Plant Physiology

1. Enoch Caryl B. Taclan
Responses of Plants to
Environmental Stress

2. • the basic concepts of plant stress, acclimation, and adaptation
• the light-dependent inhibition of photosynthesis through a process
called photo inhibition,
• the effects of water deficits on stomatal conductance,
• the effects of high and low temperature stress on plant survival
• the challenge of freezing stress on plant survival, and
• the responses of plants to biotic stress due to infestations by
insects and disease.
Chapter Overview

3. Introduction
• Has long been a central theme for plant environmental physiol
ogists and physiological ecologists.
• How plants respond to stress helps to explain their geographic
distribution and their performance along environmental gradie
nts.
• Because stress invariably leads to reduced productivity, stress
responses are also important to agricultural scientists.
• Essential in attempts to breed stress-resistant cultivars that
can withstand drought, and other yield-limiting conditions.
• They provide the plant physiologist with another very useful
tool for the study of basic physiology and biochemistry.

4. PLANT STRESS
• Energy is an absolute requirement for the maintenance
of structural organization over the lifetime of the organi
sm.
• The results in a constant flow of energy through all biolo
gical organisms, which provides the dynamic driving for
ce for the performance of important maintenance proces
ses such as cellular biosynthesis
• transport to maintain its characteristic structure and org
anization as well as the capacity to replicate and grow.
• The maintenance of a steady-state results in a meta-stab
le condition called homeostasis.

5. PLANT STRESS

6. PLANT STRESS
What is Plant Stress?
• Any change in the surrounding environment that
may disrupt homeostasis
• Environmental modulation of homeostasis may be
defined as biological stress.
• Results in adverse effect on the physiology of plant
• Involves the measurement of physiological phenomenon
in a plant species under suboptimal or stressed condition
• Plant species are highly variable with respect to their
optimum environment and susceptibility to extremes.

7. PLANT STRESS

8. PLANTS RESPOND TO STRESS IN SEVERAL
DIFFERENT WAYS
Abiotic Stress–physical (light, temperature) or chemical insult
that may be imposed by the environment in plant
Biotic Stress–biological insults in plant, such as pests and dise
ases
• The effect of injury or stress in plant varies depending on the severity
of the imposed stress
• Some plants are susceptible to stress, and others escape stress such as
those thriving in adverse environments
• Stress resistant plants have the capacity to tolerate particular stress,
hence they must exhibit the capacity to adjust, or acclimate
• Strategy is described the manner to which plants respond to
particular stress

9. PLANTS RESPOND TO STRESS IN SEVERAL
DIFFERENT WAYS
•Susceptible: plants prevent
flowering, seed formation and
induce senescence that leads to
plant death
•Acclimation: hardening induced by
gradual exposure to chilling tempera
tures.
•Ephemeral plants never experience
the stress of drought or low tempera
ture by stress avoidance.
•Germinate, grow and flower very qu
ickly following seasonal rains.
•(alfalfa, cacti)

10. TOO MUCH LIGHT INHIBITS PHOTOSYNTHESIS
What is Photoinhibition?
- Is an oxidative damage to lipids, proteins, pigments
due to excess absorption of light energy
• Light is needed in the assimilation of CO2, too much light can
inhibit photosynthesis
• At low irradiance, the rate of CO2 assimilation increases
linearly with an increase in irradiance
• More absorbed light means higher electron transport, and
increase in ATP and NADPH production
• As irradiance increases, the rate of photosynthesis is no long
er linear, and the levels become off

11. TOO MUCH LIGHT INHIBITS PHOTOSYNTHESIS

12. TOO MUCH LIGHT INHIBITS PHOTOSYNTHESIS
•Photosynthetic efficiency: max in
itial slope of light response under limit
ing condition.
•Light saturated: rate of photosynth
esis at higher light intensities.
•Photosynthetic capacity: max ligh
t saturated rate.
•Excess light: causes decrease in the
rate of photosynthesis.
•Photoinhibition: light dependent d
ecrease in photosynthetic rate.

13. TOO MUCH LIGHT INHIBITS PHOTOSYNTHESIS
•CO2 increases linearly with
irradiance.
•Upon further increase in irrad
iance, the rate of photosynthes
is said to be light saturated.
•Plants become exposed to inc
reasing levels of excess light.
•Continues exposure to excess
light decreases the rate of phot
osynthesis.

14. TOO MUCH LIGHT INHIBITS PHOTOSYNTHESIS
• Photoinhibition that results in a decrease in photosy
nthetic efficiency as well as photosynthetic capacity
usually reflects chronic photoinhibition which is
the result of photo damage to PSII.
• The decrease in PSII photochemical efficiency is us
ually paralleled by a decrease in photosynthetic effi
ciency of O2 evolution

15. TOO MUCH LIGHT INHIBITS PHOTOSYNTHESIS
•In most plants, PSII is more
sensitive to photo inhibition than
PSI.
•Photoinhibition can be assessed by:
•Monitoring changes in photosynthetic
efficiency and capacity
•Monitoring chlorophyll fluorescence
(assess changes to PSII photochemical
efficiency)

16. THE D1 REPAIR CYCLE OVERCOMES PHOTODAMAGE TO PSII
• D1 reaction center polypeptide of PSII is damaged, resulting
in the decrease of charge separation efficiency in PSII
• Decrease in PSII photochemical efficiency is usually parallel
with reduction in photosynthetic efficiency in O2 evolution.
How do plants ensure a constant supply of functional
PSII reaction centers?
Plants and green algae exhibit a single chloroplastic gene that en
codes the D1 polypeptide called psbA

17. THE D1 REPAIR CYCLE OVERCOMES PHOTODAMAGE TO PSII
• When the D1 polypeptide is damaged, it
is marked for degradation by protein
phosphorylation.
• After phosphorylation of D1, PSII is disas
sembled and the D1 polypeptide is degrad
ed by proteolysis.
• The psbA gene is transcribed and translat
ed using the chloroplastic transcriptional
and translational machinery with the sub
sequent accumulation of a new D1 polype
ptide.
• This new D1 polypeptide is inserted into t
he nonappressed, stromal thylakoids and
a new, functional PSII complex is reassem
bled

18. WATER STRESS IS A PERSISTENT THREAT TO PLANT SURVIVAL
• Can either be excess of water supply (flooding) or deficiency in
water supply (drought)
• Flooding Stress – results in the reduction of oxygen supply
in the roots
• Drought Stress – arises due to lack of rainfall and
reduced water supply
• Desiccation stress – transpirational loss from leaves.

19. WATER STRESS LEADS TO MEMBRANE DAMAGE
• Removal of water increases the solute concentration as
the protoplast volume shrinks
• This affects the structural and metabolic integrity of the
cells
• When dehydrated, cytosolic and organellar proteins
may undergo substantial loss of activity or complete den
aturation
• High concentration of cellular electrolytes accompany
dehydration of the protoplasm
• General disruption of cell metabolism upon rehydration

20. PHOTOSYNTHESIS IS PARTICULARLY SENSITIVE
TO WATER STRESS
• Photosynthesis is also affected by water stress
• Stomatal closure cuts off access of the chloroplast to atmospheric
CO2
• Low cellular water potential posts a direct effect on the structural i
ntegrity of photosynthetic machinery
• Direct effect of water deficiency in photosynthetic activity decreases
the demand for CO2 and CO2 concentration remains high
• Electron activity and photophosphorylation are reduced in chloropl
asts at low water potentials

21. PHOTOSYNTHESIS IS PARTICULARLY SENSITIVE
TO WATER STRESS
• Direct effect of water stress in photosynthesis is exacerbated
by the additional effects of light
• CO2 assimilation is inhibited, exposing the plant to excess
light
• Light absorbed by photosynthetic pigments continue to
absorb light, but cannot be processed since the electron
transport is inhibited
• Concomitant effect of exposure of plants to a water deficit is
chronic photoinhibition

22. STOMATA RESPOND TO WATER DEFICIT
• Increase in the vapour pressur
e gradient between the leaf
and the surrounding air
• Plants respond to water stress
by closing their stomata to
match transpirational water
loss in the leaf
• Stomatal closure and opening
is responsive to ambient humi
dity

23. STOMATA RESPOND TO WATER DEFICIT
• Guard cells responds directly with vapor pressure gradient.
• They become flaccid and stomatal aperture will close if rate
of evaporative water loss will exceed the rate of water regain.
• Hydropassive Closure – closure of the stomata by direct
evaporation of water from the guard cells and requires no
metabolic movement.
• Hydroactive Closure – triggered by decreasing water pote
ntial in the leaf mesophyll cells and involves abscisic acid (A
BA) and other hormones.

24. STOMATA RESPOND TO WATER DEFICIT
ABA(AbscisicAcid)
• 1960s
• Inhibitor of stomatal opening.
• Have a role in stomatal closure of plants.
• Accumulates in water-stressed leaves.
• Synthesized in the cytoplasm of leaf mesophyll cells but, accumulates in the
chloroplasts.
• At low pH, ABA exists in the protonated form ABAH, which freely permeates
most cell membranes.
• The dissociated form ABA− is impermeant; because it is a charged molecule
it does not readily cross membranes.

25. STOMATA RESPOND TO WATER DEFICIT

26. STOMATA RESPOND TO WATER DEFICIT
• Inhibition of electron transport and photophosphorylation in
the chloroplasts would disrupt proton accumulation in the th
ylakoid lumen and lower the stroma pH
• there is an increase in the pH of the apoplast surrounding the
mesophyll cells
• The resulting pH gradient stimulates a release of ABA from th
e mesophyll cells into the apoplast, where it can be carried in
the transpiration stream to the guard cells (Figure 13.10).

27. STOMATA RESPOND TO WATER DEFICIT

28. STOMATA RESPOND TO WATER DEFICIT
• Stomatal closure begins before there is any significant increase in
the ABA concentration
• The release of stored ABA into the apoplast, which occurs early
enough
• In sufficient quantity the apoplast concentration with at least
double to account for initial closure
• Increased ABA synthesis follows and serves to prolong the closing
effect
• The stomata close in response to soil desiccation before there is
any measurable reduction of turgor in the leaf mesophyll cells.

29. PLANTS ARE SENSITIVE TOFLUCTUATIONS
IN TEMPERATURE
• Plants vary response to temperature fluctuations, and plants
have unique set of optimum temperature requirement.
• When temperature reaches upper or lower limits, growth
diminishes, and may even cause death of the plant
• Several plants are adapted to extremes temperatures such as
those found in the desert, higher altitude areas, and boreal
forests
• Except in the relatively stable climates of tropical forests,
temperatures frequently exceed these limits on a daily or
seasonal basis, depending on the environment

30. PLANTS ARE CHILLING SENSITIVE
• Plants native to warm habitats are injured when exposed to low, non-
freezing temperatures and are considered to be chilling sensitive.
• Plants adapted to warm habitats are sensitive to low temperature.
• These includes corn, tomato, cucumber, soybean, cotton, and
banana.
• Signs of injury are exhibited when the temperature drops below 10 to
15̊C.
• Plants such as apple, potato, and asparagus experience injury at temp
eratures of 0 to 5 C.

31. PLANTS ARE CHILLING SENSITIVE
 Young seedling exposed to low temperatures exhibit reduced leaf
expansion, wilting, and chlorosis.
 Browning and necrosis is exhibited in extreme cases

32. PLANTS ARE CHILLING SENSITIVE
• Low temperature causes reversible changes in the
physical state of the membrane
• Membrane lipids (diacylglycerides containing 16-1
8 carbons).
• Some fatty acids are unsaturated, which means tha
t they have one or more carbon-carbon double bon
ds (—CH = CH—), while others are fully saturated
with hydrogen (—CH2—CH2—).
• Because saturated fatty solidify at higher temperat
ures than unsaturated fatty acids, the
relative proportions of unsaturated and saturated f
atty acids in membrane lipids have a strong influe
nce on the fluidity of membranes.
• Transition temperature -abrupt transition tem
perature from fluid state to gel.
• Mung bean-14°C

33. PLANTS ARE CHILLING SENSITIVE
• Transition Temperature – temperature at which an
abrupt change in membrane form from fluid state to semi
crystalline state.
• Chilling-sensitive plants tend to have more saturated fatty
acids and higher transition temperature
• Chilling-resistant species, on the other hand, tend to have
lower proportions of saturated fatty acids and, therefore,
lower transition temperatures
• During acclimation to low temperature, proportion of
unsaturated fatty acid increases and transition
temperature decreases.

34. PLANTS ARE CHILLING SENSITIVE
• Transition of membrane to semi-crystalline state disrupts the
integrity of membrane channels resulting in loss of
compartmentation.
• Respiratory assemblies, photosystems, and other membrane-
based metabolic processes are impaired
• Membranes of chilling resistant plants are able to maintain
membrane fluidity and protect critical cellular functions
against damage

35. HIGH-TEMPERATURE STRESS CAUSES PROTEIN
DENATURATION
• High-temperature limits in C3 plants are determined
by denaturation of enzymes.
• Thyllakoid membranes are most sensitive temperature
with effect on photosynthesis efficiency
• The O2 evolving complex is directly inactivated by heat ther
eby disrupting electron donation to PSII resulting in the acc
umulation of P680+ (oxidizing agent)
• The result is that an increasing portion of the absorbed ener
gy cannot be used photochemicallyby PSII, leading to
chronic photoinhibition

36. HIGH-TEMPERATURE STRESS CAUSES PROTEIN
DENATURATION
• Increase in supraoptimal temperature inhibits the synthesis
of D1, which is the PS II reaction center polypeptide
• Inhibition of the D1 repair cycle that is critically important
to overcome the effects of chronic photoinhibition
• Heat Shock Protein - new family of low mass proteins sy
nthesized when exposed to high-temperature stress

37. • High temperature stress induces synthesis o
f new family of low molecular mass proteins,
HSPs (heat shock proteins)
• Exposure in 15 min-few hrs (5°C to 15°C
above the normal growing temperature)
induce HSPs production
• 3 distinct classes based on molecular m
ass
• Ubiquitin has an important role in markin
g proteins for proteolytic degradation.
• HSPs: might have a critical role in protectin
g the cell against effects on rapid temperatur
e change.
• Chaperonins: class of proteins
• Direct the assembly of multimeric protein
aggregates

38. HIGH-TEMPERATURE STRESS CAUSES PROTEIN
DENATURATION
• HSPs are synthesized very rapidly following an abrupt increase
in temperature; new mRNA transcripts can be detected with
in 3 to 5 minutes and HSPs form the bulk of newly synthesize
d protein within 30 minutes.
• Within a few hours of return to normal temperature, HSPs are
no longer produced and the pattern of protein synthesis returns
to normal

39. HIGH-TEMPERATURE STRESS CAUSES PROTEIN
DENATURATION
• HSP70 is identical in plants and animals which
functions to prevent the disassembly and denat
uration of multimeric aggregates during heat str
ess.
• At the same time, increased ubiquitin levels
reflect an increased demand for removal of prot
eins damaged by the heat shock

40. INSECT PESTS AND DISEASE REPRESENT POTENTI
AL BIOTIC STRESSES
• Hypersensitive Reaction – usually activated
by viruses, bacteria, fungi, and nematodes
• Plants subjected to biotic stresses responds with
changes in the composition and physical properti
es of the cell walls, and biosynthesis of secondary
metabolites
• An early event in this sensing/signalling pathway
is the activation of defense-related genes and syn
thesis of their products, pathogenesis-related
(PR) proteins

41. INSECT PESTS AND DISEASE REPRESENT POTENTIAL BIOTIC
STRESSES
• Activation of defense-related genes
• Include:
• PR porteins w/ Proteinase inhibitors that disarm proteolytic enzymes s
ecreted by the pathogen
• Lytic enzymes such as β-1,3-glucanase
• Chitinase that degrade microbial cell walls
• Also activated genes that encode enzymes for the biosynthesis of isoflavono
ids and other phytoalexins that limit the growth of pathogens
• Lignin, callose, and suberin are accumulated in cell walls are believed to pr
ovide structural support to the wall
• Programmed cell death (necrosis)

42. SYSTEMIC ACQUIRED RESISTANCE REPRESENTS
A PLANT IMMUNE RESPONSE
• Secondary metabolites associated with the hypersensitive reaction
appear to constitute signal transduction pathways that prepare
other cells and tissues to resist secondary infections
• Hypersensitive reaction is limited to the few cells at the point of inva
sion
• Over a period of time, the capacity to resist pathogens gradually
becomes distributed throughout the entire plant.
• In effect, the plant reacts to the initial infection by slowly developing
a general immune capacity. This phenomenon is known as
systemic acquired resistance (SAR).

43. SYSTEMIC ACQUIRED RESISTANCE REPRESENTS
A PLANT IMMUNE RESPONSE
• One component of the signaling pathway appears to be
salicylic acid (Figure 13.14)
• Salicylic acid (2-hydroxybenzoic acid) is a natural
ly occurring secondary metabolite with analgesic proper
ties.
• It was observed that both salicylic acid and its acetyl der
ivative (aspirin), when applied to tobacco plants, induce
d PR gene expression and enhanced resistance to tobacc
o mosaic virus (TMV).
• It has been shown in a variety of plants that infection is
followed by increased levels of salicylic acid both locally
and in distal regions of the plant (Figure 13.15)

44. SYSTEMIC ACQUIRED RESISTANCE REPRESENTS
A PLANT IMMUNE RESPONSE
The first pathogens to infect the plant (primary infection) stimulate a localized hypersensiti
ve reaction(HR) and the synthesis of salicylic acid(SA). Salicylic acid is translocated through
the phloem to other regions of the plant where it prevents secondary infection by other
pathogens. Alternatively, salicylic acid may be converted to methylsalicylicacid (MSA). MSA
is moderately volatile and may function as an airborne signal.

45. JASMONATES MEDIATE INSECT AND
DISEASE RESISTANCE
• Jasmonates (jasmonicacid) & its methyl ester methyljasmon
ate also mediates insect and disease resistance.
- Occur throughout plants (young actively growing tissues)
• High concentrations of jasmonic acid have been isolated fro
m fungal culture filtrates.
• Synthesized from linolenic acid (Jasminium), led to the prop
osal that it is a second messenger.
• There are some similarities in the action of salicylic acid and
jasmonateswith respect to insect and disease resistance.
- One difference is that there is a two defensive ways specific
for both.

46. JASMONATES MEDIATE INSECT AND
DISEASE RESISTANCE
• Jasmonate action is not limited to insect and disease resistance
• Jasmonates modulate a number of other physiological processe
s.
- Seed and pollen germination, vegetative protein storage, root
development, and tendril coiling
• The jasmonates appear to work in concert with ethylene.
• This breadth of jasmonate effects has led some to suggest
that jasmonates should be elevated to the status of plant hormo
nes.