Plant Responses & Photoperiodism

Tropism

directional growth response to an external stimulus and it may be positive or negative

thigmotropism

Growth response of plants towards or away from a diectional touch

Adaptive Advantgae:

• plants stabilised with support to grow upwards, increasing chance of more light maximise photosynthesis more energy for growth

• increase chance of survival and reproducing sucessfully

Hydrotropism

Growth response of plants towards or away form a directional water source

• roots find sufficent water for cell processes (photosynthesis) produce more energy for growth

• increase chance of survival and reproducing sucessfully

chemotropism

Growth response of plants towards or away from a directional chemical concentration gradient

• roots grow towards mineral source in soul, for cell processes (growth and reproduction)

• increase chance of survival and reproducing sucessfully

phototropism

Growth response of plants towards or away from a directional light source

• Plants to grow towards light source maximise light for photosynthesis, increasing energy for growth

• increase chance of survival and reproducing sucessfully

Geotropism (gravitropism)

growth response of plants towards or away from gravity

• positive geotropism: allows radicle grow downwards, increasing chance of finding water for cell processes like photosynthesis produce more energy for growth

• increase chance of survival and reproducing sucessfully

• Negative gravitropism: allows plumule grows up, reach light maximise photosynthesis produce more energy for growth

• increase chance of survival and reproducing sucessfully

What is a Nastic Response?

• Non-directional movement in response to a non-directional stimulus.

• Rapid and reversible response to stimulus intensity.

Thigmonasty & Adaptive Advantage [AA]

• Response to touch/contact.

• [AA]: Protection & avoids grazing. Maximises energy gain (energy used for growth/reproduction, not repair). Increases survival to maturity and reproductive success.

Thermonasty & Adaptive Advantage [AA]

• Response to temperature (flowers open when temp increases, close when it drops).

• [AA]: Flowers open only when correct pollinators are active during the day. Leads to successful pollination, cross-pollination, and increased Genetic Variation improving survival if the environment changes.

Photonasty & Adaptive Advantage [AA]

• Response to light intensity (flowers open during the day, close at night).

• [AA]: Opens when diurnal pollinators are active and conserves energy/protects pollen when pollinators are absent at night.

How does folding leaves/flowers increase fitness? [AA]

• Prevents leaf tissue loss and photosynthetic damage.

• Maximises net energy gain: Energy is diverted from repair towards growth & reproduction.

• Ensures survival to maturity and maximises offspring (adaptive advantage).

Auxin Mechanism & Cell Elongation (Basics)

• Produced in shoot & root tips (apical meristems); transported to zone of elongation.

• High auxin in shoots: Promotes cell elongation.

• High auxin in roots: Inhibits cell elongation (cells don't elongate).

Positive Phototropism Mechanism (Shoot)

• Light detected by shoot tip.

• apical meristem cells transport more auxin to the zone of elongation on the shaded side of the stem

• High auxin concentration causes shaded side cells to elongate more than light side.

• Stem bends towards light.

Positive Geotropism Mechanism (Root)

• Gravity detected by root tip cells containing amyloplasts (starch-filled organelles).

• Amyloplasts sink to lower side due to gravity.

• Triggers auxin transport to lower side.

• High auxin concentration causes lower side cells to elongate less (inhibited).

• Root bends downwards.

Turgor Pressure & Nastic Movement Mechanism

• Stimulus causes cells to pump K+ ions out of the cell.

• Water concentration becomes higher inside, so water leaves the cell via osmosis.

• Cell loses turgor pressure and becomes flaccid.

• Causes rapid movement or collapse of the plant part.

Rapid changes to turgor pressure causes nastic movements

Abscisic Acid (ABA) & Seed Dormancy

• Prevents premature seed germination by promoting storage protein accumulation (keeps seed nourished, inactive).

• [AA]: Seed germinates only in consistent warmth —> optimises enzyme rates and ATP production for growth/photosynthesis —> increases , survival, and reproductive success.

Abscisic Acid (ABA) & Bud Dormancy [AA]

• Stops growth at apical meristem by inducing stiff bud scales to protect from winter damage/drying out.

• [AA]: Delays sprouting until consistent warmth —> prevents frost damage and wasted energy —> increases growth, survival, and reproductive success.

Abscisic Acid (ABA) & Stomata Closing [AA]

• Closes stomata to prevent water loss via transpiration.

• [AA]: Avoids dehydration —> maintains water for photosynthesis during drought —> increases survival and reproductive success.

Photoperiodism & Phytochrome Basics

• Photoperiodism: Plant's response to changing day length (detected by phytochrome in leaves).

• Critical day length: Specific hours of light required above/below which flowering occurs.

• 2 Forms: Pr (absorbs red light —> converts to Pfr quickly) and pfr (absorbs far-red light —> converts to pr quickly).

Phytochrome Conversions (Day vs. Night)

• Daytime: Sunlight has more red light —> Pr quickly converts to Pfr (high Pfr at end of day).

• Nighttime: Pfr slowly converts back into Pr in dark.

• Key Condition: Uninterrupted darkness determines Pfr levels—a flash of light resets the night calculation.

Short-Day Plants (SDPs) Flowering Mechanism

• Flowers when night is longer than critical night length.

• Long night gives enough time for accumulated Pfr to convert back to Pr

• Results in low levels of Pfr at night's end —> removes inhibition —> plant flowers.

Long-Day Plants (LDPs) Flowering Mechanism

• Flowers when night is shorter than critical night length.

• Short night means not enough time for Pfr to convert back to Pr.

• Results in high levels of Pfr at night's end —> promotes flowering —> plant flowers.