Mangrove Sonneratia spp.-the super Halophytes of Intertidal Zone?
Salt. Saline environment. Species. Salt Glands. Pneumatophores. Lenticels. Osmotic Adjustment. Anoxia. Compatible Solutes. NaCl. Comparmentation. Tonoplast.
The word Mangrove is considered a combination of the Portugues word "Mangue" and the English word "Grove" Mangue means salt tolerant plants while grove denote breeding, feeding, and nursery ground for many estuarine and marine organisms ( Mangroves of Andaman,2015).The term Sonneratia was named in honor of a French Ecologist, Pierre Sonnerat (1740-1844), who travelled in New Guinea, the East Indies, and China and communicated many new plants to the Botanist of Europe, thus paved the subsequent activities to classify and name these unique plants of Mangrove forest.
The adaptation to high salt concentration is a major obstacle of a plant in order to thrive and grow in a sea of salt. Only very few plants have this capacity to survive in this toxic environment. Among these plants are the Mangroves or mangal. Mangroves show a unique adaptation to high salt concentration by colonizing one of the most hostile habitats on Earth. that is the coastal environment of tropical and sub-tropical regions of the World. Maximal range of salt (NaCl} is to the tune of 15,000 mg/liter of salt water and constantly disturbed by inundation of waves that makes the environment unstable for growth and development Moreover, Mangroves are priced for its wood in shipbuilding, construction materials, floaters in fishnets, in making shoes- soles are made from cork of Mangrove, medicine as having antihemorrhagic and antibacterial properties as well as source of food, besides, Mangroves act as breeding grounds and nursery for marine life and habitats for large mammals. But, the most important ecological role of Mangrove is in the protection of coastal areas as buffers against strong winds and high tide that might otherwise erode coastal areas and endanger inland communities.
Mangroves are unique in this aspect for having the mechanism to modulate salt concentration internally and exclude salt if necessary. The Mangrove forest is made up of about 80 species, mostly trees, under the family Lythraceae. There are several genera and one of this is the genus Sonneratiaceae
Geography and distribution of Sonneratia spp.
According to Dukes,N,C and B, R. Jackes (1987), there are seven species of Sonneratia spp, extending from East Africa to Indo-Malesia, Australia, New Guinea, Western Pacific and majority of South East Asia as show in the Map. The seven species described (ibid.) are as follows:
- Sonneratia casiolaris
- Sonneratia alba
- Sonneratia lanceolata
- Sonneratia gulgani
- Sonneratia gulgani x S. alba
- S. alba x S. lanceolata
- Sonneratia ovata
Sonneratia spp. also occur throughout the Indo-West Pacific regions from East Africa to China throughout Asia and Indonesia, to New Guinea, the Western Pacific and North of Australia. The five major taxa were quite cosmopolitan in these regions while the two hybrids were quite specific to Northern Australia and North-Western Borneo.
As with other Mangrove species distribution is governed by salinity. In general, Oceans located above (30oN) and below (30oS) the equator and oceans located between the continents of North and South America, Mediterranean and Europe have higher salt content (not less than 38 parts per thousand), while lowest (34PPT) was observed in Pacific Oceans bordering South East Asia and Asia including Northern Australia as a result of High precipitation,and therefore Mangrove thrives best in these areas.
What are Halophytes?
- Halophytes are plants that thrive and grow in the high salt environment. Definitely, Mangroves are Halophytes based on the following definitions (Gregory, M, N. et al, 2010)
- Plants that can tolerate the concentration of salts in saline soils
- Halophytes are defined as those plants which grow and complete their entire life cycle in saline habitat, copi8ng with salinity ad adaptation on all levels from the autecological, the issue and the cellular level to sub-cellular and biochemical adaptations.
- Plants that occur naturally on soil or in water too salty for the average plants are usually designated as Halophytes.
- Halophytes is salt resistant or salt tolerant plants that thrive and complete their life cycle in soils or waters containing high salt concentration
Taxonomy, Morphology, and Ecology
Mangroves all the only trees that are capable of growing in salt water. they form a unique intertidal forest at the edge of land and sea. They are represented in all countries of the World with tropical and sub-tropical oceans, North and South America, Africa and the Middle East, Asia and Oceania including Australia (Kalrien, T,2013). Conversely, Mangroves does not grow at random in a Mangrove forest but rather show a pattern of Zonation.
Mangrove species have been classified into seaward, mid and landward zone according to the colonies they formed, as they occur in relation to tidal position. The seaward zone is the outfacing edged of the Mangrove forest which is freely exposed to a frequent inundation of salt water. The Sonneratias are considered superior to other species in this aspect being a pioneer species. Other species hid behind Sonneratia where they are less exposed to strong winds and waves, and changes in salt water concentration. Superior Sonneratias was able to cope with this adverse conditions while other species cannot.
Clearly variation in morphology occurs and varies among Sonneratias species and their putative hybrids.For instance, Sonneratia alba occurs in three forms, the apetalous, petalous and semi-petalous, where the number of petals is less than the calyx which is usually six. Below is the key to the classification of Sonneratia species.A thorough review of the family Sonneratia under the order Myrtales by Baker and Van Steenes (1951). Their account is made up five Mangrove tree species; Sonneratia alba, Sonneratia caseolaris, Sonneratia ovata, Sonneratia apetala, and Sonneratia griffithii. Their distribution was described as tropical and varying through the Indo- West Pacific region, but this was not complete as new information was noted in 1972. A systematic revision of the Mangrove genus Sonneratia was done in Australia that yielded seven species; five common Sonneratias and two hybrids (Dukes and Jackes, 1987) as previously discussed.
Adaptation to saline environment of Super Halophyte Sonneratia spp.
- Sonneratia species are definitely super halophytes.
- They possessed the ability to exclude, sequesters, have salt glands and salt bladders that modulate high salt concentration in their environment, grow and complete their life cycle.
- Other plants that do not have these attributes can not survive in this hostile environment. But to do this, Mangrove Sonneratia has to answer the following:
- first, How to overcome high salt concentration?,
- second, How to solve the problem of Anoxia?-the lack of O2 in the soil or substrate.
The first question- How to manage High salt concentration? Physiological and metabolic devices to manage high leaf salt
a) Ion exclusion in the roots
- Halophyte depends heavily on salt exclusion by roots to regulate their internal salt load
- `Halophyte can cope up greater load salt internally, for instance, salt concentration in xylem sap of coastal Mangrove Avicenna stays as low as 9mM despite a sediment concentration of around 500mM of NaCl that is 98% exclusion is achieved.
- Salts are prevented from entering the xylem vessels by the Casparian strips located at the endodermis of the roots.
- How the exclusion of salt is achieved in the roots, is probably by long distance signalling via the stress hormone Abscisic Acid (ABA) synthesised in the leaf that affects membrane permeability of the Casparian Strips towards NaCl ions entry.
- The leaf is the first organ to detect ion toxicity.
b) Ion excretion - salt glands and bladders in the leaf
- Many halophytes including Sonneratia carry salt glands or bladders that shift ions from mesophyll to leaf surfaces. Solutes crystallized are blown or washed away from leaf surfaces.
- Salt bladders are modified epidermal cells that accumulate salt and often burst to release their content.
Take a QUIZ?
Match Column A with B. LETTERS ONLY!
Column A Column B
- Mangroves a) Stress hormone
- Halophytes b) Photosynthetic tissue
- Sonneratia c) Breathing organ
- Sonneratia alba d) Mangrove Firefly
- Sonneratia caseolaris e) Mangrove Apple
- Abscisic Acid (ABA) f) Pierre Sonnerat
- Mesophyll g) Lack of oxygen
- Pneumatophore h) Compartmentation
- Anoxia i) Salt tolerant plant
- Tonoplast j) "Mangue"
SCORE: Excellent-10, Very Good- 8-9, Good- 6-7, Average- 5.
Did you KNow ?
- Oceans may contain as much as 50 quadrillions of dissolved solids if remove would form a layer of 500 feet thick about the height of a 40 storey building?
- That the Firefly in Mangrove trees are beetles? (Pteroptyx tener)
c) Succulence of leaf mesophyll
- Succulence offers additional storage of incoming salt ions.
- Thick and fleshy leaves contain large and highly vacuolated cells
- Succulence is a feature of many Halophytes characterized by increased in length of palisade cells and increase in the diameter of spongy mesophyll cells.
- Development of extra layer of tissue is advantageous to Sonneratia as more cells available for salts storage.
The second Question- How to solve the problem of Anoxia? Turgor Maintenance
a) Osmotic Adjustment (OA )
- Turgor is crucial to plant function and must be maintained for plant to grow despite high salt concentration
- OA is an active process and must be distinguished from a passive movement of solutes due to loss of water during drought or salination.
- Recognizing that enzymes cannot function in the high salt concentration much above 100mM, above this, extra salts are sequestered inside vacuoles so as not to interfere with metabolic processes and maintain turgidity of cells
- As the solute potential of the cells is increased water is drawn from steep gradient from an anaerobic condition of the substrate by the roots to the leaf.
- This happens when the roots are completely submerged during high tide, increasing the solute potential of the cell prevents the occurrence of Anoxia.
- Anoxia is a destructive process, lack of oxygen prevent another vital function- Respiration to occur. An energy yielding process- ATP is released, the energy currency of the cell.
- Non-respiring roots degenerate due to lack of energy and this will affect above ground functions in a non-halophyte Species
- Sonneratias and other Mangrove species are able to wear off this toxic conditions.
b) Compatible Solute (CS)
- Most plants can synthesized and store certain organic molecules without the detrimental effects on enzyme activity
- Several CS are present in halophytes, these are sugars ( glucose and sucrose), polyhydric alcohols (mannitol and inositol), amino acids (proline and asparagine), etc.
- Some of these organic compounds serve as osmotica and can be present in the cells of halophytes in high concentration.
- High solute concentration of the cells raises the osmotic potential of the cell. in this way water is drawn inside the cell despite the steep gradient imposed by the salt ions, otherwise the plants suffer from dehydration.
- The presence of CS prevents salt ions from entering the vacuoles instead they are sequestered in the Tonoplast to be excreted later through the salt glands or salt bladders of the leaf
c) Compartmentation of cell membrane- the Tonoplast
- Contrary to our belief that salt ions immediately interferes with metabolic functions of the cell is not true. In Halophytes, they are prevented from interfering by sequestration in the Tonoplast
- Tonoplast is a membrane that separate the cytoplasm and the vacuole. Salt ions are temporarily held in the Tonoplast while CS is loaded in the vacuolar sap to increase the osmotic potential of the cell.
- This device is very important since it modulate the presence of salt ions that otherwise will accumulate in toxic concentration.
- Excess salts are then excreted via the salt glands or salt bladders
d) Presence of Pneumatophores and Lenticels
- Pneumatophores and lenticels are grossly termed as the breathing organ of the plant.In Mangroves, they present a distinct taxonomic feature of the species.
- Sonneratias have cone shaped pneumatophores and numerous compared with other Mangroves species.
- This device is very crucial in storing oxygen in their specialized tissue called aerenchymatous cells. During low tide, the aerenchyma is filled with oxygen, the stored O2 is utilized during anaerobic condition at high tide when pneumatophore is submerged
- Carbon dioxide is then released at low tide as product of respiratory metabolism
- Lenticels are commonly located in the stems and branches. the is an additional venue for the acquisition of Oxygen in an aerobic condition. Sonneratias has numerous lenticels in stems and branches.
Seeds are very important propagule in the distribution of Mangroves worldwide. Mangrove Sonneratias produces a lot of seeds compared with other species like Rhizophora or Aegiceras, etc. though in Rhizophora, the seeds is released after it has germinated in the mother plant (vivipary) could be an advantage but it has to have a favorable substrate for anchorage otherwise the germinated seeds may not be able to establish and carried away by the incoming tide. A very costly endeavor since Rhizophora produces only one fruit-one seed ratio, whereas, Sonneratias produces a lot of seeds per fruit. The fruit has a persistent calyx tube that act as floaters to be carried long distance to find a suitable substrate. The degenerated pericarp released the seeds. Germination must have been triggered by salt water or methane gas released by the substrate. Though some seeds may die due to the unfavorable condition, but many will survive. The persistence and pioneering characteristics of Sonneratias can be gleaned from the superior adaptation of its seeds.
Medicinal value of Sonneratia
The antimicrobial properties of Sonneratias were explored (Mahadlek, J. et al., 2012) Traditionally, Sonneratias was used as astringent and antiseptic. The fermented juice of S. caseolaris (cork tree) can be used to treat haemorrhage whereas the half-ripe fruit can be used to treat coughs.
- Phenolic substances from seeds- Gallic Acid, Flavonoids-luteolin and luteolin-7-O-β- glycoside
- Sonneratia caseolaris (Cork tree)) - inhibit growth of Staphylococcus aureus, Candida albicans but not Escherichia coli
- Sonneratia alba ( Mangrove Apple) antimicrobial property against Staphylocuccus aureus, P. aureginosa and K.pneumonia
From the foregoing accounts, it appears that the Sonneratias are truly the super halophytes of intertidal zones based off the following, able to maintain normal metabolic and enzymatic activity despite the high salt concentration and complete entire life cycle in toxic environment. Possessed the ability to exclude and excrete excess salts through salt glands and bladders in the leaves. Prevent Anoxia in the roots by having Pneumatophores and lenticels in the stems that served as breathing organs and for the exchange of gasses in the anaerobic condition. Fruits and seeds are well equipped for long distance movement and ability to grow in the hostile environment, hence called the pioneering species.
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