Start of synthesis of chlorophyll in a plant seedling is stimulated by

Chlorophyll molecules are light trapping green pigments in plants. They are made up of a cyclic tetrapyrrole head and a long phytol tail attached to the head. One atom of magnesium is directly coordinated to the four pyrrole rings in the cyclic tetrapyrrole head. Thus, magnesium is an important structural component of chlorophyll molecules. The iron is an essential micronutrient for plants. It is required for synthesis of chlorophyll and has several other functions also. Plants also require an exposure to light in order to synthesize chlorophyll.  Apparently because in the absence of light chlorophylls are not required at all; trapping of light being there chief function. In absence of light plants will be thin and pale. Such plants are called etiolated plants.