Factors Affecting photosynthesis :

• According to Blackman‟s law of limiting factors “if a chemical process is affected by more than one factors, then the rate of the process will directly depend on the factor nearest to its minimal value”. 
• As photosynthesis is affected by number of internal (No., Size, and orientation of leaves, merophyll cells & chloroplasts etc.) and external factors (light, CO₂ Conc, temp., soil water etc.), it‟s rate will also directly depend on the factor nearest to its minimal value.
• Light intensity and quality affect rate of photosynthesis. Light of wavelengths 400 nm - 700 nm is effective for photosynthesis. With increase in light intensity, rate of photosynthesis increases, but at higher light intensity rate decreases either due to the destruction of chlorophyll or due to other factors that becomes limiting.
• Only dark reaction is affected by temperature as it is enzyme catalysed reaction and enzymes remain inactive at low temp. while become denatured at high temperatures.
• Rate of photosynthesis increases with increase in CO₂ Con. upto a certain limit (upto 500 µl/L in C₃ & 360 µl/L in C₄ plants). 
• Lower availability of soil water causes stomatal closure causing decreased supply of CO₂, hence decrease in photosynthesis.

Factors affecting photosynthesis : 

1. Light 

2. Carbon dioxide 

3. Temperature 

4. Water