plant hormones Flashcards
What are plant hormones and their general function? (3)
Chemicals produced in one region of the plant and transported through plant tissues;
To have an effect in another part of the plant;
They regulate growth and responses to environmental stimuli
What are the main functions of auxins in plants? (5)
Control cell elongation;
Prevent leaf fall (abscission);
Maintain apical dominance;
Stimulate the release of ethene;
Involved in fruit ripening and tropisms
What is indoleacetic acid (IAA), and how does it affect plant growth? (4)
Important auxin produced in the tips of shoots and roots;
It stimulates cell elongation and moves by diffusion, active transport, or via the phloem, resulting in uneven growth;
High concentrations of IAA increase cell elongation in shoots (positive phototropism);
High concentrations of IAA inhibit cell elongation in roots (negative phototropism)
How does phototropism occur in flowering plants? (4)
- IAA is produced in the shoot tip and transported evenly down the shoot;
- Light causes IAA to move from the light side to the shaded side;
- IAA concentration is higher on the shaded side, causing faster elongation on that side;
- The shoot bends toward the light
How does gravitropism occur in flowering plants? (4)
- IAA is produced in the root tip and transported evenly;
- Gravity causes IAA to accumulate on the lower side of the root;
- IAA concentration inhibits cell elongation on the lower side, while cells on the upper side elongate;
- The root bends downward, displaying positive gravitropism
What are the main functions of gibberellins in plants? (4)
Cause stem elongation;
Trigger the mobilisation of food stores in seeds during germination;
Stimulate enzyme production for food breakdown;
Stimulate pollen tube growth in fertilisation
What are the main functions of abscisic acid (ABA) in plants? (3)
Maintains dormancy in seeds and buds;
Stimulates cold-protective responses (e.g., antifreeze production);
Triggers stomatal closing
What are the main functions of ethene in plants? (2)
Causes fruit ripening;
Promotes abscission in deciduous trees
Why do deciduous plants lose their leaves in winter? (3)
To conserve water during the coldest part of the year when soil water may be frozen;
Less light for photosynthesis;
So the plant conserves energy by dropping leaves when maintaining them is not energy-efficient
How do auxins and ethene regulate leaf loss in deciduous plants? (5)
Auxins inhibit leaf loss by being produced in young leaves;
As leaves age, less auxin is produced, leading to leaf loss;
Ethene stimulates leaf loss by promoting the formation of the abscission layer;
Which separates the leaf from the plant;
Ethene causes cell expansion in the abscission layer, breaking cell walls and causing the leaf to fall off
What is the relationship between auxins and ethene in leaf loss? (2)
Antagonistic;
Auxins inhibit leaf loss, while ethene stimulates it
How do guard cells regulate stomatal closing in plants? (2)
When guard cells are turgid, stomata are open;
When they become flaccid, stomata close, reducing water loss by transpiration
How does abscisic acid (ABA) trigger stomatal closure? (5)
ABA binds to receptors on guard cell membranes;
Causing ion channels to open and allowing calcium ions to enter the cytosol;
The increased calcium ion concentration triggers other ions (e.g., K⁺) to leave the guard cells;
Raising water potential and causing water to leave the guard cells by osmosis;
Making them flaccid and closing the stomata.
What is the difference between synergism and antagonism in plant hormones? (4)
Synergism = Hormones work together to produce a greater effect;
E.g., auxins and gibberellins work together to promote tall plant growth;
Antagonism = Hormones have opposing effects;
E.g., gibberellins stimulate side shoot growth, while auxins inhibit it
What is apical dominance in plants? (3)
When auxins stimulate the growth of the apical bud (the shoot tip);
While inhibiting the growth of side shoots;
Preventing competition for light and energy
What happens to a plant when the apical bud is removed? (2)
Removing the apical bud stops auxin production;
Allowing side shoots to grow by cell division and elongation
How does experimental evidence show that apical dominance is controlled by auxins? (4)
If the apical bud is removed and replaced with an auxin source, side shoot growth is inhibited;
Showing that auxins regulate apical dominance;
Auxin concentration decreases as it moves away from the apical bud;
Allowing side shoots to grow near the bottom of tall plants
How do you set up an experiment to investigate the role of auxins in apical dominance? (7)
- Plant 30 plants of similar age, height, and weight;
- Count and record the number of side shoots on each plant;
- For 10 plants, remove the tip and apply a paste without auxins to the stem;
- For another 10 plants, remove the tip and apply a paste containing auxins;
- Leave the remaining 10 plants untreated as controls;
- Let the plants grow for 6 days, controlling other variables (e.g., light, water);
- After 6 days, count the number of side shoots on each plant
What should the results of the auxin apical dominance experiment show? (3)
Removing the tip of the shoots causes extra side shoots to grow;
Removing the tip and applying auxins inhibits side shoot growth;
Indicating auxins are involved in apical dominance
What is the evidence that gibberellins are involved in seed germination? (4)
Mutant seeds that lack gibberellins do not germinate;
But applying gibberellins externally allows normal germination;
Gibberellin inhibitors prevent germination;
But removing the inhibitor or applying gibberellins restores germination
How do you set up an experiment to investigate the role of gibberellins in stem elongation? (4)
- Plant 40 plants of similar age, height, and mass;
- Leave 20 plants untreated as controls and water them normally;
- Water the other 20 plants with a dilute solution of gibberellin;
- Let the plants grow for 28 days, measuring the lengths of the stems weekly
What should the results of the gibberellin stem elongation experiment show? (2)
The stems of the plants watered with gibberellin should grow more than the controls;
Suggesting that gibberellins stimulate stem elongation
How can you calculate the rate of growth of a plant in an experiment? (3)
Rate of growth = (Growth in cm) ÷ (Number of days);
Example: If a plant grows 10 cm in 28 days, the rate is 10 ÷ 28 = 0.36 cm/day
How is ethene used in the fruit industry? (2)
Ethene stimulates enzymes that break down cell walls, chlorophyll, and convert starch to sugar, making the fruit soft and ripe;
Bananas are harvested unripe and exposed to ethene later to ensure they ripen simultaneously
How are auxins used commercially? (4)
Selective weedkillers;
Auxins make weeds grow long stems, causing them to grow too fast, leading to death due to lack of water and nutrients;
Rooting powders;
Auxins stimulate root growth in plant cuttings, allowing them to be planted and grow into new plants
