AGRI 103: Agronomy - Plant Physiology 2

Plant Cooling: Helps in lowering the temperature of plants.
Uptake of Nutrients: Facilitates the uptake of dissolved mineral nutrients and their movement throughout the plant along with water.
Control Mechanism: Plants regulate water loss by adjusting the opening and closing of stomata.

Epidermis or Root Hairs: Water begins to enter the plant.
Root Cortex: Movement through cortical cells.
Endodermis: Passage through the innermost layer of the root's cortex.
Xylem: Movement into xylem vessels for transportation.
Leaf Mesophyll Cells: Water flows into the mesophyll cells of the leaf.
Evaporation: Water evaporates into leaf air spaces.
Exit Through Stomatal Pores: Finally, water exits the plant through stomatal pores.

Energy Source: Driven by the sun's energy and moisture-concentration gradient.
Gradient Movement: Water moves from areas of high concentration (roots) to low concentration (leaves).
Transpirational Pull: Creates suction, helping to draw water from the soil.

Soil Moisture Content: Availability of water in the soil.
Light Intensity: Higher light increases evaporation rate from leaves.
Air Temperature: Affects evaporation rates (liquid to gas transition in leaves).
Humidity: Surrounding air moisture levels; lower humidity increases transpiration.
Air Movement: Movement of air can increase the rate of transpiration.
Stomatal Characteristics: Number and size of stomata also influence transpiration.

Stomatal Location: Most stomata are found on the lower epidermis (underside) of leaves.
Leaf Surface Structures:
- Hairy Surfaces: Reduces air movement and water loss.
- Sunken Stomata: Reduces water loss by creating a microenvironment.
- Waxy Cuticle: A thick protective coating that decreases direct water loss from epidermal cells.
- Leaf Curling: A physiological response to reduce water exposure.

Importance: Nitrogen is essential for plants and often the most limiting nutrient in terrestrial environments.
Legume Family: Plants such as pulses form symbiotic relationships with Rhizobia bacteria.
Rhizobia Bacteria: This bacteria is either present in the soil or inoculated into seeds.
- Specificity: Each legume species requires a specific Rhizobial species to form these associations.
Fixed Nitrogen Variability: The quantity of nitrogen fixed can vary based on the species of legume.
- Utilization: Some of this fixed nitrogen can be transferred to other non-fixing plants or used by subsequent crops.

Invasion of Bacteria: Bacteria invade the root hairs of the plant.
Nodule Formation: Proliferation of root cells leads to nodule formation.
Conversion to Bacteroids: Inside the nodules, bacteria multiply and convert into bacteroids.
- Development of Nitrogenase: This enzyme is crucial for converting atmospheric nitrogen into a usable form for plants.
Nitrogen Transfer: Fixed nitrogen enters the plant's vascular system, thus being distributed throughout the plant.
Mutual Benefits: The plant provides energy and nutrients to the bacteria in return for fixed nitrogen.

Definition: A plant’s biochemical response to changing sunlight availability.
Involved Pigment: Phytochrome, which exists in two interconvertible forms (Pr and Pfr).
- Activation: Changes in the ratio of red and far-red light activate phytochrome and induce specific plant responses such as dormancy, germination, flowering, branching, growth direction, growth rate, and leaf abscission.

Plants can be categorized based on flowering response:
- Short-Day Plants: Flower when days become shorter than their critical photoperiod, e.g., soybeans, rice.
- Long-Day Plants: Flower when days are longer than their critical photoperiod, e.g., wheat, flax, mustard.
- Day-Neutral Plants: Flower independent of photoperiod, flowering when certain growth stages are achieved, e.g., corn, petunia, tomato, cucumber.

Day Length Data at 50°N Latitude:
- Jan 1: 6:00 h
- Feb 20: 4:47 h
- Apr 11: 3:35 h
- May 31: 2:24 h
- Jul 20: 1:12 h
- Sep 8: 12:00 h
- Oct 28: 10:47 h
- Dec 17: 9:36 h

Application in Breeding: Understanding photoperiod response assists breeders in developing short-day crop varieties that can flower and mature within the growing season.

Definition: The process where certain plant species require exposure to low temperatures during winter to trigger flowering in spring.
Effect: Without vernalization, these species will not produce flowers.
Requirements: Varied temperature and time periods of chilling process depending on the species, e.g., winter wheat.

Definition: Chemicals produced by plants that regulate growth and development.
Production Site: Often synthesized in meristems and then transported to various parts of the plant where required.
Functions: Can signal the differentiation of plant cells into specific plant organs or structures (e.g., flowers, fruit).
Applications: Hormones can be synthesized or isolated for manipulation of crop growth, including plant growth regulators and herbicides.

Growth Promoters: Hormones that promote growth.
Growth Inhibitors: Hormones that suppress growth.
Mixed Effects: Hormones that can have both promoting and inhibiting effects depending on the context.

Phototropism: Movement in response to light; typically, stems bend towards the light source and roots grow away from it. Hormone Involved: Auxins.
Gravitropism: Directional growth response to gravity; roots are positively gravitropic (grow down) while stems are negatively gravitropic (grow up).
Hydrotropism: Growth towards moisture; auxins play a role.
Thigmotropism: Response to touch, such as the climbing behavior of seedlings using tendrils.
- Examples Include: Bean plants climbing with tendrils, roots navigating around obstacles in soil, and Venus flytrap movements.
Root Functions: Roots growing downward away from light are advantageous as they are likely to find water and nutrients necessary for the plant's growth.