Photosynthesis and Plant Hormones

Week 11 Overview

• Lectured by: Stephen Trueman (s.trueman@griffith.edu.au)

• Topics Covered:

  1. Light absorption

  2. Photosynthesis:

  • C3 pathway

  • C4 pathway

  • CAM pathway

  1. Photoassimilate distribution

  2. Plant hormones:

  • Branching

  • Stem elongation

  • Stomatal opening and closing

Leaf Structure

• Components of a Leaf:

  • Upper Epidermis

  • Palisade Cells

  • Phloem Tissue

  • Xylem Tissue

  • Chloroplasts

  • Lower Epidermis

  • Stoma

  • Veins

  • Spongy Cells

  • Guard Cells (regulating stomatal pore)

Light Absorption

• Chlorophyll, absorbed light, and carbon fixation profiles in leaves:

  • Absorbed light = chlorophyll and light profiles.

  • Carbon fixation skewed towards depths.

• Chlorophyll absorbs strongly in blue and red light regions.

• Pigment-protein complexes (Chl a-PSII RC, Chl (a,b)-LHC) show specific absorption spectra.

CO₂ Diffusion to Chloroplasts

• Factors resisting CO₂ diffusion to chloroplasts:

  • Stomatal pore boundary layer resistance

  • Stomatal resistance

  • Intercellular air-space resistance

  • Liquid phase resistance

  • Boundary layer resistance

Photosynthesis Dynamics

• Light & CO₂ Effects: Photosynthetic response to light (photon irradiance) and CO₂ concentration shows slope, peak, and compensation points (e.g., in Corymbia maculata).

• Z-scheme Electron Transport:

  • H₂O splitting provides $2$ electrons.

  • Excitation of P680 (PSII) and P700 (PSI) produces $2$ H+ protons.

• Proton Motive Force:

  • H+ accumulation creates a pH gradient ($3-4$) for ATP production (photophosphorylation) in the stroma.

Photosynthesis Pathways

C3 Pathway

• Enzyme: Rubisco incorporates CO₂ into RuBP, forming two $3$-PGA.

  • $3$-PGA Fates: Sucrose export, starch production, or RuBP recycling.

• Prevalence: $\sim97\%$ of land plants (e.g., rice, wheat, trees).

• Rubisco Inefficiency:

  • Competes with O₂ (approx. $1$ mol O₂ fixed for every $3$ mol CO₂).

  • Leads to photorespiration: P-glycolate forms in chloroplasts, inefficiently producing less CO₂ via conversions in peroxisomes/mitochondria.

C4 Pathway

• Key Plants: Corn, sugarcane, sorghum.

• Features: Mesophyll and bundle sheath cells concentrate CO₂ for Rubisco, minimizing O₂ competition.

• Advantages: Thrives in high light and temperature.

• Environmental Niche: Faster at high temperatures; C3 plants better in low temperature/light.

CAM Pathway

• Crassulacean Acid Metabolism:

  • Night: Stomates open, CO₂ fixed into OAA, stored as malate in vacuoles.

  • Day: Stomates closed (H₂O conservation), malate releases CO₂ for Rubisco.

• Efficiency: Requires $6$ ATP and $2$ NADPH per CO₂ fixed; energy and CO₂ inefficient due to no spatial separation of O₂/CO₂.

• Plants: Cacti, orchids, salt-tolerant succulents (arid adaptations).

Photosynthate Distribution

• Sugars transported via phloem tubes (companion cells) to various plant parts (roots, leaves, flowers, fruits) in a bi-directional flow.

• Phloem Transport: Mass flow driven by a concentration gradient from source to sink, requiring controlled osmotic pressure.

Plant Hormones

General Characteristics

• Synthesized from common precursors.

• Multifunctional roles.

• Synthesis not restricted to specific zones.

Hormone Classes

1. Auxins

2. Cytokinins

3. Gibberellins

4. Abscisic acid

5. Ethylene

Auxins

• Main Natural Auxin: Indole-$3$-acetic acid (IAA).

• Synthesis Sites: Shoot tips, young leaves, fruit, seeds.

• Transport: Phloem and polar auxin transport.

• Key Role: Regulates apical dominance.

Cytokinins

• Variety: Many types.

• Synthesis Sites: Roots, seeds.

• Transport: Xylem stream.

• Key Role: Regulates tissue growth and division.

Gibberellins

• Types: Over $100$ types, mostly inactive.

• Key Roles: Stem growth, seed germination.

Abscisic Acid (ABA)

• Synthesis Site: Water-stressed cells.

• Key Roles: Regulates stomatal closure and drought stress responses.

Ethylene

• Gaseous hormone.

• Involved in stress responses, aging, and ripening.

Branching and Stem Elongation

Branching

• Control: Auxins (IAA) and cytokinins.

• Apical Dominance: Main shoot suppresses axillary buds, a dominant branching pathway.

Stem Elongation

• Control: Gibberellin levels influence stem growth (e.g., in peas).

• Gibberellin application can reverse dwarfism.

Stomatal Opening and Closing

• Control: ABA regulates stomatal closure during drought.

• Mechanism: Guard cells control water and gas exchange through rapid signaling pathways, including Ca²⁺.