Plant responses 5.1.5 Flashcards
types of plant responses
- response to abiotic stress
- herbivory eg. chemical defences
- folding in response to touch
role of auxin
- apical dominance
- controls cell elongation
- prevents leaf fall (abscission)
- stimulate release of ethene
- fruit ripening
- trophisms
role of giberellin
- stem elongation
- trigger mobilisation of food stores in seed
- stimulate pollen tube growth in fertilisation
role of ethene
- fruit ripening
- promotes abscission in deciduous trees
role of ABA (abscisic acid)
- maintains dormancy of seeds & buds
- stimulates cold protective responses (antifreeze production)
- stimulates stomatal closing
seed germination
- seed absorbs water
- embryo is activated and starts producing giberellins
- these stimulate the production of amylases and proteases by switching on the gene that codes for them
- these enzymes break down the food stores in the seed, producing ATP so it can grow & break through the seed coat
- ABA acts as an antagonist to giberellin
- relative levels of giberellin & ABA determine if a seed germinates or not
the experimental evidence for the role of gibberellin in the control of seed germination
+ mutant varieties of seed bred that lack gene to make giberellins so don’t germinate. If giberellins are applied externally they germinate normally
+ if you apply giberellin biosynthesis inhibitors they don’t germinate as they cannot make the giberellins needed to break dormancy. The seeds germinate if inhibitor is removed or giberellins are applied externally.
where is auxin produced?
tip of roots and shoots and in meritstems
leaf loss/abscission in deciduous plants
- caused by lengthening of the dark period
- levels of auxin fall as less light
- leaves produce ethene, which initiates gene switching on in the cells next to the abscission zone near the stalk of a leaf
- enzymes produced with digest and weaken the cell walls in the outer layer of the abscission zone (separation layer)
- vascular bundles are sealed off
- fatty material is deposited in the cells on the stem side of the separation layer. This forms a scar when the leaf falls, preventing entry of pathogens.
- cells deep in sep zone retain water and swell, putting more strain on area.
- high winds or low temps finish it off and leaf falls.
stomatal closure
- leaf cells release ABA under abiotic stress which closes stomata
- roots also produce ABA when soil water levels are low. ABA is transported to leaves where it binds to receptors of csm of guard cells, changing their ionic concentration, reducing water potential and therefore turgor of the cells. stomata close and water loss is greatly reduced.
the experimental evidence for the role of auxins in the control of apical dominance
- if you cut off the apex of a stem then the auxin producing cells are removed and the lateral shoots grow faster. If auxin is applied artificially to the cut shoot, apical dominance is reasserted and lateral shoot growth is supressed.
the experimental evidence for the role of gibberellin in the control of seed germination
+ mutant varieties of seed bred that lack gene to make gibberellins so don’t germinate. If gibberellins are applied externally they germinate normally
+ if you apply gibberellin biosynthesis inhibitors they don’t germinate as they cannot make the gibberellins needed to break dormancy. The seeds germinate if inhibitor is removed or gibberellins are applied externally.
the experimental evidence for the role of gibberellin in the control of stem elongation in internodes
- plants that have short stems produce few or no gibberellins.
- you can breed dwarf plants where the gibberellin synthesis pathways is disrupted
how does auxin control root growth?
low conc of auxin promotes root growth. up to a given conc the more auxin the more growth of roots, but high auxin conc. inhibits root growth.
auxin is produced by root tips and some also reaches roots in low conc. from the shoots. If the tip of the shoot is removed, root growth slows down a lot. artificially placing auxin on the cut shoot restores root growth.
what are internodes
the regions between leaves on a stem
