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Learn about tropisms, growth regulators, and plant defense mechanisms in response to external factors like light, gravity, and chemicals. Explore how plants adapt and protect themselves in their environment.
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Growth regulation • Some external factors that affect the growth of plants are: light intensity, day length, gravity and temperature.
Tropism A tropism is the change of the growth of a plant in response to an external stimulus. Hydrotropism Geotropism Phototropism Thigmotropism Chemotropism Different types of tropisms:
Hydrotropism Hydrotropism is the change of a plants growth in response to water
Thigmotropism Thigmotropism is the change of a plants growth in response to touch Tendrils grow around any object they touch A climbing plant has tendrils which grow in response to touch
Chemotropism • Chemotropism is the change in the growth of a plant in response to chemicals Roots grow toward minerals in the soil, but away from acids or heavy metals
Geotropism is the change in growth of a plant in response to gravity Roots usually grow toward gravity (positively geotropic) to anchor in the soil and get water and nutrients from it. Stems grow away from gravity – (negatively geotropic) to grow towards light and can make more food!
Phototropism is the change in growth of a plant in response to light Stems grow toward the light (positively phototrophic) – to get as much light as possible for photosynthesis
Plant defences • Plants can often encounter adverse conditions – HARMFUL conditions in their habitats • They cannot move around so need ways to defend themselves from being damaged or eaten!!! Protection mechanisms… 1. Spines, thorns and stinging hairs to deter animals from eating them
Plant defences 2. Toxins that cause illness or death Example – the death cap mushroom causes liver failure and death
Plant defences 3. Heat shock proteins are created when cells are exposed to high temperatures or UV light. They allow the plant to tolerate extra heat, light extra for a limited period of time, then go back to normal when the stressful conditions are over
Plant defences 3. Stress proteins are created when plants are infected by a microorganism. They may attack the microorganisms cell wall to stop the attack, or stop the infection spreading by growing specialised cell walls to keep the microorganisms out
Growth regulators • Plants control their responses by producing growth regulators- chemicals which control the growth of the plant. • Growth regulators are produced in the meristem regions of the plant – root apices and shoot apices
Growth Regulators • Growth regulators interact with each other to bring about a particular effect. • Once growth regulators are made they are then transported through the vascular system of the plant.
Growth regulators and their uses • Some growth regulators promote growth e.g. auxins Use: Artificial auxins are used in rooting powders to stimulate root formation in cuttings • Some growth regulators inhibit growth e.g. ethene and abscisic acid Use: Fruit is transported green and unripe but then can be quickly ripened by spraying it with ethene
HL: Auxin - case study Auxin is a type of growth promoter that is made in the meristems of shoots, buds and roots
The effects that auxin has on plants: 1. They cause roots to grow 2. They cause fruit formation – stimulate food to form in the fruit which surrounds the seed
3. They cause apical dominance by inhibiting the growth of axial buds
4. They cause tropisms by promoting cell elongation, growth and bending in certain regions of the plant
Mechanism of phototropism • If a plant receives light from one side only the light causes the auxins in the meristem to move down the dark side of the stem. • The extra auxins in the cells on the dark side cause them to elongate faster than the cells on the bright side. • This causes bending of the plant towards the light.
4.14 To investigate the effect of IAA growth regulator on plant tissue
Before you start get or make a copy of this Table of results for Shoots
Step 1 Label eight petri dishes 102, 101, 1, 10-1, 10-2,10-3, 10-4 and control.
Step 2 Add 10 ml of the IAA solution to the dish labelled 102.
Step 3 Using a different syringe, add 9 ml of water to each of the other dishes.
Step 4 Using a dropper, remove 1ml of solution from the 102 dish and add it to the 101 dish.
Step 5 Stir the solution using the dropper. Repeat for the remainder of the dishes (1, 10-1, 10-2, 10-3 and 10-4), using a different dropper each time.
Step 6 Discard 1ml of liquid from the dish labelled 10-4.
Step 7 Place an acetate grid on the lid of each petri dish. Place 5 radish seeds along the cross lines.
Step 8 Place filter paper on top of the seeds. Spread cotton wool on top of the filter paper.
Step 9 Put the base of each petri dish on its lid. Secure the dishes and incubate vertically at 25°C for 2-3 days.
Step 10 Measure the lengths of roots and shoots of each seed using the acetate grid.
Results & calculations Calculate the average length of the stems and roots at each concentration and the percentage stimulation or inhibition. Chart and graph the results
