Plant Physiology

Presenter: Susan Z. Herrick, PhD, Ecology and Evolutionary Biology, University of Connecticut.

Photosynthesis: Plants are responsible for nearly all land photosynthesis.
Evolutionary Background:
- Closest living relatives to land plants are green algae found near water sources.
- First land plants were small and similar to their algal ancestors.
Adaptation: Plants have adapted to colonize almost every terrestrial environment, evolving structures to deal with challenges such as desiccation.

Water Loss:
- High rate of water loss from photosynthetic surfaces is a major challenge.
Leaf Adaptations: Waxy Cuticle and Stomata:
- Leaves feature a waxy cuticle and stomata, permitting CO₂ entry while minimizing water loss.
Water Transport:
- Xylem is responsible for transporting water from the soil to leaves, allowing stomata to open without excessive drying.
Photosynthate Transport:
- Phloem distributes nutrients like carbohydrates throughout the plant for growth and respiration.
Root Functions:
- Roots utilize energy to extract nutrients from the soil and establish symbiosis with bacteria and fungi, enhancing nutrient availability.

Appearance: Terrestrial organisms differ significantly from their aquatic ancestors.
Desiccation Risk: Key risk is drying out due to exposure of photosynthetic surfaces to air, necessitating adaptations to mitigate water loss.

Bryophytes:
- Can withstand dry conditions; high surface area-to-volume ratio allows water and nutrient redistribution via diffusion and osmosis.
Vascular Plants:
- Employ bulk flow to transport water and minimize leaf water loss, allowing for greater height and photosynthesis when dry

Shoot System: Includes reproductive organs, leaves, stems, and roots.
Types of Tissues:
- Epidermis: Protective outer layer.
- Ground Tissue: Involved in photosynthesis and storage.
- Vascular Tissue: Comprised of xylem (water transport) and phloem (nutrient transport).

Primary Photosynthetic Surface: The leaf captures light energy and CO₂.
Surface Area Considerations:
- Increased surface area (more leaves or larger leaves) enhances photosynthesis but increases risk of desiccation.

Cuticle: Waxy layer that limits water loss and manages CO₂ diffusion.
Mesophyll Cells:
- Arranged to maximize light capture and facilitate CO₂ diffusion throughout the leaf.
Diffusion Dynamics: CO₂ concentration difference facilitates its movement into the leaf while evaporation leads to water loss.

Definition: The evaporative loss of water vapor from leaves, linked to CO₂ uptake.

Stomata: Small pores enabling gas exchange. Guard cells regulate their opening and closing based on internal water and solute levels.
- Mechanism: Osmotic influx of water into guard cells causes them to swell, opening the stomata.
- Closed State: Loss of solutes leads to water exiting the guard cells, closing the stoma.

Crassulacean Acid Metabolism (CAM): A method enabling plants to balance photosynthesis with water retention by capturing CO₂ at night when stomata are open, converting it to a 4-carbon organic acid.

Low Photosynthetic Rate:
- Limited capacity for carbohydrate production due to ATP requirements and storage limits of organic acids in vacuoles.

Occurrence: High oxygen concentrations can induce photorespiration, a process wasting energy and carbon.
Evolutionary Adaptations: Some plants have adapted to minimize losses associated with this process.

C4 Pathway:
- Involves capturing CO₂ in different cells compared to CAM’s temporal separation.
- 4-carbon acids are precursors for the Calvin cycle in both forms, but C4 plants are more efficient under high light and temperature conditions.

Water transport occurs in a single pathway without ATP expenditure; driven by evaporative loss from leaves.
Xylem Structure:
- Contains elongated vessels supported by lignin.
- Water enters and exits through pits, enabling flow without air ingress.
- Larger radii in xylem conduits allow for faster water transport.

Vulnerability: Xylem conduits face potential collapse due to negative pressure and cavitation (air leakage).
Environmental Stress: Freeze and thaw cycles can lead to cavitation as well.

Composition: Phloem sap is sugar-rich and transports various substances including carbohydrates, amino acids, and hormones.
Development: Sieve elements lose intracellular structures during development but play a crucial role in nutrient transport.

Mechanism: Turgor pressure in source tissues (e.g., leaves) assists in transporting phloem sap toward sinks (e.g., roots).
Pressure and Flow: A pressure difference drives nutrient flow, influenced by osmotic changes as water moves in and out of sieve tubes.

Root Branching: Extensive root networks and root hairs maximize soil contact to facilitate nutrient uptake.

Nutrient Table:
- Nitrogen: 1.5% in dry matter, essential for amino acids, nucleic acids.
- Phosphorus: 0.2% in dry matter, crucial for nucleic acids and ATP reactions.
- Other nutrients include potassium, calcium, magnesium, sulfur, and trace elements necessary for metabolism and growth.

Nutrients passively diffuse in water surrounding soil particles and roots actively excrete protons to enhance nutrient availability through chemical reactions.

Mycorrhizae:
- Ectomycorrhizae: Fungal sheaths around roots enhance nutrient exchange without penetrating the cells.
- Endomycorrhizae: Fungal threads penetrate root cells, greatly improving the uptake of carbon and nutrients.

Significance: Nitrogen is a crucial nutrient that plants cannot directly utilize from the atmosphere; it requires fixation through symbiotic relationships with nitrogen-fixing bacteria.
Function: The plant provides carbohydrates to bacteria, and in exchange, receives usable nitrogen compounds via xylem transport.