Plant-Microbe Symbioses, Photosynthesis, and Nutrient Use Efficiency

Ectomycorrhizae

Fungal hyphae form a sheath around plant roots and grow between cortical cells (Hartig net). Found mostly in woody trees (oaks, pines, birch).

Arbuscular mycorrhizae (AMF)

Fungal hyphae penetrate inside root cortical cells, forming arbuscules (tree-shaped structures) for nutrient exchange. Found in most crops and herbs.

Similarity between Ectomycorrhizae and AMF

Both are mutualistic symbioses where plants trade carbon (sugars) for mineral nutrients (especially P and N).

Difference between Ectomycorrhizae and AMF

Ectomycorrhizae = extracellular (don't enter cells). AMF = intracellular (form arbuscules inside cells).

NPQ (Non-Photochemical Quenching)

A protective process where excess light energy absorbed by chlorophyll is dissipated as heat to prevent ROS (reactive oxygen species) damage. Controlled by xanthophyll cycle and pH gradient in the thylakoid.

Photorespiration bypass

Alternative or engineered pathways that reduce the energy/carbon loss caused by Rubisco's oxygenase activity (when it fixes O₂ instead of CO₂).

Similarity between NPQ and Photorespiration bypass

Both prevent photodamage and inefficiency in photosynthesis.

Difference between NPQ and Photorespiration bypass

NPQ = prevents overexcitation of photosystems (light stress). Photorespiration bypass = prevents carbon loss due to Rubisco oxygenation.

WUE (Water Use Efficiency)

Ratio of carbon gained (via photosynthesis) to water lost (via transpiration).

NUE (Nitrogen Use Efficiency)

Ratio of plant biomass or carbon fixed per unit nitrogen absorbed.

Similarity between WUE and NUE

Both measure how efficiently a plant converts limited resources into growth.

Difference between WUE and NUE

WUE involves CO₂ uptake vs. water loss (controlled by stomata). NUE involves nitrogen uptake vs. growth (controlled by soil availability and assimilation).

H⁺ Role in Nutrient Absorption

In roots, H⁺-ATPase pumps protons out of cells, creating an electrochemical gradient that drives co-transport of nutrients like NO₃⁻, K⁺, and PO₄³⁻.

H⁺ Role in ATP Production

In chloroplasts, the proton gradient across the thylakoid membrane (lumen high H⁺, stroma low H⁺) drives ATP synthase to produce ATP from ADP + Pi.

Similarity between H⁺ Role in Nutrient Absorption and ATP Production

Both rely on proton gradients to power energy-requiring processes.

Difference between H⁺ Role in Nutrient Absorption and ATP Production

Root membrane gradient = nutrient transport. Thylakoid gradient = ATP synthesis in photosynthesis.

Fixed Nitrogen

Biologically available nitrogen (NH₄⁺, NO₃⁻) instead of N₂ gas.

Uptake from soil

Acquisition of nitrogen as nitrate or ammonium, competing with microbes and requiring energy for nitrate reduction.

Symbiotic fixation

Nitrogen fixation through associations with legumes and rhizobia, requiring energy and carbon for nodule maintenance and oxygen control.

Associations with mycorrhizae

Partnerships with fungi or organic matter mineralization to acquire nitrogen, costing carbon to fungi or enzyme production.

Biological N fixation

Process using nitrogenase enzyme, costing approximately 16 ATP per N₂.

Nitrate assimilation

Conversion process: NO₃⁻ → NO₂⁻ → NH₄⁺ → amino acids.

NADPH

Produced in the light reactions of photosynthesis, used in the Calvin cycle to reduce 3-phosphoglycerate to G3P.

Calvin cycle

Uses ATP and NADPH to convert CO₂ into G3P (sugar).

FACE Experiments

Studies showing C₃ plants increase photosynthesis under high CO₂, but effects decline over time due to nutrient limitations.

Amazon or Boreal Forest Carbon Studies

Research indicating forests store CO₂ but also release it, showing that carbon sinks are limited and reversible.

CO₂ fertilization saturation

Occurs at approximately 700 ppm.

Deforestation impact

Releases stored carbon faster than reforestation can capture it.

C₃ vs. C₄ plants

C₃ plants experience reduced efficiency at low CO₂ due to photorespiration, while C₄ plants use PEP carboxylase to fix CO₂ more efficiently.

Human CO₂ effects

Elevated CO₂ benefits C₃ plants significantly, with a smaller benefit for C₄ plants, potentially favoring C₃ species.

Thylakoid Membrane

Site of light reactions where pigments absorb light and water is split.

Thylakoid Lumen

Acidic environment that builds a proton gradient during photosynthesis.

Stroma

Location where the Calvin cycle occurs, producing G3P from CO₂, ATP, and NADPH.

Rubisco

Ribulose-1,5-bisphosphate carboxylase/oxygenase, making up 50% of leaf protein and fixing approximately 3 CO₂ per second.

Photorespiration

Oxygenase reaction by Rubisco that wastes ATP and carbon.

Light reactions

Occur in thylakoid membranes, converting light and H₂O into O₂, ATP, and NADPH.

Dark reactions

Utilize ATP and NADPH produced in light reactions to synthesize sugars.