Definition of Cell Division: It is not about creating more cells.
Definition of Auxin: A plant hormone that facilitates movement in response to light.
Mechanism: When sunlight hits one side of a plant, auxin is produced on the illuminated side and moves to the shaded side. Analogous to the ‘dark side of the moon’.
Light Side Effects: Cells on the sunlight side maintain size.
Dark Side Effects: Auxin coupled with an acid loosens bonds in the cell walls (termed 'expansins').
Result: Cells on the dark side stretch and elongate.
Outcome: Plant bends toward the light.
Directionality in Plants:
If branches lean towards the light, the plant interprets this as 'up'.
Roots grow in the opposite direction—referred to as negative phototropism (roots grow away from light).
Gravitropism
Definition: Movement of plants in response to gravity.
Function of Statoliths: Gravity-sensitive structures in roots similar to stones in the human inner ear, composed of starch.
Mechanism: Statoliths settle on the lower side of roots, signaling which direction is down.
Observations: If the plant is tipped, leaves still grow towards light while roots grow down due to statoliths' guidance.
Photoperiodism
Definition: Plant responses based on the duration of light exposure, influencing their activities at different times of the day.
Application: Seeds have planting charts indicating optimal planting times based on light needs.
Example: Tulips and daffodils require shorter daylight; roses need longer daylight for growth.
Role of Phytochrome:
A blue-green pigment that regulates growth and flowering based on light exposure.
Mechanism:
Active state of phytochrome results in a straight configuration, enabling specific hormone responses such as growth or dormancy.
Photosynthetic Processes: Inactive and active states of phytochrome lead to the production of transcription factors that initiate various growth responses. Light initiates a phosphorylation cascade leading to transcription of growth-related genes.
Hormonal Implications: Plants rely on this mechanism for development and responding to environmental conditions.
Auxin and Cytokinin Dynamics
Auxin Role: Causes cells to elongate, promoting growth in shoots and roots. If cells become too large, they need to divide.
Cytokinins Role: Promotes cell division.
Cytokinesis Explained: Literally means 'to cut' the cell, promoting division in established larger cells.
Combined Effects of Auxin and Cytokinins:
When both hormones work together, they allow cells to elongate and then divide, maintaining optimal cell size.
Ethylene and Fruit Ripening
Definition of Ethylene: A gaseous plant hormone associated with fruit ripening.
Function: Triggers decay of cells in fruit, promoting sweetness and juiciness as fruit matures.
Feedback Loop: Once produced, ethylene continues to promote ripening and if unconsumed, may lead to deterioration, ensuring dispersal post-rot.
Practical Applications: Ethylene affects surrounding fruit, promoting uniform ripening. Example: Placing bananas with avocados hastens the latter’s ripening due to ethylene emission.
Gibberellin Functions
Gibberellins Overview: Promote stem elongation, flowering, and breaking dormancy in seeds.
Comparative Mechanism: Analogous to the transitional phase from sleeping to active, symbolized by 'getting out of bed'.
Dormancy Breakdown:
Mechanism: Gibberellins cause hydrolysis within seeds, breaking down starch to sugars enabling germination and root/shoot development.
Essential Conditions: Requires water to activate gibberellins faithfully, facilitating the emergence of roots downwards and shoots upwards.
Conclusion: Plants depend on a coordination of hormones—auxin, cytokinin, ethylene, and gibberellin—to regulate growth responses to environmental factors.
Summary of Plant Hormones
Auxin: Promotes elongation and phototropism.
Cytokinin: Encourages cell division.
Ethylene: Induces fruit ripening.
Gibberellin: Stimulates growth and breaks dormancy.