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