This document discusses stress in crops caused by both biotic and abiotic factors. It defines stress and provides classifications of stress. Biotic stress results from damage by living organisms like pathogens, insects and weeds. Abiotic stress is caused by non-living environmental factors like drought, salinity, temperature extremes. Drought is a major abiotic stress that limits crop yields globally. The effects of drought on crops are discussed along with mitigation strategies like mulching, fertilizer management and foliar sprays.

1. STRESS
CLASSIFICATION

2. In today‘s climate change scenarios, crops are exposed more
frequently to occurrences of biotic (viruses, bacteria, fungi,
nematodes, insects, arachnids and weeds) and abiotic stresses
(drought, salinity, elevated temperature, submergence and
nutrient deficiencies).
These stresses limit crop production. In recent years,
advances in physiology, molecular biology and genetics have
greatly improved our understanding of crops response to these
stresses and the basis of varietal differences in tolerance.

3. DEFINITIONS OF STRESS
Physical terms
Stress is defined as the force per unit area acting upon a material, inducing
pressure and leading to dimensional change. More generally, it is used to
describe the impact of adverse forces.
Biological terms
Stress can be any factor that may produce an adverse effect in individual
organisms, populations or communities.
or
Stress is also defined as the unbearable pressure that affects the normal
functions of individual life
or
The conditions in which plants are prevented from fully expressing their
genetic potential for growth, development and reproduction (Levitt, 1980
and Ernst, 1993).

4. Agricultural terms
Stress is defined as a phenomenon that limits crop productivity
or destroys biomass (Grime, 1979).
Or
Stress in plants refers to external conditions that adversely
affect growth, development or productivity of plants.

5. CLASSIFICATION OF STRESSES
Plants are subjected to a wide range of stresses which reduces and
limits the productivity of agricultural crops.
Stresses trigger a wide range of plant responses like altered gene
expression, cellular metabolism, changes in growth rates, crop
yields, etc.
It has become importance for ecologists, physiologists and
agronomists to divide stresses experienced by plants into two major
categories i.e., biotic and abiotic.
Biotic stresses originate through interactions between organisms,
while abiotic stresses are those that depend on the interaction
between organisms and the physical environment.

6. BIOTIC STRESS
Biotic stress is stress that occurs as a result of damage done
to plants by other living organisms. It includes bacteria, fungi,
nematodes, viruses, insect-pests and weeds.
About 34% of the crop produce is lost annually due to
diseases, insect-pests and weeds on the global basis.
Out of which, 12% is lost due to diseases (caused by fungi,
bacteria or viruses), 11% due to nematodes, 7% due to
insect-pests and 3% due to weeds.

7. Despite lacking the adaptive immune system plants can counteract
biotic stresses by evolving themselves to certain sophisticated
strategies.
The defence mechanisms which act against these stresses are
controlled genetically by plant’s genetic code stored in them.
The resistant genes against these biotic stresses present in plant
genome are encoded in hundreds.
The biotic stress is totally different from abiotic stress, which is
imposed on plants by non-living factors such as salinity, sunlight,
temperature, cold, floods and drought having negative impact on
crop plants.

8. It is the climate in which the crop lives that decides
what type of biotic stress may be imposed on crop
plants and also the ability of the crop species to resist
that particular type of stress.
Many biotic stresses affect photosynthesis, as chewing
insects reduce leaf area and virus infections reduce the
rate of photosynthesis per leaf area.

9. ABIOTIC STRESS
Any adverse factor acting on physiological processes/ biochemical
activity of the plants is called as abiotic stress.
Or
The negative impact of environmental factors on plant growth and
yield.
Abiotic stresses include potentially adverse effects of-
 Salinity,
 Drought,
 Flooding,
 Metal toxicity,
 Nutrient deficiency and excess,
 High temperature and Low temperature.

10. In addition, abiotic stresses can include
 Shade,
 UV exposure
 Photo inhibition,
 Air pollution,
 Wind,
 Hail and
 Gaseous deficiency
which are often sporadic and highly localized in occurrence.

12. Plants can experience abiotic stress resulting from the
shortage of an essential resource or from the presence of high
concentrations of a toxic or antagonistic substance.
In some cases, such as the supply of water, too little (drought)
or too much (flooding) can both impose stress on plants.
In reality, abiotic and biotic stresses are often inextricably
linked.

13. Abiotic stress management is one of the most
important challenges facing agriculture.
Abiotic stress can persistently limit choice of crops and
agricultural production over large areas and extreme
events can lead to total crop failures.
Abiotic stresses adversely affect the livelihoods of
individual farmers and their families as well as national
economies and food security.

14. Major abiotic stresses which limits crop yield
1. Drought
Among the environmental stress factors, one of the most widely limiting
for crop production on a global basis is water.
According to one estimate, around 28 percent of the world‘s land is too
dry to support vegetation (Kramer and Boyer, 1995).
Drought can be defined as an extended period of deficient rainfall
relative to the statistical mean for a region.

15. Types of drought
a) Meteorological drought
It is qualified by any significant deficit of precipitation.
b) Hydrological drought
It is manifest in noticeably reduced river and stream flow and critically low
groundwater tables.
c) Agricultural drought
It indicates an extended dry period that results in crop stress and crop yield.
The impact of drought on agriculture is due to a deficit of moisture in the
soil, when the moisture in the soil is no longer sufficient to meet the needs
of growing crops.

16. Effects of drought stress on crops
 Reduced seed germination and seedling development
 Poor vegetative growth
 Reproductive growth is severely affected
 Plant height and leaf area reduced
 Significantly reduction in leaf weight
 Reduced photosynthesis
 Reduced stomatal conductance

17. Mitigation of Drought Stress
Foliar spray of 2% DAP + 1% KCl during critical stages of flowering
and grain formation
3% Kaoline spray at critical stages of moisture stress
Foliar spray of 500 ppm Cycocel
Mulching with 5 tonnes of sorghum / sugarcane trash, which saves
19- 20% of irrigation water by reducing evaporation loss of water
Split application of N and K fertilizers as in cotton at 45 and 60 DAS
Use of biofertilizers viz., Azospirillum or Phosphobacteria @ 10
packets / ha along with 25 kg of soil or FYM.

18. Seed hardening with 1% KH₂PO₄ and other salts for 6 – 8 hours
(depending upon nature of seed coat) soaked in equal volume of
water
Spray of 40 ppm NAA (4 ml of Planofix in 4.5 litres of water)
Seed treatment + soil application + foliar spray of Pink Pigmented
Facultative Methnaotrops (PPFM) @ 106 as a source of cytokinins.
Foliar spray of 0.5% zinc sulphate + 0.3 % boric acid + 0.5 %
Ferrous sulphate + 1% urea during critical stages of moisture stress
Application of PGRs namely Cytokinin (10 ppm); Brassinolide (0.5
ppm); Salicylic acid (100 ppm); Ascorbic acid (100 ppm) and CCC
(10 ppm).