Study Notes on Plant Responses to Environmental Stimuli and Hormones

Overview of Study Preparation

  • Speaker expresses uncertainty about material for an upcoming test scheduled for Wednesday.

  • Mentions having multiple subjects to study for, indicating a busy schedule.

  • Refers to studying plant responses, solutes, and hormones, particularly regarding their concentration and movement in organisms.

Solute Concentration and Water Potential

  • Distinction between solute concentration inside and outside of the cell:

    • High solute concentration inside the cell leads to low water potential.

    • Low solute concentration outside the cell leads to high water potential.

  • Water movement:

    • Water moves from areas of high water potential to areas of low water potential.

    • Example: Water moves from outside the cell into the cell if the outside has lower solute concentration.

  • Types of cells affected by water movement:

    • Plasmolysed cells (loss of water) change to turgid cells (gain of water).

Responses of Plants to Environmental Stimuli

  • Phenotypic Plasticity:

    • Definition: The ability of a single genotype to produce different phenotypes in response to different environmental conditions.

    • Importance: More pronounced in plants compared to animals due to their immobility.

  • Morphological Changes:

    • Example: Aerial leaves vs submerged leaves in aquatic plants, where leaf structure differs based on their environment to optimize for photosynthesis.

    • Aerial leaves: Broader for light interception; Submerged leaves: Dissected to reduce water resistance.

Types of Plant Responses to the Environment

  1. Nastic Movements:

    • Definition: Quick, reversible movements that do not involve growth.

    • Examples: Leaf movements in response to touch or temperature changes.

  2. Tropisms:

    • Definition: Directional growth responses towards or away from stimuli (e.g., light, gravity).

    • Types of Tropisms:

      • Phototropism: Growth towards light.

      • Gravitropism: Growth in response to gravity.

  3. Morphogenesis:

    • Definition: Large-scale developmental changes in response to environmental factors.

    • Involves changes in the overall body plan of the plant.

Plant Hormones and their Functions

  • Overview of Plant Hormones:

    • Hormones are small molecules affecting plant growth and physiology locally or transported through the phloem.

    • Five major classes:

    1. Auxins

    2. Gibberellins

    3. Cytokinins

    4. Ethylene

    5. Abscisic Acid

  • Auxin (IAA):

    • Produced in shoot apical meristems and young leaves.

    • Affects cell elongation, especially in response to light (phototropism).

    • Polar transport mechanism based on pH changes:

    • In acidic conditions, auxin is uncharged and crosses the membrane.

    • In neutral conditions, it becomes negatively charged and cannot exit freely, requiring specific transport proteins (PIN proteins) at the cell's basal side.

  • Gibberellins:

    • Involved in shoot elongation and overall growth processes, produced in regions of active growth.

  • Cytokinins:

    • Regulate cell division and shoot growth, can be transported through the plant fluids.

  • Ethylene:

    • Known as the "ripening hormone"; involved in processes like fruit ripening and stress responses.

    • Examples of ethylene effects on fruit ripening in varying concentrations and temperatures.

  • Abscisic Acid:

    • Maintains seed dormancy; promotes stomatal closure to reduce water loss.

Responses to Light: Phototropism, Action Spectra, and Receptors

  • Phototropism: Growth towards light.

  • Action Spectrum for Light:

    • Plants respond primarily to blue light for phototropic responses.

  • Types of Photoreceptors:

    • Cryptochromes (blue light receptors) control phototropism and protect against excess light exposure.

    • Phytochromes: Respond to red and far-red light, involved in seed germination and flowering.

    • Phytochromes switch between:

    • Inactive form (Pr): Responds to red light.

    • Active form (Pfr): Responds to far-red light.

  • Photoperiodism:

    • Short-day and long-day plants have different flowering triggers based on length of night and day.

    • Flashing a light during long nights can change flowering patterns (experimental evidence).

Small-scale Experiments and Observations

  • Darwin's Experiments:

    • Critical investigations into phototropism using plant tips.

  • Boys and Jensen's Gelatin Experiment:

    • Proved the existence of a substance transmitting from the tip, which was auxin, necessary for phototropic response.

Conclusion

  • Summed up the key responses and hormones discussed, highlighting their intricate roles in plant adaptation to habitats and environmental stimuli.