Plant Responses Flashcards
Ethene role in plants
Fruit ripening
Promotes abscission in deciduous trees
Auxins
Cell elongation
Prevent abscission
Stimulate release of ethene
Abscisic acid (ABA)
Maintains dormancy of seeds and buds
Stimulate cold protective responses
Gibberellin
Stem elongation
Trigger mobilisation of food stores in a seed at fertilisation
Seed germination
1 A seed absorbs water
2 Embryo is activated and produces gibberellins
3 They stimulate the produces tion of enzymes that break down the food stores found in the seed
4 The embryo uses these food stores to produce ATP for the building of materials so it grow and break out of the seed coat
5 There is evidence that suggests gibberellins switch on genes that code for atlases and proteases
6 Also evidence that ABA interferes with gibberellins
Experimental evidence for the role of gibberellins in seed germination
1 Mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins. These seeds don’t germinate. If gibberellins are applied to the seeds externally, the seeds germinate normally.
2 If gibberellin biosynthesis inhibitors are applied to seeds , they cannot germinate as they cannot produce gibberellins needed to break dormancy. If inhibition is removed, the seeds germinate.
How were gibberellins discovered?
Because they are produced by a fungus from the genus Gibberella that affects rice. The infected seedlings grew extremely tall and thin. Scientists investigated the rice and isolated the gibberellins which produce the same spindly growth in plants. It was then worked out that the plants themselves produce the same compounds. Plants with short stems produce very little gibberellins.
What is an example of an auxin?
IAA
High concentrations of auxins…
Reduce growth in lateral shoots which results in apical dominance.
Growth in main shoot is stimulated by auxin produced at the tips, so grow quickly.
Lateral shoots are inhibited by the hormone that moves back down the stem, so they don’t grow very well.
Further down the stem, the concentration of auxin decreases and so the literal shoots grow more.
Low concentrations of auxins…
Promote root growth
The more auxin in the roots, the more they grow
If the apical shoot is removed, the amount of auxin reaching the roots is reduced and root growth is slowed and stops.
What is the role of auxins?
Stimulate growth of main apical shoot.
How does auxins affect the plasticity of cell walls?
Auxin means the cell walls stretch more easily.
Auxin bind to specific receptor sites in the plant cell membranes.
Causes a fall in pH.
As cells mature, auxin is destroyed. So pH rises and enzymes maintaining plasticity become inactive.
So walls become rigid, cells can no longer expand and grow.
Physical defences of plants to herbivory
Thorns
Hairy leaves
Inedible tissue
Chemical defences of plants to herbivory
Tannins
Alkaloids
Terpenoids
Tannins
Make up 50% of dry weight of leaves
Bitter taste
Toxic to insects
Alkaloids
Bitter tasting nitrogeneous compounds
Acts as drugs, affecting the metabolism of animals that eat them
Terpenoids
Act as toxins to insects and fungi
What is a pheromone?
Chemical made by an organism which affects the social behaviour of other members of the same species
Give evidence that plants communicate by pheromones
Communicate by chemicals produced in the roots systems to tell neighbouring plants if it is under water stress
