chapt 28-external factors and plant growth

What is a tropism, and what some important tropisms in plants?

• Growth or movement towards (+) or away from (-) a stimulus

  • Gravity: gravitropism

    • Amyloplasts called statoliths in cells called statocytes serves as gravity sensors in both shoot and root

  • Touch: thigmotropism

    • Ex: circumnutation

  • Water: hydrotropism

  • Chemical: chemotropism

• Motion in response to external stimuli

  • Phototropism: follow light

    • Stimulus: phototropins more active where light is showing

    • Signal transduction: 

    • Response 

  • Phototaxis: directional movement or behavior of an organism in response to specific external stimulus

  • Photonasty: non-directional movement of plants triggered by stimuli

What plant processes show circadian rhythms, and what controls the circadian clock?

• Photosynthetic activity

• Stomatal opening

• Protein and hormone synthesis

• Cell elongation and division

• Fragrance emission

• Seed germination

• What controls it:

  • The clock is the drummer in the circadian rhythm section

  • It needs to be entrained to the 24hr cycle of the day

How does the response of flowering time to day length and cold treatment vary among plants?

• Photoperiodic: processes are affected by daylength, which is especially important in the temperate zone

• Short day plants: require the daylight to be below a critical duration to flower

• Long day plants require the daylength to be above a critical duration of flower

What conditions are needed for shoot buds to enter and break dormancy?

enter dormancy:

• shorter daylight

• cooler temp

• hormone changes

• nutrient storage

break dormancy:

• cold stratification

• increase daylight

• warmer temp

• hormonal shifts

• enough moisture/water

What is phytochrome, how does it work, and what processes are affected by it?

• Phytochromes [photoreceptor protein] the photoperiod sensor exists in two forms

• In sunlight, it rapidly achieves equilibrium between the Pf and Pfr forms, then slowly reverts to Pr in the dark

• Day length can be sensed by the ratio of the two

• Short burst of light can either cause response (red light) or stop it (far left light)

• Process phytochrome involved in?

  • Shade-avoidance reaction

  • Far red for susceptible of being filtered

• controls seed germination, stem elongation, flowering

• when exposed to red light, phytochrome switched from Pr to Pfr that triggers physiological changes

• when exposed to far-red light, reverses process converting Pfr back to Pr

What is dormancy, and what environmental cues may be necessary to break the dormancy of seeds and buds?

cold stratification or warmth

photoperiod (changes in day length) and light exposure

moisture is sufficient

oxygen is sufficient

scarification

chemical signals

How do nastic movements differ from tropisms, and what are some important examples in plants?

• Movement in response to a non-directional stimulus

  • Photonasty–light

    • ex: flowers opening up during the day

  • Thermonasty– temperature

  • thigmonasty–touch

    • venus fly trap triggered by fly

Which form of phytochrome would you hypothesize that a short-day plant requires to be below a critical threshold in order to flower?

Pfr- far red light

this would result in a lower threshold for the flower to be ready to flower (compared to Pr that has a higher threshold-more ideal for longer days)