plant responses Flashcards
1
Q
What are the differences and
similarities between chemical
coordination in plants and
humans?
A
• The key limitations on plants is that they are not mobile, and they don’t have a rapidly responding nervous system • Most plant responses are slower than animal responses, but they still respond as a result of complex chemical interactions • Plants have evolved a system of hormones
2
Q
List some plant hormones
A
- Auxins
- Gibberellin
- Ethene
- ABA (abscisic acid)
3
Q
What are the roles of auxins?
A
Control cell elongation • Prevent lead fall (abscission) • Maintain apical dominance • Involved in tropisms • Stimulate the release of ethene • Involved in fruit ripening
4
Q
What are the roles of
gibberellin?
A
Causes stem elongation • Triggers the mobilisation of food stores in a seed at germination • Stimulates pollen tube growth in fertilisation
5
Q
What are the roles of ethene?
A
• Causes fruit ripening
• Promotes abscission in deciduous
trees
6
Q
What are the roles of ABA
(abscisic acid)?
A
• Maintains dormancy of seeds and buds • Stimulates cold protective responses e.g. antifreeze production • Stimulates stomatal closing
7
Q
Why are scientists still unsure
about the details of many plant
responses?
A
• Plant hormones work at very low concentrations, so isolating them and measuring changes in concentrations is not easy • The multiple interactions between the different chemical control systems also make it difficult for researchers to isolate the role of a single chemical in a specific response
8
Q
What must happen for a plant
to start growing?
A
The seed must germinate
9
Q
Describe the process of seed
germination
A
1. When the seed absorbs water, the embryo is activated and begins to produce gibberellins 2. The gibberellins stimulate the production of enzymes that break down the food stores found in the seed 3. The embryo plant uses these food stores to product ATP for building materials so it can grow and break out through the seed coat 4. Evidence suggests that gibberellins switch on genes which code for amylases and proteases (digestive enzymes required for germination) 5. Evidence also suggests that ABA acts as an antagonist to gibberellins, and that it is the relative levels of both hormones which determine when a seed will germinate
10
Q
Where is the food store found
in seed?
A
- Cotyledons in dicot seeds
* Endosperm in monocot seeds
11
Q
Give experimental evidence
that supports the role of
gibberellins in seed
germination
A
• Mutant varieties of seeds have been bred without the gene that enables them to make gibberellins, and these seeds don’t germinate. If gibberellins are applied externally, then they germinate normally • If gibberellin biosynthesis inhibitors are applied to the seed, they don’t germinate as they can’t make gibberellins
12
Q
Describe the auxins that play a
role in the growth of a plant
shoot
A
• e.g. indoleacetic acid (IAA) • Growth stimulants produced in plants • Small quantities can have powerful effects • Made in cells at the tips of roots and shoots, and in the meristems • Can move down the stem and up the root both in the transport tissue, and from cell to cell • Effect of the auxin depends on its concentration and interaction with other hormones
13
Q
What are the effects of auxin
on plant growth?
A
• They stimulate the growth of the main, apical shoot • They affect the plasticity of the cell wall - the presence of auxins means the cell wall stretches more easily • Auxin molecules bind to specific receptor sites in the plant cell membrane, causing the pH to fall to 5 • Optimum pH needed for enzymes to keep the walls very flexible and plastic • As cells mature, auxin is destroyed and the pH rises so the enzymes maintaining plasticity become inactive • As a result, the wall becomes rigid and fixed in shape and size, and cells can no longer expand and grow
14
Q
What is the effect of high
concentrations of auxins?
A
• Suppresses the growth of lateral shoots, resulting in apical dominance • Growth in the main shoot is stimulated by auxin produced at the tip, so it grows quickly • The lateral shoots are inhibited by the hormone that moves back down the stem, so they don’t grow very wells • Further down the stem, the auxin concentration is lower, and so the lateral shoots grow more strongly
15
Q
Give some experimental
evidence for the role of auxins
in apical dominance
A
• If the apical shoot is removed, the auxin-producing cells are removed so there is no auxin • The lateral shoots grow faster • If auxin is applied artificially, apical dominance is reasserted, and lateral shoot growth is suppressed
16
Q
What is the effect of low
concentrations of auxins?
A
• Promote root growth • Up to a given concentration, the more auxin that reaches the root, the more they grow • Auxin is produced by the roots tips and also reaches the roots in low concentrations from the growing shoots
17
Q
Give some experimental
evidence for the role of auxins
in root growth
A
If the apical shoot is removed, the amount of auxin reaching the roots is greatly reduced, and root growth slows and stops • Replacing the auxin artificially at the cut apical shoots restores the growth of the roots • High auxin concentrations inhibit root growth
18
Q
Effect of different
concentrations auxin on
different tissues
A
19
Q
What role do gibberellins play
in plant growth?
A
They are important in the elongation of plant stems during growth • They affect the length of the internodes - the regions between the leaves on a stem • Plants that have short stems produce few or no gibberellins • This reduces waste, and also makes the plants less vulnerable to damage by weather and harvesting
20
Q
What is synergism?
A
When different hormones work
together, complementing each other
and giving a greater response than
they would on their own
21
Q
What is antagonism?
A
When different hormones have opposite effects, e.g. one promoting growth and one inhibiting it, the balance between them will determine the response of the plant
22
Q
Give examples of abiotic
stresses faced by plants
A
- Changes in day length
- Cold and heat
- Lack of water
- Excess water
- High winds
- Changes in salinity
23
Q
Why do deciduous plants lose
their leaves?
A
• In temperate climates a point comes when the amount of glucose required for respiration to maintain the leaves, and to produce chemicals from chlorophyll that might protect them against freezing, is greater than the amount of glucose produced by photosynthesis • Also, a tree with leaves is more likely be blown over by winter winds
24
Q
Describe daylength sensitivity
in plants?
A
Photoperiodism - plants are sensitive to a lack of light in their environment • Many different plant responses are affected by the photoperiod, including the breaking of the dormancy of the leaf buds so they open up, the timing of flowering in a plant, and when tubers are formed in preparation for overwintering • Sensitivity of plants to day length (or dark length) results from a lightsensitive pigment called phytochrome • Phytochrome exists in two forms (Pr and Pfr) - each absorbs a different type of light and the ratio changes depending on the levels of light
25
What causes abscission/leaf
| fall in plants?
```
• The falling light levels result in
falling concentrations of auxin
• Leaves respond to falling auxin
concentration by producing the
gaseous hormone ethene
• Ethene initiates gene switching in
the cells in the abscission zone,
resulting in the production of new
enzymes
• These enzymes digest and
weaken the cell walls in the outer
layer of the abscission zone,
known as the separation layer
• The vascular bundles which carry
materials into and out of the leaf
are sealed off
• Fatty material is deposited in the
cells on the stem side of the
separation layer, forming a
protective scar when the leaf falls,
to prevent the entry of pathogens
• Cells deep in the separation zone
respond to hormonal cues by
retaining water and swelling,
putting more strain on the already
weakened outer layer
• Abiotic factors e.g. low
temperatures and strong winds
finish the process
• The leaf separates from the plant
• A neat, waterproof scar is left
behind
```
26
Why do plants try to prevent
| freezing?
If cells freeze, their membranes are
| disrupted and they will dies
27
How do plants prevent
| freezing?
```
• The cytoplasm of the plant cells
and the sap in the vacuoles
contain solutes which lower the
freezing point
• Some plants produce sugars,
polysaccharides, amino acids and
proteins which act as antifreeze to
stop the cytoplasm from freezing,
or protect the cell from damage
even if they do freeze
```
28
How are the stomata
| controlled?
• The hormone ABA
• Leaf and root cells release ABA
under abiotic stress, causing
stomatal closure
29
What happens when the plant
| is under abiotic stress?
```
• Plant roots produce ABA which is
transported to the leaves
• At the leaves it binds to receptors
on the plasma membrane of the
stomatal guard cells
• ABA activates changes in the ionic
concentration of the guard cells,
reducing the water potential and
turgor of the cells
• Because of reduced turgor, the
guard cells close the stomata, and
water loss by transpiration is
reduced
```
30
What is herbivory?
The process by which herbivores eat
| plants
31
What physical defences do
| plants have to herbivory?
horns, barbs, spikes, spiny leaves,
fibrous and inedible tissue, hairy
leaves and stings
32
What are the chemical
defences plants have to
herbivory?
* Tannins
* Alkaloids
* Terpenoids
33
What are tannins?
```
• Part of a group of compounds
called phenols
• Can make up 50% of the weight of
the leaves
• Very bitter taste which puts
animals off eating the leaves
• Toxic to insects - bind to the
digestive enzymes in their saliva
and inactivate them
```
34
What are alkaloids?
```
Large group of very bitter tasting
nitrogenous compounds
• Act as drugs, affecting the
metabolism of animals that take
them in and sometimes poisoning
them
• e.g. caffeine, nicotine, morphine,
and cocaine
• Caffeine is toxic to fungi and
insects, and it prevents the
germination of other seeds in the
soil, so no competition from plant
rivals
• Nicotine is produced in the roots
of tobacco plants, and transported
to the vacuoles of the leaves,
where it is stored. It is released
when the leaf is eaten
```
35
What are terpenoids?
```
Large group compounds which
often form essential oils, but also
act as toxins to insects and fungi
• Pyrethrin, produced by
chrysanthemums, acts as an
insect neurotoxin
• Citronella, produced by lemon
grass, repels insects
```
36
What are pheromones?
```
Chemicals made by an organism
that affect the social behaviour of
other members of the same species
• Plants don’t behave socially, so
don’t rely a lot on pheromones
```
37
Give examples of where plants
use pheromones to defend
themselves
```
If a maple tree is attacked by
insects, it releases a pheromone
which is absorbed by leaves on
other branches
• these leaves them make chemicals
e.g. callose to protect themselves if
they are attacked
```
38
What are volatile organic
| compounds (VOCs)?
```
Chemicals produced by plants
which act like pheromones between
themselves and other organisms,
particularly insects. Some VOCs
may act as a ‘pheromone’ so that
neighbouring plants begin to
produce the VOC before being
attacked
• Usually only made the the plant
detects attack by an insect pest
through chemicals in the saliva of
the insects
• This may elicit gene switching
```
39
Give examples of plants
| producing VOCs
```
• When apple trees are attacked by
spider mites, they produce VOCs
which attract predatory mites that
come and destroy the apple tree
pests
• Some types of wheat seedling
produce VOCs when they have
been attacked by aphids, and
these repel other aphids from the
plant
```
40
Describe plants that fold in
| response to touch
```
Mimosa pudica
• If the leaves are touched, they fold
down and collapse
• This frightens off larger herbivores,
and dislodges small insects which
have landed on the leaves
• The leaf falls in a few seconds,
and recovers over 10-12 minutes
as a result of potassium ion
movement into specific cells,
followed by osmotic water
movement
```
41
What are tropisms?
```
Plant growth responses to stimuli
from one direction .
• Phototropism - the growth of
plants in response to light which
comes from one direction only
• Geotropism - the response to
gravity
• Chemotropism - the response to
chemicals
• Thigmotropism - the response to
touch
```
42
How are tropisms a response
| to environmental cues?
```
• Once a seed begins to germinate
in the soil, the shoot must grow up
towards the light sources for
photosynthesis, and the roots
must grow downwards into the soil
which will provide support,
minerals and water
```
43
Why does most research on
tropisms use germinating
seeds and young seedlings?
```
• They are easy to work with and
manipulate
• They are growing and responding
rapidly, so any changes show up
quickly
• Changes tend to affect the whole
organism rather than a small part,
making any tropisms easier to
observe and measure
```
44
Why are seedlings of
monocotyledonous plants
usually used?
```
• e.g. cereals like oats and wheat
• The shoot that emerges is a single
spike with no apparent leaves
known as coleoptile
• Easier to manipulate and observe
than a dicotyledonous shoot
• Coleoptiles are relatively simple
plant systems, so its important to
remember that the control of the
responses to light in an intact adult
plant may be more complex
```
45
What are phototropisms
| caused by?
```
The movement of auxins across the
shoot or root if it is exposed to light
that is stronger on one side than the
other
• The shoots of plants grow towards
the light, and the roots (if exposed)
grow away
• Shoots are positively phototropic
• Roots are negatively phototropic
```
46
What is the survival value of
| phototropism?
```
• Helps ensure that the shoots
receive as much all-round light as
possible, allowing the maximum
amount of photosynthesis to take
place
• If the roots emerge from the soil
(e.g. after particularly heavy rain)
they will rapidly turn back into the
soil
```
47
```
What is the effect of the
following?
1. Removing shoot tip
2. Lightproof cover placed over
intact tip of shoot
3. Thin impermeable barrier of
mica
4. Mica inserted on shaded side
5. Tip removed, gelatine block
inserted and tip replaced
```
```
1. No response: The tip must either
detect the stimulus or produce
the messenger (or both) as its
removal prevents any response
2. No response: The light stimulus
must be detected by the tip
3. Bends towards light: Mica on the
illuminated side of the shoot
allows the hormone to pass only
down the shaded side where it
increases growth and causes
bending
4. No response: Movement of
chemical down shaded side is
prevented by mica
5. Bends towards the light: As
gelatine allows chemicals to
pass through it, but not electrical
messages, the bending which
occurs must be due to a
chemical passing from the tip
```
48
What is the effect of unilateral
| light?
```
Where plants are partially shaded
the shoots grow towards the light
and then grow on straight towards it
1. The side of a shoot exposed to
light contains less auxin than the
side which is not illuminated
2. The light causes auxin to move
laterally across the shoot, so
there is a greater concentration
on the un-illuminated side
3. The stimulates cell elongation
and growth on the dark side,
resulting in observed growth
towards the light
4. Once the shoot is growing
directly towards the light, the
unilateral stimulus is removed
5. The transport of auxin stops, and
the shoot then grows straight
towards the light
```
49
Why do plants grow more
rapidly in the dark than when
they are illuminated?
```
If a plants in the dark, the aim is to
grow upwards rapids to reach the
light to be able to photosynthesise
• The seedlings that break through
the soil first won’t have to
compete with other seedlings for
light
• Evidence suggests that gibberellin
are responsible for the extreme
elongation of the internodes when
a plant is grown in the dark
• Levels of gibberellin fall when the
stem is exposed to light
```
50
Why is the slowing of upwards
growth when exposed to light
valuable?
• Resources can be used for
synthesising leaves, strengthening
stems, and overall growth
51
Why is geotropism important?
```
• In normal conditions, plants
always receive a unilateral
gravitational stimulus - gravity
always acts downwards
• Shoots are negatively geotropic
(grow away from gravitational
pulls)
• Roots are positively geotropic
(grow towards gravitational pull)
• Geotropism is also known as
gravitropism
```
52
List some general commercial
| uses of plant hormones
• Control of ripening
• Hormone rooting powders and
micropropagation
• Hormonal weedkillers
53
List some specific uses of
| plant hormones
```
• Auxins can be used in the
productions of seedless fruit
• Ethene is used to promote fruit
dropping in plants e.g. cotton,
walnuts and cherries
• Cytokinins are used to prevent
ageing of ripened fruit and
products e.g. lettuces, and in
micropropagation to control tissue
development
• Gibberellins can be used to delay
ripening and ageing in fruit, to
improve the size and shape of
fruits, and in beer brewing to
speed up the malting process
```
54
How is ripening controlled by
| plant hormones?
```
• The gaseous plant hormone
ethene is involved in the ripening
of climacteric fruits (i.e. fruits that
continue to ripen after they have
been harvested)
• Fruit is harvested when they are
fully formed but long before they
are ripe, because the unripe fruit is
hard and less easily damaged in
transport
• When the fruit are needed for sale,
they are exposed to ethene gas
under controlled conditions to
ensure each batch of fruit ripens at
the same rate
• Prevents a lot of wastage of fruit
during transport, and increases the
time available for them to be sold
```
55
How are plant hormones used
in rooting powders and
micropropagation?
```
• Auxin affects the growth of both
shoots and roots
• Applying auxin to cut shoots
stimulates the production of roots,
making it much easier to
propagate new plants from
cuttings
• In both horticulture and
agriculture, many plants are now
propagated on a large scale by
micropropagation, when
thousands of new plants are
grown from a few cells of the
original plant
```
56
How are plant hormones used
| in hormonal weedkillers?
```
• The interactions between the
different plant hormones is finely
balanced to enable the plant to
grow
• Weeds interfere with crop plants,
competing for light, space, water
and minerals
• Most weeds are broad-leaved
dicots
• If synthetic dicot auxins are
applied as weedkiller, they are
absorbed by the broad-leaved
plants and affect their metabolism.
The growth rate increase and
becomes unsustainable, so they
die
• Synthetic auxins used are simple
and cheap to produce, have very
low toxicity to mammals, and are
selective
```
