Morphological and Physiological Adaptations of Hydrophytes

Morphological Adapatations:

Roots are often poorly developed (e.g. Wolffia, Salvinia) or completely absent.

Root hairs have completely disappeared in some species of Ceratophyllum.

However, many hydrophytes have well developed root systems.

For example, Eichhornia and Pistia have well developed adventitious roots. In these free-floating rosette plants, the roots are at least partly responsible for preserving the stability of their rosette leaves.

The stem may be well developed (e.g., Ceratophyllum, Hydrilla), reduced (e.g. Wolffia ,Spirodela), or modified into rhizome (e.g., Vallisneria). Stem is spongy due to well developed aerenchyma. The spongy and elongated petioles of water hyacinth exhibit the development of so-called aerenchyma.

Many hydrophytes show heterophylly, i.e., production of different forms of leaves in the same plant. In submerged aquatics with free floating aerial leaves, the submerged leaves are generally linear, ribbon-shaped or finely dissected while the aerial leaves are complete and rounded or lobed.

In Sagittaria, the heterophylly seems to be due to the difference in the intensity of light in the submerged and aerial parts. More light intensity in the aerial parts favours formation of entire leaves (Fig. 2.5). The floating leaves have waxy surface so that water may not wet the surface and block stomata. The presence of mucilage on the aerial organs seems also an adaptation for protecting them from getting wet. The propagation of most hydrophtes is vegetative.

An important anatomical feature of all hydrophytes is the sponginess of their tissues. They have extensive air-spaces in their leaves, stems and roots. This helps in keeping the buoyancy of plants and facilitates exchange of gases. The epidermis usually lacks cuticle or periderm and stomata are not present in the submerged leaves. Hydrophytes have reduced vascular elements. The absence of secondary growth in thickness of stems and roots is also an important characteristic of hydrophytes.

Physiological Adaptations:

Petioles of floating- leaved hydrophytes have a great capacity for renewed growth, which is perhaps regulated by auxins (phytohormones). In lotus, the long petioles seem to adapt themselves the depth of water, thus keeping the leaf lamina on the surface of water. Many hydrophytes maintain active photosynthesis. Some carbon dioxide evolved during respiration is stored in the air spaces and utilized during photosynthesis. In hydrophytes, the osmotic concentration of the cell sap 3 equal to or slightly higher than the surrounding water.

In some hydrophytes, the initiation of the 5owering phase may depend upon nutrition. The Utricularia, a carnivorous plant, flowers when grown aseptically in inorganic nutrient medium but only when supplemented with organic nitrogenous compounds, e.g., mixture of peptone and meat extract. However, in Eichhornia, flowering seems to be influenced by temperature rather than photoperiod. In some hydrophytes (e.g., water-lily) food is stored in the rhizome, in others such as Sagittaria, tubers are formed.

Examples of Extreme Cases of Adaptations:

Organisms living in polar areas, hot springs, hot deserts, and deepest seas exhibit extreme 3ses of adaptations.

Some unique adaptations of such organisms are as follows:

The arctic and Antarctic regions, which contain polar ice, possess unique biota dominated by algae, bacteria, diatoms, and other organisms like protozoa, copepods, amphipods, nematodes, flatworms, and ice fish. Single-celled algae are abundant and are able to survive there by accumulating K⁺ as osmolyte and producing dimethyl sulphonipropionate (DMSP) and proline as antifreezes. The krill (euphausid shrimps) survive the winter by grazing on the biota at the under surface of sea ice (Willmer et al 2000).

In Antarctic ice fish, Chaenocephalus aceratus, which lacks hemoglobin, the cardiac output is very high. As there are no red blood cells, the oxygen is carried in solution X blood, which has the same oxygen- carrying capacity as sea water. To overcome the low oxygen capacity to its blood, ice fish has relatively large gills, vascular skin for respiration and large heart with exceptionally high cardiac output and very little red aerobic myotomal musculature to reduce oxygen demand.

Some other adaptations of ice fish are production of large eggs, low fecundity and delayed maturity, which is achieved after several years. Some polar marine fish possess antifreeze protein (AFP) or antifreeze glycoprotein, AFGP, (Fig 2.6) which are responsible for ‘thermal hysterisis’ and are therefore called ‘thermal hysterisis proteins (THPs)’. In such cold-hardy organisms its freezing point of the solution is lower than its melting point.

The biota of hot springs consists of a number of organisms depending on the water temperature. Near the geothermal source of a hot spring only bacteria survive, but downstream algal mats appear and are sometimes colonized by protozoans at about 65°C. Some ostracods, larval flies, rotifers and nematodes may appear as the water further cools down to about 50°C. In some hot springs, midge larvae and chironomids may be found. In India, hot springs are scattered in many parts of the country. Recently Saha and Datta Munshi (1982,1983) and Saha (1993) have described limnology of some thermal springs of Bihar.

Higher thermal tolerance of bacteria, algae and some other organisms of hot springs may be attributed to their physiological adaptations. For example, Thermoplasma acidophilus occur at 59°C and pH 1-2. It has the smallest genome so far known in non-parasitic bacteria. Certain anaerobic thermoproteales have been isolated from hot springs and volcanoes. They are extremely thermophlic, growing optimally at 85-105°C and they depend on ‘sulphur respiration'(Schlegel, 1995). They oxidise hydrogen and reduce elemental sulphur to hydrogen sulphide. Pyrodictium occulatum is a bacterium that grows at temperatures up to 110°C. It is anaerobic and grows autotrophically with H₂ as H-donor and sulphur as hydrogen acceptor.

Desert adaptations have already been discussed. Desert species have higher body temperatures (desert beetle, 33°C; desert iguana, 41°C; camel, 34°C and 41°C in the morning and afternoon respectively). Extreme desert conditions are very harsh and certain organisms survive there due to their behavioural, physiological and biochemical adaptations. In hyper-arid deserts, there is no rain, wind speed is very high, and night is extremely cold. The soil is sandy, stony and salty.

All these conditions make life almost impossible for most kinds of organism. However, some organisms survive as they live in burrows. For example, the peak day temperature on the surface may be 45°C to 65°C, but in the burrow of the scorpion, Hadrurus, it is only 32°C to 40°C (Hadley, 1970). Besides low temperature, burrows may also give a constant high humidity, which helps in osmotic regulation. In some naked mole rats the burrows are about 75 cm below surface, providing not only a low temperature (25-28°C) but also high humidity which helps minimize evaporative water loss.

Certain locomotory tricks as in sidewinder snakes and dune spider are adopted to minimize body contact with the surface (Pough et al, 1999; Henschel, 1990). Endogenous rhythms of desert invertebrates further help in their survival. Some ants and arthropods of hyper-arid deserts possess raised thermal tolerance and extremely low metabolic rate. Mechanisms for water vapour uptake in desert mites and desert cockroach, Arenivaga, are quite interesting and many desert insects use discontinuous respiration. In the kangaroo rat, Dipodomys, the expired air is cooler than the inspired air, which is a device for nasal heat exchange.

In the deepest seas organisms live in pitch dark, under high pressure and low temperature. In deep sea hydrothermal vents, the water is rich in silicates, hydrogen sulphide and sulphide minerals containing magnesium and iron, which crystallise as the hot water meets the cold sea water, forming ‘black smokers’, ‘chimneys’ and other mineral deposits. The vent water contains no oxygen, and high sulphide and ammonium levels. The organisms, especially prokaryotes, which occur in deep-sea thermal vents, are extremophiles having highly specialized enzymes.

The vent fauna consists of annelid and pogonophoran tubeworms, bivalve molluscs and some decapod crustaceans. The ‘Pompeii worm’, Alvinella pompejana, occurs on vent chimneys. It is so far the most eurythermal organisms on record; at the tube mouth the water temperature is around 22°C but in other parts of the tube it may be upto 82°C, showing a gradient of 60°C along the length of the worm’s body.

The deep-sea crustacean, Rimicaris exoculata, which also occurs around vents, has no eyes, but has a ‘thoracic eye’ used to detect the radiation emitted by the very hot vent water (up to 350°C). The vent animals also possess other physiological adaptations in nutrition and respiration to cope with toxic hydrogen sulphide.

Their sulphide-binding proteins play an important role by drawing free sulphide into general circulation. In pogonophoran worm, Riftia pachyptila, the symbiotic bacteria in trophosome possess adaptations to protect the worm from sulphide toxicity. These bacteria can probably use nitrate in addition to oxygen, producing ammonia and nitrate as end products. The nitrate respiration enables the bacteria to function in an environment which is very low in oxygen (Willmer et al, 2000).

In deepest parts of the ocean the effects of depths and pressure are very obvious. In all deep- sea forms, the rate of oxygen consumption decreases rapidly with increasing depth and they use various methods to increase buoyancy. Some other adaptations of deep-sea animals are chemoreception, bioluminescence, electroreception, and echolocation.