s.p pillai aswathy viswanath
TRANSCRIPT
STRESS PHYSIOLOGY
By,Pillai Aswathy viswanathPG 2 BotanySt. Thomas college kozhencherry
INTRODUCTION Stress is usually defined as an
external factor that exerts a disadvantageous influence on the plant
In 1972 , Jacob Levitt proposed a definition of biological stress
He suggested that biological stress is any change in environmental conditions that might reduced or adversely change a normal plant’s growth or development
The current concept of stress in plants has been well developed over the past 60 years.
Any unfavourable condition or substance that affects or blocks a plant's metabolism, growth, or development is regarded as stress.
Stress are of two types abiotic and biotic stress
ABIOTIC STRESS Abiotic stress is defined as the
negative impact of non-living factors on the plants in a specific environment.
Abiotic stress factors, or stressors, are naturally occurring, that may cause harm to the plants
Abiotic stress is essentially unavoidable. but plants are especially dependent on environmental factors, so it is particularly constraining.
Abiotic stress:- Water stress Temperature stress Salt stress Oxygen deficiency Heavy metals Air pollution
Water Stress
Today water stress is the most frequently encountered environmental stress
Drought conditions are more common than the flooding ,which are usually created by non – availability of water in the soil due to the delayed or in frequent rains
The productivity of the plants depends on the total amount of water available in the soil
Ecologists classify the plants according to their response to water
Hydrophytes grow where water is always available as in ponds or marshy
Mesophytes grow where the water availability is intermediate
Xerophytes grow where water is scarce
When water deficit develops slowly enough to allow changes in developmental process
Water stress has several effects on the growth, one of which is a limitation in leaf expansion
Leaf area is important because photosynthesis is usually proportional to it
Inhibition Of Gas Exchange And Photosynthesis
The water stress have a negative effect on the stomatal opening
The stomata in most species start closing under drought conditions
Hence the gas exchanges in the process of transpiration , respiration and photosynthesis are usually reduced during drought condition
The closing of stomata in turn is belived to be due to the accumulation of absicisic acid on the cell wall
Also leaf abscission is also seen in a few sps. Which may due to the accumulation of absicisic acid under drought condition
Growth Inhibition And Visible Injury The most visible effect of prolonged
drought condition is the inhibition of plants growth , which is seen in both reduced in length and in decreased biomass of the plants
The most drastic reduction is seen in the leaves
The total leaf area of a plants does not remain constant after all the leaves have matured
The visible injury is seen in the form of a wilting , which may be temporary or permanent depending upon the duration of the drought condition
Premature Flowering Extended period of drought
causes premature flowering and the production of smaller fruits in bean and tomato
Root Symbiosis Water stress causes the reduction
in microflora The rhizobium – legume
association and efffective nodulation is also inhibited by the water stress
Wax Deposition On The Leaf Surface
A common developmental response to water stress is the production of a thicker cuticle that reduces water loss from the epidermis
A thicker cuticle also decreases CO2 permeability
Temperature Stress Plants growing extremely hot
and dry climate suffer from temperature stress
Temperature stress include 1. Low temperature 2. High temperature
Low Temperature
Low temperature of chilling injury is common to plants of tropical or sub tropical tropical region
In some plants like rice, chlorosis develops at low temperature
The leaves are white and once these leaves have developed ,they remain so even if the plants are brought to normal temperature
This is apparently due to aberration in the cell growth and plastid development at low temperature
Chilling sensitive plants show decreased root growth , increased in ethylene production at temperature below 10 C
When the plants growing at relatively warm temperature are cooled to 10 C to 15 C,chilling injury occurs
Growth is slowed,decoloration or lesions appear on leaves
Leaves from plants injured by chilling show inhibition of photosynthesis ,lower the respiration rate , inhibition of protein , increased the degradation of existing proteins
High Temperature
If the temperature drops below 150C plants experience low temperature stress and if it is above 450C , plants are subjected to high temperature stress.
Both photosynthesis and the respiratory rates are inhibited at high temperature
In most of the sps. Short term exposure to high temperature causes reduction in root and shoot growth
An exposure to a temperature above 50 C for 10 minutes can kill most herbaceous sps.
Seed germination and seedling growth also inhibited at high temperatures
Also increase in temperature causes an early maturity and early senescence of plant parts
Salt Stress Most mesophytes when exposed to
salinity in the soil responds to it as a stress factor
Halophytes plants are adapted to saline water and show various adaptation to with stands in the salinity environment
Excess of salt in water not only lower the absorption of water but also decreases the productivity of plants and overall growth of plants. both shoot and root growth are inhibited (growth inhibition)
Inhibition of gas exchange and photosynthesis:-
Inhibition of transpiration , respiration and photosynthesis by salinity has been reported in many sps. Which is considered to be due to stomatal closing as has been observed in bean cotton spinach etc.
Nitrogen asimilation :- Inhibition of nitrogen assimilation by
salinity has been reported in several cases
The inhibition of nitrate reductase the enzyme reducing nitrates to nitrite
Salinity also induces the reallocation of nitrate assimilation from shoot and root and there by causing reduction in growth rates
Mineral uptake and transport :- Reduced the uptake of K and other
monovalent cation has been reported in a few sps.
The inhibition of mineral transport from shoot has also been reported , specially in bean
This may due to the competitive effects of Na and Cl
Heavy Metals Heavy metals is widely used in
agriculture The increase in the concentration in the
heavy metals in the soil may adversely affect the plant life
Lead may exists in the atmosphere as dusts fumes and in the soil as minerals
Mercury may present in the soil as the salts
Plants absorbed and accumulate large quantities of these heavy metals from the contaminated soils
This accumulation causes various types of physiological and metabolic disorders in plants
Inhibition of seed germination and seedling growth :-
The inhibition is seen usually at high concentration of heavy metals
At high concentration of heavy metals usually the do not grow beyond seedling stage and they killed
In mature plants , the inhibition of root growth is more apparent than the shoot growth
Inhibition of photosynthesis and chlorophyll synthesis :-
Carbon dioxide assimilation and many components of light reaction of photosynthesis are usually inhibited by heavy metals
The heavy metals cause disruption of chloroplast and also inhibit chlorophyll biosynthesis in greening leaves
Senescence and abscission:- Senescence and abscission of
leaves by mercury has been observed in pistia,citrus etc.
Air Pollutants The pollutants may not cause any visible
injury, but they may affect physiological and biochemical process .such disturbance is termed hidden injury
The hidden injury may ultimately lead to reduced plant growth and productivity
In higher concentration of pollutants cause visible injury which is usually seen in the forms of chlorotic and necrotic spots on the leaves.
Some times abnormal pigmentation in the leaves is also be seen
Reduction in biomass production :- The total biomass of plants is
often reduced in polluted atmosphere
When there is reduction in plant biomass,the reduction in root growth is more pronounced than in shoot growth
Gases and vapours enter the leaves and plants through open stomata, although cuticle also permeable to gaseous diffusion to some extent
Opened and functional stomata are necessary for the gas exchange
Air pollution may induce stomatal closing or some times opening also
The nitrogenous air pollutant often increase protein and total organic nitrogen contents of the plants
The enzyme involved in the nitrate assimilation are also activated during exposure
MECHANISMS OF RESISTNACE TO ABIOTIC STRESS
The capacity of plants to cope with unfavorable environments is known as stress resistance
There are two types of resistance mechanisms operating in plants – stress avoidance and stress tolerance
The plants to avoid the stress by certain adaptation whereas the latter can withstand it.
Drought resistance can be caused by avoidance or tolerance
The succulents avoid drought injury by means of several adaptation
They conserved water by reducing the loss of water
Their stomata are closed during the day and are open during the night
Their thick cuticle and closed stomata during the day contribute to their resistance to drought
In temperature resistance plants, the moist cell of plants are killed above 40C
Dehydrated cell can tolerate about 140C
Plants resistant to high temperature have high level of bound water
Resistance to salt stress by halophytes has been found to be developed by accumulating salt within their cell so that water can be absorbed by osmotic process
Excess of salt has also been seen in some halophytes to be exuded on the leaf surfaces or are alternatively leached.
REFERENCE Verma .V, (2008), plant physiology,
published by Ane Books India. Salisbury and Ross,(1986),plant
physiology,CBS publishers Taize and Zeiger , (2003) , plant
physiology, panima publishing corporation
Srivastava H.S,(1999),plant physiology