Why do leaver not test positive for starch early int he morning but they at the end of the day?

Leaves do not test positive for starch early in the morning because they use stored starch for energy during the night when photosynthesis is not occurring. As daylight returns, photosynthesis resumes, allowing leaves to convert carbon dioxide and sunlight into glucose, which can then be stored as starch. By the end of the day, the accumulation of starch from photosynthesis leads to a positive test result. Thus, the timing of photosynthesis determines the presence of starch in leaves.