Plant Responses

Question 1

Explain why plants need to respond to their environment.

Answer 1

• They must cope with abiotic stresses such as lack of water, but also protect themselves from herbivores and pathogen (biotic threat).
• They also respond to cues such as light and gravity by directional growth.

Question 2

List the different types of response they have with examples of each.

Answer 2

1) Plant defences against herbivory (can be chemical or physical).
2) The control of growth (phototropism/ geotropism).
3) The triggering of specific events (leaf loss/ seed germination, stomatal closure, preventing freezing).

Question 3

Define the term “tropism”.

Answer 3

A growth response by a plant in response to a unidirectional stimulus.

Question 4

Name, and give an example of, 4 different types of tropism.

Answer 4

1) Phototropism - plant growth in response to presence of light.
2) Geotropism - in response to gravity.
3) Chemotropism - in response to chemicals.
4) Thigmotropism - in response to touch.

Question 5

Define the term “plant hormone”.

Answer 5

Chemical signals to co-ordinate growth and responses in plants. They do not always act in the same way as animal hormones.

Question 6

Define the term “target tissue/ target cell”.

Answer 6

Cells or tissues which have receptors to specific to the plant hormone or chemical messenger in question.

Question 7

Outline how plant hormones can be transported around plants and have their effect.

Answer 7

They are released by cells and travel the plant by diffusion/ active transport across and between cells and by mass flow in the xylem or phloem. They then trigger a response in target cells or tissues.

Question 8

List 5 plant hormones and describe the effects of each of them

Answer 8

1) Auxins - control cell elongation, prevent leaf fall, maintain apical dominance, involved in tropisms, stimulate the release of ethene, involved in fruit ripening.
2) Gibberlin - cause stem elongation, trigger the mobilisation of food stores in a seed at germination, stimulate pollen tube growth in fertilisation.
3) Ethene - causes fruit ripening, causes abscission in deciduous trees.
4) ABA (abscisic) - maintains dormancy of seeds and buds, stimulates cold protective responses (e.g. anti-freeze production), stimulates stomatal cloning.
5) Cytokinins - promote cell division.

Question 9

Define the term “meristem”.

Answer 9

Tissue found at regions of growth in plants, contains stem cells.

Question 10

Name the 4 types of meristem in plants and describe where they are and their effect on the growth of the plant.

Answer 10

1) Apical meristem at the tips of roots and shoots.
2) Lateral bud meristem giving rise to side shoots.
3) Lateral meristems in cambium, responsible for roots and shoots getting wider.
4) Intercalorie meristems located between the nodes, where the leaves and buds branch off the stem. Cause shoots to get longer.

Question 11

Describe what auxins are and how they behave.

Answer 11

• An example of auxins is IAA, these are growth stimulants which are produced in plants.
• They are made in the cells at the tip of the roots and shoots and in the meristems. A small amount can have a powerful effect.
• Auxin can travel down the stem and up the root in the vascular bundles or cell to cell.
• Its effect depends on its concentration and the interactions it has with any other hormones.

Question 12

How do auxins stimulate growth in the apical shoot?

Answer 12

• Auxin molecules bind to specific receptor sites in the plant cell membrane. This triggers a second messenger system and cells pump H+ ions out which causes a fall in PH to about 5.
• Low PH breaks H-bonds in the cell wall’s cellulose.
• This is the optimum PH for the enzymes that keep the cell wall very flexible and plastic.
• As cells mature, auxin is destroyed. As the hormone level falls, the PH rises so the enzymes maintaining plasticity become inactive.
• As a result, the wall becomes rigid and more fixed in shape and size and the cells can no longer expand and grow.

Question 13

How do auxins suppress the growth of lateral buds?

Answer 13

• High concentrations of auxin suppress the growth of the lateral shoots, this results in apical dominance.
• Growth in the main shoot is stimulated by auxin that is produced in the tip.
• The lateral shoots are inhibited by the auxin that moves back down the stem, so they do not grow very well.
• Further down the stem, auxin concentration is lower and so lateral shoots grow more strongly.
• If the apical shoot is removed, the auxin producing cells are removed so lateral shoots grow more strongly. Apical dominance is lost.

Question 14

How do auxins promote root growth?

Answer 14

• Low concentrations of auxin promote root growth.
• Up to a certain conc, the more auxin that reaches the roots the more they grow.
• Auxin is produced by the root tips and auxin also reaches the roots in low concentrations from the growing shoots.
• If the apical shoot is removed, then the amount of auxin reaching the roots is greatly reduced and the root growth slows and stops.
• Replacing the auxin artificially at the cut apical shoot restores the growth of the roots.
• High auxin concentration inhibits root growth.

Question 15

Describe the mechanism/ results of phototropism.

Answer 15

• Phototropisms are the results of movement of auxins across the shoots or roots, if exposed to light on one side that is stronger than the other.
• If plants are grown in bring, all-round light in normal conditions of gravity they grow straight upwards.
• If exposed to light that is brighter on one side, then the shoot of the plants will grow towards the light, and the roots, if exposed, will grow away (providing balance).
• Shoots positively phototrophic and roots negatively phototrophic.
• Allows plant to maximise photosynthesis + if shoot should emerge from the soil they would rapidly turn back into the soil.

Question 16

Describe how a plant can grow towards light.

Answer 16

• Presence of light that is stronger on one side than the other causes the auxin to move laterally across the shoot so that there is a greater concentration on the unilluminated side.
• This stimulates cell elongation and growth on the dark side, resulting in observed growth towards light.
• Once the shoot is growing directly towards light, the unilateral stimulus is removed.
• The transport of auxin stops and the shoot then grows straight towards the light.

Question 17

Experiment 1 - Describe what happens when a plant with a normal apical shoot tip is exposed to unilateral light.

Answer 17

• The shoot is positively phototrophic. Bending occurs behinds the tip, shoot bends towards the light.

Question 18

Exp 2 - Describe what happens when a plant shoot with its shoot tip is removed is exposed to light.

Answer 18

• No response.
• We conclude that the tip must either detect the stimulus or produce the messenger or both because its removal has prevented any response.

Question 19

Exp 3 - When light-proof cover is placed over intact tip of shoot.

Answer 19

• No response.

- Conclude that the light stimulus must be detected by the tip.

Question 20

Exp 4 - When a thin impermeable barrier of mica placed just beneath the shoot tip, but only on the side facing the sun (see pp.452).

Answer 20

• Shoot bends towards light because auxin is still able to move down shaded side.
• Auxin able to promoted cell elongation and cause bending on shaded side.

Question 21

Exp 5 - Mica barrier inserted on shaded side of shoot tip.

Answer 21

• No response.

- Mica prevents auxin from travelling to shaded side.

Question 22

Exp 6 - Shoot tip removed, gelatin block inserted and tip replaced.

Answer 22

• Bends towards light.
• Auxin is able to diffuse through gelatin block and reach shaded side.
• Gelatin is permeable to chemical messengers but not electrical, shows response must be due to chemical passing through the tip.

Question 23

What experiment can be used to show the movement of auxin across the shoots and tips?

Answer 23

1) Leaving shoot intact and splitting it in half. Shoots kept in the dark. Total auxin produced approximately the same whether shoot split or not.
2) Shoot intact and shoot in half. Shoots left in unilateral light. Total auxin produced the same whether split or not.
3) Shoot in unilateral light but undivided. Auxin accumulates in higher concs on the dark side and lower concs on the lit side when the shoot is intact. When the shoot is divided the auxin concentration is approximately the same on both sides. This suggests that normally auxin is transported across the shoot in unilateral light from lit side to dark side.

Question 24

Design an experiment to investigate one factor affecting phototropism in seedlings/coleoptiles.

Answer 24

1) Germinate and grow seedlings in different conditions of dark, all-round light, and unilateral light. Observe, measure and record patterns of growth. Could use time-lapse photography.
2) Germinate and grow seedlings in unilateral light with different colour filtres to see which wavelenghts of light trigger the phototrophic response.
3) Repeat some of the classic experiments and cover tips with foil, remove tips, place auxin-impregnanted agar jelly blocks on decapitated shoots, place auxin-impregnanted agar jelly blocks on decapitated shoots on only one side.

Question 25

Describe what happens when plants grow in the dark.

Answer 25

• Plants grow more rapidly in the dark than when they are illuminated. This is because if a plant is in the dark its biological imperative is to grow rapidly upwards to reach the light source.
• The seedlings that break through the soil first will not have to compete with other seedlings for light.
• Gibberellins are likely to be responsible for the extreme elongation of the internodes when a plant is grown in the dark.
• Once a plant is exposed to the light, a slowing of upward growth is valuable. Resources can be used instead for synthesising leaves, strengthening stems and overall growth. Gibberellin concs fall when leaf is exposed to light.

Question 26

Describe the mechanism of geotropisms.

Answer 26

• In normal conditions, plants always receive a unilateral gravitational stimulus - gravity always acts downwards.
• The response of plants to gravity can be seen in the lab using seedlings placed on their sides either in all-round light or in the dark.
• Shoots are normally negatively geotrophic and roots are positively geotrophic.
• This adaptation ensures that roots grown down into the soil and the shoots grow up to the light.

Question 27

Design an experiment to investigate one factor affecting geotropism in shoots or roots.

Answer 27

1) - Investigate the geotrophic response can be investigated in shoots and using a rotating drum known as a clinostat.
- The plants can be grown on a slowly clinostat so the gravitational stimulus is applied evenly to each side.
- The root and shoot grow straight.
2) - Alternatively, the seeds can be placed in petri dishes stuck to the wall of lab, and the dishes are rotated 90oC at intervals as the seedlings grow.