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Plant growth and development
The need to explain tropisms:
re-direction of growth in response to
light,
PHOTOTROPISM
gravity,
GEOTROPISM
touch,
THIGMOTROPISM
The need to explain patterns of development:
production of flowers, PHOTOMORPHOGENESIS
development of fruits,
senescence of foliage,
response to wounds
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
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.
Experimental analysis of shoot phototropism
A sequence of experiments started by the Darwins and continued
by Boysen-Jensen
Fig. 33.1C
Components of experiments
An experiment has:
A statement predicting alternative responses:
1. Hypothesis
2. Treatment
“If this is done that will happen otherwise it will not.”
A specific, designed, manipulation
3. Measurement
4. Control
sufficiently accurate to detect response to the
treatment
The same measurement is made but the treatment is not
applied. This provides the essential contrast.
5. Replication
Enables the degree of response to be defined and helps
to protect against obtaining results by chance
And must be
6. Repeated
Required to establish the degree of certainty that can be
attributed to a result, e.g., repetition with the same and
different species
The initial observation
The first experiment
to remove the effect
1. Hypothesis
2. Treatment
3. Measurement
4. Control
5. Replication
And must be
6. Repeated
Deficiencies?
1. Hypothesis
The initial observation
Expt 1
Expt 2
Expt 3
2. Treatment
3. Measurement
Expts 2 and 3
4. Control
5. Replication
And must be
6. Repeated
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?
What type of an
experiment is this?
The initial observation
1. Hypothesis
2. Treatment
Expt4
3. Measurement
4. Control
5. Replication
And must be
6. Repeated
Boysen-Jensen’s experiment
The initial observation
1. Hypothesis
2. Treatment
3. Measurement
4. Control
5. Replication
And must be
6. Repeated
Has the postulate changed from that of Expt 4?
What are the improvements over Expt4 ?
Which is the treatments and what are the controls?
Went’s experiments
Is this really a
control?
Fig. 33.1D
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
Five plant growth substances and their functions
Major Functions
Where Produced
Stimulates cell elongation; involved in
phototropism, gravitropism, apical
domincance, and vascular differentiation;
stimulates ethylene synthesis and induces
adventitious roots on cuttings
Meristems of apical buds,
embryo of seed, young
leaves
Cytokinin
Stimulates cell division, reverse apical
dominance, involved in shoot growth,
delay leaf sequence
Synthesized in roots and
transported to other organs
Ethylene
Stimulates fruit ripening, leaf and flower
senescence, and abscission
Tissues of ripening fruits,
nodes of stems, senescent
leaves and flowers
Abscisic Acid
Inhibits growth, stimulates stomatal
closure, maintains dormancy
Leaves, stems, green fruit
Stimulates shoot elongation, stimulates
bolting and flowering in biennials,
regulates production of hydrolytic
enzymes in grains
Meristems of apical buds
and roots, young leaves,
embryo
Auxin
Gibberellin
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
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
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
The acid growth hypothesis
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
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.
The effect of day length on flowering
Autumn flowering plants,
e.g., chrysanthemums
Summer flowering plants,
e.g., iris
Fig. 33.11
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
Phytochrome
The control of flowering is determined by a substance called
phytochrome that exists in two forms.
660 nm
730 nm
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
Control through the relative amounts of different
Plant Growth Substances
Abscissic acid as a growth inhibitor
gibberellin as a promoter
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
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.
Sections you need to have read
33.1 through 33.12
Courses that deal with this topic
Botany 371/372 Plant physiology laboratory