Module 5.5 - Plant And Animal Responses - Plants Flashcards
Cytokinins
Promote cell division
Delay leaf senescence (loss of chlorophyll)
Overcome apical dominance
Promote cell expansion
Abscisic acid
Inhibits seed germination and growth
Cause stomata to close in low water availability
Inhibit bud growth
Auxins
Promote cell elongation
Inhibit growth of side shoots
Inhibit leaf abscission
Gibberellins
Promote seed germination
Growth of stems
Ethene
Causes leaf abscission
Causes fruit ripening
Why do plants need to respond to their environment?
Avoid abiotic stress
Maximise photosynthesis by obtaining more light/water/minerals
Avoid herbivory/grazing
Ensure germination occurs in suitable conditions
How do plants avoid abiotic stress?
Higher temperatures - thicker waxy layer
Very windy - more lignification
Drought - root growth slows, stomata close (abscisic acid)
How can plants maximise photosynthesis?
Tropisms Phototropism Geotropism Hydrotropism Chemotropism Thigmotropism
How can plants avoid herbivory/grazing?
Thigmonasty - folding in response to touch Chemical defences (e.g. tannins - make leaves taste bad and defend roots against pathogens; alkaloids - make tips of roots and shoots and flowers taste bitter; pheromones - can be produced when one leaf is eaten to communicate with other leaves to produce chemical defences)
Process of plant hormones action
Made in many plant tissues and act on very wide range of target tissues
Move in xylem or phloem tissue by mass flow up and down the plant
Move by diffusion or active transport from cell to cell
Bind to complementary receptors on plasma membranes
Binding causes series of enzyme controlled reactions (sometimes causing genes to be switched on/off) to bring about the response
Commercial uses of auxins
Plant cuttings dipped in rooting powder containing low concentrations of auxin to promote root growth
Weed killer - promotes rapid shoot growth - plant can’t support itself so falls and dies
Help make seedless fruits
Commercial uses of cytokinins
Prevent yellowing of lettuce leaves (senescence)
Added in tissue culture to promote many side shoots - can be grown into new plants - higher production of new plants
Commercial uses of gibberellins
Elongate internodal cells in grape stalks - grapes spread out and get bigger
Elongation of internodal cells in sugar cane - more sugar
Beer production needs malt - gibberellins added to encourage barley seed to make amylase so starch is broken down into maltose - germination then stopped by drying
Commercial uses of ethene
Speed up ripening
Promote fruit abscission
Cold conditions, little oxygen and high carbon dioxide prevents ethene synthesis so can prevent fruit ripening during shipping
Experimental evidence for the role of auxins in the control of apical dominance
Tip cut off plant caused lateral growth, implies auxins are found in tip and cause apical dominance
To prove that another hormone wasn’t made which promotes lateral growth, auxin transport inhibitors applied below the tip to stop auxins moving down the stem
Side shoots still grew even though tip wasn’t removed
Suggests that auxins cause apical dominance
Experimental evidence for the role of gibberellins in the control of stem elongation
If GA is applied to a normally very short plant, the internodal regions of the stem grew, suggesting that gibberellins cause stem elongation
In naturally occurring plants, there are higher levels of GA found in tall plants than in dwarf plants of the same species, suggesting that gibberellins cause stem elongation
Experimental evidence for the role of gibberellins in the control of seed germination
Application of gibberellins to a seed can cause early germination
Mutants lacking gibberellin can’t germinate on their own but can be induced to germinate by by applying gibberellins
Abscisic acid inhibits gibberellins - if abscisic acid is added during fruit ripening, seeds inside fruit will begin germinating during ripening
Auxin mechanism for elongation
Auxins are produced at apex of shoot and diffuse down shoot to zone of elongation and bind to receptors on cells
Causes H+ ions to be actively transported into cell wall
Low pH causes wall loosening enzymes to work by breaking bonds with cellulose, making walls more flexible
Water enters cell and flexible wall allows cell to elongate
Auxin mechanism for phototropism
Auxins produced at apex of shoot
If more light is coming from one side phototropin enzymes are activated more on this side
Phototropins cause PIN proteins to transport more auxins to shaded side
Auxins bind to receptors on cells on shaded side, causing H+ ions to be actively transported into cell walls
Low pH causes wall loosening enzymes to work by breaking bonds with cellulose, making walls more flexible
Water enters cells and flexible walls allow cells to elongate
Cells on shaded side of shoot elongate more quickly, causing shoot to bend towards light
What did Darwin’s work on phototropisms show?
Tip was responsible for phototropism
What did Boysen-Jensen’s work show about phototropisms?
Shows a substance responsible doe phototropism (auxins) must pass from tip down to cause response Used gelatin (permeable) and mica (impermeable) blocks to separate tip from rest of shoot
Auxin mechanism for geotropism
Auxins produced at apex of shoot
If roots are lying flat (e.g. seed has been deposited on side), auxin collects on lower side
Auxin inhibits cell elongation in roots so lower side cells do not elongate but upper side cells do so root bends downwards
Auxin promotes elongation in shoots so lower side elongates faster and shoots bend up
