Plant Biology
Plant Responses and Hormones
5th Year · 6th Year (Leaving Cert)
- ✓Define tropisms and distinguish between positive and negative phototropism and gravitropism.
- ✓Explain the role of auxins in plant growth and their specific mechanisms in phototropism and gravitropism (HL).
- ✓Describe the essential conditions required for successful seed germination and explain the importance of each.
- ✓Relate plant responses to environmental stimuli to their survival and adaptation.
Key concepts
Tropisms are growth responses of plants to external stimuli, where the direction of growth is determined by the direction of the stimulus. They are vital for plant survival, allowing plants to grow towards resources (e.g., light, water) and away from adverse conditions (e.g., excessive gravity for shoots).
Phototropism is a growth response to light. Positive Phototropism: Growth towards a light source (e.g., shoots, stems, leaves). This maximises light absorption for photosynthesis. Negative Phototropism: Growth away from a light source (e.g., roots). This helps roots grow into the soil for anchorage and water absorption.
Gravitropism is a growth response to gravity. Positive Gravitropism: Growth towards the force of gravity (e.g., roots). This ensures roots grow downwards into the soil for stability and water/mineral uptake. Negative Gravitropism: Growth away from the force of gravity (e.g., shoots, stems). This ensures shoots grow upwards towards light.
Auxins are a group of plant hormones (phytohormones) primarily involved in cell elongation and growth regulation. They are produced mainly in the apical meristems of shoots, young leaves, and root tips. Key Functions: Promote cell elongation in shoots. Inhibit cell elongation in roots at high concentrations. Maintain apical dominance (inhibition of lateral bud growth by the apical bud). Promote root formation on stem cuttings. Influence fruit development. Mechanism in Phototropism (HL): When light strikes a shoot unevenly, auxins migrate from the illuminated side to the shaded side. The higher concentration of auxins on the shaded side promotes greater cell elongation there, causing the shoot to bend towards the light. Mechanism in Gravitropism (HL): In Shoots: When a shoot is horizontal, gravity causes auxins to accumulate on the lower side. This higher concentration of auxins on the lower side promotes cell elongation, causing the shoot to bend upwards (negative gravitropism). In Roots: When a root is horizontal, gravity also causes auxins to accumulate on the lower side. However, root cells are much more sensitive to auxins. The high concentration of auxins on the lower side inhibits cell elongation, while the upper side elongates normally, causing the root to bend downwards (positive gravitropism).
Germination is the process by which a seed embryo develops into a seedling. For successful germination, specific environmental conditions are required. Water: Essential for rehydrating the seed, activating enzymes, and transporting dissolved nutrients. It also softens the seed coat, allowing the radicle to emerge. Oxygen: Required for aerobic respiration, which provides the energy (ATP) necessary for cell division and growth of the embryo. Suitable Temperature: Each seed type has an optimal temperature range for germination. Temperature affects the rate of enzyme activity involved in metabolic processes. Too low or too high temperatures can inhibit or prevent germination.
Key facts to remember
- 1Tropisms are directional growth responses of plants to external stimuli.
- 2Phototropism is a response to light; gravitropism (geotropism) is a response to gravity.
- 3Shoots exhibit positive phototropism (towards light) and negative gravitropism (away from gravity).
- 4Roots exhibit negative phototropism (away from light) and positive gravitropism (towards gravity).
- 5Auxins are plant hormones produced in meristems, primarily promoting cell elongation in shoots (HL).
- 6In roots, high concentrations of auxins inhibit cell elongation (HL).
- 7Unilateral light causes auxins to migrate to the shaded side of a shoot, causing it to bend towards the light (HL).
- 8Gravity causes auxins to accumulate on the lower side of roots, inhibiting elongation there and causing the root to bend downwards (HL).
- 9The three essential conditions for seed germination are water, oxygen, and a suitable temperature.
Worked examples
Example 1
Explain, with the aid of a labelled diagram, how a plant shoot exhibits positive phototropism.
Answer
A plant shoot exhibits positive phototropism due to the uneven distribution of auxins. Auxins, produced in the shoot tip, migrate to the shaded side when light is unilateral. This higher concentration of auxins on the shaded side promotes greater cell elongation compared to the illuminated side. The differential growth causes the shoot to bend towards the light source, maximising light absorption for photosynthesis.
A clear diagram showing auxin distribution and differential growth is crucial for full marks.
Example 2
Describe the role of auxins in the positive gravitropism of a plant root.
Answer
Auxins, produced in the root tip, play a key role in positive gravitropism. When a root is positioned horizontally, gravity causes auxins to accumulate on its lower side. Unlike shoots, root cells are extremely sensitive to auxins, and this higher concentration on the lower side inhibits their cell elongation. The cells on the upper side, with a lower auxin concentration, continue to elongate normally. This differential growth causes the root to bend downwards, growing into the soil.
It is vital to remember that high auxin concentrations inhibit root cell elongation, which is the opposite effect seen in shoots.
Example 3
List and explain the three essential conditions required for seed germination.
Answer
The three essential conditions for seed germination are: 1. Water: Rehydrates the seed, activating metabolic enzymes and allowing for the transport of dissolved nutrients. It also softens the seed coat, facilitating the emergence of the radicle. 2. Oxygen: Required for aerobic respiration, which releases the energy (ATP) needed for cell division, growth, and other metabolic activities within the developing embryo. 3. Suitable Temperature: Each seed has an optimal temperature range for germination. Temperature influences the rate of enzyme activity, which drives all biochemical reactions necessary for growth. Temperatures outside this range can inhibit or prevent germination.
These three conditions are external factors. Internal factors like viability and dormancy are also important but are not 'conditions required' in the same sense.
Common mistakes
- ✗Confusing the effect of auxins on shoots (promote elongation) with their effect on roots (inhibit elongation at high concentrations).
- ✗Incorrectly identifying positive vs. negative tropisms (e.g., stating roots show positive phototropism).
- ✗Forgetting one of the three essential conditions for germination (water, oxygen, temperature).
- ✗Not explaining why a particular tropism is beneficial for the plant's survival.
- ✗Failing to mention the role of gravity in gravitropism, or light in phototropism, when explaining auxin distribution.
Exam tips
- ★Always define key terms like 'tropism', 'phototropism', 'gravitropism', and 'auxin' clearly.
- ★When explaining tropisms, explicitly state the stimulus, the part of the plant responding, and the direction of the response.
- ★For Higher Level questions on auxins, clearly distinguish their effect on cell elongation in shoots versus roots.
- ★Practice drawing and labelling diagrams for phototropism and gravitropism, showing auxin distribution and differential growth.
- ★Ensure you explain why each germination condition is necessary, not just list it.
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