<span>Radioactive H will show up in the sugar, C6H12O6, and radioactive oxygen will show up in the O2 emitted to the air.</span>
In short, plants require sunlight because that is how they make their food.
Longer version: Plants use photosynthesis to produce glucose for food with O₂ as a by-product. Without sunlight, photosynthesis cannot happen, and so would have no food to use.