Putting Halophytes to Work - From Genes to Ecosystems

 
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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.