List the four external factors affecting photosynthesis.
1. Concentration of carbon dioxide: Carbon dioxide is one of the raw materials of photosynthesis. Thus, the rate of photosynthesis is directly proportional to the availability of carbon dioxide in the atmosphere.
2. Temperature: Increase in temperature to certain level increases photosynthesis. Further increase in temperature above 40 °C reduces photosynthesis.
3. Water content: It is one of the raw materials of photosynthesis. Thus, the rate of photosynthesis is directly proportional to water availability until the optimum requirements.
4. Intensity of sunlight: Sunlight is used as an energy source for photosynthesis. Thus, under optimum conditions, the rate of photosynthesis increases with the intensity.