Bentham & Hooker's Classification. along with the merits and demerits of the ...
Lectures 1 4
1. ACS 104: CROP PHYSIOLOGY
INSTRUCTOR: Dr. C. M. Onyango
2. Intended learning outcomes
By the end of the course it is expected that you will be able to:
1. Describe the plant cell, the parts of a plant cell, their structure and
functions
2. Describe the physiology of green plants in relation to plant structure
and environmental factors.
3. Explain the principles of photosynthesis and respiration including
their relationship to crop yield.
4. Explain the mechanisms of water movement through plants, relate
this to mineral nutrition
5. Outline the stages of plant development, the mechanisms by which
these stages are regulated, plant senescence and dormancy
7. Plastids
• Proplastids - small, precursors to the other plastid
types, found in young cells, actively growing
tissues;
• Chloroplasts - sites of photosynthesis (energy
capture).
• Chromoplasts - non-photosynthetic, colored
plastids; give some fruits (tomatoes, carrots) and
flowers their color;
• Amyloplasts - colorless, starch-storing plastids;
• Leucoplast - another term for amyloplast;
• Etioplast - plastid whose development into a
chloroplast has been arrested (stopped).
10. What distinguishes green plants from animals?
• Autotrophism!
• An autotroph makes its own food (energy-rich organic
compounds) from simple, inorganic materials in the
environment.
• Plants use light as their energy source, hence they are
photosynthetic
11. CONSEQUENCES OF A STATIONARY LIFESTYLE
A non-motile organism is unable to move to a more favorable
location to carry out its vital functions.
Thus, plants have at least three major problems to contend
with:
i) Environmental Positioning/Location - or, Getting Started
in the Right Spot.
i) The need to sense & respond to the environment -plants,
like other organisms, must be able to respond to changes in
their environment
iii) Physical and biological dangers in the environment - a
non-motile organism cannot flee when conditions get tough.
It must "fight" it out. Both the physical environment and
biological environment threaten the well being of plants.
12. ENVIRONMENTAL POSITIONING
• Axis orientation:-Once a seed germinates in a favorable
environment it must determine which way is up/down to
insure that the roots grow down and shoots up.
• Fine Tuning:-Even non-motile organisms need to "fine-
tune" their position in the environment. Thus plants have a
variety of mechanisms that enable them to optimize their
position in the environment including:
- Phototropism; Skototropism; Thigmomorphogenesis; Solar
tracking; Leaf mosaics; Etiolation;
14. SKOTOTROPISM: growth of vines toward a darkened
region of the environment. Mechanism by which some
tropical vines find a support to grow up
15. THIGMOMORPHOGENESIS - response to touch in
which the plant is shorter with thicker stems - prevents
plants from getting too spindly and reduces risk of
breaking in wind
16. SOLAR TRACKING: (e.g. flowers follow the
movement of the sun) - keeps pollen dry, maximize
photosynthesis.
18. THE NEED TO SENSE & RESPOND TO THE
ENVIRONMENT
• Plants typically respond by various growth movements
to unpredictable, usually short-term environmental
fluctuations, like changes in temperature or light. E.g.
flowers that are temperature sensitive open when it is
warm and close when it is cooler.
• Plants responds to the environment. Light is one of the
most important environmental signals for plant
development. This phenomenon is called
photomorphogenesis and one classic example is
etiolation.
19. PHYSICALAND BIOLOGICAL DANGERS IN
THE ENVIRONMENT
• A non-motile organism cannot flee when conditions
get tough. It must "fight" it out. Both the physical
environment and biological environment threaten the
well being of plants
Physical dangers - wind, water (flood), drought, cold
(winter) are among the physical dangers that a plant
faces
Biological dangers - predators (=herbivores) and
competitors (=other plants)
20. PHYSICAL DANGERS - how do plants cope?
• In general, plants cope with these by dormancy,
senescence, and even death
• The evergreen and deciduous lifestyle are in part a
response to adverse conditions
• Evergreens are much better able to tolerate cold, dry
conditions. They also do better in poor soil because they
don't loose as many leaves
• Plants also respond to environmental challenges
morphologically - for example, reduce their Surface
area/Volume ratio to minimize water loss
22. Mimicry- "tricking"predators. E.g. lithops in S. African deserts look likepebbles -
are stonemimics. mistletoeleaves look just likethehost tree leaves to avoid being
eaten.
Someplants, likewild tobacco, when attackedby herbivores releasevolatile
chemicalsthat summon predatory insects to the damaged plants. Theseinsectsin
turn, kill theherbivores.
23. • Produce toxic chemicals. Allelopathy is chemical
warfare between plants.
• Phytoalexins are chemicals produced by plants to
resist microbial infection.
25. Properties and functions of water
Properties
•a liquid at physiological temperatures (i.e., between 0-100
C) - water has a high boiling point and a high melting point
•has a high heat of vaporization - it takes a lot of energy to
convert water from a liquid to a gas; Water resists evaporation.
•has a high specific heat (heat capacity)- It takes a lot of
energy to raise the temperature of water. Thus, water is slow to
heat up and cool down, i.e. water resists temperature changes.
•has a high heat of fusion- It takes a lot of energy to convert
water from a solid to a liquid, or put another way, water resists
freezing
26. • has a high surface tension - It takes a lot of energy to
break through the surface of water Thus, water acts as
though it has a skin.
• The density of water decreases on crystallization
• a universal solvent
• has high tensile strength and incompressibility
• transparent to light
• chemically inert
• dissociates into protons and hydroxide ions - basis for
the pH system
• Water affects the shape, stability & properties of
biological molecules
27. FUNCTIONS OF WATER
• is a major component of cells
• is a solvent for the uptake and transport of materials
• is a good medium for biochemical reactions
• is a reactant in many biochemical reactions (e.g.,
photosynthesis)
• provides structural support via turgor pressure (i.e., leaves)
• is the medium for the transfer of plant gametes (sperms
swim to eggs in water, some aquatic plants shed pollen
underwater)
• offspring (propagule) dispersal (think "coconut")
• plant movements are the result of water moving into and
out of those parts (i.e., diurnal movements, stomatal
opening, flower opening)
• cell elongation and growth
• thermal buffer
30. • There are two major ways to move molecules in plants.
These are:
i) Bulk or mass flow: is the mass movement of molecules
in response to a pressure gradient. The molecules move
from hi → low pressure, following a pressure gradient
i) Diffusion: This is the net, random movement of
individual molecules from one area to another. The
molecules move from high → low, following a
concentration gradient.
32. Specialized forms of diffusion
Osmosis: it represents the diffusion of a solvent
(typically water) across a membrane.
33. Dialysis: another specialized case of diffusion; however is
different from osmosis because it involves the movement
of a solute and not a solvent as is the case with osmosis. It
is the diffusion of solute across a semi-permeable
membrane
e.g.
- consider a cell containing a sugar dissolved in water. If
water (the solvent) moves out of the cell into the
surroundings it moves osmotically; if the sugar (solute)
moves into the surroundings, it is an example of dialysis
34. Factors influencing the rate of diffusion
• A. Concentration Gradient. - from an area of high
concentration to one of lower concentration.
Fick’s Law - is an equation that relates the rate of diffusion to
the concentration gradient (C1 – C2) and resistance (r).
Diffusion rate, also called flux density (Js, in units of mol
m-2 s-1) can be expressed in the simplified version of Fick's
equation as: Js = (C1 - C2) / r
• Molecular Speed
• Temperature - increases the rate of molecular movement,
therefore, increases the rate of diffusion
• Pressure - increases speed of molecules, therefore,
increase the rate of diffusion
• Solute effect on the chemical potential of the solvent-
Solute particles decrease the free energy of a solvent.
35. WATER POTENTIAL
Definition: Water potential is a measure of the energy state
of water. This is a particularly important concept in plant
physiology because it determines the direction and
movement of water.
• Equation for water potential (must account for the
factors that influence the diffusion of water)
Ψw= Ψp + Ψs + Ψg
• Where:
Ψw = water potential; Ψp = pressure potential; Ψs = solute or
osmotic potential; Ψg = gravity potential
36. Important terms
• Solute (or osmotic) potential (Ψs): This is the
contribution due to dissolved solutes. Solutes always
decrease the free energy of water, thus their contribution
is always negative
• Pressure Potential; (Ψp): Due to the pressure build up in
cells. It is usually positive, although may be negative
(tension) as in the xylem.
• Matric potential: This is the contribution to water
potential due to the force of attraction of water for
colloidal, charged surfaces. It is negative because it
reduces the ability of water to move. However, it can be
very important in the soil, especially when referring to
the root/soil interface.
• Gravity (Ψg): Contributions due to gravity which is
usually ignored unless referring to the tops of tall trees.
37. Terms used for Solutions
• Hypertonic Solution: a higher solute concentration
outside the cell than the cytoplasm. Water diffuses out
of the cell. Protoplast and vacuoles will shrink causing
the plasma membrane to pull away from the cell wall -
called plasmolysis
38. Isotonic solution: has the same concentration inside
and outside the cell. No net movement of water.
39. Hypotonic solution: refers to a solution that contains less
Solute (more water) outside compared to the inside of the of
the cell
43. •We defined Autotrophs as organisms that are able to
synthesize organic molecules (A molecule that contains
both carbon and hydrogen) from inorganic materials
•Photoautotrophs - are organisms that use light as an
energy source and carbon dioxide as its carbon source;
absorb and convert light energy into the stored energy of
chemical bonds in organic molecules through a process
called photosynthesis.
44.
45. • Plants, algae, and bacteria known as cyanobacteria are
known as oxygenic photoautotrophs - they
synthesize organic molecules from inorganic materials,
use water as an electron source, and generate oxygen
as an end product
• Some bacteria, such as the green and purple bacteria,
are known as anoxygenic phototrophs - do not use
water as an electron source during photosynthesis and,
as a result, do not evolve oxygen
• The electrons come from compounds such as hydrogen
gas, hydrogen sulfide, and reduced organic molecules
46. In this section on photosynthesis, we will be concerned
with the oxygenic phototrophs
• Photosynthesis is composed of two stages:
i) the light-dependent reactions
ii) the light-independent reactions.
47. • The light-dependent reactions convert light energy into chemical
energy, producing ATP and NADPH- occur in the thylakoids
• The light-independent reactions use the ATP and NADPH from the
light-dependent reactions to reduce carbon dioxide and convert the
energy to the chemical bond energy in carbohydrates such as glucose
- Occur in the stroma
ATP- Adenosinetriphosphate
NADPH is the reduced form ofNADP
(Nicotinamideadeninedinucleotide
phosphate)
48. The overall reaction for photosynthesis
Note that carbon dioxide is reduced to produce glucose
while water is oxidized to produce oxygen
49. Before the photosynthetic reactions- need to understand
the electromagnetic spectrum and chloroplasts
• Visible light constitutes a very small portion of a spectrum
of radiation known as the electromagnetic spectrum.
• All radiations in the electromagnetic spectrum travel in
waves and different portions of the spectrum are
categorized by their wavelength.
• At one end of the spectrum are television and radio waves
with longer wavelengths and low energy.
• At the other end of the spectrum are gamma rays with a
very short wavelength and a great deal of energy.
• Visible light is the range of wavelengths of the
electromagnetic spectrum that humans can see, a mixture
of wavelengths ranging from 380nm to 760nm. It is this
light that is used in photosynthesis.
50. • Light and other types of radiation are composed of
individual packets of energy called photons (A particle of
electromagnetic radiation; a single unit of radiant energy)
• The shorter the wavelength of the radiation, the greater
the energy per photon
• When photons of visible light energy strike certain atoms
of pigments during photosynthesis, that energy may push
an electron from that atom to a higher energy level where
it can be picked up by an electron acceptor in an electron
transport chain
51. Interaction between a Photon and an Atom
ATP can then be generated by chemiosmosis which is the
production of ATPutilizing the energy released when hydrogen
ions flow through an ATP synthase complex
53. • Chloroplasts are surrounded by an inner and an outer
membrane.
• The inner membrane encloses a fluid-filled region
called the stroma that contains enzymes for the light-
independent reactions of photosynthesis.
• Infolding of this inner membrane forms interconnected
stacks of disk-like sacs called thylakoids, often
arranged in stacks called grana.
• The light-dependent reactions of photosynthesis occur
in the thylakoids
54. Pathways used in the synthesis of carbohydrates
• Different plants follow different pathways in the
synthesis of carbohydrates.
• These pathways are:
- C3 (Calvin cycle),
- C4 and
- the CAM pathways
55. The Calvin cycle (C3 pathway)
• Most plants use the Calvin (C3) cycle to fix carbon
dioxide
• C3 refers to the importance of 3-carbon molecules in
the cycle
• There are three stages to the Calvin cycle:
- CO2 fixation
- Production of G3P
- Regeneration of RuBP
56. Stage 1: CO2 fixation
• To begin the Calvin cycle, a molecule of CO2 reacts
with a five-carbon compound called ribulose
bisphosphate (RuBP) producing an unstable six-
carbon intermediate which immediately breaks down
into two molecules of a three-carbon compound-
phosphoglycerate (PGA
• The carbon that was a part of inorganic CO2 is now part
of the carbon skeleton of an organic molecule.
• The enzyme for this reaction is ribulose bisphosphate
carboxylase (Rubisco)
57. Stage 2: Production of G3P from PGA
• The energy from ATP and the reducing power of
NADPH (both produced during the light-dependent
reactions) is now used to convert the molecules of
PGA to glyceraldehyde-3-phosphate (G3P), another
three-carbon compound
• For every six molecules of CO2 that enter the Calvin
cycle, two molecules of G3P are produced
58. Stage 3: Regeneration of RuBP from G3P
• 10 of every 12 G3P produced - are used to regenerate the
RuBP so that the cycle may continue
• Ten molecules of the three-carbon compound G3P
eventually form six molecules of the four-carbon
compound ribulose phosphate (RP)
• Each molecule of RP then becomes phosphorylated by
the hydrolysis of ATP to produce ribulose bisphosphate
(RuBP)
59. THE C4 PATHWAY
• The C4 pathway is designed to efficiently fix CO2 at low
concentrations and plants that use this pathway are known
as C4 plants
• This occurs in cells called mesophyll cells. First, CO2 is
fixed to a three-carbon compound called
phosphoenolpyruvate to produce the four-carbon
compound oxaloacetate
• The enzyme catalyzing this reaction, PEP carboxylase
60. • The oxaloacetate is then converted to another four-
carbon compound called malate in a step requiring the
reducing power of NADPH
• The malate then exits the mesophyll cells and enters the
chloroplasts of specialized cells called bundle sheath
cells. Here the four-carbon malate is decarboxylated to
produce CO2, a three-carbon compound called pyruvate,
and NADPH
• The CO2 combines with ribulose bisphosphate and goes
through the Calvin cycle while the pyruvate re-enters
the mesophyll cells, reacts with ATP, and is converted
back to phosphoenolpyruvate, the starting compound of
the C4 cycle.
61. THE CAM PATHWAY
• CAM plants live in very dry condition and, unlike other
plants, open their stomata to fix CO2 only at night
• Like C4 plants, they use PEP carboxylase to fix CO2,
forming oxaloacetate.
• The oxaloacetate is converted to malate which is stored
in cell vacuoles. During the day when the stomata are
closed, CO2 is removed from the stored malate and
enters the Calvin cycle
