Question

On putting iodine solution,why the portion near starch turns blue?

Answer 1

Hope this will help you -  plant consists of two types of molecules, amylose - a soluble starch  and amylopectin.  When starch is mixed with iodine in water, an intensely blue colored starch/iodine complex is formed.  Many of the details of the reaction are still unknown.

Apparently, the Iodine gets stuck in the coils of beta amylose molecules. The starch forces the iodine atoms into a linear arrangement in the central groove of the amylose coil. There is some transfer of charge between the starch and the iodine. That changes the way electrons are confined, and that in turn, changes spacing of the energy levels. The iodine/starch complex has energy level spacings that are just right for absorbing visible light- giving the complex its intense blue color.

This Iodine Test for Starch is used to determine the presence of starch in biological materials.

Answer 2

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