Why does the candle go out in the vessel with the seedling?

All plants are living organisms and during their life they have all the main processes: growth, nutrition, respiration, reproduction, etc. So in the process of breathing they absorb oxygen necessary for chemical reactions, and release carbon dioxide, and in the process of photosynthesis, on the contrary, release oxygen and absorb CO2. If the plant is covered and a candle is lit, then after a while there will be little oxygen in the container and the candle will go out, because the rate of O2 combustion is faster than its formation.