After reading this article you will learn about the meaning and types of halophytes.
Meaning of Halophytes:
The term halophyte literally means salt plant. According to Jennings (1975) halophytes are defined as the native flora of saline soils and has assumed the salt concentration with osmotic pressure of at least 3.3 bars. The non-halophytes are called glycophytes (sweet plants).
Types of Halophytes:
Ecologists have sub-divided halophytic plants into various categories e.g. Van Eijk (1939), Toopa (1939), but in so far as they did not indicate what was or was not a saline soil, the categories are not satisfactory. Iversen (1936) also published a classification of halophytes but it is unsatisfactory in that the lower limit for halophytic vegetation is taken as 0 01% NaCl in the soil water, a value that is certainly too low.
In 1942 Chapman modified the Iversen scheme in order to incorporate the general thesis that 0 5% sodium chloride or sodium sulphate present in the soil water represents the critical limit between glycophytic and halophytic conditions.
Chapman (1966) classified the halophytes into the following:
i. Miohalophytes (Plants of low salinity):
Plants that grow in habitats with a range of 0-01 to 10% NaCl, Na2SO4 or Na2CO3 in the soil water. Such plants are clearly capable of tolerating more than the limiting value of 0-5% salt.
ii. Euhalophytes (Plants of high salinity):
These are sub-divided into following:
Plants that grow in habitats with a salinity of 0 5 to 10% NaCl.
Plants that grow in habitats with a salinity range of 0-5 to more than 10%.
Plants that do not occur where the soil water concentration is less than 1-0%. Such habitats are mainly to be found bordering the salt lakes and seas.
Halophytes may be classified on the basis of degree of moisture in the soil into following:
Plants grow in areas with more or less (but continuously) wet salt soil with a very high water table.
Plants grow in saline soils which are not continuously wet. The water content in the soil is very low and this results in a high salt concentration of the soil solution. The xerohalophytes are Atriplex nummularia, Zygophyllum species, Mesembryanthemum species and others.
Steiner (1934) distinguished three types of halophytes with regard to regulation of their salt concentration.
1. Regulation by increase of succulence and hence dilution of the salt concentration in the cell sap.
2. Regulation by salt excretion with the salt glands in the desalting halophytes.
3. Regulation absent, leading to a constant but slow increase in the salt concentration during the vegetative periods, e.g. Juncus.
Waisel (1972) classified the halophytes on the basis of plant – soil relationship, salt resistance mechanism and internal salt relationship into the following:
1. Salt-requiring halophytes:
A. Obligatory halophytes:
Plants survive only in saline environment e.g. Salicornia spp., Aphanothece halophytica (Blue green alga).
B. Preferential halophytes-:
Plant whose growth and development improved in the presence of salt, e.g. Suaeda spp. Aster spp.
2. Salt-resisting halophytes:
A. Salt-enduring or salt tolerant halophytes. Plant enduring a high protoplasmic salt content.
B. Salt-excluding halophytes:
Plant accumulating salts in special hair e.g. Atriplex spp. plant secreting salts from their shoot (Tamarix spp.); plants re-transporting salts from the shoot into root.
C. Salt evading halophytes:
Plant evading salt uptake (Rhizophora); evading salt transport into the leaves (Prosopis foxta).
II. Pseudo halophytes:
Plants behave like halophytes, e.g. ephemerals.
Halophytes can be clearly distinguished on the basis of salt composition in their ash into the following:
1. Sulphate halophytes:
These are characterised by high sulphate content in their ash which exceeds the chloride content, e.g. Salsola rigida, S. dendroids Korelinia caspia.
2. Chloride halophytes:
These are having ash with low sulphate content which may in part originate from organic compounds; e.g. Salicornia europaea, Anabasis aphylla, Salsola furcomanica.
3. Alkaline halophyte:
Halophyte which contains much higher equivalents of cations (Na+ and K+) than of inorganioanions (Cl– and SO42- ) e.g. Suaeda microphylla, Haloxylon aphyllum.
4. Desalting halophytes:
Plants excrete salts (mainly NaCl) through special organs, the salt glands. These contain considerably less Na+ and Cl– in their total dry matter than non-desalting halophytes, and are non-succulent, e.g. Tamarix laxa, T. ramosissima; Aeluropus litoralis.
The halophytes of inland saline’s of Western Rajasthan can be classified into the following:
Plant species which attain optimal growth on soil having NaCl above 0-5% e.g. Aeluropus lagopoides, Cressa cretica, Haloxylon recurvum, Heliotropium curassavicum, Salsola baryosoma, Spurobolus helvolus, Suaeda fruiticosa, Zygophyllurn simplex.
2. Transitional halophytes:
Plants growing only at the transition of saline and non-saline areas e.g. Cassa italica, Haloxylon salicornicum, Sporobolus marginatus, Salvadora persica. Tragus racemossus.
3. Salt accumulating halophytes:
These thrive well in rising salt concentration by increasing their permanent water content. They are highly succulent. This type of halophytes in Thar Desert are Haloxylon recurvum, Salsola baryosoma, Sesuvium sesuviodes, Trianthema triquetta, Zygophyllurn simplex.
4. Salt secreting halophytes:
These halophytes have special salt secreting glands. The salt glands act as desalinization device, which maintains the salt balance in the leaves by the secretion of excess of salts. The salt glands are present in Aeluropus lagopoides, Chloris virgata, Cressa cretica, Sporobolus helvolus, Tamarix spp. etc.
Characters of Halophytes:
The word halophyte means salt plant. The plants can grow in the high salt concentrations. The characters of halophytes are following:
1. Shore building:
The unique mangroves grow in places where other trees and shrubs cannot survive in the wet, salty world of tide washed tropical and subtropical seas. Along such coasts, mangroves have created virtual jungles on stilts.
A halophyte seedling floats horizontally in the water, like a well-designed canoe. The seedling can float for months, unaffected by salt water, scorching sun, and battering waves and even continue to grow. Its sharp root tip turns downward.
3. Prop roots:
From seedling new roots emerge in tiers that extent and nut and downward from the trunk, forming arches called prop roots. The prop roots may send up new trunks where they touch the ground. The drop roots form efficient anchor of plants in the loose sandy soil.
4. Pneumatophore or respiratory root:
Root respiration is difficult in water logged and saline soil. In such situations halophytes send root up from beneath the soggy ground. These roots are erect, branched or un-branched, asparagoid, bent knee-like and knee projecting above water. These ‘air roots’, which may be a 30 cm or more high absorb oxygen from the air when the underground roots are covered. –
The rate of growth of halophytes is slow. In 20 or 30 years, the halophyte reaches its maximum height (about 10 metre).
6. Swamp formation:
The profusion of prop roots and new tree trunks form a dense, interlacing mesh that traps sediment, plants and debris. Soon a swamp is formed at the edge of the sea. Gradually, as more mangroves sprout up, new land is created. Every year, the land advances a few cms into the sea.
Vivipary is an extra-ordinary phenomenon i.e. germination of seed while it is still attached to the parent tree. The seed germinates without undergoing any rest period. The seeds of Rhizophora, Bruguiera, Aegiceras, Avicennia and Ceriops sprout while they are still attached to the flowering twig.
A root bursts through the fruit and forms a long, dagger-like point that may reach 20 cms in length. When the seed finally detaches from the branch, it falls, often planting itself upright in the soft, muddy bottom.
All littoral halophytes show very wide areas of distribution. This is because of medium and uniform temperature. The air containing spaces in the integument or in other parts helps fruits, seeds and seedling of mangroves in floating and in doing so are not robbed of their discriminating power.
9. Osmotic potential:
The salts which are absorbed by halophytes accumulate in transpiring organs. The presence of salt in cell vacuoles of these organs causes a decrease in osmotic potential and will not affect the hydration of the protoplasm as by water stress.
10. Chloride accumulation and succulence:
The degree of succulence for halophytes grown in nutrient solutions with added NaCl parallels NaCl concentration. The increase in leaf NaCl concentration results in an increase in succulence.
11. Salt uptake:
The salt concentration in the soil and in the cell sap changes seasonally. The highest uptake of NaCl into the plants occurs ring when plants grow fastest.
12. Nutritional imbalance:
In saline environment the toxic ions influence the physiological and bio-chemical processes of biological system, excess of certain ions disrupt the membrane and other basic metabolic processes which are basic to biological system or plant.