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
25
Q
What causes abscission/leaf
fall in plants?
A
• 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
47
Q
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
A
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
Q
What is the effect of unilateral
light?
A
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
