5.5 - Plant Responses Flashcards
Explain why plants need to be able to respond to their environment
- To cope with changing conditions
- Avoid abiotic stress
- To maximise photosynthesis
- To obtain more light / water / minerals
- Avoid herbivory
- To ensure germination in suitable conditions
Give examples of abiotic stresses
- Change of day length
- Temperature
- Water levels
- Wind
- pH
Summaries the physiological adaptations plants show to cope with abiotic stress
- Thick waxy cuticles
- Fine hairs on leaves
- Sunken stomata
- Wilting in hot, dry conditions
Name the type of plant that keeps their leaves all year round
Coniferous
Name the type of plant that lose their leaves in winter
Deciduous
Explain why deciduous trees lose their leaves in winter
- Rate of photosynthesis decreases as day length reduces and temperatures fall
- Amount of glucose produced by photosynthesis decreases
- Amount of glucose needed increases
- Needed for respiration to maintain leaves through winter and produce chemicals to
prevent damage from freezing - More efficient to lose leaves and become dormant until days lengthen and temperatures
increase in spring
What is photoperiodism?
Response of plant to lack of light (length of period of darkness)
What are phytochromes?
- Pigments in leaves
- Detect light levels
Describe the different types of phytochromes
Pr (inactive form)
Pfr (active form)
Pr abundant in darkness, Pfr abundant in light.
Ratio of Pr:Pfr enables plant to detect how long days are.
Give examples of plant responses affected by length of darkness
- Dormancy of leaf bud
- Timing of flowering
- Tuber formation in preparation for winter
Define abscission
- Leaf fall
- Occurs in deciduous trees in autumn
Which plant hormones are involved in abscission?
- Auxin
- Ethene
Describe the process of abscission in deciduous trees
- Triggered by falling light levels
- Decreased concentration of auxin
- Leaves produce ethene
- Initiates gene switching in abscission zone at base of leaf stalk
- Gene switching causes production of new enzymes
- Enzymes weaken cell walls in outer layer of abscission zone (‘separation layer’)
- Vascular bundles sealed off on stem side of separation layer
- Layer forms protective scar when leaf falls preventing pathogen entry
- Cells in separation zone retain water and swell
- Puts more strain on outer layer
- Further abiotic factors finish process (e.g. strong wind)
- Strain too much and leaf separates from plant leaving waterproof scar
Explain how plant hormones protect plant cells in freezing conditions
- Hormones (e.g. abscisic acid) trigger gene switching
- Plants produce more sugars and proteins
- Lower the freezing point of the cytoplasm
- Protect cells against damage by ice crystals if they do freeze
How else can plants protect themselves from freezing?
- Water in intercellular spaces freezes
- Energy released raises temperature of cells
- Solute concentration in cytoplasm and vacuoles maintains lower freezing point
How do stomata respond to abiotic stress?
They close
Name the hormone responsible for the closure of stomata
Abscisic acid (ABA)
Explain how ABA causes stomatal closure
- ABA produced by roots
- In response to low water levels
- ABA transported to leaves
- Binds to receptors on plasma membrane of guard cells
- Causes ions to diffuse out guard cell
- Water follows by osmosis
- Guard cells become less turgid
- Change in shape leads to closure of stomata
Define herbivory
Process by which herbivores eat plants
Outline the physical defences some plants have against herbivory
- Thorns
- Spiny leaves
- Fibrous, inedible tissue
- Stings
- Hairy leaves
Summarise the various chemical defences plants have against herbivory
- Tannins
- Have bitter taste
- Toxic to insects
- Alkaloids
- Affect metabolism of animals, acting as poison
- Prevent germination in plants
- Terpenoids
- Act as toxin to insects and fungi
How can plant chemical defences be used by humans?
- Tannins
- Flavour tea and red wine
- Alkaloids
- Used in coffee (caffeine)
- Contained in cigarettes (nicotine)
- Terpenoids
- Insect repellent
Define pheromone
- Chemical made by an organism
- Affects social behaviour of other members of same species
Why is it necessary for pheromones to be volatile?
Must travel through the air to carry message