Some plants grow and complete their life cycle in the habitats with a high salt content. They are known as salt plants or halophytes.
According to Stocker (1933), the critical level of salinity for plants is 0.5% of the dry weight.
Though the fact that only a small group of higher plants can grow in the saline habitats was recognized many hundred years ago yet the name “halophyte” was assigned to such plants by Pallas in the early nineteenth century.
Such plants are commonly found near sea-shores where mesophytes and fresh water hydrophytes cannot thrive well. Although these plants grow in the areas which are well saturated with water yet they cannot avail of the water because of high concentration of salts in the soils. Thus, the halophytes are plants of physically wet but physiologically dry habitats. Plants cope with the problems of salinity in various ways, some of them avoid salinity, some evade salinity or resist it, and a few others tolerate salinity.
Salinity avoidance is usually accomplished by limiting germination, growth and reproduction to specific seasons of the year as well as by growing roots into non-saline layers and limiting salt uptake. Plants evade or resist salinity by accumulating salts in their cells as well as by secretion of excess salts. In the salt tolerant, the protoplasm functions normally and endures a high salt concentration without apparent damage.
Plants occupying only local non-salty ecological niches in an overall saline environment or those which appear in such habitats only for short periods, i.e., during rainy season are called psuedohalophytes or false halophytes (Yoave Waisel, 1972). Saline habitats are not restricted to sea coasts only but they may also be found in many dry places far away from the sea coasts.
In India, certain places in Rajasthan and in many other deserted places, the soils are very salty because of presence of sodium chloride, calcium sulphate, sodium bicarbonate, potassium chloride, etc. In these dry and salty habitats generally succulent xerophytes, e.g., Chaenopodium album, Suaedafructicosa, Haloxylon salicorneum, Salsola foestida, Tamara articulata grow very successfully and form micro-edaphic formations.
Classification of Halophytes:
Classification of Saline Habitats which is as Follows:
(a) Habitats affected by salt spray (maritime)
(b) Habitats affected by salt dust (salt desert)
On the basis of contact between salt and plant in different habitats halophytes can be grouped into the following categories:
1. In terrestrial saline habitat, the contact occurs between the plant roots and soil-terrestrial halophytes.
2. In aquatic-haline habitat contact occurs:
(a) Between salt and plant roots-emerged halophytes or hygrohalophytes, and
(b) Between salt and entire plant body—submerged halophytes or hydro-halophytes.
3. In aero-haline habitat, the contact occurs:
(a) Between aerial organs of plants and air borne salt droplets (in coastal regions), and
(b) Between aerial organs and salt droplets (in dust deserts)—Aero-halophytes.
Iversen (1936) classified the haline habitats on the basis of their salt contents. Different habitats and plants found therein are given below:
Besides above-mentioned three types of halophytes, there are some other halophytes which grow in habitats with wider salinity ranges. Plants occurring in both oligohaline and mesohaline habitats are called oligo-mesohalophytes and those existing in all the three types of habitats are called euryhalophytes.
According to Chapman (1942), halophytes have been classified into the following categories:
(i) Miohalophytes—Plants growing in the habitats of low salinity (below 0.5% NaCl).
(ii) Euhalophytes—Plants of highly saline habitats.
They have been further sub-divided into the following groups:
(a) Mesohalophytes—Plants of habitats with salinity range of 0.5 to 1%.
(b) Mesoeuhalophytes—Plants of habitats with salinity range of 5% and higher.
(c) Eneuhalophytes—Plants of habitats with salinity range of 1% and up.
Van Eijk (1939) classified halophytes into the following two main categories on the basis of their distribution and their responses to saline habitats.
(i) Salt enduring halophytes which show optimum development in non-saline habitats but can tolerate salts.
(ii) Salt resistant halophytes which show optimum development in saline habitats.
Tsopa (1939) classified halophytes into the following four groups on the basis of their response to salinity.
1. Obligatory halophytes:
Plants requiring salinity throughout their life.
2. Preferential halophytes:
Plants show optimum growth in saline habitats, despite their appearance in non-saline habitats.
3. Supporting halophytes:
Non-aggressive plants which are capable of growing in saline
4. Accidental halophytes:
Plant which grow in marsh saline habitats only accidentally.
Steiner (1935) classified salt marsh plants into the following three types:
(a) Succulent halophytes:
Plants which can tolerate high concentration of chloride in their cell sap due to increased succulence (as for example, Salicornia herbacea).
(b) Non-succulent halophytes:
Plants resisting salts by desalinization of their tissues and secreting excess salts through salt glands (e.g., Spartina alterniflora).
(c) Accumulating type:
Plants without any special mechanism of salt removal. Salt concentration in such plants goes on increasing until the death of plants (e.g., Juncus gerardii, Suaeda Jructicosa).
Coastal angiospermic halophytes have been divided into the following groups:
(i) Submerged marine halophytes—hydrohalophytes,
(ii) Low coast plants—hygrohalophytes,
(a) Swamp halophytes, the mangrove, and
(b) Marsh halophytes,
(iii) High coast plant —aerohalophytes.
Mangrove is a West Indian name given to formation of trees and shrubs inhabiting the coasts and river estuaries of tropical or subtropical seas. Plants occurring in mangrove vegetation belong to several families. Common mangrove plants are Rhizophora mucronata, Kandeha rheedii, Ceriops roxburghiana, Bruguiera gymnorhiza, and many other plants belonging to the family Rhizophoraceae, Avicennia officinalis of the family verbinaceae, Acanthus ilicifolius, (Acanthaceae), Ardesia humiles and Aegiceras majus of family Myrisinaceae, Cassula species (a very common plant of rice field in northern India belonging to the family compositae), Cometes surathensis (Caryophyllaceae) which is commonly found in western Asia, Avicennia, Sechium edule (Aroideae)—an Indomalayan plant, Cryptaeorine ciliata (Aroideae), a common plant growing in the muddy banks of Ganga, Poa, Festuca, Melacanna, and Oryza patnai (rice) of family Gramineae, Scirpodendron of Cyperaceae and a number of other plants belonging to the families Palmae, Parkeriaceae Chaenopodiaceae, Plumbaginanceae, Ficoideae, Euphorbiaceae.
Mangrove plants are very commonly found on some saline soils of Indogangetic plains (Bay of Bengal, Sunder-bun and Assam), in western India near the sea coasts of Mumbai and Kerala, in the banks of Gaumati and Godavari in south India,, particularly in the regions where rivers meet the ocean, and in Andaman and Nicobar Islands.
Mangrove vegetation of Gangetic estuary, particularly of Sunder-bun region, according to Prain, is localized in the following three geographical zones:
1. Southern coastal strip and south-western part,
2. Central zone, and
3. North-eastern part.
The Central zone is characterized by climax forest of Heritiera. Ceriops is one of the dominant plant species on the higher grounds near the sea and at certain places it may form pure forest in Sunder-bun.
Mangrove vegetation in the western sea coast of India is represented by the following plant communities:
(i) Avicennia alba and Avicennia officinalis community.
(ii) Acanthus ilicifolius and Avicennia alba community.
(iii) Suaeda fructicosa populations,
(iv) Pure community of Salvadora species,
(v) Sesuvium portulacastrum population, and
(vi) Aeluropus repens population.
Godavari delta in Andhra Pradesh shows characteristic mangrove vegetation of Avicennia alba Avicennia marina, Rhizophora, Bruguiera, Ceriops, Sonneratia. Acanthus ilicifolius, Myriostachya weightiana, Clerodendron inerme, etc.
The ecological conditions which are essential for the development of mangrove vegetation or halophytes are:
(a) Shallow water with thick mud,
(b) Water logged saline soil or sandy or loose soil or heavy clays containing large amount or organic matter,
(c) High rainfall, and
(d) High humidity in the atmosphere and cloudy weather.
Important Characters of Halophytes:
As the water in the habitat is not such as can easily be absorbed by the plants, the halophytes develop in them almost all important xerophytic devices for water economy.
A great majority of halophytes in the tropical and subtropical regions are shrubs, but a few of them are herbaceous, for example. Acanthus ilicifolius. In temperate zones, halophytic vegetation is purely herbaceous. The shrubs are generally dome-shaped in appearance because of their cymose branching.
(i) Halophytes develop many shallow normal roots. In halophytes, in addition to normal roots, many stilt or prop roots develop from the aerial branches of stem for efficient anchorage in muddy or loose sandy soil. These roots grow downward and enter the deep and tough strata of the soil. In some plants, e.g., Rhizophora mucronata, the stilt roots may be strong and extensively developed, but in others they may be poorly developed (Rhizophora conjugata). In some plants, the stilt roots may not at all develop (Fig. 10. 1).
(ii) Sometimes, a large number of adventitious root buttresses develop from the basal parts of tree trunks. These root buttresses provide sufficient support to the plants.
(iii) The soil in coastal region is poorly aerated and it contains very small percentage of oxygen because of water logging. Under such conditions, the roots of halophytes do not get sufficient aerahon. In order to compensate this lack of soil aeration, the hydro halophytes develop special type of negatively geotropic roots, called pneumatophores or breathing roots (Fig. 10. 2).
The pneumatophores usually develop from the underground roots and project in the air well above the surface of mud and water. They appear as peg-like structures. The tips of these respiratory roots may be pointed. They possess numerous lenticels or pneumathodes on their surface and prominent aerenchyma enclosing large air cavities internally. The gaseous exchange takes place in these roots through the lenticels.
The aerenchyma helps in the conduction of air down to the subterranean or submerged roots. In some plants, e.g., Bruguiera, the horizontal roots grow above the surface of mud and then again bend downwardly and enter deep into the mud. In this way, they form knee-like structures. The aerial surface bears a number of pores which facilitate the exchange of gases (Fig. 10. 3). Pneumatophores do not develop in some species of Rhizophora. In those cases, the upper aerial parts of descending stilt roots probably take up the respiratory activity.
Stems in several halophytes develop succulence. Salicornia herbacia (Fig. 10.4) and Suaeda maritima may be quoted as familiar examples for it. According to Arnold (1955) the succulence depends on the ratio of absorbed to free ions in the plant cells rather than absolute amounts of sodium chloride or sulphate present. Succulence is induced only after the accumulation of free ions in an organ increases above a critical level.
According to Pokrovskaya (1954, ’57) salinity inhibits the cell division and stimulates cell elongation. Such effects cause decrease in the cell number and increase in cell size, so typical of succulents. Repp et al. (1959) are of the opinion that succulence is directly correlated with salt tolerance of plants and the degree of their development can serve as an indicator of the ability of plants to survive in highly saline habitats. The temperate halophytes are herbaceous, but the tropical ones are mostly bushy and show dense cymose branching. Submerged marine angiosperms are among the very few species of halophytes that do not become succulent.
The leaves in most of the halophytes are thick, entire, succulent, generally small-sized, and are often glassy in appearance. Some species are aphyllous. Stems and leaves of coastal aero halophytes show additional mode of adaptation to their habitats. Their surfaces are densely covered with trichomes. Leaves of submerged marine halophytes are thin and have very poorly developed vascular system and frequently green epidermis. They are adapted to absorb water and nutrients from the medium directly.
(d) Fruits, Seeds and their dispersal:
The fruits and seeds are generally light in weight. Fruit walls have a number of air chambers and the fruits, seeds, and seedlings which can float on the water surface for pretty long time are dispersed to distant places by water current. Mangrove vegetation’s of tropical sea-shores from Australia to East Africa include approximately the same species of plants. Similarly, the mangroves of West Asia show considerable resemblances with those of East Asia and East Africa.
It is due, in part, to the fact that medium and temperature remain uniform throughout and partly due to the efficient means of dispersal or migration of plants. A littoral species of Spinifex (S. quarrosus), a member of Gramineae commonly growing in the sandy saline sea shores in Andhra shows peculiar type of fruit dispersal. In this plant, female inflorescence is spherical in shape and consists of many spikelets (Fig. 10.5).
A number of stiff bractiolar bristles of the inflorescence help in the dancing and somersaulting of the inflorescence. When the seeds mature the globular and hairy inflorescence becomes bodily detached from the creeping plant and trails on the sandy substratum dropping its seeds at places. Finally the inflorescence with rest of fruits becomes buried in the mud.
(e) Viviparous mode of seed germination:
Halophytes or mangrove plants growing in the tidal marshes are met with the phenomenon of ‘vivipary’ which is defined as the germination of seeds while the fruits are still attached to mother plants (Fig. 10.6).
In Rhizophora plants, when the embryo reaches advanced stage of development the massive club-shaped hypocotyl and terminal radicle pointing downwardly emerge out of the fruit. When the hypocotyl attains a length of several centimeters (about 50-80 cms), the seedling falls vertically down. Thus, the radicle and a part of hypocotyl become fixed in the mud and the remaining upper part of hypocotyl along with other embryonal parts, such as plumule and cotyledons remains above the surface of mud or water.
Within a few hours the radicle develops a tuft of roots and plumule also starts growing rapidly. Sometimes, seedlings fall in deep water and they float on the surface of water vertically with hypocotyl pointing downwards. On reaching shallow areas, the radicle becomes fixed in the soft mud and the plant starts growing rapidly.
It is noticed that high degree of salinity in the soil or water checks the germination of seeds. So the viviparous germination is a very significant adaptation in these plants to avoid the retarding effects of salinity on seed germination. Species of Rhizophora, Aegiceras, Avicennia, Cassula, Ranansatia vivipara are some of the common examples for vivipary.
3. Anatomical Features:
The appearance and structures which characterize certain groups of plants sum up to a great extent their ecological and physiological means of adaptation. Halophytes are no exception to this rule because of specific land typical structural characteristics which make them distinguishable from other groups of plants.
1. Large cells and I small intercellular spaces,
2. High elasticity of the cell walls,
3. Extensive development of water storing tissues,
4. Smaller relative surface area (surface/volume ratio),
5. Small and fewer stomata, and
6. Low chlorophyll content.
Anatomy of halophytes reveals a number of xerophytic features in them. These are as follows:
(i) Presence of thick cuticle on the aerial parts of the plant body. The epidermis of xerosucculents and coastal halophytes is characterised by a cover of waxy layers in addition to thick cuticle (Uphof, 1941) (Fig. 10.7).
(ii) Leaves may be dorsiventral or isobilateral. They develop protected stomata which are not deeply sunken. Epidermal cells are thin-walled. The palisade consists of several layers of narrow cells with intercalated tannin and oil cells (Fig. 10.8).
(iii) Stems in the succulent plants possess thin-walled water storing parenchyma cells in them. Mucilage cells may be found in abundance. Epidermal cells of various mangrove species contain large quantities of tannins and oil droplets. Cortex is fleshy, several cells thick and in old stems it may become lacunar. Salinity causes extensive lignification of stele (Fig. 10.7).
(iv) The leaves and stems of coastal halophytes are abundantly covered with various types of simple and branched trichomes giving the plants a greyish appearance (Fig. 10.9).
The trichomes may exert a protective function in plants by:
(a) Affecting water economy,
(b) Affecting the temperature of the leaves, and
(c) Preventing sea water droplets from reaching the live tissues of leaves.
(v) Leaves of many species of mangrove are dotted with local cork formation “cork warts”. Leaves of Sonneratia and Aegiceras and Nitraria (a desert shrub), Suaeda monoica contain well developed aqueous tissue (Figs. 10.8, 10.9, 10.10) (Mullan, 1932). Salt secreting glands may be found in some halophytes.
(vi) The stilt roots of mangrove plants show normal features with periderm on the surface, aerenchymatous cortex containing sclereids, normal endodermis, secretory pericycle, radially arranged xylem and phloem and extensively developed pith (Fig. 10.11). Pneumatophores develop a number of lenticels on their surface. The cortex is spongy and consists of extensively developed aerenchyma enclosing large air chambers.
Highly developed air chambers are continuous with the stomata of leaves and with the cortex and primary phloem of the stems. Pneumatophores show variations in their internal structures. Generally, they show conjoint, collateral vascular bundles with endarch xylem at maturity. Very young pneumatophores, however, show root features, i.e., exarch xylem and radial arrangement of vascular tissues (Fig. 10.11). Generally the negatively geotropic breathing roots show features of stem and not of roots (Fig. 10.12).
4. Physiological Adaptations in Halophytes:
Morphology and anatomy of the halophytes clearly show xeromorphic features in them. Now, these plants are growing in an environment where water is available to the plants in abundance then why xeromorphy develops in halophytes? Previously physiological drought was believed to be the main cause of developments of xeromorphy in halophytes, but recent physiological experiments on these plants have proved that xeromorphism in these plants is, apparently, an example of purposeless adaptation. Physiological experiments make it clear that the halophytes do not experience difficulties, whatsoever, in absorbing too saline water.
This point is concluded taking into consideration the following reasonable facts:
(i) They show high rates of transpiration,
(ii) They show exudation of sap that contains dissolved salts, and
(iii) They develop many shallow absorbing roots.
Saline conditions are not essentially “dry” for all plant species. Under saline conditions sometimes higher transpiration rates have been observed in halophytes than in neighbouring salt hating plants (Delf, 1911; Braun-Blanquet, 1931).
It should, therefore, be admitted that the halophytes show xeromorphism for enduring high salinity of soil water and also for absorbing water with perfect ease. The significance of succulence is not so clearly understood. Probably, it is induced by accumulation of salts in cytoplasm. It seems reasonable because of the fact that sodium salts if present in the soil water will definitely stimulate succulence even in non-halophytes and characteristic succulence of some plants may disappear if they are grown on the soil lacking in these common salts.
Excessive accumulation of sodium does not harm these plants. Halophytes grow in saline habitats not because they are salt loving but because they tolerate high concentration of salts better than other plants of non- saline habitat. Active accumulation of salts also increases the osmotic concentration of cell sap in these plants and thus makes them able to absorb salty water very easily.
Succession of Mangrove Vegetation in Sea Coast:
The distribution of a halophytic community appears to be limited by salinity and depth of water table, as well as the competitive ability of the members of next community in the halosere (Reed, 1947). The aggressiveness of plant communities in saline habitat is due to changes in the salinity level.
In coastal region, nature of vegetation is greatly affected by the gradual elevation sea coast Succession of mangrove formation in coastal regions may take place very slowly in the following sequence:
(1) In deep water generally true mangroves, e.g., the species of Avicenma grow.
(2) When the bottom of the sea is slightly raised up, Avicennia, Rhizophora, Ceriops, Bruguiera etc. form mixed mangroves vegetation in the shallow water.
(3) As the ground is exposed, true mangroves disappear and other halophytes, e.g., of Aegiceras, Icoecaria, etc. gradually invade the land within short period. These halophytic communities are interspersed by salt tolerating succulents and the grasses make the soil fit for the cultivation of some crop plants. Several varieties of paddy, such as Oryza sativa var, achra, wild species of Oryza coarctata grow very well in these areas.
Succession of angiospermic halophytes varies also in different habitats in accordance with other ecological conditions besides salinity. Thus, in reality there is no general trend for development of various halophytic plant communities around the world and local variations are encountered in each specific site (Yoav Waisel, 1972).