Light Quality: Wavelength & Color
Plants absorb light selectively. Chlorophyll a and b absorb strongly in the blue (430–460 nm) and red (640–680 nm) regions, while reflecting green light — which is why leaves look green. Carotenoids extend absorption into the 400–500 nm range.
The action spectrum of photosynthesis closely mirrors the combined absorption of photosynthetic pigments. Red and blue light drive photosynthesis most efficiently; green light is mostly reflected or transmitted.
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Light Intensity: The Light Response Curve
Net photosynthesis (An) increases with photosynthetic photon flux density (PPFD) but saturates at high light. The shape of this curve is governed by the maximum photosynthetic rate (Amax), the quantum yield (α), and dark respiration (Rd).
The light compensation point is where photosynthesis exactly balances respiration (An = 0). Above the light saturation point, more light gives diminishing returns.
CO₂ Concentration: The A–Ci Curve
The relationship between net assimilation (An) and intercellular CO₂ (Ci) reveals two key limitations: at low Ci, Rubisco carboxylation limits photosynthesis; at high Ci, electron transport (RuBP regeneration) becomes limiting.
At current ambient CO₂ (~420 ppm), C3 plants are not CO₂-saturated. Enriching to 800–1000 ppm typically boosts photosynthesis 30–50%. This is the basis for CO₂ enrichment in greenhouses.
Temperature: Enzyme Kinetics & Thermal Optimum
Photosynthetic enzymes have temperature-dependent activity. As temperature rises, reaction rates increase (Arrhenius kinetics) until an optimum, beyond which enzyme deactivation causes a steep decline. Different species have different thermal optima.
The photosynthetic optimum reflects a balance: Rubisco activity rises with temperature, but so does photorespiration. High temperatures also denature proteins. Cool-season crops (lettuce, spinach) peak at 20–25°C; warm-season crops (tomato) peak at 30–35°C.
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Water Availability & Stomatal Conductance
Stomata are the gatekeepers: they open to let CO₂ in for photosynthesis but lose water vapor in the process. When vapor pressure deficit (VPD) is high or soil water is low, stomata close to conserve water — at the cost of reduced photosynthesis.
As soil dries, stomata close → less transpiration → leaf loses evaporative cooling → leaf temperature rises above air → more photorespiration + enzyme damage. The red shaded area (top plot) shows leaf heating. The red gap (bottom plot) shows how much MORE photosynthesis drops because of this heating — beyond what stomatal closure alone would cause.
Photorespiration: The Rubisco Problem
Rubisco can fix either CO₂ (carboxylation → photosynthesis) or O₂ (oxygenation → photorespiration). At current atmospheric levels (~21% O₂, 0.04% CO₂), about 25% of Rubisco reactions fix O₂ instead of CO₂ in C3 plants, wasting energy.
Photorespiration increases with temperature because the CO₂/O₂ specificity of Rubisco decreases. At 2% O₂ (lab conditions), photorespiration is nearly eliminated and the CO₂ compensation point (Γ*) drops dramatically.
Photosynthetic Mechanisms: C3, C4, and CAM
C3 plants fix CO₂ directly via Rubisco. C4 plants use a CO₂-concentrating mechanism (PEP carboxylase) to boost Rubisco efficiency. CAM plants open stomata at night to store CO₂ as malic acid, then fix it during the day with closed stomata to save water.
C3 (lettuce, wheat, rice): efficient at cool temps, low light. C4 (corn, sugarcane): dominant at high temps, high light — virtually no photorespiration. CAM (pineapple, cacti): extreme water conservation at the cost of slow growth.
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Source–Sink Relationships & Carbon Partitioning
Sources are organs that export carbohydrate (mature leaves producing more sugar than they need). Sinks are organs that import carbohydrate (roots, fruits, seeds, growing tips, young leaves). Carbon partitioning — how the plant distributes its photosynthetic products — changes with development stage and carbon availability.
Under carbon limitation, plants prioritize survival (roots, leaves) over reproduction (fruit). Below a minimum carbon threshold, fruit set fails entirely. Growers manipulate this: pruning directs carbon to remaining fruits; removing flowers early builds root reserves; fruit thinning increases individual fruit size; CO₂ enrichment boosts source strength.