Parameter Tree:
EARTH SCIENCE > BIOSPHERE > VEGETATION > PHOTOSYNTHETICALLY ACTIVE RADIATION

Definition:
Photosynthetically Active Radiation is the light in the whole wavelength band from 400 nm (deep violet) to 700 nm (dark red) used by plants in photosynthesis.

Reference:
Open University. 1989. Seawater: Its Composition, Properties and Behavior. Pergamon Press. England.