Study Notes on External Growth Factors in Plants

Definition: The phototropic response refers to the bending of a plant towards light.
Historical Context: This concept was introduced in the previous chapter, particularly highlighted in Darwin's publication on phototropism.
Auxin's Role: The bending response is facilitated by auxins, plant hormones that promote cell elongation.

Gwen's Experiment:
- Utilized oak coleoptiles (the protective sheath covering a germinating seed) to demonstrate auxin diffusion.
- Placed oxin-impregnated agar blocks on coleoptiles, showing that auxin causes bending towards light.
Briggs' Experiment:
- Sought to illustrate lateral movement of auxin in response to light using mica glass splits.
- Results:
- Under dark and light conditions, coleoptiles exhibited equal concentrations of auxin on agar blocks.
- When the agar tip was exposed, auxin concentrated on the shaded side, supporting the directional growth towards light.
- Conclusion: Auxin travels transversely across the stem away from light, accumulating on the shaded side, influencing phototropic bending.
Discovery of Blue Light Response: Later research indicated that phototropic response is specifically triggered by blue light, within the wavelength range of 435-440 nm.

Definition: An action spectrum plots the effectiveness of different wavelengths of light in driving a biological process, such as phototropism.
Relevant Figure: Curvature per photon is plotted against wavelength, revealing absorption peaks.
Phototropin: A specific photoreceptor responsible for detecting blue light for phototropic responses.

Experiment Observations:
- In wild-type Arabidopsis, stem tips bend toward blue light.
- Mutated plants lacking phototropins do not exhibit bending, confirming the necessity of these receptors for blue light responses.

Blue light also regulates stomatal opening in plants, which typically opens during the day and closes at night.
Relative Effectiveness of Stomatal Aperture: Similar action spectrum as photo-responsive growth.
Photoreversible Response: Upon exposure to a blue light pulse followed by green light, stomata remain responsive to the blue light, showing that green light can reverse the blue light response.

Definition: Gravitropism is the response of plants to gravity.
Shoot Response: Negative gravitropism as shoots grow upward against gravity; roots exhibit positive gravitropism by growing downward.
Starch Statolith Hypothesis:
- Statoliths are gravity sensors hypothesized to be amyloplasts containing starch. They help perceive gravity direction.
Experimental Evidence: In experiments with Arabidopsis, mutants that lacked proper amyloplasts displayed abnormal growth patterns, illustrating the importance of statoliths in gravitropic response.

Auxin distribution stimulates growth inhibition on the lower side of roots when a root is placed on its side, directing growth downward.
Cytokinins are involved at the root tip, collaborating with auxins in the gravitropic response.

Definition: Thigmotropism is the growth response of plants to touch.
Observations: Plants like vines and tendrils exhibit differential growth when in contact with structures.
Memory of Response: Tendrils retain a memory of thigmotropic responses even when kept in the dark over time.
Mechanism: The response is believed to be due to ion fluxes, potentially involving opsins.

Definition: Gnastic movements occur in response to a stimulus, independent of stimulus direction.
Types:
- Nyctonastic Movements: Daily up-and-down movements (e.g., leaves of some plants close at night and open during the day).
- Thigmomastic Movements: Rapid closure of leaves upon mechanical stimulation (e.g., Mimosa pudica).
Examples:
- Mimosa leaves protect against predators by closing up when touched.
- Venus flytraps close in response to vibrations from captured insects due to touch.

Role of Pulvinus: The swollen base of leaves contains vascular tissue that changes turgor pressure, facilitating movements.
Ion Fluxes: Changes in ion concentration and water movement cause rapid closure in both mimosa and Venus flytrap.

Definition: Altered development in response to touch, resulting in morphological changes in plants.
Experimentation: Arabidopsis subjected to continuous touch exhibited stunted growth compared to controls, suggesting negative impacts from frequent mechanical stimulation.
Gene Expression: Touch activates gene expression of calmodulin-binding proteins involved in growth regulation, linking calcium as a secondary messenger.

Definition: The ability of certain plants to orient towards light during the day (e.g., sunflowers, lupines).

Definition: Daily rhythms characterized by the opening and closing of leaves (circadian rhythms).
Example: Wood sorrel shows a regular pattern of opening during the day and closing at night due to an internal biological clock.
Circadian Rhythms: Governed by endogenous genes, including "tick" and "tock" genes in plants, maintaining rhythmic patterns even in darkness.

Definition: The response to the duration of light and darkness, influencing flowering responses.
Types of Plants:
- Short Day Plants: Require less than a critical value of light to flower (e.g., chrysanthemums).
- Long Day Plants: Require more than a critical value of light for flowering (e.g., irises).
- Day Neutral Plants: Flower regardless of light duration.
Critical Values: Short day plants like chrysanthemums flower only under reduced light conditions, while long day plants flower under extended light.

Photoreceptors involved in detecting light wavelengths.
Types: Phytochrome Red (PR) absorbs around 666 nm and Phytochrome Far Red (PFR) absorbs around 730 nm.
Mechanism of Action: PR converts to PFR upon red light absorption and triggers flowering; PFR reverts to PR under far-red light.

Florigen: Originally hypothesized substance from leaves responsible for flowering; identified as Flowering Time (FT) protein.
Vernalization: A cold exposure treatment that induces flowering in certain species (e.g., cattails), demonstrating the importance of prolonged cold conditions for flowering preparedness.
Inhibitory Role of FLC: The Flowering Locus C gene serves as a flowering suppressor that must be turned off to enable flowering.

Definition: A period of growth rest in plants, regulated by environmental factors such as moisture and temperature.
Mechanism: Seasonal changes result in dormancy through physiological adaptations, allowing plants to survive harsher conditions until favorable growth resumes.
Examples: Many trees exhibit dormancy in winter, adapting biochemically to withstand cold.

The interplay of short and long day flowering plants maintains a diverse rhythm across various seasons, including dormancy during colder months.