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🌱 Photosynthesis Light Response

Interactive photosynthesis light-response simulator. Adjust PAR irradiance, CO₂, temperature and plant type; watch the light-response curve, compensation and saturation points, and which Blackman factor limits net assimilation.

Agronomy & Soil Physics3DModerate60 FPS

About this simulation

This simulation models a leaf's light-response curve — the non-rectangular hyperbola of Thornley (curvature θ = 0.8), θ·Ag² − (φ·I + A_max)·Ag + φ·I·A_max = 0 with A = Ag − R_d, which links net CO₂ assimilation A to incident PAR irradiance I. (A rectangular hyperbola, θ = 0, would only reach 90 % of A_max at about 4500 µmol/m²/s — more than twice full sunlight — so real leaves are modelled with a curved knee.) Quantum yield φ sets the curve's initial slope, A_max (itself scaled by CO₂ availability and a Gaussian temperature-optimum curve) sets its plateau, and dark respiration R_d shifts the whole curve down so photosynthesis must first overcome respiration before net carbon gain becomes positive. Four plant presets — C3 wheat, C4 maize, a shade plant and a sun plant — carry different real physiological parameters, illustrating Blackman's law of limiting factors in action.

🔬 What it shows

A live light-response curve with a draggable orange operating point, plus a bar chart showing which of three factors — light, CO₂ or temperature — currently limits net assimilation (the lowest bar, per Blackman's law). A small animated leaf releases O₂ bubbles at a rate proportional to the current assimilation rate, and the readout panel reports net A, gross photosynthesis, dark respiration, and the light compensation and saturation points.

🎮 How to use

Choose a plant type (C3 wheat, C4 maize, shade plant, sun plant) to load its quantum yield, A_max, thermal optimum and respiration rate. Drag the PAR irradiance slider or click/drag directly on the curve to move the operating point, and adjust CO₂ (200-1000 ppm) and Temperature (5-40°C) sliders to see A_max rise or fall and watch which factor becomes limiting in the bar chart below.

💡 Did you know?

C4 plants like maize evolved a CO₂-concentrating pump around Rubisco, which is why the C4 preset here has a much lower CO₂ half-saturation constant (120 ppm vs 300 ppm for C3 wheat) and a higher A_max — C4 photosynthesis stays productive even when stomata partly close to conserve water in hot, dry conditions.

Frequently asked questions

What is the light compensation point?

The light compensation point is the irradiance at which gross photosynthesis exactly balances dark respiration, so net CO₂ assimilation A equals zero. Below this irradiance the leaf is a net CO₂ source (respiration exceeds photosynthesis); above it, the leaf becomes a net carbon sink. Shade-adapted leaves typically have a lower compensation point than sun leaves because they are adapted to photosynthesise efficiently at low light.

What is Blackman's law of limiting factors?

Blackman's law states that when a process depends on several factors, its rate is limited by whichever factor is most scarce relative to the others — not by the average of all factors. In this simulation, photosynthesis can be capped by insufficient light, insufficient CO₂, or non-optimal temperature, and only the single most limiting factor determines the actual rate at any moment, exactly as the bar chart's lowest bar indicates.

Why does the curve plateau at high light instead of continuing to rise?

At low irradiance, the rate of photosynthesis is limited by how fast light energy can be captured, so assimilation rises almost linearly with light (the initial slope is the quantum yield). At high irradiance, the biochemical machinery — Rubisco carboxylation capacity and the CO₂ supply — becomes the bottleneck instead, so extra light no longer increases the rate. This transition produces the characteristic saturating hyperbola shape.

Why do C4 plants like maize have a higher photosynthetic capacity than C3 wheat?

C4 plants use a two-stage carbon-fixation pathway that first concentrates CO₂ in specialised bundle-sheath cells using PEP carboxylase, before handing it to Rubisco. This CO₂-concentrating mechanism nearly eliminates the wasteful process of photorespiration that limits C3 plants, allowing C4 species to sustain much higher assimilation rates and to keep functioning efficiently even with partially closed stomata in hot, dry, high-light environments.

Why does temperature have an optimum rather than photosynthesis simply increasing with heat?

Photosynthetic enzymes, especially Rubisco, have activity that rises with temperature up to an optimum and then falls sharply as heat begins to denature proteins and destabilise thylakoid membranes. The simulation models this with a Gaussian (bell-shaped) temperature factor centred on each plant's optimal temperature, reflecting why C3 crops like wheat generally peak around 25°C while C4 crops like maize, evolved in warmer climates, peak at a higher temperature.

⚙ Under the hood

Drag the light-response curve to find compensation and saturation points, then raise CO₂ to see whether light, CO₂ or temperature is limiting.

AgronomyPhotosynthesisLight ResponseLimiting FactorsC3 C4 Plants

3D · Three.js / WebGL renderer · 60 FPS target · runs fully client-side, no install