Plants have the ability to respond to their environment.
- Stimulus: anything that causes a response in an organism.
- Response: activity of an organism or part of an organism as a result of a stimulus.
- Growth regulator: chemical that controls the growth of a plant.
- Tropism: growth response of a plant to a stimulus.
- Phototropism: growth of a plant in response to light.
- Geotropism: growth response of a plant to gravity.
- Thigmotropism: growth response of a plant to touch.
- Hydrotropism: growth response of a plant to water.
- Chemotropism: growth response of a plant to chemicals.
Auxin is a growth promoter. An example of an auxin is indole acetic acid (IAA).
- Auxins are produced in the meristematic tissue of shoot tips and root tips.
- Stimulates cell elongation
- Stimulates cell division
- Differentiation of meristem cells into xylem and phloem
- Apical dominance
- Delaying of fruit ripening
- Phototropism and geotropism
Mechanism of a plant tropism – phototropism
- IAA (auxin) is produced in the apical meristem of the shoot.
- This diffuses down the shaded side of the stem.
- This causes cell elongation on the shaded side.
- The shaded side of the stem grows more quickly than the exposed side of the stem.
- This causes the shoot to bend towards the source of light.
Uses of plant growth regulators
- Naphthalene acetic acid (NAA) is used as a commercial rooting powder.
- Ethene is used as a ripening agent for fruit.
Plant adaptations for protection:
- Epidermis – protects against pathogens entering the plant.
- Guard cells – protect against excess water loss.
- Some plants have bark – to protect against herbivores.
- Cacti have evolved to have no leaves (to protect against water loss) and spikes (to protect against herbivores).
- Corn lily produces a toxin called cyclopamine to protect itself against herbivores
- Many plants produce alkaloids that protect against insects and herbivores.
- Poison ivy produces a chemical called urushiol that protects against herbivores.
- Conifers produce monoterpenes that protect against many insect herbivores.
Practical activity: to investigate the effect of IAA on the growth of plant tissue.
- Cress seeds
- Petri dishes
- Cotton wool
- Filter paper
- Acetates with printed grid
- IAA stock solution (100 mg/L)
- Tissue paper
- Lab coat, gloves and safety goggles
- Set up 8 Petri dishes labelled A-H.
- Make up a stock IAA solution (100mg/L) by first dissolving IAA in 2-3 ml of ethanol and then making up to 1 L using distilled water.
- Add 10 ml of the stock IAA to dish A.
- Take 1 ml from A and place in dish B.
- Add 9 ml distilled water to dish B and mix.
- Using a new pipette take 1 ml from dish B and place in dish C.
- Add 9 ml distilled water to dish C and mix.
- Repeat this procedure for dishes D to G.
- Place 9 ml distilled water dish H (control).
- In the lids of each dish place an acetate grid along with 5 radish seeds in a straight line.
- Place a filler paper on top of each set of seeds in the lid of each dish.
- Place cotton wool on top of each filter paper.
- Pour the contents of each dish into the cotton wool in the lid of each dish.
- Close each dish and tape shut.
- Attach all dishes together with tape making sure the line of seeds in each dish are in the same orientation.
- Place the dishes in an incubator for approximately 2 weeks.
- At the end of 2 weeks, observe results (see photo below) and carefully remove each seedling and measure the lengths of each root and each shoot from each seed.
The following results tables should be filled in: (NOTE: The first row in each table is Dish H (control), the second row is Dish G and so on until the last row which corresponds to Dish A (102 mg/L).
Note in the dishes with high concentrations of IAA, there should be little to no growth. Therefore, there will probably be zeros in the last row of each table (dish A)
To calculate percentage stimulation or inhibition, use the following formula:
Plot and draw a graph of percentage stimulation or inhibition (on the vertical axis and IAA (auxin) concentration on the horizontal axis – your graph should look something like the one below (taken from Leaving Certificate Higher Level Biology Examination Q14 (b), 2005).