-
Plants
and Environment
- Hormones are used by plants to control
- Growth
- Development
- Responses to environmental stimuli
- Plants have adapted to many aspects of
- their environment
- Physical, or abiotic,part
- of environment such as light, temp, nutrients
- Biotic such as predators, pollinators
-
Hormones
- Hormones are found in all multicellular
- organisms
- Chemical “coordinators” of organisms
- Required in only minute amounts
- Produced in one part of the organism and
- are then transported to another part where they trigger a response
-
Plant Hormones
- Auxin
- Cytokinins
- Gibberellins
- Abscisic Acid
- Ethylene
- Florigens
-
Auxin
- Controls growth and response to light
- Promotes or inhibits cell elongation
- Stimulates cell differentiation in xylem
- and phloem of developing plants
- Stimulates maturation of fruits
- Influences timing of germination
- Auxin is produced in the apical meristem
-
Phototropism
- Growth toward light
- Controlled by auxin
- production in shoot tip
- Increase in auxin
- causes cells in the stem to elongate
- If stem is vertical, auxin is
- evenly distributed and cell growth is equal
-
Phototropism and Gravitropism
- Work together to encourage vertical plant
- growth
- Auxin causes roots to grow toward gravity
- because at very high concentrations it inhibits cell elongation
- Root cells contain specialized plastid
- cells with starch grains, called
statoliths - As statoliths fall, auxin accumulates
- Cell growth is inhibited on that side
-
Cytokinins
- Stimulates cell division, budding
- Produced in actively growing tissues
- Roots, embryos, fruits
- Cytokinins travel through plant via xylem
- It is the ratio of cytokinins
- to
auxin that- determines cell division and differentiation
- Ratio helps a
- plant maintain “weight” at the level of soil
-
Apical dominance occurs when high auxin- suppresses the formation of branches
- Removing the apical meristem results in
- more branching
- High cytokinins in roots inhibits root growth
- Cytokinins prevent cell death
-
Gibberellins
- Produced by the embryo and in young
- plants’ roots and leaves
- Activates the genes for enzymes that
- break down the starch in the seed
- Sugars are used by embryo for energy
- Promotes growth of leaves and stems
- Stimulates growth of flower stalk
- Encourages fruit growth (fig 45-10)
-
Abscisic Acid
- Slows/suspends plant growth
- Prepares plant for winter
- Promotes dormancy of seeds (i.e. over the
- winter)
- Helps plant cope with stress
- Water loss triggers abscisic
- acid, which triggers the closure of stomata
-
Ethylene
- Senescence- irreversible changes leading
- death
- Can happen at the cell, structure, or
- plant level
- Leaves dropping in fall, break down of
- chlorophyll
- Flowers dying
- Fruit ripening
- Cell walls break down making fruit
- “softer”
-
Florigens
- Leaves produce signaling hormones to
- indicate lengthening day
- A single leaf can detect changing light
- and induce flowering on other areas of the plant
- The actual molecule responsible is still
- unknown
- Scientists refer to these hypothetical
- flowering hormones as
florigens
-
Biological Clocks and Rhythms
- Circadian rhythm
- Many plant functions change throughout
- the day
- Depends on environmental conditions
- Ex: opening/closing of stomata
- Photoperiodism
- Increasing day length signals flowering
- Varies by plant species
- Plants can be:
- Day-neutral, long-day, short-day
-
Photoperiod
- It is night length that is important
- Interrupting night will prevent flowering
- of short day plants, even if the day length is appropriate
-
Phytochromes
- Pigments in plants that detect varying
- wavelengths of red light
- Ratio of red (666nm) to far red (730nm)
- wavelengths informs plant about light availability
- Example:
- Sunlight has a R:FR ratio of 1.2
- Light under a canopy of leaves has a R:FR ratio
- of
0.13 - Light under 5 mm of soil has a R:FR ratio of
- 0.88
- Cue hormones that control daylight
- dependent reactions
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