Photosynthetic Rate Calculator
Free Photosynthetic rate Calculator for gardening & crops. Enter variables to compute results with formulas and detailed steps.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
P = Pmax x [I / (I + Km)] x [CO2 / (CO2 + Kc)] x exp(-0.5 x ((T - Topt) / sigma)^2)
Where P = photosynthetic rate, Pmax = maximum rate (20 umol CO2/m2/s), I = light intensity, Km = half-saturation constant for light (200), CO2 = carbon dioxide concentration, Kc = half-saturation for CO2 (300 ppm), T = temperature, Topt = optimal temperature (25C), and sigma = temperature sensitivity (10C).
Worked Examples
Example 1: Optimal Conditions Photosynthesis
Problem:A plant leaf (50 cm2) is exposed to 800 umol/m2/s light, 400 ppm CO2, at 25 degrees C. Calculate the net photosynthetic rate.
Solution:Light factor = 800 / (800 + 200) = 0.80\nCO2 factor = 400 / (400 + 300) = 0.571\nTemperature factor = exp(-0.5 * ((25-25)/10)^2) = 1.0\nGross rate = 20 * 0.80 * 0.571 * 1.0 = 9.14 umol CO2/m2/s\nRespiration = 2.0 umol CO2/m2/s\nNet rate = 9.14 - 2.0 = 7.14 umol CO2/m2/s
Result:Net photosynthetic rate: 7.14 umol CO2/m2/s
Example 2: Elevated CO2 Greenhouse Scenario
Problem:A greenhouse enriches CO2 to 1000 ppm with supplemental lighting at 1200 umol/m2/s and 28 degrees C. Calculate the improvement over ambient conditions.
Solution:At 1000 ppm CO2: CO2 factor = 1000/(1000+300) = 0.769\nAt 1200 light: Light factor = 1200/(1200+200) = 0.857\nTemp factor at 28C = exp(-0.5*((28-25)/10)^2) = 0.956\nGross = 20 * 0.857 * 0.769 * 0.956 = 12.61\nVs ambient (400 ppm, 800 light): Gross = 9.14\nImprovement = (12.61 - 9.14) / 9.14 = 38%
Result:CO2 enrichment + high light increases gross photosynthesis by ~38%
Frequently Asked Questions
What is the photosynthetic rate and how is it measured?
The photosynthetic rate is the speed at which a plant converts carbon dioxide and water into glucose and oxygen using light energy. It is typically measured in micromoles of CO2 fixed per square meter of leaf area per second (umol CO2/m2/s). Scientists measure it using infrared gas analyzers (IRGAs) that detect changes in CO2 concentration as air passes over a leaf enclosed in a chamber. Typical rates range from 5 to 40 umol CO2/m2/s depending on the plant species and environmental conditions.
Why does temperature affect photosynthetic rate?
Temperature affects photosynthesis through enzyme kinetics. As temperature rises, enzymatic reactions speed up, increasing the rate. However, above an optimum temperature (typically 25-35 degrees C for most plants), enzymes begin to denature and the rate drops sharply. Additionally, photorespiration increases at higher temperatures in C3 plants, reducing net carbon fixation. Very low temperatures slow enzyme activity and can damage the photosynthetic apparatus. The temperature optimum varies by species: tropical plants peak at 30-35 degrees C, while temperate species peak at 20-25 degrees C.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy