Temperature as a Limiting Factor

Temperature affects photosynthesis mainly through enzymes. The light-independent reaction (Calvin cycle) is catalysed by enzymes such as rubisco, so it is the stage most affected by temperature. The light-dependent reaction is less affected, because absorbing light is a physical process.At low temperatures, molecules have little kinetic energy, so there are few successful collisions between enzymes and substrates and few enzyme-substrate complexes form. The rate of photosynthesis is low. As the temperature rises, the rate increases up to an optimum.Above the optimum the rate falls, for two reasons:1. Enzymes begin to denature: hydrogen bonds holding their tertiary structure break, the active site changes shape and the substrate no longer fits.
2. Plants may close their stomata to reduce water loss in hot conditions, which reduces carbon dioxide uptake.So a graph of rate against temperature rises to a peak and then falls, unlike the light intensity and carbon dioxide graphs, which level off.Example: On a sunny but cold spring morning there is plenty of light and carbon dioxide, so temperature is the limiting factor. As the day warms, the rate rises.Different species have different optimum temperatures, depending on the climate they are adapted to. High temperatures also increase the rate of respiration, so the plant's net gain of sugar can fall even before photosynthesis reaches its peak.