Project
Take a plotted plant with broad leaves. Take two strips of black paper and cut out a small square in the centres. Cover a part of two leaves with these papers and secure them with paper clips. Keep the plant in sunlight for 2-5 days. Observe the difference in the colour of the covered and the uncovered portions on the leaf. Perform iodine test on this leaf. Did the two parts show different results? Now take second leaf. Remove the strip and expose the covered part to the sunlight 2-3 days and do the iodine test again. Describe your observations.
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When a potted plant with broad leaves is taken, two strips of black paper were used to cover the tips of two leaves. The plants were kept in sunlight for 2–5 days. The difference is observed in the color of the covered and uncovered portions of the leaf. When an iodine test was performed on this leaf, the two parts showed different results. The second leaf was taken, and the strip was removed and exposed to the covered part to sunlight for 2-3 days, and the iodine test was repeated. 

The presence of starch in leaves can be tested through the Iodine test.

Extra information:

Test for starch in plants:

1. Fill the beaker or jar with ethyl alcohol. Keep it in a saucepan full of water. Heat the pan until the ethyl alcohol begins to boil.

2. Dip each part of the leaves (uncovered and covered leaf portion) in the hot water for 60 seconds and then in the ethyl alcohol for 2 minutes, or until they turn almost white. Keep them aside in a separate shallow dish.

3. Cover the leaves with iodine solution and observe.

When the part of the leaf is boiled in alcohol, the chlorophyll in the leaf becomes inactivated or removed. Add two drops of iodine solution to this boiled leaf. If the color changes to bluish-black, it indicates the presence of starch.

On boiling the green leaf in alcohol, it loses its green color and removes the chlorophyll in the leaves. On treating it with iodine, the blue-black color indicates starch's presence. To avoid the interference of the green color, the leaf is treated with alcohol.

Reaction: The hot water kills the leaf, and the alcohol breaks down the chlorophyll. This removes the green color of the leaf. When we put iodine on the leaves, one of them will turn blue-black and the other will be reddish-brown. Because the leaf that was in the light performed photosynthesis and produced starch. Iodine is an indicator that turns blue-black in the presence of starch.

Precaution: Keep ethanol away from naked flames. Wear eye protection while working with ethanol and iodine solution.

Observation: The hot water kills the leaf, and the alcohol breaks down the chlorophyll. This removes the green color of the leaf. When iodine is reacted with both the parts of the leaves (covered and uncovered), one of the parts will turn blue-black and the other will be reddish-brown.

Because the leaf that was not covered (exposed to light) performed photosynthesis and produced starch. Iodine is an indicator that turns blue-black in the presence of starch.

The part of the leaf which was covered (which cannot perform photosynthesis) cannot produce starch and, thus, iodine cannot react with starch to turn a blue-black color, instead producing a reddish-brown color.

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Updated on: 10-Oct-2022

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