Why Must a Plant Be Destarched Before Photosynthesis Investigations?

Starch formation in plants is a by-product of photosynthesis. Therefore, when a photosynthesis investigation is conducted the plant must be destarched in order to have a metric to determine if photosynthesis took place. The most common method of destarching a plant involves placing it in a dark setting for 24 to 48 hours.

During photosynthesis, a plant produces glucose molecules. The glucose that is not used immediately by the plant is stored in the chloroplast and cytoplasm as starch. This starch can later be hydrolyzed back into glucose when needed. Placing a plant in dark conditions prevents photosynthesis form taking place and forces the plant to use its stored starch. In order to determine if a plant has been properly destarched, it must be chemically tested. The most common way to do so involves taking a leaf from the destarched plant, boiling it in water, then boiling the leaf in ethanol in a hot water bath. Following this, a simple iodine test reveals if starch is still present in the leaf.

Carbon dioxide deprivation can also be used to destarch a plant. This involves placing the plant in an enclosure, such as a large jar and sealing it shut. A sodium hydroxide solution or granular soda lime can be placed in the enclosure along with the plant to accelerate the process.