other organisms, plants are also subjected to various environmental stresses
such as water deficit, drought, cold, heat, salinity and air pollution. The
study of functioning of plants under adverse environmental conditions is called
stress physiology. Jacob Levitt (1972)
first used the term biological stress in relation to plants
and according to him stress is “any change in environmental condition that
might adversely change the growth and development of a plant”.
reaction of plants facing stress is called strain.
For example, if a normal plant growing under
favourable light conditions is subjected to low light intensity, its
photosynthesis is reduced. Thus, low light intensity is referred as stress and
reduction of photosynthesis is referred as strain. Biological strains are of
two types; Elastic biological strain and Plastic biological strain. If the
reaction of plant function is temporary and when it returns to its original
state it is called elastic biological
strain. Example: Temporary wilting.
If the reaction is permanent and the
plant function does not return to the normal state it is called plastic biological strain. Example:
Permanent wilting. Some plants get adapted to stress condition and are not
adversely affected by stress. Such plants are called stress resistant or stress
tolerant plants. Example:
Mangroves. Some plants cannot face stress and they pass their adverse period in
dormant state and so they are called stress
enduring plants. Ephemeral plants
are short lived desert plants, which
complete their life cycle during the seasonal rains before the onset of dry
season. These ephemeral plants are called stress
escapers. Stress in plants can be classified as given in figure 15.31.
adverse effects on plants caused by other living organisms such as viruses,
bacteria, fungi, parasites, insects, weeds and
Biotic environmental stress is also caused due to the activity of man
by cutting herbs and trees, twigs for fodders, fuels and agricultural purposes.
The biotic stresses caused by bacteria, fungi and nematodes that are ever
present in the environment are called potential biotic stresses. These are
divided into two types. They are:
organism producing one or more biochemical substances that greatly influence
the germination, growth and reproduction of other organisms is called Allelopathy. These biochemicals are
known as allelochemicals. They are
beneficial (positive allelopathic) or detrimental (negative allelopathic).
These allelochemicals are obtained from leaf after leaching on the ground and
also from roots. The term allelopathy is from Greek words allelon-each other and pathos-to
suffer and first used in 1937 by Hans Molisch. Allelopathic effect may occur with weeds on crops and vice versa (Figure 15.32).
One of the most famous allelopathic plants is Black walnut (Juglans nigrum). The chemical which is present in Black walnut is Juglone and it is a respiratory inhibitor. Solanaceous plants such as tomato, capsicum and eggplant are susceptible to juglone. These plants when exposed to these allelochemicals exhibit symptoms such as wilting, chlorosis and death.
heaven (Ailanthus altissima) is a
recent addition to the list of allelopathic trees. Ailanthone an allelochemical extracted from the root of Ailanthus acts as potent herbicide. In Sorghum plant the allelochemical
sorgolone possess allelopathic activity. It is found in root exudates of most Sorghum species. Root exudation of maize
inhibits the growth of some weeds such as Chenopodium
album and Amaranthus retroflexus.
The seed exudates of oat (Avena fatua)
affect the germination of wheat seedling.
effect of microbes that cause diseases in plants. Example: Xanthomonas citri
stress may occur due to an atmospheric condition (atmospheric stress) or soil
condition (edaphic stress). Atmospheric stresses may occur due to excess and
deficient levels of light temperature and air pollutants.
limits the distribution of species. In low light intensity Sciophytes (shade loving plants) develop, while in high light
intensity Heliophytes (high light
loving plants) develop. In low light intensity, stomata do not fully open hence
there is less diffusion of gases. As a result, there is less photosynthesis and
the chlorophyll synthesis is also affected. High light intensity also inhibits
photosynthesis. Change in photoperiod inhibits flowering.
are adapted to a particular region and they face temperature stress in another
temperature causes soil and atmospheric drought. Plants are subjected to
permanent wilting in soil drought and temporary wilting in atmospheric drought.
Plants generally die above temperature of 44oC. However, some
organisms like Mastigocladus (a
cyanobacterium) grow well at 85 oC
to 90oC in hot springs. At 42oC synthesis of normal
protein declines and new protein called Heat Shock Proteins (HSPs) appears.
These proteins were discovered in fruit fly (Drosophila melanogaster) and since then they have also been
discovered in animals, plants and microorganisms. At high temperature all
physiological processes decline. Photosynthesis decreases and respiration
increases. So, plants face a shortage of organic substances.
temperature stress is quite harmful to plants and the temperature near freezing
point causes irreversible damage so that the plants fail to survive under
extreme cold conditions. However, some plants growing in alpine and arctic
regions can survive under low temperature and such plants are said to be cold resistant. Stress due to freezing
temperature is called frost stress. Temperature below 10oC,
decreases root growth, increases
leakage of ions and ethylene production.
atmospheric pollutants prevalent in the Indian sub-continent are CO2,
CO, SO2, NO2, O3 , fluoride and H 2S.
These pollutants do not cause visible injury but cause hidden injury. If the
concentration of these pollutants increases visible injury like chlorotic and
necrotic spots appear on leaves as well as inhibit photosynthetic carbon
metabolism and biomass formation. Some pollutants at low concentration
stimulate plant growth. Example: SO2, NO2 and NO.
Respiration and photorespiration are sensitive to air pollutants. If the
concentration of air pollutants is high it inhibits respiration whereas at
lower concentration stimulates respiration. Nitrogenous air pollutants under
chronic exposure increases chlorophyll content while NO2 reduces
pigment content at acute exposure.
divided into two types. They are water
stress and salt stress:
stress condition arising from lack of water or excess of water is called water stress. The abundance of water leads to a stress called flood stress and scarcity of water
leads to a stress called drought stress.
temporary inundation of plants and its parts by flooding causes oxygen
deficiency to the roots and soil borne microorganisms. Effects of flooding are
as follows: Nitrogen turnover in the soil is reduced; Abscisic acid, ethylene
and ethylene precursors are formed in larger amount; Stimulation of partial
stomatal closure, epinasty and abscission in leaves; Cellular membrane systems
break down, mitochondria and microbodies disintegrate and enzymes are partially
inhibited. Flood tolerant plants include those found on permanently wet soils.
Examples: Marsh plants, shore plants and hydrophytes. Tree species found dominant
in flooded sites are also tolerant. Examples: Taxodium disticum, Mangroves and palms are tolerant to flood stress.
‘drought’ denotes a period without appreciable precipitation, during which the
water content of the soil is reduced to such an extent that plants suffer from
water deficiency. Effects of drought are as follows: Decrease in cellular
growth and synthesis of cell wall components cause the cells to become smaller
in size; Nitrogen fixation and its reduction are decreased by decreasing the
activity of certain enzymes; Increase in abscisic acid level ultimately closes
down the stomatal apparatus to the minimum, hence, transpiration declines;
Protochlorophyll formation is inhibited and photosynthetic process declines; Levels
of proline increases; Respiration and translocation of assimilates decreases;
Loss of water leads to increase in the activity of hydrolytic enzymes, followed
by destruction of RNA and disruption of protein; Wilting in mature leaves is
associated with carbohydrate depletion due to mobilization export, followed by
are well adapted for drought either because,
protoplasm of such plants does not die when it faces extreme or prolonged desiccation
(dehydration) hence, it tolerates or endures such conditions. Example: Creosote
bush (Larrea tridentata) can survive water content drops upto 30% whereas, in most plants the lethal level is below
50–70% or these plants are able to avoid or postpone the lethal level of
desiccation because they have developed structural or physiological
adaptations. Plants that avoid or postpone desiccation have evolved an
alternative path by developing following mechanisms: Improved water uptake by
roots which penetrate deep down up to the water source; Efficient water
conduction by increasing and enlarging the conductive tissues in terms of
producing more number of xylem elements, dense leaf venation and reducing the
transport distance (short internodes); Restriction of transpiration brought
about by stomata present only on the lower epidermis and covered by dense
trichomes; Rolling of leaves also help to reduce water uptake by minimizing the
transpiring surface; Water storage in succulent tissue of Agave americana and other
CAM plants have been found to use
drought stress an essential protection mechanism that stabilizes the cell
structure is induced gene expression of stress
protein (dehydrin and osmotin). These proteins protect the macromolecules
in the cytoplasm and in the nucleus, the cytoskeleton (biomembranes) against
denaturation. High desiccation tolerance implies that the protoplasm rehydrates
when water becomes available. Plants growing in deserts and arid regions are
usually drought resistant.
of high salt concentration in the soil restricts the growth and development of
plants. Most commonly the plants which are present near the seashore and
estuaries are subjected to salt stress. According to an estimate about one
third of irrigated land on earth is affected by salt stress. Na+, Cl-,
K+, Ca++ and Mg++ ions usually contribute to
soil salinity. Plants growing in such areas face two problems:
Absorption of water from the soil with negative
Interaction with high concentration of toxic sodium
carbonate and chloride ions.
basis of salt tolerance, they are grouped into two categories:
Non-halophytes or glycophytes
are native to saline soils. The halophytes which can resist a range of salt
concentration are called as euryhaline
and those with narrow range of resistance are called stenohaline. Non-halophytes cannot resist salts as the halophytes. Helianthus annus tolerates high Mn2+ ions. Those which are
present in salt regions face two problems:
One is high concentration of salts in soil water
leads to decrease in water potential so they grow in opposite direction.
Injuries in salt affected plants caused by both
osmotic effects and specification effects. Accumulation of chloride ions
reduces water absorption and transpiration.
stress due to deficiency of mineral elements (K, P, S, Fe, Mo, Zn, Mg, Mn)
causes physiological disorders which lead to reduced growth and yields.
Salt accumulators absorb and store salts so that
the osmotic potential of their cells continues to remain negative throughout
the growing region.
In some salt hardy plants, the excess salt is
excluded on the surface of leaves. Some plants have salt glands which secrete
salt (mostly NaCl). The exuded salt absorbs water hygroscopically from the
Some plants lose their excess salt by leaching into
the soil or by dropping their salt filled leaves.
Salt tolerant plants (true halophytes)
synthesizelargeamountsoftheamino acid proline, galactosyl glycerol and some
organic acids which function in osmotic adjustments.
plants growing in salty habitats like halophytes face the problem of excessive
dissolved salts in the solution. Excess of salt creates comparatively more
negative osmotic potential so that the plants tend to lose water into
surrounding medium. Under such conditions the plants tend to lose water only
when their water potential becomes more negative. It is possible only if they absorb
excess of salt and accumulate it in their cell saps to maintain the same or
higher concentrations as those of outside plants.
Salt accumulates in the vacuoles
The plants become succulents
Accumulated salt dehydrates the cytoplasm
Sodium chloride cannot be tolerated in the
cytoplasm and it denatures several enzymes
absorption and accumulation of inorganic salts fail to solve the problem. The
plants however tolerate the salt stress by synthesizing organic compounds that
can exist at high salt concentrations without denaturing the enzymes. These
organic compounds are called nontoxic
organic osmotica. Examples: Proline and Betalin (osmoregulators).