1. Chapter 26: Response in the flowering plant
Leaving Certificate Biology
Higher Level

2. Growth Regulation Growth of plants is controlled by growth regulators
Growth regulator levels in the plant are controlled by tropisms
A tropism is the response of a plant to a change in its external environment or to a specific stimulus

3. Phototropism Geotropism
Phototropism is the growth of a plant in response to light
Example is when plants bend towards a window
Geotropism
Geotropism is the growth of a plant in response to gravity
Example: as soon as young root emerges from seed it grows towards gravity regardless of what orientation the seed was in

4. Hydrotropism Chemotropism Thigmotropism
Hydrotropism is the growth of a plant in response to water
Example: roots grow towards water
Chemotropism
Chemotropism is the growth of a plant in response to chemicals
Example: roots grow towards minerals and away from heavy metals
Thigmotropism
Thigmotropism is the growth of a plant in response to touch
Example: ivy growing up a wall or around a tree

5. Growth Regulators
A growth regulator is any chemical that controls the growth of a plant
Growth regulators are produced in extremely small amounts usually in the meristematic areas of a plant
Growth regulators are thought to be transported by vascular tissues to other areas of the plant but can also diffuse through ground tissue

6. Growth Regulators
The effects of growth regulators depend on various factors:
The area where it is located
The concentration (high concentrations can have opposite effects to very low concentrations)
Growth regulators can cause growth or inhibit growth depending on interactions with other growth regulators

7. Examples of Growth Regulators
Growth promoters:
Auxins (indole acetic acid [IAA] – cause stem and root growth
Growth inhibitors:
Ethene (ethylene) – causes fruit to ripen
Abscisic acid – helps plants deal with harmful conditions:
causes stomata to close in very dry conditions despite presence of light)
Inhibits germination until ideal conditions are present

8. Phototropism

9. Anatomical & Chemical Protection
Bark/dermal tissue/cuticle: functions in protecting plant
Stomata and guard cells: control water loss
Chemical:
Heat shock proteins: produced during times of stress to protect the enzymes within plant cells
Phytoalexins: produced when plant is infected with microorganisms and help kill microorganisms, prevent further spread of the invader, and warn nearby plant cells

10. Mandatory Experiment: to Investigate Effect of IAA on Plant Tissue
Leaving Certificate Biology
Higher Level

11. Apparatus: Graduated cylinder Cotton wool Weighing scales Filter paper
Beakers
Masking tape
Stirrers
Ruler
Spatula
IAA( Indole Acetic Acid)
Petri dishes
Ethanol
Pipettes
Cress seeds/Mustard seeds/Radish seeds
Deionised water

12. Method: Dissolve 100 mg IAA in 2 ml ethanol
Transfer to 1 L graduated cylinder and top up to 1 L with deionised water to give 100 mg/L IAA stock solution
Transfer 10 ml IAA stock to first petri dish labelled A
Transfer 1 ml of this solution in A to B and top up with 9 ml deionised water
Mix B and transfer 1 ml of B to C
Repeat procedure until dish G
Dish H is control with 9 ml deionised water

13. Method (continued)
Place 5 cress seeds in line in the lid of each petri dish
Cover seeds carefully with filter paper
Transfer the serially-diluted solutions to each lid
Place layer of cotton wool into each lid and place base on
Seal each dish with masking tape and stand on sides in incubator set at 25 ˚C

14. Results After 2 – 3 days remove petri dishes and seedlings
Using ruler measure length of root and shoot for each concentration of IAA and record in table
Calculate % increase or % decrease for each concentration

15. Conclusion
IAA stimulates growth of roots and shoots up to a certain concentration
High concentrations inhibit growth