Chapter 26 Terms and Ideas Flashcards
Dormant
development of the embryo is stopped.
Seeds maintain dormancy by:
- Exclusion of water or oxygen by an impermeable seed coat
- Mechanical restraint of the embryo by a tough seed coat
- Chemical inhibition of germination
Advantages of dormancy:
- Ensures survival during unfavorable conditions
- Results in germination when conditions are most favorable
- Helps seeds survive long-distance dispersal, allowing plants to colonize new territory.
Germination
seeds begin to grow or output
Imbibition
First step of germination: seeds take up water if seed coat is permeable (e.g., a germination inhibitor might be washed away by rain).
Steps 2, 3 and 4 of germination
- Enzymes are activated, cellular respiration increases, and other metabolic pathways start up.
- Embryo grows using food stored in the cotyledons or endosperm.
- Germination is completed when the radicle (embryonic root) emerges.
The plant is then called a seedling.
Monocots
growing shoot is protected by a sheath of cells called a coleoptile
Eudicots
growing shoot is protected by cotyledons
Hormones
chemical signals that act at very low concentrations at sites often far from where they are produced.
Photoreceptors
pigments associated with proteins that absorb light.
Two key plant hormones
Gibberellins and Auxin
Gibberellins
- Stem elongation
- Fruit growth
- Seed germination—trigger hydrolysis of stored food molecules
Auxin
allows for the plant to bend towards the light
Polar transport
- Auxin enters cell by passive diffusion.
- Proton pumps move H+ out of cell; sets up a chemiosmotic gradient to drive transport.
- Auxin (indole-3-acetic acid) is ionized in higher pH of the cytosol.
- Ionized auxin exits via auxin anion efflux carriers that are concentrated at the basal end of each cell.
Negative gravitropism
upward gravitropic response of shoots
Positive gravitropism
downward gravitropic response of roots
Abscission
removal of old leaves from the stem
Acid growth hypothesis
protons are pumped into cell wall and activate expansins, which catalyze changes that loosen the cell wall (allows for expansion)
Excessively tall plants vs. Dwarf plants
Excessively tall plants always have the hormone response turned on while dwarf plants always have it off
Repressor for excessively tall plants is never attached to the complex while the repressor for dwarf plants is always attached
Ethylene gas
promotes leaf abscission and senescence; speeds ripening of fruit
Ethylene also causes an increase in its own production. Once fruit ripening begins, more and more ethylene forms
Apical hook of eudicot seedlings
maintained by asymmetrical production of ethylene, which inhibits elongation of cells on the inner surface
Cytokinins
- Induce proliferation of cultured plant cells
- In cell cultures, high cytokinin-to-auxin ratio promotes formation of shoots; low ratio promotes formation of roots.
- Cause some light-requiring seeds to germinate even if kept in darkness.
- Usually inhibit elongation of stems, but also cause lateral swelling of stems and roots (e.g., radishes).
- Stimulate axillary buds to grow into branches; auxin-to-cytokinin ratio controls extent of branching
- Cytokinins delay senescence of leaves
Brassinosteroids
- Enhance cell elongation and cell division in shoots
- Promote xylem differentiation
- Promote growth of pollen tubes
- Promote seed germination
- Promote apical dominance and leaf senescence
Abscisic Acid
- Prevents seed germination when seeds are still on parent plant
- Promotes seed dormancy; inhibits initiation of germination events
- Mediates responses to environmental stresses and pathogens (e.g., closure of stomata to prevent water loss)
