Plant Responses Flashcards
What are the three types of plant chemical defences?
• Tannins
• Alkaloids
• Pheromones
What are tannins and what do they do?
• Part of a group of compounds called Phenols produced by plants
• Found in the upper epidermis of the leaf
• Have a very bitter taste
• Toxic to microorganisms and insects
• Found in tea and red wine
• Bind to digestive enzymes produced in saliva and inactivate them
• In some herbivores, they can bind to proteins in the gut, making the plant hard to digest
What are alkaloids and what do they do?
• Large group of compounds derived from amino acids
• Very bitter taste, noxious smells or poisonous characteristics
• E.g. nicotine
• Many act as drugs which affect the metabolism of the animals that ingest it, sometimes poisoning them
• Found in growing tips and flowers, and peripheral cell layers of stems and roots
Give some examples of alkaloids
• Caffeine (toxic to insects and fungi)
• Nicotine (released when herbivores graze)
• Atropine (toxin produced by deadly nightshade)
• Capsaicinoids (make chilli peppers hot)
What are pheromones and what do they do?
• Chemicals released by one individual which can affect the behaviour and physiology of another member of the same species
• Capable. Of acting like hormones outside of the body of the secreting individual
• E.g. maple trees release a pheromone when attacked by insects, absorbed by leaves on other branches, causing them to make chemicals to help protect them when attacked
Give a brief summary of chemical plant defences
• Alkaloids - make plants taste bitter
• Pheromones - chemical signal transmitter between individuals of the same species
• Tannins - toxic to microorganisms/herbivores, make the leaf taste unpleasant
What is a mastic response?
A non-directional response to stimuli (e.g. thigmonasty - plat responding to touch by, for example, folding its leaves)
What is a tropism?
A directional growth response in which the direction of the response is determined by the direction of the external stimulus (e.g. light or gravity) - may be positive or negative.
Give examples of plant tropisms
• Phototropism - light
• Geotropism - gravity
• Chemotropism - chemicals
• Thigmotropism - touch
• Hydrotropism - water
What are the two important plant hormones involved in growth?
Auxins and gibberellins
How do hormones control leaf loss (abscission)?
• Auxins inhibit leaf loss - auxins are produced by young leaves, but less is produced by old leaves
• Ethene stimulates leaf loss - ethene is produced by ageing leaves
• A layer of cells (abscission layer) develops at the bottom of the leaf stalk, which separates the leaf from the rest of the plant
• Ethene stimulates the cells in the abscission layer to expand, breaking the cells walls and causing the leaf to fall off
How do hormones cause stomata closure?
• ABA binds to receptors on the guard cell membranes, causing specific ion channels to open
• Calcium ions enter the cytosol from the vacuole, creating an increased concentration of calcium ions in the cytosol, causing other ion channels (e.g. K+) to open
• These ion channels allow ions to leave the guard cells, raising the water potential of the cells
• Water then leaves the guard cells by osmosis, causing the guard cells to become flaccid and close the stomata
What are auxins and what do they do?
• A group of hormones produced by the tip of a shoot or root
• In the shoots, auxin stimulates cell elongation
• In the roots, auxin inhibits cell elongation
• Also inhibit the growth of lateral buds
Explain how IAA (an auxin) causes phototropism
• IAA (an auxin) increases the flexibility of the cell wall to promote plant growth via cell elongation
• Auxin activates a proton pump in the plasma membrane, causing the secretion of H+ ions into the cell wall
• The resultant decrease in pH of the cellulose cell wall is the optimum pH for the enzyme ‘expanase’
• Expanase breaks the bonds in the cellulose fibres, making the cell wall more flexible
• The cell can now take in more water and become turgid, stretching/elongating the cell
• IAA moves towards the shaded side for phototropism to occur
• IAA sinks to the bottom of a horizontal shoot/root, causing gravitropism
What is apical dominance?
Inhibition of lateral bud growth due to hormones produced by the apical bud.
