If radio labelled carbon in carbon dioxide is supplied to potato plants in an experiment was seen in the tuber, it is due to the following:

When the potato plant carries out photosythesis using the CO2 which is radiolabelled, it forms Oxygen and glucose (C6H12O6) where the carbon in the glucose molecule has the radiolabelled carbon present. This glucose when is converted to sucrose takes the radioactive carbon which is transferred when the form changes. This molecules of sucrose moves via the phloem to the tuber where it is converted into starch which gets radiolabelled due to the same radiolabel carbon which is present and this starch is stored. The process of measuring is by autoradiography which detects the radioactive carbon and traces the components along with the movement in the plant body.