What factor would least affect photosynthesis?

The factor that would least affect photosynthesis is the presence of certain non-essential nutrients, such as trace minerals like copper or zinc. While these minerals can play a role in overall plant health, they are not directly involved in the photosynthesis process itself. In contrast, factors like light intensity, carbon dioxide concentration, and temperature have a much more significant impact on the rate of photosynthesis. Therefore, while essential nutrients are critical, non-essential ones have a minimal effect.