This article throws light upon the three main stresses adaptation in plants and animals. The stresses are: 1. Stress Adaptation in Plants 2. Genetic Response to Environmental Stress 3. Stress Adaptation in Animals.
Stress # 1. Stress Adaptation in Plants:
“Stress” is an external factor that exerts a disadvantageous influence on the biota (plants, animals or microbes).
The stress influence is measured as variety of biotic changes viz., survival, growth, assimilatory processes and yield.
The concept of stress is intimately associated with that of “stress tolerance”, which is the plants fitness to cope with an un-favourable environment.
Under both natural and agricultural conditions, plants are frequently exposed to stress. The major environmental stress factors are water, temperature, salinity, air pollutants and toxic metals of soil. It is well known that various categories of plants are capable of adapting to environmental changes through variety of changes, thus they are designated as hydrophyte, xerophytes, halophytes, metallophyte, and or thermophytes/thermophilic forms. All these adapted plant forms have their own characteristic adaptive strategies in various forms viz., morphological, anatomical, physiological or genetical means.
(a) Water Stress:
A large number of higher plants are capable of adaptation in water deficit condition. They are designated as drought resistance plant forms. The resistance strategies depends on a variety of causatives viz., decreased leaf area, enhanced root growth, higher accumulation of abscisic acid growth hormones induced stomatal closure reduced photosynthesis, higher cell sap osmotic pressure, increased wax deposition, induced CAM metabolism and also increased resistance to liquid phase water flow.
(b) Temperature Stress:
Identically low temperature generally induce chilling injury in most of the higher plants. There are a number of structural biochemical and also metabolic changes induced in low temperature tolerant plant of alpine and subalpine region. There is remarkable rise of ratio (>3) of unsaturated to saturated fatty acid in chill resistant plant species. The resistance to freezing temperatures involves super-cooling and slow dehydration.
Turning to the other temperature extreme, to what extent are plants able to resist excessively high temperatures? Few higher plant species survive a steady temperature above 45°C. Of course non- growing cells or tissues of higher plants that are dehydrated (seed and pollen) can survive much higher temperatures than hydrated, vegetative, growing cell can. But there are a good number of fungi, actinomycetes, bacteria which can withstand up to 110°C.
A good number of succulent (CAM) higher plants, such as Opuntia, is adapted to hot conditions and can tolerate tissue temperatures of 60 to 65°C during intense solar radiation in summer. Because CAM plants keep their stomata closed during the day, they cannot cool by transpiration instead, they depend on reemission of long wave (infrared) radiation and loss of heat by conduction and convection to the surrounding air.
In environments with intense solar radiation and higher temperatures, plants avoid excessive heating of their leaves by decreasing their absorption of solar radiation. The anatomical and physiological adaptations that accomplish this task are similar to the adaptations to water stress that decrease water use by energy dissipation.
In addition, in a number of temperature tolerant plants, increase in an ambient temperature induce synthesis of heat shock proteins. The heat shock proteins (HSPS) helps cells withstand heat stress. It is a low molecular weight (15 to 30 kda) protein or even sometime molecular weight is high (60 to 90 kda).
(c) Salt Stress:
In general salt accumulation in soil impair plant function and soil structure. It is further known that the salinity depresses growth and photosynthesis. But there are salt tolerant plants that are capable of avoiding salt injury by excluding salt from meristem. A number of metabolites provide resistance to salt stress—these includes, free acids, vit C, proline or flavonoids Table 11.1.
(d) Air Pollution Stress:
The fuel burning or industrial process emission leads to considerable rise of air pollutants viz., SOx, NOx, CO2, CO, SPM, NH3, HF, C2H4, HC and several other pollutants. Some of them produces photochemical smog in urban environment. In general, the responses of plants to polluting gases can also be affected by other ambient conditions, such as light, humidity, temperature and the supply of water and minerals.
The polluting gases and dusts (SPM) inhibit stomatal movements, and thus impart in photosynthesis and growth. Apart from screening out sunlight, dust on leaves block stomata and lowers their conductance to CO2, simultaneously interfering with photosystem II.
Polluting gases such as SO2 and NOx enter leaves through stomata, following the same diffusion pathway as CO2.NOx dissolves in cells and gives rise to nitrite ions (NO-2) and nitrate ions (NO-3) that enter into nitrogen metabolism.
Identically, in the cells, SOx dissolves to give bisulfite and sulfite ions, which is toxic at higher concentration. But in urban environment these pollutants may be present in such high concentrations that they cannot be detoxified rapidly enough to avoid injury.
There are some enzyme like superoxide dismutase’s (SODs) that helps in protection of these kinds of damages. Polluted acid rain which often cause severe damage to plants as the pH of rain is less than 3. This is of course a consequence of high level of industrial pollution.
Stress # 2. Genetic Response to Environmental Stress:
Early attempts to identify stress induced genes in plant begins when investigators noticed the appearance of new proteins after exposure to different stress. Now most of the genes that were identified by various screening procedures as stress induced gene expression, that require much more detailed examination. A variety of genes in CAM plant like Mesembryanthemum crystallinum are induced by osmotic stress.
However, in recent years the multi-genic nature of stress tolerance and the simultaneous activation of many genes that are involved in stress tolerance suggest that stress tolerance genes have common regulatory features. A general scheme for a signal transduction pathway that mediates stress tolerance is shown in Fig. 11.3.
The transfer of genes suspected to mediate stress tolerance to a test plant where they are over-expressed or altered in someway has given scientists an important new tool with which to study genes involved in stress tolerance. Results of the first study in which transgenic plants were reported to show altered stress tolerance after transfer of a foreign gene appeared in 1993.
The transferred gene encoded the enzyme mannitol dehydrogenase and its expression lead to the accumulation of mannitol in the transgenic plants. However, elucidation of the function of stress- activated genes should facilitate breeding for and genetic engineering of stress resistant plants.
Stress # 3. Stress Adaptation in Animals:
Diverse groups of animal adapted to varied environmental stress, in course of evolution. Animal species which are unable to adapt with climatic oscillation become extinct and fossilized. The details of animal adaptation to terrestrial habitats and aquatic habitats are given below.
Adaptation to Terrestrial Habitats:
Various animal groups have invaded land at different times. Insects, terrestrial arachnids and tetrapod’s, for example, were found first in the Devonian period. At a later time animal groups such as the operculate gastropods, the opisthobranchiates, the isopods and crabs have colonised the land. Identically many animals that migrated to terrestrial environment continue to live in damp areas or nearer to water source.
However, there are other animals which later migrated to semiarid and arid environments. The animals have brought about morphological, physiological, and behavioural adaptations to continue their vital activities on land.
All these adaptations help in reducing the loss of water and salts. The skin of mammals is less keratinized than that of reptiles. Yet the loss of water through the skin of mammals is as low of in the reptiles. By regulating metabolic processes specially respiration, and body temperature, the water loss can be adjusted substantially in terrestrial animals. Mammals are liable to suffer from heat stroke if they are exposed to higher temperatures.
The Kangaroo rat (Dipodomys sp) which is well adapted to desert life has developed novel mechanisms to conserve water.
The water loss is considerably reduced due to the following factors:
1. Reduction of evaporation loss through skin and lungs,
2. Production of concentrated urine,
3. Production of dry faeces,
Absence of sweat glands is an additional factor in reducing the loss of water through the skin. Because of the absence of sweat glands the cooling mechanism is inefficient but the animal developed tolerance to temperatures up to 41 °C, i.e. 6°C above its normal temperature.
However, higher day temperature are avoided by the Kangaroo rat by developing behavioural habits such as living in relatively cool and humin burrows during the day, and foraging during nights (nocturnal) when the ambient temperature falls down to a comfortable level. In addition, Kangaroo rat conserved water by absorbing water from urine and stool too.
Another desert animal Camel can go without water for long periods. Camel loses water by evaporations through the skin and the lungs. Water is also lost through the faeces and urine. Camel evolved mechanisms to minimise water loss through special mechanism as well as stored for adverse periods.
During winter months Camel meets its water needs by browsing on bushes and succulent plants. Therefore, it goes without drinking water for periods longer than two months and still shows no signs of dehydration. When access to water Camel drinks enough water within minutes. Camel has a fat store, which on metabolism provides water required for metabolism even without drinking water.
Normally Camel hump stored fat of about 100 pounds which can supply 13 gallons of water. Like Kangaroo rat Camel also have reduced urination and dry stool. Camel has also extreme adaptability to adjust to high summer temperature up to 60°C.
By and large all terrestrial animals have varied degree of water and temperature stress tolerance. Very little information about their tolerance to air pollutants. However, some animals have salt tolerance too. For instance marine reptiles and sea birds do drink sea water, their kidneys are less efficient than those of man and yet they suffer no ill effects.
Adaptation to Aquatic Habitats:
Analysis of the body fluids from a number of marine animals has resulted in the following generalisations:
(a) The body fluid concentrations are similar to that of sea water.
(b) They differ from sea water in relation to the ionic composition.
(c) Considerable variation exists in the ionic regulation by various groups of animals.
(d) Considerable variation exists in terms of uptake of salts and waters and excretion of salt and water exists among the aquatic animals.
Animals living in low salinity i.e. brackish water habitat showed special mechanism of osmoregulation. Most of the brackish water arthropods maintain their blood concentrations greater than that of medium and hence water tends to enter by osmosis and ions tend to escape by diffusion.
To maintain hyperosmotic condition of blood, the water entered is returned to the medium and the ions escaped are actively transported back into the blood. The water is removed partly as urine, and partly as external water.
Freshwater animals have body fluid hyperosmotic to their medium. They have osmotic problems similar to those facing the brackish water animals, but more extreme in their nature. Freshwater animals meet these osmotic problems by improving upon some of the osmotic and ionic regulatory mechanisms of the type existing in brackish water animals.
The permeability of the body surface of these animals is far less than that of brackish water animals. But freshwater molluscs have more permeable body surface, a feature favourable for the influx of water. However, the influx of water is greatly reduced because the blood concentration of these molluscs is far lower than that of other freshwater animals.
The freshwater fishes and the freshwater invertebrates have similar osmotic conditions and regulatory mechanisms. Both Lampreys and teleost’s have hyperosmotic blood. The concentration of blood in the freshwater species is maintained at a fairly constant level. Due to hyperosmotic nature of these forms, water tends to enter their body via general body surface, gill and mouth epithelia.
Aquatic life first to be affected by lake acidification are crayfish, shrimps, snails, mussels and some species of mayfly. The fish most sensitive to a fall in pH are minnows, salmon, roach and trout, as shown in Fig. 11.4. Once a lake pH falls to 4.5-5.0, all that usually remains are bog moss, certain plankton species and the hardiest insects.
Aquatic life is generally sensitive to temperature of water. They can not withstand the temperature above 45°C. However, in hot spring the- water temperature lies above 75°C, where selected blue green algae and bacteria can survive.
Industrial hot-water discharge-above > 35°C is usually not permitted due to threat of aquatic life in the receiving water bodies. Chemical pollutants often causes determination of aquatic life. That is why, aquatic biotic forces are often used as bio-indicators of water quality.