The major difference between action spectrum and absorption spectrum is that inaction spectrum the plant growth and metabolism are occurred and are given only by Chlorophyll A while in the absorption spectrum electromagnetic radiation or light absorbs in plants. it is given by all pigments.
the absorption spectrum is used to measure the absorption of the various wavelengths of light, on the other hand, the action spectrum works as an indicator by which the wavelength of the light can be used to allow photosynthesis in plants.
The action spectrum of photosynthesis is similar to the absorption spectrum of chlorophylls. There are some peaks and valleys in the absorption of light and consumption of CO2.
However, the action spectrum of photosynthesis does not exactly parallel the absorption spectrum of chlorophyll. The peaks of the action spectrum of photosynthesis are comparatively broader than the absorption spectrum of chlorophylls. Similarly, the valleys of the action spectrum are narrow and not deep.
It indicates that photosynthesis in the most absorbed range (in blue and red) is more than the absorption of these wavelengths by chlorophylls. Similarly, photosynthesis in 500 — 600 nm (green light) is more than the absorption of green light by the chlorophylls.
This difference occurs due to accessory pigments like carotenoids. These accessory pigments absorb light in these zones and transfer some of the absorbed light to chlorophyll. This chlorophyll then converts this light energy to chemical energy. When equal intensities of light are given, there is more photosynthesis in the red part than in the blue part of the spectrum.