Question

why the part of leaf is black when leaf is exposed to sun​

Answer 1

Answer:

Iodine is an indicator that turns blue-black in the presence of starch. The leaf that was in the light turns blue-black, which demonstrates that the leaf has been performing photosynthesis and producing starch. Try the test again with a variegated leaf (one with both green and white) that has been in the sunlight.

Answer 2

Answer:

The sun's ultra violent rays turns it black