What are Halophytes?
Halophytes and soils
Halophytes and water requirements
Depending on their tolerance and demands for sodium salts one distinguishes obligate and facultative halophytes. Obligatory means that they need some salt, facultative means they can live also under freshwater conditions.
Further divisions are hydro-halophytes and xero-halophytes. Hydro-halophytes grow in aquatic conditions or on wet soil. Most mangroves and saltmarsh species along coastlines are hydrohalophytes.
xerohalophytes may grow in habitats where the soil is always saline but where the soil may dry out so much as to cause problems with water availability for the plant. Most species in sabkhas of desert areas are xero-halophytes. Many of them are succulent.
Still another division is morphologically based. One distinguishes succulent halophytes, halophytes with salt bladders on the leaf surface, and those which excrete the salt with evaporation water, where the salt crystals remain visible on the leaf surface. Under lower salinity levels some plants are able to exclude the salt otherwise taken up by the roots.
Many plants fall under several of the above listed categories. They all possess genes which allow them to master the respective salinity under which they must operate.
The overall definition of a halophyte may be therefore:
Halophytes are plants which are able to live under elevated salinities in their growth media. The salinity level in which they grow varies from slight to brackish to medium to severe and to ASWAS (above seawater salinity, see salinity table).
The genetic and physiological properties which enable them to cope with the salt concentration is presently subject of intense research, part of which will be demonstrated during the display of the EXPO 2000.
For our future studies on the utilisation potential of halophytes more precise data are required about the physiology of salinity tolerance and their genetic background present to master the saline environment.
While the salinity tolerance level of all species must be precisely analysed, it is also important to know the demands for climatic and soil parameters.
Halophytes occur in all climates where plants can grow. The partners of the Concerted Action investigate halophytes in hot desert climates with low seasonality as well as in more moderate or even cool and moist climates with strong seasonality. The plants are adapted to temperate levels as well as to seasonal patterns. It is important, therefore, to compare the climate of natural occurrence of a species with the climate where a species is intended to be used. In climates with strong seasonality it might well be that an annual plant can be used during that part of the year where the temperature range is nearly equal to the temperature of the growing season where the plant occurs naturally.
Climate diagrams from Pakistan, Amsterdam,
Most halophytes are genetically adapted to their present habitat. A.
tripolium from Siberia differs substantially in its growth conditions
from populations growing on the North Sea Coast.
The texture of the soil is also important. On the seashore stretches of coarse sand which moves with the waves exerts additional force upon the plants. Its water holding capacity is very low and gets full aeration twice a day with the tides.
Fine sediments at the coasts are either silt or clay which hold the water much longer than the sandy soils.
The chemical composition of the solid particles is also important. Seashore sand may consist up to 90% of broken shells and other structural enforcements of animals. This is mainly CaCO3. In contrast inland sand dunes consist mainly of SiO2 and other ground earth crust material. Saline soils are classified as Solonchak or Solonetz.
Preparation of a lacquerprofile on a Solonchak soil.
As far as farming, cropping or feed production is concerned, the choice of the best irrigation method is as important as choosing the optimal fertilizing technique.
There is a wide span of halophyte utilisation. In most cases amount and quality of the irrigation water as well as economic parameters control any possible utilisation. The key point for irrigation is the demand on salinity of each species chosen for utilisation. There are wide ranges of salinity tolerances among species and even within species wide margins of salinity tolerance as well as requirements. Some of them are given in the listing of the 2600 species by Menzel and Lieth (1999). Because of the importance of this parameter, the CA has developed quick checks for salinity tolerance levels.
For practical applications of halophyte utilisation we need to pay attention to the quality of the irrigation water (e. g. the wastewater qualities from agricultural irrigation systems). Halophytes tolerate more influx of heavy metals and large organic molecules than glycophytes (see table).
Choukr-Allah et al. (1996) describe various applications of urban waste waters. Together with the experiences resulting from saline irrigation experiments at many places around the world we can draw the conclusion that halophyte production systems can be established in an ecologically sustainable way if surface salinity increase is prevented. This requires intermittent irrigation and good drainage.
Most waste waters and even seawater usually contains a sizable amount of fertilizer. Since ion concentration and composition are different at each site, a routine chemical analysis of water and plant material is essential.