Plant growth and development

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Plant growth and development
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The need to explain tropisms
re-direction of growth in response to light,
PHOTOTROPISM gravity, GEOTROPISM touch, THIGM
OTROPISM
The need to explain patterns of development
production of flowers, development of
fruits, senescence of foliage, response to wounds
PHOTOMORPHOGENESIS
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Phototropism
The cress shoot was grown in unidirectional light
from the right. There is a rapid curving
response in the apical region which moves down
the stem. Straightening of the upper stem in the
later stages of the sequence makes the shoot
appear to stop curving, but close examination of
the lower stem shows there is still a response
towards the light source.
180 frames filmed over 4 hours at 0.013 f.p.s.
http//www-users.york.ac.uk/drf1/tropism/tropisms
.htm
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Negative gravitropism in a shoot
A young, vertically growing, sunflower seedling
shoot placed in a horizontal position. The shoot
curves until it is once again growing vertically.
This gravitropic response is first observed in
the apical region of the shoot. Rapid curving at
the tip progresses along the whole stem. The rate
of curvature is not necessarily constant along
the stem and is complicated by subsequent
straightening of curved areas known as
autotropism.
180 frames filmed over 5 hours at 0.01 f.p.s.
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Experimental analysis of shoot phototropism
A sequence of experiments started by the Darwins
and continued by Boysen-Jensen
Fig. 33.1C
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Components of experiments
An experiment has
A statement predicting alternative responses If
this is done that will happen otherwise it will
not.
1. Hypothesis
2. Treatment
A specific, designed, manipulation
sufficiently accurate to detect response to the
treatment
3. Measurement
The same measurement is made but the treatment is
not applied. This provides the essential
contrast.
4. Control
Enables the degree of response to be defined and
helps to protect against obtaining results by
chance
5. Replication
And must be
Required to establish the degree of certainty
that can be attributed to a result, e.g.,
repetition with the same and different species
6. Repeated
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The first experiment
The initial observation
Deficiencies?
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Expts 2 and 3
The initial observation
Does the postulate change for Expt 2 and 3?
What are the improvements over the first
experiment?
What are the treatments and what the controls in
Expt2 and Expt3?
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Expt4
What type of an experiment is this?
The initial observation
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Boysen-Jensens experiment
The initial observation
Has the postulate changed from that of Expt 4?
What are the improvements over Expt4 ?
Which is the treatments and what are the controls?
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Wents experiments
Is this really a control?
Fig. 33.1D
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Chemicals are produced in small quantities that
change the rate at which growth takes place
and/or the types of cells that are produced.
These chemicals are usually produced by
meristematic tissue and are actively transported
from that tissue. The chemicals influence the
development of cells according to the
concentration that accumulates in the developing
cells.
Plant growth substances, the text book calls them
hormones
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Five plant growth substances and their functions
Major Functions
Where Produced
Auxin Cytokinin Ethylene Abscisic
Acid Gibberellin
Meristems of apical buds, embryo of seed, young
leaves Synthesized in roots and transported to
other organs Tissues of ripening fruits, nodes
of stems, senescent leaves and flowers Leaves,
stems, green fruit Meristems of apical buds and
roots, young leaves, embryo
Stimulates cell elongation involved in
phototropism, gravitropism, apical domincance,
and vascular differentiation stimulates ethylene
synthesis and induces adventitious roots on
cuttings Stimulates cell division, reverse
apical dominance, involved in shoot growth, delay
leaf sequence Stimulates fruit ripening, leaf
and flower senescence, and abscission Inhibits
growth, stimulates stomatal closure, maintains
dormancy Stimulates shoot elongation, stimulates
bolting and flowering in biennials, regulates
production of hydrolytic enzymes in grains
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Auxin and phototropism
Auxin, indole acetic acid (IAA), is transported
downwards from the shoot apex. In a seedling
exposed to light from the side the concentration
of IAA is greater on the shaded side of the shoot
Fig. 33.1B
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Results of experiments that applied IAA
Fig. 33.3B
1. The chemical may have different effects at
different concentrations
2. It can affect different tissues differently
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Polar transport of auxin
Transport at 1 cm/hr implies active transport
Picks up a hydrogen ion at the acid wall
environment
Passes across membrane as a neutral molecule
Gives off the H into the cell which induces the
proton pump
Auxin can only exit the cell at its basal end
where there are specific carrier proteins
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The acid growth hypothesis
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Gibberellins

• Reverses dwarfism the first discovery of
  gibberellin
• Seed Germination--Barley de novo amylase
  synthesis (Varner 1964)
• Can cause bolting in biennials
• Control of sex expression
• Can enhance fruit growth e.g., seedless grapes
• Delays Senescence

Gibberellins act in the elongation of intact
plants as opposed to stem section elongation by
auxin. Much research on plant gibberellins has
been possible due to gibberellin sensitive
mutants. They have adequate levels of GA1 (the GA
species most likely to be responsible for stem
elongation) but can not respond to it. This may
be due to lack of receptor protiens.
Transport is non polar, has been found in both
the transpiration and translocation stream. It
can occur more rapidly, 5 cm/hr, than auxin
There are many gibberellins closely related
chemically
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Bioassay
Using an organism or other living material to
determine the level of an environmental condition
1. Establish pattern of response under
standardized conditions
2. Use the established pattern in analysis of
treatments
Advantages cost, useful where the treatment
has not been precisely defined yet, e.g.,
substances like gibberellin.
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The effect of day length on flowering
Autumn flowering plants, e.g., chrysanthemums
Summer flowering plants, e.g., iris
Fig. 33.11
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Flowering response can be manipulated by short
periods of red or far-red radiation applied
during the dark period of a long night regime
Fig. 33.12A
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Phytochrome
The control of flowering is determined by a
substance called phytochrome that exists in two
forms.
All plants contain phytochrome but they may
respond differently to the relative amounts of
the two forms
Phytochrome is involved in other plant growth
process in addition to flowering
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Control through the relative amounts of different
Plant Growth Substances
For cells growing in culture Cytokinins added
have no effect on their own. Cytokinins plus
auxin cause cells to divivde. If the
concentrations are about equal the cells continue
to grow and form a callus but there is no cell
differentiation. If there is more cytokinin than
auxin then shoot buds develop. If there is more
auxin than cytokinin then roots develop
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Are plant growth substances hormones?
A hormone is a regulatory chemical that travels
in the blood from its production site and affects
other sites in the body often at some distance.
Hormones are made and secreted by organs called
endocrine glands.
This hormone concept as developed for animals has
some distinct differences from what we know of
the production, distribution and function of
plant growth substances.
1. Transport in plants is very different from
that in animals. Its is frequently polar
2. The range of plant growth substances that we
know are produced and possibly distributed in
different ways
3. Plants no equivalent to the central nervous
system that integrates and co-ordinates
physiological activities.
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Sections you need to have read
33.1 through 33.12
Courses that deal with this topic
Botany 371/372 Plant physiology laboratory