Question

In the experiment "light is essential for photosynthesis" why does the uncovered part of the leaf turn blue black after putting iodine solution?

Answer 1

Because starch is produced in the uncovered area of leaf which turns blue black in the presence of iodine solution.

Answer 2

In the experiment "light is essential for photosynthesis" the uncovered part of the leaf turn blue black after putting iodine solution. It is because, in the region which does not cover during the process of photosynthesis and it leads to the formation of Starch.  

Hence, the uncovered region is now composed of starch. When iodine solution is added to this region, the starch and iodine reacts with each other leading to the formation of blue black color in the uncovered region.