14.2 - Plant Growth Factors Flashcards
(18 cards)
What external stimuli do plants respond to?
- Light: Shoots grow towards light (positive phototropism) for photosynthesis.
- Gravity: Roots grow in the direction of gravity’s pull (positive gravitropism) for firm anchorage in soil.
- Water: Roots grow towards water (positive hydrotropism) to absorb it for metabolic processes and support.
What are plant growth factors, and how do they function?
Plant growth factors are hormone-like substances that:
- Affect growth and may be made by cells throughout the plant, not specific organs.
- Can affect the tissues that release them, unlike animal hormones that act on distant organs.
- Are produced in small quantities and regulate processes like cell elongation.
What is indoleacetic acid (IAA) and its role in plants?
IAA is a plant growth factor from the auxin group, controlling processes like plant cell elongation, which is essential for tropisms (directional growth responses).
What is phototropism, and how does it affect flowering plants?
- Phototropism is the growth of a plant in response to light.
- Shoots exhibit positive phototropism, growing towards light for photosynthesis, while roots often display negative phototropism, growing away from light.
Describe the 6 steps involved in the phototropic response of flowering shoots to unilateral light.
- IAA is produced by cells in the shoot tip and transported.
- IAA is initially distributed evenly throughout the shoot.
- Light causes IAA to move to the shaded side of the shoot.
- A higher concentration of IAA builds up on the shaded side.
- IAA stimulates greater elongation of shoot cells on the shaded side.
- The shoot bends towards the light due to differential elongation.
How does IAA influence the growth of roots in response to light?
High concentrations of IAA inhibit root cell elongation. In roots:
- More IAA accumulates on the shaded side.
- The light side elongates more than the shaded side.
- This causes the root to bend away from the light (negative phototropism).
What is gravitropism, and how does it influence plant growth?
- Gravitropism is the growth response of a plant to gravity.
- Roots grow towards gravity (positive gravitropism), while shoots grow upwards, away from gravity (negative gravitropism).
Describe the steps involved in the gravitropic response of roots to gravity.
1) Cells in the root tip produce IAA, which is transported along the root.
2) IAA is initially distributed evenly across the root.
3) Gravity causes IAA to accumulate on the lower side of the root.
4) Higher concentrations of IAA on the lower side inhibit cell elongation.
5) Cells on the upper side elongate more, causing the root to bend downwards.
How does IAA influence the gravitropic response in shoots?
- In shoots, IAA increases cell elongation.
- The greater concentration of IAA on the lower side causes more elongation there, leading to upward growth, away from gravity (negative gravitropism).
How is IAA transported, and what effect does it have on plant cells?
- IAA is transported one directionally, from the tips of shoots and roots.
- It increases the plasticity of young cell walls, allowing them to elongate.
- Older cells, being more rigid, do not respond to IAA.
What is the acid growth hypothesis, and how does it explain cell elongation?
- The acid growth hypothesis suggests that IAA stimulates the active transport of hydrogen ions from the cytoplasm into the cell wall.
- This acidifies the cell wall, increasing its plasticity, and allows cell expansion and elongation.
How does uneven distribution of IAA cause bending in plant stems and roots?
- Uneven distribution of IAA (due to environmental stimuli like light or gravity) causes differential cell elongation on one side of the stem or root.
- This leads to bending towards or away from the stimulus.
How do environmental stimuli cause plants to respond using IAA?
- Environmental stimuli like light or gravity cause IAA to accumulate unevenly in plant tissues.
- This uneven distribution triggers differential cell elongation, leading to rapid responses such as bending towards light (phototropism) or gravity (gravitropism).
How does the concentration of IAA affect cell elongation in roots versus shoots?
- In shoots, a high concentration of IAA increases cell elongation, promoting growth towards light or away from gravity.
- In roots, a high concentration of IAA inhibits cell elongation, causing growth away from light or towards gravity.
How does gravity affect the distribution of IAA in plants?
- Gravity can influence the movement of IAA from one side of a root or shoot to the other, resulting in a higher concentration on the lower side.
- This leads to differential elongation and bending towards or away from the gravitational pull.
Explain how the movement of IAA in shoots helps a plant to survive.
- More IAA moves towards the shaded side of shoots than the light side when the light is unidirectional.
- In response to this uneven distribution of IAA, the cells on the shaded side elongate faster than those on the light side and the shoot bends towards the light.
- This ensures that the shoot and the leaves attached to it have a greater chance of being well illuminated.
- As light is essential for photosynthesis, the process by which organic material for respiration is manufactured, the plant has a greater chance of survival.
Suggest two advantages to a plant of having roots that respond to gravity by growing in the direction of its force.
- Response ensures that roots grow down into the soil, anchoring the plant firmly and bringing them closer to water (needed for photosynthesis).
Consider the following facts about IAA:
-They are easily made synthetically
- They are readily absorbed by plants
- They are not easily broken down
- They are lethal to some plants in low concentrations
- Narrow-leaved plants are less easily killed than broad-leaved plants.
Suggest ways in which these facts might be relevant to agricultural practice.
- The fact that IAA is readily absorbed, easily synthesized and is lethal to plants in low concentrations makes it useful as a herbicide.
- The fact that it more readily kills broad-leaved plants than narrow-leaved ones is an advantage because many agricultural crops are narrowleaved while the weeds that compete with them are broad-leaved.
- As a result, application of IAA at appropriate concentrations will kill only the weeds with little, or no, harm to the crop.
- As IAA is not easily broken down means it will persist in the soil and continue to act as a selective weedkiller for some time.
- This may prevent a broad-leaved crop being grown on the land for some time after application of IAA.
- There is also a danger that IAA might accumulate along food chains with possible harm to animals in those chains.
