Chapter 16 Plant Hormones

Question 1

What is a tropism?

Answer 1

Directional growth in response to environmental cues such as gravity and light

Question 2

What are the general roles of auxins in plants?

Answer 2

(IAA is an example of an Auxin)

Cell elongation- ph, plasticity
Apical Dominance
Tropisms

Inhibit abscission by inhibiting ethene production increasing
Fruit ripening

Question 3

What are the general roles of gibberellins in plants?

Answer 3

Stem elongation
Mobilisation of food stores in seeds at germination
Pollen tube growth

Question 4

What are the general roles of ethene in plants?

Answer 4

Fruit ripening
Abscission ( leaf loss)

Question 5

What are the general roles of ABA in plants?

Answer 5

Maintain dormancy
Cold responses
Stomatal closing

Question 6

Why is it difficult to understand the role of hormones in plants?

Answer 6

Some effects are hormonal and environment- hard to isolate what is causing the change
Work at very low concentration

Question 7

How do hormones affects seed germination?

Answer 7

When a seed absorbs water, it becomes activated
Gibberellins switch on genes for amylases and proteases, needed for germination, digest food stores
ABA antagonist to Gibberellins

Question 8

What is the experimental evidence for the role of gibberellins within germination?

Answer 8

Mutants without the gene for gibberellins won’t germinate unless external gibberellin is added
If gibberellin synthesis inhibitors are added, germination cannot continue

Question 9

Chemically, how do auxins stimulate the main growth of the apical shoot?

Answer 9

Auxin binds to cell surface receptors, causing the pH to fall to 5
Optimum pH for enzymes which keep the cell wall flexible so expand
As cells mature, auxin is destroyed, pH rises, enzymes less efficient less stretchy so cannot grow
So only near top can grow as there is where the auxin is made and diffuses down from

Question 10

Physically, how do auxins enable apical dominance, with respect to other shoots? and what evidence is there?

Answer 10

Inhibit growth of lateral shoots, move down the stem
Near the bottom, lower conc of auxin so lateral can grow
Evidence: if apical is removed, lateral shoots can grow. If auxin applied, apical

Question 11

How does auxin affect roots? How does this compare to lateral buds and the apical shoot

Answer 11

Low concentration promote growth
There comes a point when if the concentration is too high, the auxins will inhibit growth rather than promote it
The same thing will occur to lateral buds and the apical shoot, however, the concentration of auxins must be far higher to inhibit growth, more for apical shoot

Question 12

How do gibberellins affect growth of the plant?

Answer 12

Increases the length of the region which is effectively the stems
Without gibberellins, plants are much shorte

Question 13

What is synergism and antagonism?

Answer 13

Synergism- hormones compliment each other, giving a greater response
Antagonism- hormones have opposite effects, with the balance determining the response

Question 14

Why do trees lose their leaves?

Answer 14

In winter there are short days and long nights. And low temperatures
There comes a point where the amount of glucose respires to maintain leaves and anti freeze is greater than the amount of glucose produced by photosynthesis
And trees with leaves are more likely to be damaged by winter gales
So trees lose their leaves

Question 15

What is photoperiodism and how does it work within plants?

Answer 15

A sensitivity to a lack of light due a pigment of phytochrome
Phytochrome r and Phytochrome Fr
Ratio of the two changes depending on the level of light

Question 16

What is the mechanism of abscission?

Answer 16

Lower light levels means a lower concentration of auxin
In response, cells produce and secrete ethene
Ethene has target cells in the abscission zone, two layers of cells between the base of the stalk and the leaf
Ethene turns on the genes to produce digestive enzymes to digest the cells walls of the outer layer of the abscission layer, called the separation layer
The vascular bundles are sealed and fatty material deposited on the stem side, when leaves fall, a protective scar will form
Cells within the separation layer retain more water due to hormonal cues
With wind, the strain is too much and the leaves fall

Question 17

How does cold protection within the leaves work?

Answer 17

The cytoplasm and vacuole contain solutes which lower the freezing/melting point, as impurities effectively
Sugars, polysaccharides and amino acids act as antifreeze
Sustained cold weather activates genes to produce chemicals which make the plant hardy and tough

Question 18

What is the mechanism for stomatal closure?

Answer 18

In response to abiotic stress, the leaves and roots can release ABA
Bind to receptors in the cytoplasm of guard cells, inhibiting proton pumps, preventing H+ leaving the guard cells
Also allows Ca2+ to enter the cell, acting as secondary messengers
-Open channels to allow negative ions to leave, and others to allow K+ to leave and shut those that allow K+ in
Lower solute concentration, water leaves via osmosis, flaccid

Question 19

What is herbivory? What are some physical defences to herbivory?

Answer 19

The process by which plants are eaten by animals

Thorns, Barbs, Spikes, Inedible tissue, hairy leaves

Question 20

What are some examples of chemical plant defences?

Answer 20

Tannins- bitter, toxic to insects as they bind to digestive enzymes and deactivate them
Alkaloids- bitter, nitrogenous compounds like caffeine and morphine, can affect metabolism
Terpenoids- essential oils but toxins to insects and fungi

Question 21

What are pheromones? What do plants use instead?

Answer 21

Chemicals which affect the social behaviour of members of the same species

Plants use volatile organic compounds to signal to other species

Question 22

What is an example of how a plant uses VOCs?

Answer 22

Cabbages attacked by caterpillar
Plants secrete a compound which attracts parasitic wasps which lay eggs in the caterpillars, killing them

Question 23

How does folding in response to touch help plants?

Answer 23

Dislodges insects, frightens herbivores
Potassium ion movement into cells means water osmoses in, returns to original placement

Question 24

What is a phototropism? What is a geotropism? What is a chemotropism? Why do tropisms occur?

Answer 24

Response in growth to light, gravity, chemicals
Enables maximum use of environmental conditions e.g keep growing within the light, roots anchored in

Question 25

Why type of plants are used to investigate tropisms?

Answer 25

Seedlings- changes tend to be more noticeable, affecting the whole organisms, responds quickly
Generally monocotyledonous like oats and wheat, as easier to work with than dicots
Coleoptile- single spike with no apparent leaves

Question 26

How does auxin cause phototropism?

Answer 26

When there is unilateral light, auxin moves across to the shaded side, laterally move
Greater auxin come on the unlit side, stimulates cell elongation on that side
More growth on shaded side, so points towards the light
If unilateral light removed, movement of auxin stops, growth upwards

Question 27

In what type of light do plants grow the best?

Answer 27

Grow faster in dim light than bright light
Need to maintain apical dominance

Question 28

What does removing the tip of the plant do and provide evidence for? What does covering the tip prove?

Answer 28

Removing tip- no response, growth upwards, suggests auxin produced in the tip or detects light
Covering tip- growth up, unable to detect light

Question 29

What does covering the shoot in black card on either side prove?

Answer 29

If on the shaded side, doesn’t do anything, auxin moves across, growth towards the light
If on the lit side, unable to detect light, auxin cannot move across, no tropism

Question 30

What does mica suggest about tropism?

Answer 30

Tropism occurs
Chemical not electrical growth

Question 31

How has the role of auxin been proved?

Answer 31

When a shoot is cut in half in unilateral light, the auxin the concentration is the same on either side
In a normal shoot, the auxin concentration is higher on the shaded side, suggesting auxin moves across to the shaded side instead of being destroyed

Question 32

Why do plants grow faster in the dark? How has this been used by farmers?

Answer 32

The dark provides a biological imperative to reach light, and to photosynthesise against competition
Extreme elongation in the dark is caused by gibberellins, triggered by a fall in light
In light, growth is slower but used for synthesis leaves, strengthening stems and growth
So plants in darkness thin, but faster growth, used by farmers. Called etiolated plants

Question 33

How does auxin enable positive geotropism?

Answer 33

Inhibits growth in roots
Unilateral auxin, as gravity pushes to lower side, higher side grows more, so in the ground

Question 34

How is ethene used in industry?

Answer 34

Fruit ripening in climacteric fruits, continue to ripen after harvest (non climacteric do not continue to ripen after harvesting)
Allows fruit to arrive unripe- survive transport, then ripen
Fruit dropping from trees

Question 35

How does ethene ripen fruit?

Answer 35

Triggers break down of the cell wal
Converts starch into sugar

Question 36

How is auxin used commercially?

Answer 36

Rooting powder: cut shoots, place in auxin, stimulates root product, micropropagation, cloning
Producing seedless fruit
Weed killer: generally broadleaves dicots, selective to these, increases metabolism to the point where it’s growth becomes unsustainable so they die

Question 37

How can gibberellins and cytokinins be used commercially in plants?

Answer 37

Gibberellins: delay ripening, improve size and shape
Cytokinins: prevent ripening, control tissue development, micropropagation

Question 38

What are nastic movements?

Answer 38

Ones which the direction of movement is not directly related to the stimulus
E.g thigmatropism, although something needs to touch the plant, the place it is being touched doesn’t affect the way it moves, so it is not a tropism

Question 39

Where is auxin made?

Answer 39

The TIP of a shoot, not the shoot itself