Question

when does plants start to photosynthesis

Answer 1

Photosynthesis starts as soon as the reaction centres start getting light. Reaction centres are the proteins that contain the green chlorophyll pigments that absorb the light the plant uses

Answer 2

During photosynthesis, a plant translates $CO_{2}$, water and light into $O_{2}$. I assume the carbon $C$ is further used for the growing process. I wonder how the plant grows before the time where photosynthesis is possible, i.e. before there are even leaves, in which photosynthesis occurs.