Download Chapter 29 Control Systems in Plants

Chapter 29
Control Systems
in Plants
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Unlike animals, plants possess no nervous
system and thus their co-ordination is achieved
almost entirely by hormones.
Also plant hormones almost affect some aspect
of growth which may lead to movements of
plant parts, although these are relatively slow
responses compared to those of animals.
29.1 Plant Responses - Plant movements are performed by
growth responses
29.1.1 Tropisms
- a directional growth movement made by a part of a
stationary plant in response to a unilateral stimulus such as
water, gravity or light.
When the plant part grows towards the stimulus: positive
When the plant part grows away from the stimulus: negative
- The growth movement is caused by different rates of
growth on the two sides of the plant under the influence
of the unilateral stimulus.
The part bends to that side which grows more efficiently.
Actively growing structures show tropisms more obviously,
thus young seedlings are used to show tropisms.
- According to the nature of the stimuli, tropisms are
classified into
(1) phototropism (light),
(2) geotropism (gravity),
(3) hydrotropism (water),
(4) chemotropism (chemicals),
(5) thigmotropism (touch),
(6) aerotropism (air).
1. Hydrotropism - growth movement in response to water
- roots are positively hydrotropic
2. Geotropism
- growth movement in response to gravity
- roots: positively geotropic shoots: negatively geotropic
- If a plant is subjected to two external stimuli such as
water and gravity, to which stimulus will the plant
respond? Which stimulus has greater influence?
The plant will respond to water which has a greater
influence.
3. Phototropism
- growth movement in response to light
- roots are negatively phototropic ; they grow away from
light
- shoots and leaves are positively photopropic ; they grow
towards light
4. Chemotropism
- growth response to chemicals, e.g. pollen tubes
growing towards chemicals at the micropyle
4. Chemotropism
- growth response to chemicals, e.g. pollen tubes
growing towards chemicals at the micropyle
5. Thigmotropism
- growth response to touch, e.g. tendrils of peas twine
around support
6. Aerotropism
- growth response to air, e.g. pollen tubes grow away
from air
29.1.2 Taxes
- a directional movement made by the whole plant (usually
motile) in response to a unilateral stimulus
- Examples:
(1) Phototaxis
Chlamydomonas (a motile unicellular alga) swims
towards unilateral light for photosynthesis.
They are scattered evenly in water in complete darkness.
(2) Thermotaxis
Green algae or bacteria swim towards regions of
optimum temperature
(3) Chemotaxis
Antherozoids of mosses are attracted by chemicals
secreted by the archegonia
29.1.3 Nasties
- a non-directional movement made by part of a stationary
plant in response to an external stimulus.
The direction of movement is not determined by the
direction of the stimulus.
This response is reversible and repeatable.
Examples:
- thigmonasty (Mimosa, Venus flytrap), photonasty,
thermonasty, etc
29.2 Plant Hormones
- Characteristics of plant hormones:
1. They are organic substances which regulate growth
& other physiological functions of plants
2. They lack specificity
3. They act on parts of the plant other than the part that
produced them.
29.2.1 Auxins
1. Auxins are produced in small amounts at the tips of
growing shoots and roots.
2. Auxins diffuse from the tip to the region of elongation
where they exert their effects:
cell walls become more easily stretched & cells elongate.
3. Transport of auxins is one direction (polar), i.e. away from
tips by diffusion or by phloem
From Figure:
- Shoots show an increased growth rate as the
concentration of auxins increases. Once auxins
concentration reaches a maximum, the growth of the
shoot will be inhibited.
The growth of a root is promoted by a lower concentration of auxins
than the growth of a shoot.
The concentration suitable for shoot growth has an inhibitory effect
on root growth.
The concentration that stimulates maximum root growth has no
stimulating effect on shoot growth.
MECHANISM OF
PHOTOTROPISM OF
SHOOTS AND ROOTS
- more auxins on shaded side
MECHANISM OF GEOTROPISM
OF SHOOTS AND ROOTS
- more auxins on lower side by gravity
Other effects of auxins:
1. Stimulate cell division, e.g. calluses formation
2. Help to maintain the structure of cell walls, thus
inhibits abscission
3. Inhibit growth in high concentrations in apical
dominance
4. Stimulate fruit development without fertilization
(parthenocarpy)
Effect of gibberellins on
plant growth
29.2.2 Gibberellins
1. Reverse of genetic dwarfism, e.g. maize
2. Promote cell elongation, e.g. length of internodes
3. Break dormancy of buds & seeds --> see past paper
on seed germination**
4. Stimulate fruit development
5. Remove the need for cold in vernalization
6. Affect flowering
29.2.3 Cytokinins
1. Promote cell division
2. Delay leaf senescence
3. Stimulate bud development
4. Break dormancy
29.2.4 Abscissic Acid - not required in syllabus
29.2.5 Ethene - not required in syllabus
Commercial Applications of Synthetic Growth Regulators
Auxins as weedkillers;
help to increase fruit yields;
produce seedless fruits (parthenocarpy)
Gibberellins keep leaves of crops,
vegetables from yellowing when picked;
with cytokinins to initiate germination
Apple sectioned to show
effects of auxin production
29.3 Control of Flowering
- affected by daylength & temperature
29.3.1 The Phytochrome System
Phytochrome is a photoreceptor for absorbing light.
It is a blue-green pigment existing in two forms:
Phytochrome 660 (P660) which absorbs red light
Phytochrome 730 (P730) which absorbs far-red light
A short exposure to the
appropriate
light
wavelength causes the
conversion of one
form into the other.
These conversions may
also be brought about
by
daylight
and
darkness (with slow
conversion).
Phytochrome comprises a
protein & a pigment.
It is distributed in the plant
in minute quantities,
being more concentrated
in growing tips.
The actions of these two
forms of phytochrome
are usually antagonistic:
TABLE 29.4 Summary of the effects of red light and far-red light
Red light effect
Far-red light effect
TABLE 29.4 Summary of the effects of red light and far-red light
Red light effect
P660  P730
Far-red light effect
P730  P660
TABLE 29.4 Summary of the effects of red light and far-red light
Red light effect
Far-red light effect
P660  P730
P730  P660
Stimulate germination of some
seeds, e.g. lettuce
Inhibit germination of some
seed, e.g. lettuce
TABLE 29.4 Summary of the effects of red light and far-red light
Red light effect
P660  P730
Far-red light effect
P730  P660
Stimulate germination of some
Inhibit germination of some
seeds, e.g. lettuce
seed, e.g. lettuce
Induces formation of anthocyanins Inhibits……
TABLE 29.4 Summary of the effects of red light and far-red light
Red light effect
P660  P730
Far-red light effect
P730  P660
Stimulate germination of some
Inhibit germination of some
seeds, e.g. lettuce
seed, e.g. lettuce
Induces formation of anthocyanins Inhibits……
Stimulate flowering in long-day
plants
Inhibits ……
TABLE 29.4 Summary of the effects of red light and far-red light
Red light effect
P660  P730
Far-red light effect
P730  P660
Stimulate germination of some
Inhibit germination of some
seeds, e.g. lettuce
seed, e.g. lettuce
Induces formation of anthocyanins Inhibits……
Stimulate flowering in long-day
plants
Inhibits flowering in short-day
plants
Inhibits ……
Stimulate…..
TABLE 29.4 Summary of the effects of red light and far-red light
Red light effect
P660  P730
Far-red light effect
P730  P660
Stimulate germination of some
Inhibit germination of some
seeds, e.g. lettuce
seed, e.g. lettuce
Induces formation of anthocyanins Inhibits……
Stimulate flowering in long-day
plants
Inhibits flowering in short-day
plants
Inhibits ……
Elongation of internode is
inhibited
….promoted
Stimulate…..
TABLE 29.4 Summary of the effects of red light and far-red light
Red light effect
P660  P730
Far-red light effect
P730  P660
Stimulate germination of some
Inhibit germination of some
seeds, e.g. lettuce
seed, e.g. lettuce
Induces formation of anthocyanins Inhibits……
Stimulate flowering in long-day
plants
Inhibits flowering in short-day
plants
Inhibits ……
Elongation of internode is
inhibited
Induces increase in leaf area
….promoted
Stimulate…..
Prevents …..
TABLE 29.4 Summary of the effects of red light and far-red light
Red light effect
P660  P730
Far-red light effect
P730  P660
Stimulate germination of some
Inhibit germination of some
seeds, e.g. lettuce
seed, e.g. lettuce
Induces formation of anthocyanins Inhibits……
Stimulate flowering in long-day
plants
Inhibits flowering in short-day
plants
Inhibits ……
Elongation of internode is
inhibited
Induces increase in leaf area
….promoted
Causes epicotyl hook to unbend
Maintains epicotyl hook bent
Stimulate…..
Prevents …..
29.3.2 Photoperiodism
- affects flowering
1. Long-day plants:
plants which flower when the period of daylight
exceeds a critical minimum length
2. Short-day plants:
plants which flower when the period of daylight is
shorter than a critical maximum length
3. Day neutral plants:
plants which flower regardless of the length of daylight
The length of the dark period is crucial in determining flowering:
Short-day plants require a long dark period while
long-day plants require a short dark period.
Interrupting a long dark period with red light is as effective as daylight in
stopping short-day plants flowering. Far-red light has no effect and shortday plants flower as if the dark period had been continuous.
These & other experiments suggest that phytochrome is
the photoreceptor detecting different light wavelengths
and ultimately determining whether or not a plant
flowers.
The light stimulus is detected by the leaves to induce
flowering, with a hormone called florigen to travel from
the leaf to the apex.
Mechanism:
Vernalization
Many plants grow vegetatively but fail to flower if they are not
exposed to a period of low temperature.
This phenomenon is called vernalization.
A hormone called vernalin is responsible.
Importance:
1.To ensure flowering at specific times of the year to suit the
occurring of the particular pollinating agent, e.g. insects
2. To ensure the species produces flowers at the same time for cross
pollination.
Since photoperiod is consistent regardless of climatic conditions,
thus a more reliable means of achieving synchronized flowering.
Application:
Growers can control temperature and lengths of light periods and
produce the plant continuously throughout the year.