The correct option is (b).
Explanation
The iodine test is a standard biochemical procedure used to detect the presence of starch (a complex carbohydrate) in plant tissues. Since starch is the stored form of glucose produced during photosynthesis, this test is indirectly used to demonstrate that photosynthesis has occurred.
Statement-wise Analysis:
- Statement 1 is Incorrect: The iodine test is primarily used to detect the presence of starch, not chlorophyll. In fact, during the experimental procedure, chlorophyll is removed from the leaf by boiling it in alcohol (ethanol) so that the green pigment does not mask the colour change caused by the iodine reaction.
- Statement 2 is Correct: Iodine solution (usually potassium iodide) reacts with starch (specifically the amylose component) to form a polyiodide complex. This complex exhibits a distinct blue-black colour. If starch is absent, the solution remains yellowish-brown.
- Statement 3 is Incorrect: Leaves kept in the dark for several days undergo a process called destarching. In the absence of light, photosynthesis ceases, and the plant consumes the stored starch for energy. Consequently, a destarched leaf will test negative for starch and will not turn blue-black; instead, it will retain the brownish colour of the iodine solution.
Key Takeaway:
The iodine test confirms the occurrence of photosynthesis by detecting stored starch. A blue-black colour indicates a positive result (presence of starch), while a yellowish-brown colour indicates a negative result (absence of starch due to lack of light or chlorophyll).