At what temperature does photosynthesis take place at a maximum rate?

• A. 18-33 C
• B. 10-20 C
• C. 25-40 C
• D. 5-15 C

Answer: (A)

Photosynthesis runs best when temperature supports fast enzyme activity without causing stress to the plant. As temperature rises from low values, enzyme reactions speed up, increasing the rate of carbon fixation. But beyond a certain point, enzymes start to lose stability and processes like photorespiration rise, and stomata may close to conserve water, reducing CO2 intake. The balance of these factors for many plants falls in a moderate warm window, roughly 18–33 °C, where the rate is at or near its maximum. Cooler ranges slow the reactions too much, while very hot ranges can stress the system and lower the rate.