The major difference between action spectrum and absorption spectrum is that inaction spectrum the plant growth and metabolism are occurred and is given only by Chlorophyll A while in absorption spectrum electromagnetic radiations or light absorbs in plants. it is given by all pigments.
the basorption spectrum is used to measure absorption of the various wavelengths of light on the other hand, action spectrum works as an indicator by which wavlength of the light can be used to allow photosynthesis in plants.
However, the action spectrum of photosynthesis does not exactly parallel to 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 the photosynthesis in the most absorbed range (in blue and red) is more than the absorption of these wave lengths 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.