Autoradiography of a green leaf of summer squash (upper panel) showed import of ${ }^{14} \mathrm{C}$ carbon from the source over a period of time. A similar experiment was carried out with an albino tobacco leaf (lower panel) which do not photosynthesize. Shaded portions denote ${ }^{14} \mathrm{C}$ labelling.


Based on these observations the correct option(s) is(are)

• A

  In the early stages of development, the leaf acts as a source.
• Correct answer
  B

  Mature leaf gains the ability to load and export sugar.
• Correct answer
  C

  The import to export transition is dependent on the developmental stage of leaves irrespective of photosynthesis.
• Correct answer
  D

  Import cessation and export initiation are two separate events.

Hence, the correct answer is options: 'B', 'C', 'D'.