Pigment acting as a reaction centre during photosynthesis is

Pigment acting as a reaction centre during photosynthesis is P₇₀₀.
The process always begins when energy from light is absorbed by proteins called reaction centers that contain green chlorophyll (and other coloured) pigments/chromophores.
The excited electrons lost from the reaction center (P₇₀₀) of photosystem I are replaced by transfer from plastocyanin, whose electrons come from electron transport through photosystem II.
P700 absorbance and gas exchange measurements reveal detailed information about e.g. the photosystems, quantum efficiency and the CO₂ assimilation rates.