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

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

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

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

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

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

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

Phototropism

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

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

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

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

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

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

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