Take two potted plants of the same kind.Keep one in the dark (or in a black box)for 72 hours and the other in the sunlight. Perform iodine test with the leaves of both the plants.Now leave the pot which was earlier kept in the dark ,in the sunlight for 3-4 days and perform the iodine test again on its leaves.
In first iodine test of the leaves placed in darkness the result will be negative as no starch is present on leaves. In darkness the plant failed to perform photosynthesis and could not produce starch. When that plant pot is kept in sunlight and iodine test is done for the second time, the leaves become dark blue due to the reaction of iodine with starch. This means the plant has produced starch in the sunlight by the process of photosynthesis. This experiment proves that photosynthesis occurs in presence of sunlight and produces starch.