but more important for
the plant is the salt or ion concentration in the soil water.
The definition often used by Jennings (1976) refers to
halophytes as the natural flora on salt sites. Already Warming
(1909), who first defined the term halophyte, wrote: "A certain
amount of soluble salts must be present before halophytic
vegetation is formed; the type of salt appears to be of no
significance. Mann et al. (1983) described the sites as follows:
"[...] contain salt water with an osmotic pressure of over 3.3
bar", which corresponds to a concentration of 70 mM of
monovalent salts.
Plants that do not survive at salt sites, even under exclusion
of competition, are often called glycophyts in German-speaking
countries. This expression is derived from the word "freshwater"
and is found exclusively in the German language. More
appropriate, but less commonly used terms for the same term are
halophobic plants or simply non-halophytes.
Salt-tolerant organisms are generally referred to as halophilic
if, due to evolutionary salt resistance, they are no longer able
to exist in the absence of salt. This salt resistance has
developed independently several times. Therefore, salt plants
are not a monophyletic kinship group, although they occur more
frequently in some plant genera and families.
Among the higher plants, halophytes are only found among the
angiosperms. There are about 1,500 known halophyte species
Families with numerous representatives are the Chenopodiaceae,
Aizoaceae, Frankeniaceae, Plumbaginaceae and among the mangroves
the Rhizophoraceae, Lythraceae, Avicennioideae within the
acanthus family, Combretaceae and Myrsinaceae. There are also
halophytes among the sweet and sour grasses and the Juncaceae.
There are 54 halophytes in Germany. In addition, there are some
transitional forms and intermediate species, which rarely
develop halotolerance or are regarded as bastards of littoral
(Ammophila arenaria) and inland (Calamagrostis epigeios)
species. The existence of such special littoral forms of
otherwise widespread halophytic or sand-dwelling grasses is the
best proof that they must have taken possession of their present
location for a very long period of time and produced the
corresponding forms here.
Classification according to salt dependency
Obligate Halophytes
The obligatory halophytes (obligatory), also called
Eu-halophytes, are bound to their salty environment. Without a
certain concentration of salt as the basis of life, these plants
would not be able to thrive and germinate because they have
largely adapted to the extreme conditions of this environmental
factor. The tolerance range of the obligatory halophytes to salt
is accordingly very large, so that they can survive even in
constant flooding with sea water. The best known native genera
are Queller (Salicornia), Binsen (Juncus), Salzmelden (Suaeda)
and Schlickgräser (Spartina).
Optional halophytes
The narrow-leaved beach lilac (Limonium angustifolium) is one of
the facultative halophytes.
The facultative halophytes (facultative = optional) have the
ability to grow at salt sites, but are not bound to them.
Representatives such as the cinquefoil (Potentilla anserina),
the beach aster (Aster tripolium), the beach plantain (Plantago
maritima) and the beach mugwort (Artemisia maritima) can also
occur in maritime areas. However, they only achieve their
optimum vital functions on soils that are predominantly
salt-free or have only a slight salt content. As they
increasingly encounter competition from other plants in these
areas, these salt plants are often at a disadvantage compared to
the freshwater plants found there. These are better adapted to
their environment and reproduce faster. Optional halophytes have
a larger limited tolerance range to the salinity of the soil
than obligatory halophytes.
Indifferent halophytes
So-called site-differentiated halophytes form a transitional
form to freshwater plants and can usually only be found in
salt-free areas. Their tolerance range is relatively small, but
they can still cope with salt soils with a lower concentration.
In these cases the habitus of the corresponding plant changes in
a variety of ways and deviates from the basic appearance.
Representatives of this group are: Red fescue (Festuca rubra
litoralis), white ostrich grass (Agrostis stolonifera), toad
rush (Juncus bufonius), creeping buttercup (Ranunculus repens)
and wall pepper (Sedum).
Classification by type of salt exposure
Salt can affect the plant in various ways, according to which
halophytes can be basically described as air halin (aerohalin),
water halin (hydrohalin) or terrestrial halin. The last two
categories are intertwined, which is why the term
hydroterrestric halin is often used.
Air halines
Spray of waves on the coast (Calp)
Breaking waves and spray on the oceans lead to the release of
small droplets (sea spray) in the air through a dispersion
process (bulk to particle conversion). Substantial parts of this
are transported upwards by the turbulence of the marine boundary
layer and can partially dry. The aerosol created by such surf
sputtering, which is collectively referred to as sea salt
aerosol, has an effect on plants in areas near and far from the
sea as a result of the high salt air concentration. Many
lufthaline species are at the same time also water haline, so
that there is a closer connection between the two categories.
The purely aerohaline species live as a transition from the
maritime to the terrestrial range in remote surroundings to the
sea and absorb salt almost exclusively via the leaf surface.
Thus the salt content of the leaves on the windward side can be
up to ten times higher than that of the leaves of the same plant
in the slipstream. The yellow horned poppy (Glaucium flavum) is
insensitive to salt dust and splash water, but cannot tolerate
salt in the soil.
In the vicinity of evaporation basins, i.e. certain inland salt
lakes that dry out regularly in droughts and leave a salt desert
behind (desertification), there are also numerous species of
salt plants. The salt present in the air is attributable to such
evaporation processes of the salt water and is absorbed by the
flora living there from the aerobic environment.
Water halines
The maritime species live in the immediate vicinity of both
salt and brackish water and can be found in inland salt waters
from the tidal zone via river estuaries into the interior of the
country. Hydrohaline plants are all those species that are
either perfect or semi-baquatile, i.e. have their centre of life
in or near the water. If the soil is dry and sandy, in the
narrower sense on beaches and dunes, the arenicolous halophytes
there have usually adapted to their environment with a further,
deeper root system. On muddy, often flooded ground, which
ensures direct water uptake, plants with smaller, but also
stronger, non-washable roots are more likely to grow. Some
closed plant formations have obviously specialised in diluted
seawater from river deltas and river mouths as habitats, while
others live in a pronounced sand gap flora on salt-concentrated
lakes and inland seas such as the Dead Sea. The salt content of
thalassohaline sites varies over a wide range and can correspond
to that of seawater (3.5 %) up to the salt content of a
saturated sodium chloride solution (30 %). The transitional
forms to terrestrial and aerohaline salt plants form more
diverse areas remote from salt water, which have settled on
normal soil or rock debris (preferably sedum species) above the
water level usually reached at high tide and are normally only
reached by the salty spray.
Terrestrial Haline
Terrestrial Haline is the name given to all terrestrial species
that have specialised in inland salt sites. They absorb salt
exclusively via the soil. To the salt-plants not occurring at
the beach belong for example the rock-mugwort (Artemisia
rupestris), slit-leaved mugwort (Artemisia laciniata) or the
land-riding grass (Calamagrostis epigeios). Also in desert and
steppe regions appearing plants, like species of the genus
Atriplex are to be understood as such from the water remote
forms, which can be found preferentially under
location-indifferent Halophyten.