Assertion :Oxygen is compulsory for growth.
Reason :Oxygen is must for releasing metabolic energy essential for growth.

Plants use oxygen for cellular respiration as the final electron acceptor in the electron transport chain. Photosynthesis produces some ATP, but this can not happen at night. The light-independent reactions i.e Calvin cycle produces sugars from NADPH and ATP. But to actually get energy from the sugars and for growth, the plant must consume the sugars in cellular respiration, which requires oxygen.

So, the correct option is 'Both Assertion and Reason are correct and Reason is the correct explanation for Assertion'.