For photosynthesis to occur, both sunlight and CO₂ are required. The plant forms glucose (later on converted to starch) through photosynthesis. The part Q of leaf was covered and not exposed to light and part R was kept in absence of CO₂. Thus, both parts of leaf did not form starch and gave a negative test for starch. (Note - iodine solution gives a positive test for starch and turns blue-black in colour)