biol.622-23.ppt

• Jimma University
• College of Natural Sciences
• Department of biology
• Botanical Science Stream
• Course Title: Advanced Plant Physiology
• Course Code: Biol.622
• For 1st year MSc students
• March, 2023; Semester II
1

• Introduction: Some areas of research in plant physiology:
• possible to study the whole plant or plant parts through measuring:
 plant heights
 root collar diameter
 root to shoot ratios (below ground and above ground biomass),
fresh weight, dry weight ( through destructive and non destructive
methods)
 leaf number, size, area, seedgermination, germination rate &
percentage, etc
 Tissue culture techniques
2

Photosynthesis
• It is not easy being not green for plants!
• The Sun- the primary source of energy for nearly all life.
• Photosynthesis-a process of introducing sun’s energy into the
biosphere; is ‘synthesis with the help of light’
• vital process by which green plants synthesize organic matter
• sometimes known as carbon assimilation
• depends on a set of complex protein molecules that are located in
and around THYLAKOID membrane.
4

• An example of the 1st Law of Thermodynamics –
• an energy conversion process - converts light energy to chemical
energy (carbohydrate).
• Three major types of energy conversions during photosynthesis:
i) Radiant energy (sunlight) → electrical energy (passage of
electrons via a series of carrier).
• is part of the light-dependent reactions (z-scheme, non-cyclic
electron flow)
ii) Electrical energy → changed to unstable chemical energy (ATP,
NADPH; not readily stored).
5

• ATP and NADPH - produced as the end result of non-cyclic
electron flow.
iii) Unstable chemical energy → Stable chemical energy
(carbohydrate).
• This is the light-independent reactions / Calvin-Benson cycle.
Significance of Photosynthesis:
• Maintains equilibrium of O2 in the atmosphere
• provides food directly or indirectly
• provides vast reserves of energy in the form of coal, oil, peat, wood
and dung.
6

1.1The photosynthetic apparatus
 Are-leaves along with the chloroplasts, Or
 Part of the leaf or plant cell which contains the ingredients
for absorbing and channeling the energy of the excited
pigment molecules into a series of chemical and
enzymatic reactions.
• In higher plants & green algae- the chlorophyll is
contained & synthesized in a cellular plastid-
chloroplasts
7

• the chloroplasts of higher plants are discoid or ellipsiodal in shape
• 4-6µ in length and 1-2µ thick.
• Chloroplast-internally filled with a hydrophilic matrix - stroma in
which grana are embedded
• Each granum has a diameter of 0.25-0.8µ
• The disk shaped grana lamellae placed one above the other like the
stack of coins.
• these lamellae are paired to form sac like structures - thylakoids.
• Each thylakoid encloses a space named as the lumen.
8

• Thylakoid membranes and
stroma lamellae both are
composed of lipid bilayer
and proteins.
9

• Chlorophylls and other photosynthetic pigments are found in the
form of protein pigment complexes mainly in thylakoid
membranes of grana.
• Some of the protein-pigment complexes are also found in stroma
lamellae.
• Thylakoids are sites of primary photochemical reaction.
• Inside the chloroplast, chlorophyll is further confined to a system
of membranes - the 'thylakoid membranes'.
• they exist as photosynthetic pigment-protein complexes.
13

• Chloroplasts have necessary enzymes and some ribosomes and
DNA which give them a partial genetic autonomy.
• Different pigments absorb light of different wavelengths
• The chlorophyll molecules together with specific integral
membrane proteins & carotenoids form a structured
environment to absorb photons
• The pigment-protein complexes are arranged into arrays of
hundreds of pigment molecules –photo-systems.
• Most pigments function as light-harvesters and transfer the
excitation energy to other pigments.
14

• Three major classes of photosynthetic pigments:
(1) Chlorophylls-insoluble in water and can be extracted only with
organic solvents.
• Give plants green colour
• Constitute about 4% of chloroplast dry matter
• occupy about 8% of the total cell volume in leaves
• Contains a porphyrin’ head’ and a phytol ‘tail’.
• Porphyrin is polar; water soluble, made up of tetrapyrrole ring and
a magnesium atom.
• Sandwiched between protein and lipid layers of chloroplast
lamellae 15

• The porphyrin part of the molecule is bound to the protein while
the phytol chain extends into the lipid layer (is soluble in lipids).
Pheophytins -are chlorophylls without central Mg atom
 have been identified as constituents of the photosynthetic
electron transport chain.
(2) Carotenoids- yellow or orange pigments, in leaves their
color is masked by chlorophyll. include carotenes and
xanthophylls
16

• Have absorption spectra in the region from about 400-550 nm.
• Insoluble in water and accessary photosynthetic pigments
• Perform two functions:
(a) harvest light energy and transfer the quanta absorbed to
chlorophyll molecules
(b) protect chlorophyll from photo-damage
(3) Phycobillins- common in blue green algae and red marine algae;
are soluble in water
17

1.2Light harvesting pigment protein complexes
• Photosystem: is functional and structural units of protein
complexes involved in photosynthesis
• carry out the primary photochemistry of photosynthesis
• an array of light-harvesting pigments containing a reaction center
that carries out the necessary photochemistry.
• Two separate photosystems:
i)Photosystem II (PS II) -
– reaction center contains chlorophyll ‘a’ molecules (P680) that
have peak absorbance (λmax) of 680nm.
20

ii) Photosystem I (PS I) - reaction center contains chlorophyll ‘a’
molecules (P700) that have λmax of 700nm.
• each is associated with different proteins in their respective
reaction centers.
• Their function is to harvest light energy and transfer it to a small
number of pigment-protein complexes –( reaction centers).
• In the reaction center, the energy from the photon is used to excite
an electron to a higher energy level.
21

• The e- can be transferred to an acceptor molecule that has a higher
energy level than the original reaction center.
• The acceptor molecule is then considered to be reduced.
• In this way, the energy from photons is used to 'pump' electrons
to higher energy levels.
• Each time the reaction center loses an electron, it becomes
oxidized and is able to accept electrons from an external
source.
22

• Plants evolved to use electrons from water and transfer them to
electron carrier like NADPH.
• plants use two different photosystems coupled in a series to excite
the electrons with two consecutive photons.
• a photon is first captured by the pigments of Photosystem II and
the excitation energy transferred to an electron in P-680 (a
pigment in the reaction center).
23

• The high-energy electron in excited P-680 (P-680*) is next
transferred to plastoquinone (PQ) via a protein component.
• This electron is eventually transferred to Photosystem I.
• A second photon is similarly captured by Photosystem I and the
energy transferred to the pigment (P-700) in its reaction center.
24

• The electron excited in P-700 is transferred to NADP+ through the
protein ferredoxin (Fd) to produce NADPH.
• When P-680* and P-700* donate their high-energy electrons, they
become oxidized.
• To bring the system back to neutrality electrons are channeled from
water to P-680, thereby forming O2, and electrons from PQ are
transferred to P-700 via cytochrome and plastocyanin
intermediates. (see the images)
25

1.3 Light and the primary act of photosynthesis
• Photosynthesis consists of two series of biochemical reactions:
i) the light dependent reactions
ii) light independent reactions
• The light rxns- use light energy absorbed by chlorophyll to
synthesize structurally unstable high-energy molecules.
• The LI rxns- use high-energy molecules to manufacture
carbohydrates.
28

• Do not require light but occur in the light because they are
dependent upon the light reactions.
• The carbohydrates are stable structures that can be stored
by plants.
• In the light rxns of photosynthesis:
 light energy excites photosynthetic pigments to higher energy
levels, and this energy is used to make two high energy
compounds, ATP & NADPH.
29

• ATP and NADPH are consumed during the subsequent dark rxns in
the synthesis of carbohydrates.
• The light rxn requires the cooperation of two photosystems
• The primary rxns of photosynthesis are mediated by three
protein complexes embedded in the thylakoid
membranes of chloroplasts.
These are: a)PSII
b) the cytochrome b6f complex (Cytb6f), and c) PSI
30

 all are connected in series through the photosynthetic electron
transport chain.
• Light energy captured by LHS of PSII and PSI -is transferred to the
reaction center chlorophylls to create a charge separation across the
membrane.
• This leads to the formation of a strong oxidant on the donor side of
PSII capable of splitting water into molecular O2, protons, and
electrons.
31

• The electrons are transferred stepwise from PSII to the
plastoquinone pool, Cytb6f, plastocyanin, and PSI where a
second charge separation creates a strong reductant capable of
reducing ferredoxin that subsequently reduces NADP+ to NADPH.
• Besides there is a cyclic pathway in which electrons are cycled
around PSI through the Cytb6f complex.
32

• The electron transfer rxns are coupled to proton pumping
into the lumen space which drives the ATP synthase
for ATP production.
• Ultimately NADPH and ATP are used for CO2 assimilation by the
Calvin-Benson cycle.
• A remarkable feature of photosynthetic organisms is their
ability to adapt to changing light conditions
• It is done through changes in the organization of the
photosynthetic complexes.
34

1.4THE CALVIN CYCLE
 The process of photosynthesis is completed in two broad steps
 light dependent and light independent
• In the light rxn: energy rich molecules such as ATP and reduced
coenzymes (NADPH) are synthesized
In the light independent rxn:
 energy rich molecules are used up for the synthesis of
carbohydrates.
35

 The LI reaction of photosynthesis occurs in stroma.
• three different types of LI rxn pathways are operated in
different plants.
• they are named on the basis of the components of these
pathways. These are:
• C3 plants
• C4 plants, and
• CAM plants
36

• C3 plants - higher Plants, algae & photosynthetic bacteria which
uses C3 cycle (Calvin cycle)
• the first described light independent rxn pathway.
• Named after its discoverer, Melvin Calvin
• Majority of the plants (~95%) on earth are C3 type
• The first stable product formed in C3 cycle is a three carbon (3C)
compound, hence the name.
• The photosynthetic efficiency is comparatively less due to the
high rate of photorespiration.
37

• The sugar first produced by the Calvin Cycle is a 3-carbon sugar
known as glyceraldehyde-3-phosphate or G3P.
For the Calvin cycle (C3 cycle):
• the ATP provides the energy, and
• the NADPH supplies the electrons which converts CO2 to sugar.
• The Calvin Cycle has three phases:
38

I. Carbon Fixation-the first committed enzymatic step to
generate two molecules of a 3-carbon intermediate (3-
phosphoglycerate).
 The conversion of inorganic carbon from CO2 into the carbons
of an organic molecule, G3P, is carbon fixation
II. Reduction
– A phosphate group (from ATP) is attached to each 3-phospho
glycerate, forming 1,3-phosphoglycerate
39

– NADP-H leaps in and reduces 1,3-phosphoglycerate, which
loses its phosphate group to become G3P.
– ADP and NADP+ formed during the Calvin Cycle are carried
back to the photosystems to be "recharged" with energy, and
converted to ATP and NADP-H, respectively.
III. Regeneration of RuBP
• Five molecules of G3P from the Calvin Cycle's Reduction phase
pass through a complex series of enzymatic reactions to yield
three molecules of RuBP.
• This costs the cell 3 more molecules of ATP, but provides new
"machinery" for the Calvin Cycle to continue spinning
40

1.4.2 C4 Plants-
 use C4 cycle or Hatch-Slack Pathway for the dark rxn of
photosynthesis.
• The first stable product is a 4 carbon (4C) compound– Oxalo-acetic
Acid (OAA).
• C4 plants -abundant in tropical conditions
• commonly seen in dry areas; less in number (~5%).
• Leaves show Kranz Anatomy.
• Examples of C4 plants: Maize, Sugarcane, Sorghum, etc
42

• the bundle sheath cells contain chloroplasts.
• the carbon dioxide fixation takes places twice (one in
mesophyll cells, second in bundle sheath cells).
• Possess two CO2 acceptors (primary acceptor and
secondary acceptor).
• The first acceptor (atmospheric CO2 acceptor) is PEP
(phosphoenol pyruvate)
• The secondary acceptor (metabolic CO2 acceptor) is RuBP.
43

• In C4 plants, the mesophyll cells will only do the initial steps of C4
cycle.
• Subsequent steps are carried out in bundle sheath cells.
• Chloroplasts dimorphic: in the bundle sheath large agranal and in
mesophyll small and granal.
• Can do photosynthesis even in the closed stomata condition
• The optimum temperature for photosynthesis is high.
• require an optimum temperature range of 32 –55o
C
44

• Growth of C4 plants begins when the soil temperature is 16 –21oC
• more efficient when the temperature increases.
• The CO2 fixation is comparatively faster; more efficient in
photosynthesis.
• Photorespiration is altogether absent in C4 plants (if present very
little).
• The rate of translocation of end-products of photosynthesis is very
high. C4 cycle is comparatively recent in origin.
45

CO2 concentrating mechanism-CAM plants
• Crassulacean Acid Metabolism” (CAM plants)
• comes from the Crassula plant, which was the first plant that CAM
metabolism was discovered and studied.
• originally studied extensively in the family Crassulaceae
• Found in about 23 families of angiosperms e.g. Cactaceae,
Euphorbiaceae, Agavaceae, Polypodiaceae, Welwitschaceae, etc
• Unique in conserving water
• Live in extremely dry or xerophytic habitats
• Characterized by thick fleshy leaves
47

• a method of carbon fixation evolved by some plants during dry
circumstances.
• Most plants open their stomata during the day because that is
when energy is received from the Sun.
• In plants living in very dry environments much amounts of water
can be lost if the stomata are open during the hot, dry days.
• At night: Stomata are Open
– Co2 uptake mainly occurs
– plants fix Co2 into a 4-carbon product
– 4-carbon product stored overnight in vacuole
48

• During the day: stomata are closed
– CO2 is released from the 4-carbon produced
– normal light and dark reactions occur without stomata opening
– allows the plants to conserve water during the day
• Examples of CAM Plants
– Cacti,
– Succulents Crops including Pineapple, Agave, etc
49

1.5 Photorespiration
• a wasteful metabolic pathway that occurs when the Calvin cycle
enzyme rubisco acts on oxygen rather than carbon dioxide.
• known as the oxidative photosynthetic carbon cycle, or C2
photosynthesis
• Respiration driven by light energy
• Discovered when some plants have faster respiration rate in light
than in dark
• begins when rubisco grabs O2 rather than CO2.
• Tends to happens when plants close their stomata to reduce water
loss.
52

• High temperatures make it even worse.
• favored by high O2, low CO2 and warm temperatures
• O2 has an inhibitory effect upon photosynthesis
• in the presence of elevated O2 levels photosynthesis rates are
lower (expt)
• “wastes” CO2 which is converted into sugars
• may significantly limit the growth rate of some plants
• protects against light damage
• Result in a loss or no net gain of dry matter for the Plant
• Less ATP is produced
53

• in the "normal" reaction, CO2 is joined with RUBP to form 2
molecules of 3PGA;
• in photorespiration only one molecule of 3PGA is formed
• The majority of plants are C3 plants, which have no special
features to combat photorespiration.
• The appearance of C4-type plants seems to be an evolutionary
mechanism by which photorespiration is suppressed
• C4 plants minimize photorespiration by separating initial CO2
fixation and the Calvin cycle in space, performing these steps
in different cell types.
54

• Crassulacean acid metabolism (CAM) plants minimize
photorespiration and save water by separating these steps in time,
between night and day.
• Biologists have a dream to increase the production of certain crop
plants ( by changing their metabolic pathways) such as wheat,
that carry on C3-type photosynthesis by genetically re-engineering
them to perform C4-type photosynthesis.
• Would you mind the dream will be realized?
55

Factors influencing photorespiration
• O2:Co2 ratio-if cells have higher O2 and lower CO2-
photorespiration dominates
• light intensity- increasing light intensity will increase energy for
the photorespiration process
• Temperature: aerobic respiration and photorespiration increase
with temp. plants have optimum, minimum and maximum temp
ranges
• Net photosynthesis or net assimilation rate
– C4 plants generally have net assimilation rates about 2 to 3
times that of C3 plants
– C4 plants are often called efficient plants and C3 plants called
non-efficient plants
58

2. Concepts of Action spectra and Absorption spectra
Action spectrum
• the rate of a physiological activity plotted against
wavelength of light. i.e.,
• the overall rate of photosynthesis at each wavelength of light
 Spectrum of light most effectively used for photosynthesis.
 is the part of the light spectrum that does the work.
59

• range of wavelengths capable of driving a particular biological
process
• Describes the efficiency with which specific wavelengths
produce a photochemical reaction
• Describes the wavelengths that actually drive photosynthesis
• is most important in plant growth and metabolism
• Some reactants are able to use specific wavelengths of light more
effectively to complete their reactions.
60

• For e.g., chlorophyll “a, & b” is much more efficient at using the
red and blue spectrums for photosynthesis.
• The action spectrum graph would show spikes above the
wavelengths representing the colors red and blue.
• E.g. Engelmann split light into its components by the prism and
then illuminated Cladophora placed in a suspension of
aerobic bacteria.
• He found that bacteria accumulated in the region of blue and red
light of the split spectrum. 61

• Thus he discovered the effect of the different wavelengths of light
on photosynthesis and plotted the first action spectrum of
photosynthesis.
• To obtain an action spectrum for some particular response, we
could expose the system to the same photon flux density at each
of a series of wavelength intervals and measure the resulting
effect or action.
62

• The action could be : the amount of O2 evolved, the portion of
seeds germinating, or some other measured change.
• We could then plot the responses obtained as a function of their
respective wavelength intervals to see which wavelengths are
most effective in leading to that “action.”
An absorption spectrum-
• Is the spectrum of electromagnetic radiation, or light, plants absorb.
63

• range of wavelengths absorbed by a particular pigment
• dependent on the cellular and molecular build-up of the plant and
therefore differ depending on species
• Pigments absorb light as a source of energy for photosynthesis.
• But light absorption and light use are two different phenomena.
• The absorption spectrum- indicates the wavelengths of
light absorbed by each pigment (e.g. chlorophyll).
65

• Each pigment has a specific absorption spectrum, and in
living systems, pigments never exist alone.
• Pigments are always bound to proteins and this shifts their
absorption spectrum.
• This phenomenon explains why wavebands are absorbed rather
than a single wavelength.
• The absorption of radiation by a substance can be
quantified with an instrument- a spectrophotometer
66

Based on the study made on living plants, the probability of a pigment
absorbing light depends on:
1) the specific protein that the pigment is bound to;
2) the orientation of the pigment-protein complex within the cell;
3) the forces exerted by the surrounding medium on the pigment-
protein complex.
 There are a wide range of experimental approaches for measuring
absorption spectra.
 The most common is to direct a generated beam of radiation at a
sample and detect the intensity of the radiation that passes
through it.
 The transmitted energy can be used to calculate the absorption.
67

• The action spectrum indicates the overall rate of
photosynthesis at each wavelength of light
• There is a strong correlation between the cumulative
absorption spectra of all pigments and the action spectrum
• Both display two main peaks – a larger peak at the blue region
(~450 nm) and a smaller peak at the red region (~670 nm)
• Both display a trough in the green / yellow portion of the visible
spectra (~550 nm)
70

3. Methods in the study of plant water relations
Measuring water potential of cells and tissues
• Liquid water is absolutely necessary for life
• water is the solvent and reaction medium of all living cell
• a reactant in many metabolic processes
• forms part of the structure of protoplasm
• Cells contain about 75–90% water by weight
71

• The physicochemical properties of water are unique
• Water provides the transport medium in plants
• One of the raw materials for photosynthesis
• produces turgor pressure which gives mechanical rigidity to
thin-walled tissues
• Movements of some plant organs occur as a result of turgor
pressure changes.
• Plant cell expansion is also driven by turgor pressure
Water potential: Difference in water energy between two regions
containing water.
72

• Indicates the tendency of water to move from one area to another
due to osmosis, gravity, mechanical pressure, or matrix effects
such as capillary action
• useful in understanding and computing water movement within
plants, animals, and soil.
• expressed in potential energy per unit volume and is represented by
the Greek letter ψ.
• water behind a dam
• water in your bladder
• Water moves into plants, in terrestrial plants mainly from the soil
73

• There is much movement of water within plants
• water moves out of plants, mainly into atmosphere
• Movement implies the involvement of energy.
• Metabolism is driven by changes in free energy.
• Water movement is driven by energy levels.
• Water will move from an area of higher free energy to lower free
energy.
• In order to predict the direction of movement of water into/out of
plants cells we need a measure for the free energy of water
74

• This measure is the water potential, denoted by the Greek letter ψ
(psi), or ψw
• Water moves along gradients of water potential, from higher to
lower water potential.
• Ψw is basically a measure of free energy, it is expressed in
pressure units,
• since hydrostatic pressures and tensions (negative pressures)
contribute to water potential and play a very important part in the
water relations of plants.
75

• The pressure unit is the pascal (Pa) or larger one mega pascal
MPa (106 Pa)
• The free water can be accessed without exerting any energy.
• The soil water can only be extracted by expending energy.
• Water potential expresses how much energy you would need to
expend to pull water out of the soil sample.
76

Forces determining cellular water potential
• three kinds of forces which affect the free energy of cellular
water.
• Pressure potential: hydrostatic pressure in excess of atmospheric
increases the free energy and raises water potential;
• thus the pressure potential (ψp) is a positive value.
Osmotic (solute) potential: exert osmotic forces, which decrease the
free energy and lower the water potential
• the osmotic potential (ψp) –has a negative value
77

Matric potential (ψm)-forces exerted by colloids, decrease the free
energy of water and lower the water potential;
 In vacuolated cells of high water content, the matric potential has
minor contribution and, for such cells, the water potential is
often given simply as ψ = ψS + ψp
• The overall water potential of a plant cell is the sum of these three
quantities:
• ψ = ψ p +ψ S + ψ m
78

Soil water status
• The soil acts as a reservoir for water, making it available
for plants as it is needed.
• Soil water is very important to the entire soil system, because the
nutrients required for plant growth are present in the
soil solution.
• Most of the important soil reactions (weathering, cation
exchange, organic matter decomposition, fertilization) take
place in the soil solution.
• Thus, it is evident that the moisture status of a soil is a key
property.
79

• Plant water status may be described basically by two parameters
viz.
• the amount of water and the energy associated with the forces
which hold the water in the soil.
• The amount of water is the water content and the energy state of
water is the water potential.
• Plant growth, soil temperature, chemical transport and ground
water recharge are all dependent on the state of water in the
soil.
i) plant water content – usually expressed as relative to that of full
saturation, i.e. relative water content (RWC) 80

• indicates how much water is present in the plant.
• can be used to estimate the amount of stored water in a profile
Energy status of the plant water:
• usually expressed as the total plant water potential
• The total plant water potential is the difference between the
chemical potential of plant water and that of pure free water.
• Soil water status is related to energy and the forces that hold and
move water within the soil.
• The best description of soil water includes an analysis of the
energy involved.
81

• Three major forces are involved in the movement of soil water.
• Gravitational potential -(describes the force gravity has on
water),
• Matric potential- (describes the surface attraction of soil particles
for water), and
• Osmotic (solute) potential- (the difference in energy between pure
water and water containing dissolved salts )- comprise the
total soil water potential
• soil water potentials give a measure of the differences in energy
status between soil water and pure, standing water.
82

• Pure water has a potential of zero.
• Soil water movement occurs when there is a difference in total
potential between two points in the soil.
• The direction of water movement will be in the direction of the
point having the lowest potential.
• A dry soil absorbs water from a wet soil and soil water moves
toward an absorbing plant root.
• Plant water relationships comprises measurement of both plant
water content and total plant water potential
• measurements of other plant responses to plant water deficit such
as: 83

• stomata openings
• leaf resistance to gas diffusion
• gas exchange
• growth rate, etc.
Plant water status strongly influences plant growth through:
• influence on gas exchange and expansion of leaves and roots.
• Leaf water deficit results in stomatal closure limiting the CO2
uptake, and net photosynthesis.
• plant water deficit may negatively affect the process of
photosynthesis itself
84

Measuring Water Potential
• Plant physiologists have expended great effort in devising accurate
and reliable methods for evaluating the water status of a plant.
• Four instruments have been used to measure Ψ, Ψs , and Ψp.
• Psychrometer- can be used to measure the water potentials of both
excised and intact plant tissue.
• Pressure chamber- (Pressure bomb) - a steel chamber that can be
pressurized, usually with nitrogen.
• The sample is placed in the chamber with the petiole or surface
exposed through a hole in the lid.
85

• The sample is pressurized and the pressure that is required to force
water to appear on the cut surface is assumed to be equivalent
to the water potential of the tissue.
• Cryoscopic osmometer- measures the osmotic potential of a
solution by measuring its freezing point
• Pressure probes- measure turgor pressure via displacement
86

A diagram showing the setup of a Pressure bomb
87

Measuring Soil Moisture
• A wide range of tools are available for determining soil moisture.
• are not much expensive and are straight forward to operate.
• Tensiometers- devices that measure soil moisture tension
• Electrical resistance blocks (gypsum blocks)- measure soil
water tension.
• Time Domain Reflectometry (TDR) - a newer tool that sends an
electrical signal through steel rods placed in the soil and
measures the signal return to estimate soil water content.
Water potential measurement technique matrix
88

Method Measures Principle Range (MPa) Precautions
Tensiometer
(liquid equilibration)
soil matric potential internal suction balanced
against matric potential through
porous cup
+0.1 to -0.085 cavitates and must be refilled if minimum
range is exceeded
Pressure chamber
(liquid equilibration)
water potential of plant
tissue (leaves)
external pressure balanced
against leaf water potential
0 to -6 sometimes difficult to see endpoint; must
have fresh from leaf;
in situ soil psychrometer
(vapor equilibration)
matric plus osmotic
potential in soil
same as sample changer
psychrometer
0 to -5 same as sample changer psychrometer
in situ leaf psychrometer
water potential of plant
tissue (leaves)
same as sample changer
psychrometer
0 to -5 same as sample changer; should be
shaded from direct sun; must have good
seal to leaf
Dewpoint hygrometer
matric plus osmotic
potential of soils, leaves,
solutions, other materials
measures hr of vapor
equilibrated with sample. Uses
Kelvin equation to get water
potential
-0.1 to -300 laboratory instrument. Sensitive to
changes in ambient room temperature.
Heat dissipation
(solid equilibration)
matric potential of soil ceramic thermal properties
empirically related to matric
potential
-0.01 to -30 Needs individual calibration
Electrical properties
(solid equilibration)
matric potential of soil ceramic electrical properties
empirically related to matric
potential
-0.01 to -0.5 Gypsum sensors dissolve with time. EC
type sensors have large errors in salty soils
89

4. Methods in the study of stomatal physiology
The stomata: the primary control mechanisms that plants use to
reduce water loss
• Sensitive to the environmental cues that trigger them to open or
close.
• allow carbon dioxide entry to drive photosynthesis and the
exit of water as it evaporates, cooling the leaf.
• Have two specialized cells - ‘guard cells’ make up each stoma
(stoma is singular for stomata).
90

• Two distinct types of guard
cells exist, kidney shaped and
dumb-bell shaped .
• Kidney-shaped are found in
dicotyledons
• dumb-bell-shaped are found
in grasses.
• Dumb-bell shaped guard cells
are more advanced in
evolutionary terms and more
efficient physiologically
91

• Plants have many stomata (up to 400 per mm2) on their leaf
surfaces, and found usually on the lower surface to minimize
water loss.
• Stomatal aperture is tightly regulated by divergent exogenous
stimuli, such as light, drought stress, pathogens, temperature
and CO2 concentration.
• These stimuli are sensed and signaled to the guard cells via
endogenous signaling molecules including: phytohormones,
hydrogen peroxide (H2O2) and Ca2+ & K+ ions
92

• Stomata: open in the light and close in the dark.
• can close in the middle of the day if water is limiting, CO2
accumulates in the leaf, or the temperature is too hot.
• If the plant lacks water, stomata will close because there will not
be enough water to create pressure in the guard cells for
stomatal opening;
• this response helps the plant conserve water.
93

If the leaf’s internal concentration of CO2 increases:
• the stomata are signaled to close because respiration is releasing
more CO2 than photosynthesis is using.
• There is no need to keep the stomata open and lose water if
photosynthesis is not functioning.
• Alternatively, if the leaf’s CO2 concentration is low, the stomata
will stay open to continue fueling photosynthesis.
• High temperatures will also signal stomata to close and increase
the water loss from the leaf
• With less water, guard cells can become flaccid and close.
94

• During high temperatures respiration rates rise above
photosynthesis rates causing an increase of CO2 in the leaves;
high internal CO2 will cause stomata to close.
• Some plants may open their stomata under high temperatures so
that transpiration will cool the leaves
• The opening and closing of stomata is a fine-controlled
masterpiece of plant evolution
• It is driven by the transition of a chemical signal into a mechanical
movement.
95

• Stomata regulate leaf temperature, water evaporation and gas
exchange - processes essential for plant survival and growth
• is done by changing the osmotic pressure in the guard cells
• stomata participate in providing a carbon source for
photosynthetic reactions
• stomatal transpiration of water is essential for nutrient uptake from
soil
96

• Abscisic acid (ABA) is among the major role players in terms of
stress related stomatal closure
• Other phytohormones, such as ethylene, jasmonates and salicylic
acid, function in the regulation of stomatal aperture.
• Signaling pathways triggered by hormones, as well as by pathogen
attack, often involve the generation of second messengers like
NO and H2O2.
• Treatment of plants with exogenous H2O2 alone can trigger
stomatal closure
• the measurement of stomatal aperture is difficult and depends on
various environmental factors 97

Stomatal conductance: is a measure of the degree of stomatal
opening and can be used as an indicator of plant water status
• is a measurement of how open the stomata are in a leaf or plant,
and infers plant transpiration.
• is the rate of CO2 entering, or water vapor exiting through
stomata.
• Mechanically, a high stomatal conductance value can come from
many open stomata per unit leaf area, many small open
stomata, or fewer, larger open stomata.
• i.e. a function of stomatal density, stomatal aperture, and stomatal
size 98

• The measurement can be made by a mass flow porometer.
• The mass flow porometer exerts a pressure on a unit area of leaf
clamped in a small hand-held chamber and records the
pressure loss per unit time.
• A rapid decrease in pressure would indicate that many stomata are
open, and no decrease in pressure would suggest that the stomata
are closed.
• The actual mechanism responsible for entry and exit of water to
and from the guard cells has been explained by several theories.
99

• The most important theories are
i. The starch-sugar inter conversion theory –
ii. Active K+ transport
iii. pH theory
iv. Proton-potassium pump theory-
• There are some factors that affects the opening and closing of the
stomata
• Light, temperature, CO2 concentration and availability of
water
100

5.Techniques in the study of ion transport
• Ion transport- movement of salts and other electrolytes in the
form of ions from place to place within living systems. Or
• the transfer of ions across biological membranes in the cells and
tissues of organisms.
• may occur by any of several different mechanisms:
electrochemical diffusion, active-transport requiring energy, or
bulk flow
• extremely important in the vital activity of cells
102

• The transport of ions and gases into and out of tissues and cells is
central to the life of plants.
• Ions are absorbed from the soil by both passive and active
transport.
• Specific ion pumps in the membranes of root hair cells pump ions
from the soil into the cytoplasms of the epidermal cells.
• Evidence indicates that active transport is used in ion transport.
• The active uptake of ions is partly responsible for the water
potential gradient in roots, and therefore for the uptake of ions
and water by osmosis
103

• Measurement of specific ion fluxes has contributed to the
characterization of transport systems.
• Progress in molecular genetics is allowing gene identification
and controlled expression of transporter molecules.
• Ion transport and the plasma membrane transporters
themselves have been studied using a variety of
techniques.
104

• There are about four techniques
1.Ion selective electrode technique
2.Ion exchange chromatographic technique
3.Atomic absorption technique-
4.Isotopic technique-
1.An ion selective electrode-( a specific ion electrode)
 a sensor that converts the activity of a specific ion dissolved in a
solution into an electrical potential, which can be measured
by a voltmeter or pH meter.
105

 consists of a thin membrane across which only the intended ion can
be transported.
 The transport of ions from a high concentration to a low one
through a selective binding with some sites within the
membrane creates a potential difference.
Types of ion selective electrode
 Glass membrane electrode- are responsive to univalent cations
such as H+, Na+
 Solid state electrode (Crystalline membranes)-selective
primarily for anions
 it may be a homogenous membrane electrode or heterogenous
membrane electrode 106

 Liquid membrane electrode- used for direct measurement of
polyvalent cations (Ca ions) as well as a certain anions.
 Gas sensing electrode-available for the measurement of NH3, CO2
& NO
 ISEs are used in a wide variety of applications for
determining the concentrations of various ions in solutions.
• The main areas in which ISEs have been used are:
 Pollution monitoring-CN, F, S, Cl, NO3 etc in effluents and
natural waters
 Agriculture-NO3, Cl, NH4
+ , K, Ca, I, CN in soils, plant material,
fertilizers and feedstuffs 107

 Food processing-NO3, NO2 in meat preservatives
 Salt content of meat, fish, dairy products, fruit juices, brewing
solutions
 F in drinking water & other drinks
 K in fruit juices and wine making
 Corrosive effect of NO3 in canned foods
 Detergent manufacture: Ca, Ba, F for studying effects on water
quality
 Paper manufacture: S and Cl in pulping and recovery cycle liquors
 Explosives: F, Cl, NO3 in explosive materials and combustion
products 108

 Biomedical laboratories: Ca, K, Cl in body fluids ( blood, plasma,
serum, sweat)
 F in the skeletal and dental studies
 Ca in dairy products and beer
2.Ion exchange chromatographic technique
 a technique that is commonly used in biomolecule purification.
 involves the separation of biomolecules on the basis of their
charge.
 The crude sample containing charged molecules is used as the
liquid phase.
109

 When it passes through the chromatographic column, molecules
bind to oppositely charged sites in the stationary phase.
3. Atomic absorption technique- an analytical technique that
measures the concentrations of elements
• The technique makes use of the wavelengths of light specifically
absorbed by an element
4.Isotopic technique- used to track the passage of an isotope (an atom
with a detectable variation in neutron count) through a
reaction, metabolic pathway, or cell.
110

6. Photomorphogenesis -responding to light
• plant development is regulated by four factors:
– Plants sense and respond to environmental cues.
– use receptors (photoreceptors) which absorb light, to sense
some environmental cues.
(Photoreceptors are protein molecules that absorb light).
– Chemical messages (hormones) -mediate the effects of the
environmental cues.
– Enzymes, which are encoded by the plant’s genome, catalyze
the biochemical reactions of development.
111

• Plants have the capability to detect and interpret a variety of
environmental signals
• One important environmental signal is light
• plants can sense light direction, quality (wavelength), intensity and
periodicity.
• Light induces phototropism, photomorphogenesis, chloroplast
differentiation and various other responses such as flowering
and germination.
• Light quality is mainly sensed by the presence of different light
receptors specific for different wavelengths.
112

Photomorphogenesis: light-induced control of plant growth
and differentiation. or
• light regulating development – (photo = light, morpho =
form, genesis = origin)
• The use of light to control structural development- dependent upon
the presence of specialized photoreceptors- (chemical
pigments capable of absorbing specific wavelengths of light).
• an integral element in the normal development of all higher plants
113

• Certain wave lengths function as a signal causing the
generation of an information within the cell that is used for
the selective activation of certain genes.
• Plants can’t change their environment or location
• must be able to avoid adverse conditions
• Plants know where they are
• where to maximize photosynthesis
• how to measure the passing of season, etc in relation to their
habitats
114

• For e.g. seed germination and survival of the emerged seedlings
depends on conditions in their immediate environment.
• Plants can detect light gradient & differences in spectral
composition
• The response to light is a central theme in plant development;
for instance, the ability of a plant to maximize photosynthetic
output depends on its capacity to sense and respond to
changes in the amount and direction of light.
115

Thus plants are able to determine whether in shade or in full
sun
• able to mark the beginning and end of the day.
• involves the inhibition of: stem elongation, the differentiation of
chloroplasts and accumulation of chlorophyll, and the expansion of
leaves.
• Thus the same stimulus causes opposite effects on cell elongation
in leaves and stems.
• Photomorphogenesis can be induced by red, far red and blue light.
116

• Light regulates many aspects of plant development,
inhibiting internode elongation
• promoting leaf expansion (dicotyledons) or leaf unrolling
(monocotyledons),
• promoting chlorophyll synthesis and chloroplast development
• stimulating the synthesis of secondary products such as
anthocyanin pigments
117

Growth in dark and in light
118

• The germination of many seeds is influenced by light
• Light is used as information
• plants use this information in many different ways to direct their
growth, form, and reproduction
• To acquire and interpret the information provided by light, plants
have developed a sophisticated system of photoreceptors and
signal transduction pathways
• A photoreceptor by selectively absorbing different
wavelengths of light, reads the information contained in the
light and interprets that information for the cell in the form of
a primary action. 119

Primary action may involve:
• A conformation change in proteins
• A photochemical redox-reaction, etc
• Most photomorphogenic responses in higher plants appear to be
under control of one of three classes of photoreceptors:
• Light is perceived by a series of photoreceptors, the best studied of
which are:
1)The phytochrome (red and far-red)-bluish chromoprotein (a blue
light protein pigment)
• is ubiquitous in plants
• Pigment that absorb red, far red and blue light 120

• can exist in 2 state
• absorption maximum in the red (R, 665nm)
• absorption maximum in the far red (Fr, 730nm)
• has a role from germination to flowering & in every stage of plant
development
• Regulates - Gene expression, circadian rhythms, membrane
potentials, and ion fluxes
• enables plants to sense shading
• short exposure to red light causes early pollen tube germination
• etiolated seedlings undergo de-etiolation when exposed to light
121

• one physiological effect of phytochrome is presence of red/far red
reversible pigments which helps plants to adjust to their
environment
• Affect all aspects of plant development
Chemical nature of phytochrome
• is synthesized as the Pr form, which accumulates in dark-grown
tissue and is generally considered to be physiologically inactive
• In etiolated plants phytochrome is present in a red light absorbing
form – Pr
122

• Due to protein conformation change phytochrome can interconvert
between Pr and Pfr forms
• When Pr absorbs red light, it is converted to the Pfr form, which is
physiologically active form of the pigment for most known
responses
• Thus, physiologically active form of phytochrome - Pfr
• The other two classes of receptors detect blue and UV-A light
2) Cryptochromes- are photolyase, recognizes blue, green and UV-A
light
• play major roles during seedling development and flowering
• mediates blue light inhibition of hypocotyl elongation
123

• photoperiodic control of floral elongation
• regulate many light processes, such as circadian rhythm, stomatal
opening, guard cell development, cell cycle, programmed cell
death, apical dominance
3) Phototropin- perceives blue light;
• mediates differential growth in a light gradient
• All of the three photoreceptors are chromoproteins
• Chromoproteins are molecules made up of two parts
• Chromophore light absorbing moiety and Apoprotein-a protein
124

Chapter 7. Seed physiology
Seed: a small package produced in a fruit or cone
• Seeds are mature, fertilized ovules
• the beginning and the end of most plant (seed lants) life cycle
• an embryo plant, contains within itself virtually all the materials
and energy to start off a new plant
• a basic unit of dispersal
• a source of staple food in most parts of the universe
125

• agents for transmitting the legacy of one generation to the next
• are critically important for agricultural success.
• Containing highly concentrated nutrient stores
• indispensable value in human nutrition and the development of
civilization
• most dramatic innovations of the vascular plants
• Originated about 360 million years during Devonian period
126

• Used by plant physiologists to study the influence of various
environmental factors such as: temperature, moisture, oxygen, light
and other factors on germination and seedling emergence.
• In Angiosperms seed contains three parts:
1)Testa (protective covering)
2) Embryo (new sporophyte)
3) Food supply (endosperm and/or cotyledons)
127

Testa (seed coat): Outer protective layer of the seed
• developed from the integuments of the ovule
• diploid maternal tissue
Food supply (Endosperm/ cotyledons):
• presents in the mature seed
• serves as food storage organ
• The amount of endosperm in mature seeds is highly species-
dependent
• Testa and endosperm are the two covering layers of the embryo.
128

• An embryo is the future
young plant
• consists of the epicotyls (
develops to plumule & shoot
systems),
• hypocotyls, one or two
cotyledons or endosperm
(nutritive tissue)
129

Role of the seed
• Serve as sole food storage organs as in the case of pea (Pisum
sativum) (Source of food & spices)
• A basic tool for food security
• Survival mechanism as seed bank
• dispersal mechanism
• reproduction mechanism/propagation
• segregation and recombination of the genetic material
(A carrier of new technologies)
130

• Seed producing plants have two stages
i) Vegetative stage
• Stems and roots elongation
• increase in diameter
• has specific stage
ii) Reproductive stage
• flower induction
• Seed and fruit production
133

Seeds viability and life span
 a given seed is either viable or nonviable
• Viability means capability of a seed to germinate.
• It denotes the degree to which a seed is alive, metabolically
active, and possesses enzymes capable of catalyzing
metabolic rxns needed for germination and seedling growth.
• Seeds do not remain viable indefinitely
134

• lose their viability first gradually and finally completely.
• The life span of seeds may be from few weeks to many years
depends upon: species, the environmental conditions
prevailing during seed storage
• Numerous tests exist for determining seed viability and quality
such as:
• Seed weight/appearance
• seedling growth rate/vigor
• germination %/rate
• morphological appearance of seeds/seedlings
135

Chemical compositions of seeds
• Carbohydrates and oils are main storage foods.
• Proteins belong to either group
• no seeds are known in which the predominant storage material is
protein
• exceptions, such as soybean containing 66% protein
Variability between seeds-
 Vary in size, shape, form, texture, colour and chemical
composition
136

• Variability in seed shape exists within a given species and is
referred to as seed polymorphism.
• Polymorphic seeds differ in:
• Shape, or
• colour
• germination behavior, and dormancy
Reading Assignment
• Read on Flower, Floral parts, Pollination, Fertilization, seed
formation, maturation and seed dispersal methods
137

– Physiology of germination
• Germination is-resumption of growth
• the process in which a seed or spore emerges from a period of
dormancy.
• the emergence of the radicle through the seed coat
• Common example of germination is the sprouting of a seedling
from a seed of an angiosperm or gymnosperm.
Necessary factors for germination
• several factors affecting seed germination.
• External factors and internal factors
138

External factors ( environmental factors)
• Most important factors affecting germination are:
• Water/Optimal moisture
• Temperature/ proper temperature
• oxygen
• Light - may or may not be needed
Internal factors
• Seed dormancy
139

Water: is required for germination
• enough water is needed to moisten the seeds but not enough to
soak them.
• the uptake of water by seeds is - imbibition,
• leads to the swelling and the breaking of the seed coat.
• most plants store a food reserve with the seed, such as starch,
proteins, or oils.
• this food reserve provides nourishment to the growing embryo.
• when the seed imbibes water, hydrolytic enzymes are activated
140

O2: is required for metabolism.
• used in aerobic respiration, the main source of energy.
• found in soil pores
• Some seeds have impermeable seed coats that prevent oxygen from
entering the seed
• when the seed coat is worn away it allows gas exchange and water
uptake from the environment.
141

Temperature: may cause physiological dormancy.
• affects cellular metabolism and growth rates.
• a wide range of temperatures is required for seed germination
• Seeds will not germinate above or below this range.
• Many seeds germinate at temperatures slightly above room-
temperature 16-240c
• others germinate just above freezing, and
142

• others germinate only in response to alternations in
temperature between warm and cool.
• some seeds germinate when the soil is cool -2 - 4 0 c
• some when the soil is warm 24-320 c.
• some seeds require exposure to cold temperatures (vernalization)
to break dormancy
Light or darkness: may or may not be needed
• is a type of physiological dormancy
143

• Mostly required for seedling growth
• Specific light quality (wavelength or intensity) or quantity
(photoperiod) is required for germination
• most seeds are not affected by light or darkness
• But some seeds need light to germinate
• Others need darkness, and light prevents sprouting.
• if light is required, sow on the surface
• if darkness is needed, cover seed well.
144

 Gibberellic acid-3 (ga-3): pre-soaking seeds in ga-3 will often
cause rapid germination of many highly dormant seed
 Hot water soak: for 10 seconds to 3 minutes.
• Dry heat: the seeds are baked dry in an oven at 140° to 220°f for
4 - 10 hours, or
• are microwaved for 30 seconds to 4 minutes.
• Warm moist treatment: many seeds need 1-4 months of
warm moist treatment, followed by cold treatment
145

• in some, the root sprouts during the warm period, but the
shoot in a cold period
 Smoke treatment: helps germination of plants from fire-
prone environments ( mediterranean-climate plants).
 Hard seeds-chipping: prevent moisture being absorbed by the
seed.
• the outer surface to be scratched to allow water to pass through.
Hard seeds-soaking: is beneficial in two ways:
• can soften a hard seed coat, and leach out any chemical inhibitors
in the seed.
146

stratification (cold treatment): artificially stimulated by placing the
moistened seed in a refrigerator for a certain period of time
(usually 3- 5 weeks at around 410F).
Germination - Stages
Phase 1-Activation: imbibition of water
• At this time synthesis of enzymes amylase is initiated
• breaks storage material which are utilized by the embryo for
germination
 It also softens inner tissues, and causes swelling and seed coat
rupture
 At the end of activation cell elongation and radicle emergence
occurs 147

Phase 2 - Digestion and Translocation
• activated enzymes begin to break down storage material into
simple compounds
• are translocated to the embryo axis or plumule and root or radicle
• The plumule will grow and develop as cells elongate and divide.
Phase 3 - Seedling Growth
• The germinating seed continues to undergo metabolic changes
culminating into a seedling
• two types of germination based on the position of the cotyledons
during germination
148

a)Epigeal Germination –
• cotyledons are raised above the ground
• characteristic of bean and pine seeds
• considered evolutionarily more primitive
b) Hypogeal Germination-
• cotyledons or comparable storage organs remain beneath the soil
• characteristic of pea seeds
• all grasses such as corn, and many other species.
149

Germination rate:- describes how many seeds of a particular plant
species, variety or seed lot are likely to germinate.
• usually expressed as a percentage
• e.g., 70% germination rate means about 70 out of 100 seeds will
germinate.
• useful for calculating the seed requirements for a given area or
desired number of plants.
151

– Biology of seed dormancy
Seed Dormancy is: a mechanism to prevent germination during
unsuitable ecological conditions, or
• the physiological inability of a seed to germinate even under
favorable condition
• may become dormant in dry or cold seasons that are unfavorable
for growth
• Environmental signals both initiate and end dormant phases in the
life of a plant.
152

when the probability of seedling survival is low:
• most seeds delayed germination
• this allows time for dispersal & prevents germination of all the
seeds at the same time
 Delayed germination safeguards some seeds and seedlings from
suffering damage/ death from:
• short periods of bad weather, or
• transient herbivores
• allows some seeds to germinate when competition for resources is
less intense
153

• Some seed passes a period of rest or inactivity before
germination
• Many kinds of seeds can germinate immediately after
maturation
• The seeds in some species of citrus may germinate in the fruit
Duration of dormancy
• may last for weeks, months, years, or even centuries
• Dormancy may be:
154

Primary dormancy- present at dispersal-
• exogenous, endogenous and combinational
• have primary dormancy when they are shed from the plant
Secondary dormancy/ Induced dormancy
• develops after dispersal
• obtains dormancy due to external unfavourable conditions
caused by conditions that occur after the seed has been
dispersed
155

• The Primary seed dormancy is divided in to three
1. Exogenous-outside the seed-(related to the seed coat or the
surrounding tissues)
• imposed by factors outside the embryo
• three types of exogenous dormancy:
i) Physical dormancy (seed coat dormancy):
• seed covering may become hard and impermeable to water &
gases.
156

• prevents the physiological processes initiating germination.
• preventing leaching of inhibitor from the embryo and
supplying inhibitor to the embryo.
• very common in drupe fruits
Physical dormancy is broken by several factors such as:
• high temperatures and fluctuating temperatures, fire, etc
• freezing/thawing
• drying or passage through the digestive tracts of animals. 157

• Scarification -removing the seed coat mechanically using sand
paper, small stones, etc
• Soaking in concentrated sulfuric acid for an hour (chemically)
ii) Mechanical dormancy:
• occurs when seed coats or other coverings are too hard to allow
the embryo to expand during germination.
• E.g. In some fruits seed covering restricts embryo expansion &
development, resulting in dormancy of seeds.
158

iii) Chemical dormancy: in some seeds chemicals that inhibit
germination are accumulated and remain with the seed after
harvest.
• Includes growth regulators that are present in the coverings
around the embryo
• e.g. fleshy fruits such as citrus, cucurbits, stone fruits, pear, grapes
and tomatoes
2. Endogenous-embryo- lacks the growth potential
• Growth potential is the force exerted by the radicle to penetrate the
seed coat.
159

• Physiological dormancy (endogenous)- prevents embryo growth
and seed germination until chemical changes occur
• Prevents germination. e.g. Immature embryo, Hormonal inhibition.
Morphological dormancy-the embryo is underdeveloped or
undifferentiated
• Immature embryos – seeds released before the tissues of the
embryos have fully differentiated, and the seeds ripen after they
take in water while on the ground, germination can be delayed for
weeks and months.
160

• Morpho-physiological dormancy- occurs when seeds with
underdeveloped embryos, also have physiological
components to dormancy.
• These seeds therefore require dormancy-breaking treatments as
well as a period of time to develop fully grown embryos.
3. Combined- seed coat + embryo dormancies=double
dormancy
161

Biological Importance of Seed Dormancy
1.Perennation: allows seeds to pass through drought, cold and other
unfavorable conditions.
• The dormant seeds can remain alive in the soil for several years.
• provide a continuous source of new plants even when all
the mature plants of the area have died
2. Dispersal: helps the seed to get dispersed over long distances
162

3.Germination under favourable conditions: when there is
sufficiency & less stress
4. Storage: transport to the areas of deficiency and to make
available throughout the year
• for later use by animals and man
163

– Dormancy breaking mechanisms (Natural & Artificial)
• Increased range of conditions in which germination will occur
In nature seed dormancy is broken automatically due to:
• Development of growth hormones to counter growth inhibitors
• Leaching of germination inhibitors
• Maturation and after-ripening of embryo
• Weakening of impermeable and tough seed coats by microbial
action, abrasion, passage through digestive tract of animals, etc.
165

Artificial means of Breaking Seed Dormancy
1) Scarification: used to soften the hard seed coat
• to improve its permeability to water and gases.
• can be done chemically or mechanically by cracking the seed coats
• is given a beating without injuring the embryo
• can be treated with dilute acid solution for 3-4 hours to few
minutes depending upon hard seed coat.
166

2) Stratification/ vernalisation (cold treatment) –
 a cold, moist period that breaks seed dormancy
 the placing of seeds close together in layers in moist sand or peat
to preserve them or to help them germinate
• Seeds exposed to variable period at low temperature
3) Counteracting Inhibitors: leaching of chemicals from the seed
coats that inhibit germination
 removed by dipping seeds in KNO3, thiourea, ethylene
chlorohydrin, and gibberellin.
167

4. Shaking and Pressure: used to weaken seed coats.
• The activity of microorganisms in soil help to soften seed coats.
• Fire melts waterproof waxy coatings.
• Scorching by fire may break down chemical inhibitors
• Growth regulators & other Chemicals: application of low level
of growth regulators (i.e. Gibberellins, Cytokinins and
Ethylene, etc) may break the seed dormancy.
168

8. Different aspects of plant stress physiology
• What is stress? any change in environmental conditions
that might adversely change a plant’s growth or
development
• a threat to homeostasis, which refers to the physiological balance
of systems critical to survival, or
• Any factor that acts on an organism so as to impair its
functions, or a significant deviation from the optimal
conditions for life
• At first reversible or may be permanent (duration)
169

• Organisms bodies work optimally within a very narrow range of
parameters, such as pH, temperature, metabolism, and so on.
• Stress systems operate to maintain and restore balance when these
parameters become unbalanced and left unchecked (would
lead to dysfunction and death).
170

• Being sessile organisms, plants cannot move away from a
stressful situation
• Stress-can be regarded as a functional state or, as the dynamic
response of the whole organism.
• The stress response is a race b/n the effort to adapt & the
potentially lethal processes in protoplasm.
171

The dynamics of stress comprises:
• a destabilizing, destructive component as well as countermeasures
promoting, re-stabilization and resistance.
• Constraint, adaptation and resistance are interconnected
parts of the whole events.
• The relative success of the harmful and protective reactions
determines whether:
• stress causes only slight & temporary deviations from the normal
state, or severe and permanent injuries.
172

• The nature and intensity of response of individual plants to a
particular stress factor may vary depending upon:
• age
• Degree of adaptation
• On seasonal, and diurnal activity
• The sessile nature of plants has naturally evolved sophisticated
molecular mechanisms to sense and respond to stress
conditions
• This can result in stress tolerance or stress avoidance
173

• What causes a plant to be stressed?
• Factors such as drought, salinity, extreme temperature,
inadequate or excessive light conditions, ozone,
pollution, radioactivity and some others
• Hence, stresses can originate from the surrounding environment
1) abiotic/ nonliving stresses; or,
2) biotic stresses-from living organisms
174

• Plants are subjected to external stresses under natural conditions
which limit their: growth, productivity, reproductive capacity,
Species distribution, and survival.
Terminology and concepts-
Stress resistance (hardiness): ability to endure an externally applied
stress, e.g. ability to survive a low external temp (water potential).
• Resistance to a stress can be achieved by:
• avoidance, tolerance, or a combination of both to various degrees.
175

Stress avoidance: the ability to prevent an externally applied stress,
e.g. the ability to maintain high cellular water content even when
the external temperature is low.
Stress tolerance: ability to survive an internal stress, e.g. the
ability to survive low cellular water content.
Abiotic factors- non-living factors affecting growth and
productivity of living organisms
• divided into two main categories:
(1) physical stressful factors such as drought, flooding, extreme
temperature, and inadequate light quality or intensity; and
176

(2) Chemical stressful factors e.g. salinity, ozone, elevated CO2
level, and heavy metal pollution.
• Some plants may be injured by a stress, which means that they
exhibit one or more metabolic dysfunctions.
• The occurrence of one abiotic stress may affect the plant
functioning mechanisms through the induction of several
interrelated changes at:
• morphological, anatomical, physiological, and biochemical levels.
• Speed at which the stressful factor installs as well as the intensity
and duration of stress determines the beneficial or injuring
effect of stress. 177

• Abiotic stress elicits a complex of responses:
• beginning with stress perception, initiates a signal transduction
pathway (s) and is manifested in changes at the cellular,
physiological, and developmental levels
• all plants have encoded capability for stress perception, signaling,
and response
The set of responses observed depends upon:
• severity and duration of the stress,
• plant genotype, developmental stage, and
• Environmental factors providing the stress.
178

Water stress
- most important abiotic stresses affecting plants life.
• required by a plant for its optimal survival
• too much water (flooding stress) can cause plant cells to swell and
burst;
• too little water (drought stress) can cause plant to dry up, a
condition termed desiccation.
• Either condition can be deadly to the plant.
• Water stress either inhibit (-) or promote (+) the followings:
179

• Cell elongation(-)
• Cell wall synthesis(-)
• Protein synthesis(-)
• Chlorophyll synthesis(-)
• Seed germination(-)
• Stomatal opening(-)
• CO2 assimilation(-)
• respiration(-)
• ABA synthesis (+)
• Proline accumulation (+)
180

• Mechanisms of resistance to drought and the methods to increase
the resistance
1）Morphology- increase in water absorption and transportation,
declination of transpiration.
a. Developed root system and higher ratio of root to shoot
b. Thick leaf, smaller leaf area and thick cuticle。
c. Developing smaller and more stomata
2) Physiology and biochemistry
a. Stomatal regulation
• ABA accumulation→stomatal closure →
181

b. Increase in capacity of resistance to dehydration of cytoplasm
c) Rapid accumulation of osmolytes:
• Proline
• Glycine betaine
• LEA protein
• dehydrin
• osmotins, and ions, etc.
Temperature Stress-can cause disorder to a plant life.
• a plant has an optimal temperature range at which it grows and
performs best.
182

• If temperature is too cold for the plant, it can lead to cold stress-
chilling stress.
• Extreme forms of cold stress can lead to freezing stress.
• Cold temperatures can affect the amount and rate of uptake of
water and nutrients, leading to cell desiccation and starvation.
• Under extremely cold conditions, the cell liquids can freeze
outright, causing plant death.
• Hot weather can affect plants adversely, too.
• Intense heat can cause plant cell proteins to break down, a
process called denaturation.
183

• Cell walls and membranes can "melt" under extremely high
temperatures, and the permeability of the membranes is affected.
• T- stress can result in damaged membranes and enzymes
• any abiotic factor that alters membrane properties can disrupt
cellular processes.
• Photosynthesis and respiration are both inhibited
• Typically, photosynthetic rates are inhibited by high temperatures
to a greater extent than respiratory rates
• Plants may experience physiological stress when abiotic factor is
deficient or in excess (referred to as an imbalance).
• The deficiency or excess may be chronic or intermittent. 184

• Imbalances of abiotic factors in the environment cause primary
and secondary effects in plants.
• Primary effects such as reduced water potential and cellular
dehydration directly alter the physical and biochemical
properties of cells, which then lead to secondary effects.
• These secondary effects, such as reduced metabolic activity, ion
cytotoxicity, and the production of reactive oxygen species,
initiate and accelerate the disruption of cellular integrity, and
may lead ultimately to cell death.
• Different abiotic factors may cause similar primary physiological
effects because they affect the same cellular processes.
185

• E.g. water deficit, salinity, and freezing, cause reduction in
hydrostatic pressure(turgorpressure) and cellular dehydration.
 Imbalances in soil minerals
• can affect plant fitness either indirectly, by affecting plant
nutritional status or water uptake, or directly, through toxic effects
on plant cells.
• Soil mineral content can result in plant stress in various ways
• Several anomalies associated with the elemental composition of
soils can result in plant stress
• including high concentrations of salts (e.g., Na+ and Cl-) and toxic
ions (e.g., As and Cd), and
• low concentrations of essential mineral nutrients, such as Ca2+, Mg
2+, N, and P.
186

• The term salinity is excessive accumulation of salt in the soil
solution
Salinity stress has two components:
• nonspecific osmotic stress that causes water deficits, and
• specific ion effects resulting from the accumulation of toxic ions,
which disturb nutrient acquisition and result in cytotoxicity.
• Salt-tolerant plants genetically adapted to salinity are
halophytes, while less salt-tolerant plants that are not adapted to
salinity are termed glycophytes.
• Soil salinity occurs naturally and as the result of improper water
management practices 187

Radiation stress
• Excess quantities of photosynthetically active radiation and
increased absorption of uv radiation produce radiation stress
in plants
• Extremely high irradiance destroys photosynthetic pigments and
thylakoid structures (photo-damage)
• The primary site of attack in radiation stress (strong light) is the
reaction center of photosystem II
• Electron transport is interrupted
• Efficiency of photosystem II is lessened
188

• Under strong light aggressive oxygen species accumulate
• These can destroy chloroplasts pigments and membrane lipids
• e.g. oxidoreductases (super oxide dismutase, peroxidases,
catalases)
• can be mitigated by escape movements e.g. slanting position of
leaves, by rolling up the shoots, dense coverings of trichomes on
the upper surfaces of leaves
Biotic Stresses- resulting from plant interaction with other
organisms, or the damage to plants via other living organisms such
as bacteria, fungi, nematodes, protists, insects, weeds, viruses,
viroids, plant competition, and allelopathy 189

• If the stress is moderate and short term, the injury may be
temporary and the plant may recover.
• If the stress is severe enough, it may prevent flowering, seed
formation, and induce senescence that leads to plant death.
• Such plants are considered to be susceptible.
• Some plants escape the stress altogether, such as ephemeral, or
short-lived, desert plants.
• Fungi cause more diseases in plants than any biotic stress
factors.
• Over 8,000 fungal species are known to cause plant disease.
190

• about 14 bacterial genera cause economically important diseases
in plants
• Viruses also cause biotic stress to plants.
• a few plant pathogenic viruses exist, but they are serious
enough to cause as much crop damage worldwide as
fungi
• Microorganisms can cause plant wilt, leaf spots, root rot, or seed
damage.
• Insects can cause severe physical damage to plants & act as a
vector of viruses and bacteria
191

Multiple Stresses: Occurrence and Interaction
• The occurrence of only one stress at the same time in the
field or in the natural conditions is rare.
• For example, increase of plant transpiration resulting from
pollutants such as sulfur dioxide (SO2 ) may expose plant to
drought stress.
• two or more stresses are simultaneously or successively
associated.
• For e.g., drought stress is closely related to high temperature and
luminosity in hot climate arid and semi-arid areas.
192

• The multiple stress combination may lead to a modification in the
plant stress susceptibility.
• The occurrence of one environmental stress may expose plant to a
second stress.
• In the particular case of biotic and abiotic stresses
association, the occurrence of abiotic stress can enhance or
reduce plant resistance to a pest or pathogen and vice versa.
Physiological mechanisms and morphological
adaptations to stress factors
• Plants can modify their life cycles to avoid stresses
• One way to do this is through modification of their life cycles. 193

• For e.g., annual desert plants have short life cycles: they complete
during the periods when water is available, and are dormant
during dry periods.
• Deciduous trees shed their leaves before the winter so that
sensitive leaf tissue is not damaged by cold temperatures.
• Phenotypic changes in leaf structure and behavior are important
stress responses
194

• Plants have evolved various mechanisms that enable them to avoid
or mitigate the effects of stresses to leaves.
• these mechanisms include changes in leaf area, leaf
orientation, trichomes, and the cuticle.
• Plants can regulate stomatal aperture in response to
dehydration stress
• Osmotic adjustment is the capacity of plant cells to
accumulate solutes and use them to lower water potential
during periods of osmotic stress.
195

• Many plants have the capacity to acclimate to cold
temperature
• Plants survive freezing temperatures by limiting ice
formation
• Specialized plant proteins, termed antifreeze proteins, limit the
growth of ice crystals through a mechanism independent of
lowering of the freezing point of water.
• Synthesis of these antifreeze proteins is induced by cold
temperatures.
• The proteins bind to the surfaces of ice crystals to prevent or slow
further crystal growth. 196

• Cold-resistant plants tend to have membranes with more
unsaturated fatty acids
• A large variety of heat shock proteins can be induced by different
environmental conditions
• cells that have previously experienced one condition may
gain cross-protection against another
197

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