Which of the following external factors can influence photosynthesis?

All of the above

The correct answer is Option (4)- All of the above

The rate of photosynthesis is a critical factor that influences plant yield, especially in crop plants. Photosynthesis is regulated by various factors, both internal (related to the plant itself) and external.

Internal factors include characteristics like leaf number, size, age, orientation, mesophyll cell and chloroplast abundance, internal CO₂ concentration, and chlorophyll content. These internal factors are influenced by the plant's genetics and growth.

External factors encompass elements such as sunlight availability, temperature, CO₂ concentration, and water availability. During photosynthesis, all these factors act together and can collectively affect the rate of photosynthesis or CO₂ fixation. However, usually, one of these factors plays a more dominant role or becomes the limiting factor, determining the overall rate of photosynthesis. Therefore, at any given time, the rate of photosynthesis will be constrained by the factor that is available at sub-optimal levels.