When a plant produces glucose in excess, it can be converted into starch and stored, usually in the roots and seeds of the plant, where it is kept as a long-term energy reserve for the plant. Typical starch components found in plants are amylose, which is linear in structure, and amylopectin, which is branched. Both these components are polysaccharides comprised of thousands of sugar molecules and are stored in granules called plastids within plant cells.
Starches are found in seeds because they help feed plants in their embryonic stages, but are most often located in roots. Trees store their food in the tissues of trunks and branches in order to sustain themselves over winter when their leaves have dropped off and food cannot be generated through photosynthesis.
Root vegetables like beets, radishes, carrots, and potatoes, are starch-laden because the plants keep their energy stores underground, out of sight of animals hunting for a meal.
Soil also provides a stable environment for energy storage, as it is not as affected by changes in weather, which is why perennial plants tend to store starch below ground, where it can maintain its energy source through winter months until blooming in spring. Plant starch has become a major part of the human diet and is sought after by wildlife as well. Plants also store starches for future food creation, to provide sustenance in case of cloudier weather when photosynthesis cannot be used to generate glucose.
In this case, the plant metabolizes starches in its roots to produce glucose and continue producing energy. In some plants, starch is stored in cell organelles called amyloplasts. Some plant roots and embryos, in the form of seeds and fruit, also serve as storage units for starch. Cells in plant leaves produce starch in the presence of sunlight. To test for presence of starch, apply tincture of iodine to a cut surface of a fruit or vegetable. To test solid parts of plants like leaves and stems, pulverize them with a mortar and pestle.
Then, use drops of a tincture of iodine added to a test tube containing the crushed plant parts and sap. If starch is present in the plant's juices, the iodine will change color from dark brown to dark bluish-purple or black. After harvest, the glucose in the kernels of an ear of corn convert into starch over time, making the corn lose its flavor.
Each year, new hybrids of sweet corn are produced that allow the kernels in an ear of corn to retain their sweetness for a longer period post-picking. Genetic researchers are studying ways to increase the quality and quantity of starch in plant cells.
The food processing industry continues to see great demand for plant starch used in things like high fructose corn syrup and other foods. Scientists are studying the way plant cell walls are built. This starch is stored by special organelles, or cell subunits, called amyloplasts. Plant starch begins as glucose, a primary product of photosynthesis, or the process by which plants produce food from sunlight.
Glucose is difficult for plants to store, however, and is converted either to sucrose or starch through a process called polymerization. The polymerization and storage process in plants is performed by special cell parts—the amyloplasts. These non-pigmented organelles take glucose, turn it into starch and move it to another part of the cell, called the stroma. The stroma is the colorless, spongy cell matrix that supports the plant cell itself.
In tubers, rhizomes and other starch-storing plant organs, it also acts as a place to store food for later use.
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