Temperature

Temperature affects photosynthesis mainly through enzymes. The light-independent reactions of the Calvin cycle are catalysed by enzymes such as RuBisCO, so they are the most temperature-sensitive stage. The light-dependent reactions depend largely on light absorption and are affected much less.At low temperatures, molecules have little kinetic energy, so enzyme-substrate complexes form slowly and the rate of photosynthesis is low. As temperature rises, enzyme and substrate molecules move faster and collide more often, so the rate increases. Over the lower part of the range, a 10 °C rise roughly doubles the rate (a temperature coefficient, Q₁₀, of about 2).The rate reaches a maximum at the optimum temperature, which is about 25 °C to 30 °C for many temperate plants. Above the optimum, the rate falls quickly because hydrogen bonds and other bonds holding the enzymes' tertiary structure break. The active sites change shape and the enzymes denature.High temperatures reduce photosynthesis in two further ways. Plants close their stomata to reduce water loss, which cuts carbon dioxide uptake. RuBisCO also fixes more oxygen instead of carbon dioxide as temperature rises (photorespiration), which wastes RuBP.Worked example: a plant photosynthesises at 12 units at 15 °C. If the rate doubles for every 10 °C rise, the rate at 25 °C is 12 × 2 = 24 units.Because temperature acts on the enzymes of the Calvin cycle, it becomes the limiting factor when light intensity and carbon dioxide concentration are high, for example on a bright but cold day.