This document discusses the physiology of crops under biotic stress. It begins by defining biotic stress as stress caused by living organisms such as insects, bacteria, fungi, viruses, nematodes, and weeds. Biotic stress affects key physiological processes in plants like photosynthesis, transpiration, respiration, and reproduction. The document then examines specific examples of how different types of biotic organisms like insects and pathogens impact these physiological functions through tissue damage, nutrient deprivation, and disruption of transpiration and nutrient translocation. It concludes by discussing how abiotic and biotic stresses can interact and combine their negative physiological effects on crops.

1. PHYSIOLOGY OF CROPS
UNDER BIOTIC STRESS
R.Gowthami M.Sc. (Ag) Plant
Physiology
Assistant Professor

2. INTRODUCTION
• Stress is usually defined as an external factor that exerts a disadvantageous
influence on the plant. It may be due to abiotic factors or biotic factors
• In contrast to abiotic stress such as drought and heat, biotic stress agents
directly deprive their host of its nutrients leading to reduced plant vigour
and in extreme cases, death of the host plant.
• Biotic stress in plants is caused by living organisms, specifically insects,
bacteria, fungi, viruses, nematodes, and weeds.
• Many biotic stresses affect photosynthesis, by chewing insects reduce leaf
area and virus infections reduce the rate of photosynthesis per leaf area.
• Global crop losses due to weeds, pests and diseases can be up to 40%
(Mitchell et al.,2016)

3. BIOTIC STRESS - INSECT
Host-Pest Interaction:
The insect herbivore perceives plant-derived cues, optical and/or olfactory
& responds.
The plant is found and the plant surface is examined by contact-testing
The plant may be damaged
The plant is accepted (one or more eggs being laid or continued feeding) or
is rejected, resulting in the insect’s departure
The herbivorous are distinguishable based on the degree of food-plant
specialization in monophagous, oligophagous, and polyphagous.
(HANDBOOK OF PLANT AND CROP STRESS, Mohammad Pessarakli)

4. Lepidopteron and Coleopterans:
• Caterpillars, beetles, weevils
• subterranean herbivore feeds on plant roots
Hemipterans:
• Plant bugs penetrate epidermal cells and ingest cell contents
• Aphids suck from the sap flow in phloem (phloem sieve elements)
• Spittlebugs and cicadellinine leafhoppers often tap the xylem
• Leaf-mining insects that live and feed during their larval stage between the
upper and lower epidermis of leaf blade and damage the parenchymal tissues
(HANDBOOK OF PLANT AND CROP STRESS, Mohammad Pessarakli)
Major pest Orders that causing damage to crops

6. Effects on Plant Physiological function
• Photosynthesis
 Insect herbivore damage is assessed by surveying the amount of tissue removed
from foliage.
 Over 50% of all plant–insect interactions resulted in a loss of photosynthetic
capacity
Direct and Indirect Reduction of Photosynthetic Capacity
Direct : The removal of leaf tissue by herbivores represents a “direct” reduction of
photosynthetic capacity
Indirect:
1. Altered sink demand,
2. Defense-related autotoxicity
3. Defense-induced down-regulation of photosynthesis

7. 1. Alteration of Sink Demand:
S.no. Type of
insect
Damage Reference
1 Gall
forming
insects in
tree
saplings
(Apple)
Reduced photosynthetic efficiency, but
increased NPQ, indicating a down regulation
of the PS II reaction centers in the area
around galls
Aldea et al.,
2006
Decline in leaf temperature near galls due to
increased transpiration rate resulting in
increased fluid and nutrient transport near
the point of damage
Macfall et al.,
1994
2 Defoliators
in Sour
Cherry
Defoliation, which improve photosynthesis in
remaining leaf tissue by increasing
carboxylation efficiency and the rate of RuBP
regeneration
Layne and
Flore, 1992

8. 2. Autotoxic Effect of Defensive Compounds
on Photosynthesis
• Plants run the risk of autotoxicity because of the biocidal properties of
many secondary compounds.
• The autotoxic effect of defensive compounds on photosynthesis is highly
species-specific
Example: The presence of furanocoumarins in oil tubes of wild parsnip
which will reduce the photosynthetic efficiency and gas exchange
Many plant species contain essential oils with allelochemical properties, yet
the extent to which these same chemicals can be autotoxic is unclear.

9. 3. Down-Regulation of Photosynthesis
• Photosynthetic proteins are often down-regulated by insect attack.
• i.e., there is reductions in ribulose-1,5-bisphosphate carboxylase/oxygenase
(RuBisCO)
• Jasmonates play a central role in regulating plant defense responses to
herbivores but while jasmonates induce defenses, they also inhibit growth
and photosynthesis
• The slower growth and down-regulation of photosynthetic-related genes by
herbivore elicitation may be required to free up resources for defense-related
processes.

10. BIOTIC STRESS - PATHOGEN
• While pathogens infect plants depending on the kind of
pathogen and on the plant organ and tissue they infect,
pathogens interfere with the different physiological function
• Very little consideration has been given to the mechanisms
involved in the pathogenic process
• Although there are many kinds of biotic stress, the majority of
plant diseases are caused by fungi

11. Effects on Plant Physiological function
Photosynthesis:
• Any disruption or interference in photosynthesis can create and induce stressful
conditions in plants.
• The overall chlorophyll content of leaves is reduced, but the photosynthetic activity
of the remaining chlorophyll seems to remain unaffected
• The toxins produced by the pathogens inhibit some of the enzymes that are
involved directly or indirectly in photosynthesis
Pathogen Symptoms
Fungi Chlorosis and necrosis
Leaf spots and blight
Rust
Bacteria Leaf spot, blight
virus Chlorosis
Stunting

12. A) Spots on barley leaves caused by the fungus Rhynchosporium sp. (B)Pumpkin leaves infected heavily
with the downy mildew caused by Pseudoperonospora cubensis. (C)Wheat plant infected with the stem
rust fungus Puccinia graminis f.sp. tritici. (D)Angular leaf spots on cucumber leaf caused by the
bacterium Pseudomonas lacrymans. (E) Reduced chlorophyll in yellowish areas of virus-infected plants,
such as cowpea infected with cowpea chlorotic mottle virus or (F)Stunting and yellowing of rice plants
infected with the rice tungro virus.
(Plant Pathology, AGRIOS,2005)
B E
C DA
F

13. Translocation of water and nutrients
• Plants absorb water and inorganic (mineral) nutrients from the soil through
their root system.
• When a pathogen interferes with the upward movement of inorganic
nutrients and water or with the downward movement of organic
substances, diseased conditions result in the parts of the plant denied these
materials.
• Influence of Translocation of water and nutrients in the host plant through
Interference with Upward Translocation of Water and Inorganic
Nutrients
Interference with Absorption of Water by Roots
Interference with Translocation of Water through the Xylem
Interference with Transpiration
Interference with Translocation of Organic Nutrients through the
Phloem

14. Interference with Upward Translocation of Water and
Inorganic Nutrients
• Pathogens may interference with upward translocation
through,
Affecting the integrity or function of the roots, causing them to absorb
less water;
By growing in the xylem vessels and interfere with the translocation of
water through the stem
In some diseases, pathogens interfere with the water status of the plant
by causing excessive transpiration through their effects on leaves and
stomata.

15. Interference with Absorption of Water by
Roots
• Many pathogens cause an extensive destruction of the roots before any
symptoms appear on the aboveground parts of the plant
• Example: Damping-off fungi, Root-rotting fungi and bacteria
(A) Destruction of roots of young seedlings by the damping-off oomycete Pythium sp. (B)
Roots and stems of pepper plants killed by Phytophthora sp. (C) Infection of crown and
roots of corn plant with the fungus Fusarium.
C
(Plant Pathology, AGRIOS,2005)

16. • Some bacteria and nematodes cause root galls or root knots, which
interfere with the normal absorption of water and nutrients by the roots.
(A) Numerous galls caused by the bacterium Agrobacterium tumefaciens on roots of a
cherry tree. (B) Root knot galls caused by the nematode Meloidogyne sp. on roots of a
cantaloupe plant.
(Plant Pathology, AGRIOS,2005)
A B

17. Interference with Translocation of Water
through the Xylem
• Fungal and bacterial pathogens that cause damping off, stem rots, and
cankers may reach the xylem vessels in the area of the infection
(A) The stem of a cantaloupe plant infected with the fungus Phomopsis sp. (B) Canker on
an almond tree caused by the fungus Ceratocystis fagacearum.
(Plant Pathology, AGRIOS,2005)

18. (A) Vascular wilt of tomato caused by the fungus Fusarium. (B) Discoloured vascular tissues of a tomato stem
infected with the same fungus. (C) Wilted tomato plants infected with the vascular bacterium Ralstonia
solanacearum. (D) Discoloured vascular tissues of a tomato plant infected with the same bacterium.
A DCB
(Plant Pathology, AGRIOS,2005)

19. • The pathogen can reduce the flow of water through its physical presence in
the xylem as mycelium, spores, or bacterial cells and by the production of
large molecules (polysaccharides) in the vessels
Microscopic images: (A) Pseudomonas bacteria clogging a xylem vessel of a young plant
shoot. (B) Bacteria of the xylem-inhabiting Xylella fastidiosa in a vessel of a grape plant. (C)
Xylella bacteria in a cross section of a xylem vessel of an infected grape leaf.
B C
(Plant Pathology, AGRIOS,2005)

20. • In all cases, however, in infected hosts the flow of water is
reduced through
Reduction in the size or collapse of vessels due to infection
Development of tyloses in the vessels
Release of large molecule compounds in the vessels as a result of cell
wall breakdown by pathogenic enzymes
Reduced water tension in the vessels due to pathogen-induced
alterations in foliar transpiration
(A) Tyloses in a xylem vessel.
A
(Plant Pathology, AGRIOS,2005)

21. Interference with Transpiration
• In plant diseases in which the pathogen infects the leaves,
transpiration is usually increased.
• Due to destruction of part of the protection afforded the leaf
by the cuticle.
Disease Effect on Transpiration
Rust Formation of numerous pustules and break up the epidermis
Leaf spots The cuticle, epidermis and all the other tissues, including xylem may
be destroyed in the infected areas
Powdery
mildews
Epidermal cells are invaded by the fungus
Scab The fungus grows between the cuticle and the epidermis

22. A B
D
C
E
(A) The wheat leaf rust pathogen Puccinia recondita produces innumerable lesions (uredia) on wheat
leaves. (B) Uredospores breaking the epidermis and emerging from the surface of an infected leaf. (C)
Grape berries infected with the powdery mildew fungus Uncinula necator, the mycelium of which
penetrates and forms haustoria in almost every epidermal cell. (D) The apple scab fungus Venturia
inaequalis grows between the cuticle and the epidermis, causing the cuticle to break in numerous places,
allowing transpiration to occur. (E) Tomato leaves with numerous lesions caused by the fungus Septoria
sp. and through which excessive transpiration occurs.
(Plant Pathology, AGRIOS,2005)

23. Interference with Translocation of Organic
Nutrients through the Phloem
• Plant pathogens may interfere with the
 Movement of organic nutrients from the leaf cells to the phloem
 Translocation of organic nutrients through the phloem elements
 Movement from the phloem into the cells that will utilize them
(A) Young canker caused by the
fungus Nectria in the bark of
the branch.
(B) Two advanced Nectria
cankers in which both the
xylem and the pholem have
been killed by the fungus.
(C) Blister canker on a pine tree
in which the bark and
phloem have been killed by
the fungus Cronartium
ribicola.
A CB
(Plant Pathology, AGRIOS,2005)

24. • In diseases caused by phytoplasmas and phloem-limited fastidious bacteria, they exist
and reproduce in the phloem sieve tubes, thereby interfering with the downward
translocation of nutrients.
• In several plants propagated by grafting a variety scion onto a rootstock, infection of
the combination with a virus leads to formation of a necrotic plate at the points of
contact.
(A) The graft union of a pear
grafted on oriental pear
rootstocks, which results in the
death of pear phloem.
A
(Plant Pathology, AGRIOS,2005)

25. Respiration
• The energy produced through respiration is utilized by the plant for all types of
cellular work and because of this the respiration of plant tissues is one of the
first functions to be affected when plants are infected by pathogens.
Respiration of Diseased Plants
• When plants are infected by pathogens, the rate of respiration generally
increases.
• Due to use up of reserve carbohydrates by infected tissue faster than healthy
tissues.
• The activity or concentration of several enzymes of the respiratory pathways
seems to be increased.
• Increased respiration in diseased plants is also accompanied by an increased
activation of the pentose pathway, which is the main source of phenolic
compounds.

26. • In diseased plants, oxidative phosphorylation becomes less
effective. In that case, no utilizable energy (ATP) is produced
through normal respiration, despite the use of existing ATP and the
accumulation of ADP, which stimulates respiration.
• The energy is then produced through other less efficient ways,
including the pentose pathway and fermentation.
• The increased respiration of diseased plants can also be explained
as the result of increased metabolism.

27. Plant reproduction
• Pathogens that attack various organs and tissues of plants weaken
and often kill these organs or tissues, thereby weakening the
plants.
• Effects: smaller in size, Production of fewer flowers, Reduced fruit
and seed set, Inferior vigour
(A)The flowers of apricot tree which have been killed by the brown rot fungus Monilinia fructicola.
(B) A mixture of barley kernels (whitish-yellow) and ergot sclerotia (the larger black bodies) produced by
the ergot fungus Claviceps purpurea
(C) A mixture of intact healthy wheat kernels and somewhat darker, broken wheat kernels filled with
spores of the common bunt (covered smut) fungus Tilletia sp.
(D) Ear of corn having some of the corn kernels replaced by galls containing spores of the fungus Ustilago
maydis.
A B C D
(Plant Pathology, AGRIOS,2005)

28. Abiotic and biotic stress - Combined effect
• Based on the number of interacting factors, stresses
can be grouped into three categories: single, multiple
individual, and combined stresses
• One of the important diseases, dry root rot known to
be caused by high temperature and water deficit
conditions and by the fungus R. bataticola
• Long periods of drought accompanied with warm days
and cool nights generally favour powdery mildew in
sugar beet caused by the fungus Erysiphe betae.
• In both the cases the physiology is affected, decrease
in translocation in case of dry root rot and reduced
leaf photosynthetic area in case of powdery mildew
(Pandey et al.,2017)

29. Case study

30. Damage caused by biotic agents on leaf of
different hardwood saplings
(Aldea et al., 2006)
(a, b) fungal infection of a Cercis Canadensis
(c, d) leaf damage caused by Caryomyia midge galls on a Carya glabra leaf
(e, f) skeletonizer damage to a Quercus alba leaf

31. The indirect effects of three classes of biotic
damage (fungal, gall, chewing) on leaves
(Aldea et al., 2006)
Undamaged leaves
Unaffected portions of
damaged leaves
Damaged leaves
PSII Efficiency

32. The effects of release of endogenous oils
produce autotoxicity
2nd
(Gog et al., 2005)

33. Effect of razor cut on reduction in photosynthetic
capacity of wild parsnip, parsley and rough lemon
(Gog et al., 2005)

34. Survival of grapevines upon infection by
virus
ELISA results on the presence (+) or absence (–) of grapevine fan leaf virus (GFLV), grapevine fleck virus
(GFkV) and grapevine leaf roll-associated virus (GLRaV) virus in potted ex vitro acclimatized
noninfected (NI) and infected (VI) Vitis vinifera plants, and in field-grown young lowly infected (LI), old
highly infected (HI) and HI plants sprayed with nutrient solution (HI + N)
(Sampol et al., 2003)

35. Effect of virus infection on physiological
parameters in Grapevine
Leaf water potential (Ψ), chlorophyll (Chl), carotenoid and total soluble protein contents of
leaves, light-saturated net CO2 assimilation (AN), stomatal conductance (gs), electron
transport rate (ETR), ETR to AN ratio, ETR to gross photosynthesis (AG*) ratio. % Change
indicates the percentage of change (–, decrease; +, increase) of VI in respect to NI plants for
each parameter. *Statistically significant differences between treatments at P < 0.05; ns,
non significant differences.
(Sampol et al., 2003)

36. The relationship between net CO2 assimilation (AN) and substomatal CO2
concentration (Ci) in potted-grown Noninfected (NI, closed triangles) and
virusinfected (VI, open triangles) Vitis vinifera plants.
(Sampol et al., 2003)

37. Effect of virus infection on physiological
parameters in Grapevine
Leaf water potential (Ψ), total soluble protein contents of leaves, light-saturated net CO2
assimilation (AN), stomatal conductance (gs), electron transport rate (ETR), ETR to AN ratio, ETR
to gross photosynthesis (AG*) ratio, and mesophyll conductance (gmes) in field grown lowly
infected (LI), highly infected (HI) and highly infected plus nutrients (HI + N) plants. % Change
indicates the percentage of change (–, decrease; +, increase) of HI or HI + N with respect to LI
plants for each parameter.
(Sampol et al., 2003)

38. The relationship between net CO2 assimilation (AN) and substomatal CO2 concentration (Ci)
in field-grown lowly infected (LI, closed squares), highly infected (HI, open squares) and
highly infected plus nutrient (HI + N, closed circles) Vitis vinifera plants.
(Sampol et al., 2003)

39. Initial and total Rubisco activity and activation state in noninfected (NI) and
virus-infected (VI) potted plants
Initial and total Rubisco activity and activation state in field-grown lowly infected (LI),
highly infected (HI) and highly infected plus nutrient (HI + N) plants
(Sampol et al., 2003)

40. Effects of TuMV on Leaf number, plant height and leaf
area in mustard
Leaf number, plant height and leaf area at different
stage in TuMV infected (o) and control (•) plants.
(Guo et al.,2005)

41. Fig1
Fig2
Fig1:Chlorophyll a, chlorophyll b, total
chlorophyll content, and chlorophyll a/b at
different stage in TuMV infected (o) and
control (•) plants.
Fig2:The net photosynthetic rate (Pn),
transpiration rate (E), stomatal
conductance (Gs) and intercellular CO2
concentration (Ci) in TuMVinfected (o)
and control (•) plants at different stage.
(Guo et al.,2005)

42. Plant Growth Regulator in mitigating Biotic
Stress
• Exogenous application of salicylic acid through root feeding and foliar spray could
induce resistance against Fusarium oxysporum f. sp. Lycopersici (Fol) in tomato.
• Exogenous application of 200 µM SA to tomato plants through hydroponics medium
could significantly elevate foliar SA levels and activate systemic resistance
Fig1:(a) Vascular browning (b) Leaf yellowing wilting
Fig1: Fig2:
(Mandal et al.,2009)

43. Role of nutrients in controlling plant
diseases
Mineral
nutrient
Role
N Decrease the severity of the infection
K Decreases the susceptibility of host plants up to the optimal level for
growth
Mn Can control a number of diseases through lignin biosynthesis, phenol
biosynthesis
B Reduce the severity of many diseases
Cl Can enhance host plant resistance to disease
Si Creates a physical barrier which can restrict fungal hyphae penetration
In particular, nutrients could affect the disease tolerance or resistance of plants to
pathogens. However, there are contradictory reports about the effect of nutrients on
plant diseases and many factors that influence this response are not well understood.
(Dordas,2007)

44. • Management of Mung bean yellow mosaic using mineral nutrients –
Sulphur(S), boron(B), molybdenum(Mo).
• S-20kg/ha, B-2kg/ha, Mo-1.5kg/ha
(Pramanik et al.,2001)

45. Scanning electron micrographs of maize (a), rice (b)and wheat (c)
sheath surfaces showing silica cell form and deposition.
(Alhousari et al.,2018)
• Silicon, an important plant nutrition in plant defence.
• Silicic acid (4ml/l) as a foliar spray has effect against pest (stem borer & plant
hopper in rice)

46. Role of plant hormones in plant defence
responses
• Three phytohormones—SA, JA and ET, are known to play major roles in
regulating plant defence responses against various pathogens, pests and
abiotic stresses such as wounding.
• However, the underlying molecular mechanisms are not well understood
and several questions remain to be answered.
(Bari et al.,2009)

47. CONCLUSION
• Abiotic stress conditions such as drought, high and low temperature and salinity
are known to influence the occurrence and spread of pathogens, insects, and
weeds.
• These stress conditions also directly affect plant–pest interactions by altering
plant physiology and defense responses.
• Biotic stress in plants affects photosynthesis, respiration, translocation of
nutrients and water.
• There is an increased respiration rate and decreased photosynthesis, water &
nutrient transport in both pest and disease attack.
• Some traits for screening genotypes for tolerance to pathogen are Root System
Architecture, Leaf Pubescence, Cuticular wax
• Further, Plant traits that confer herbivore resistance typically prevent or reduce
herbivore damage through expression of traits that deter pests from settling,
attaching to surfaces, feeding and reproducing, or that reduce palatability to be
improved