The correct substance to add to the water in order for the bubbles to reappear is carbon dioxide.
During photosynthesis, plants utilize carbon dioxide (along with sunlight and water) to produce glucose and oxygen. The small bubbles observed on the plant's leaves are likely oxygen bubbles released as a byproduct of photosynthesis. If the scientist wants to stimulate the photosynthetic process again and observe the bubbling, adding carbon dioxide would be necessary.