External factors: Light, carbon dioxide, temperature, water, mineral and pollutants.
1. Light:
i. Energy for photosynthesis comes only from light.
ii. Photooxidation of water and excitation of pigment molecules are directly controlled by light .
iii. Stomatal movement leading to diffusion of `CO_(2)` is indirectly controlled by light .
(a) Intensity of light :
i. Intensity of light plays a direct role in the rate of photosynthesis .
ii. Under low intensity the photosynthetic rate is low and at higher intensity photosynthetic rate is higher .
iii. It also depends on the nature of plants . Heliophytes (Bean Plants) require higher intensity than Sciophytes (Oxalis ) .
(b) Quantum of light : In plants which are exposed to light for longer duration (Long day Plants) photosynthetic rate is higher .
(c) Quaity of light :
i. Different wavelengths of light affect the rate of photosynthetic because pigment system does not absorb all the rays equally .
ii. Photosynthetic rate is maximum in blue and red light . PAR Photosynthetically Active Radiation is between 400 to 700 nm .
(iii) Red light induces highest rate of photosynthesis and green light induces lowest rate of photosynthetic .
Carbon dioxide : i. `CO_(2)` is found only 0.3 % in the atmosphere but plays a vital role.
ii Increase in concentration of `CO_(2)` increases the rate of photosynthetic . `(CO_(2)` concentration in the atmosphere is 330 ppm).
3 Oxygen :
i. The rate of photosynthesis decreases when there is an increase of oxygen concentration .
Water :
i. Photolysis of water provides electrons and protons for the reduction of NADP , directly .
ii Indirect roles are stomatal movement and hydration of protoplasm.
iii . During water stress , supply of `NADPH^(+) H^(+)` is affected .
