New research shows that to prevent starvation at night, plants perform accurate arithmetic division. The calculation allows them to use up their starch reserves at a constant rate so that they run out almost precisely at dawn. Plants feed themselves during the day by using energy from the sun to convert carbon dioxide into sugars and starch. Once the sun has set, they must depend on a store of starch to prevent starvation.

 

Starch is a carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store. It is the most common carbohydrate in the human diet and is contained in large amounts in such staple foods as potatoes, wheat, maize (corn), rice, and cassava.

 

During the night, mechanisms inside the leaf measure the size of the starch store and estimate the length of time until dawn. Information about time comes from an internal clock, similar to our own body clock. The size of the starch store is then divided by the length of time until dawn to set the correct rate of starch consumption, so that, by dawn, around 95% of starch is used up.

 

The scientists used mathematical modelling to investigate how such a division calculation can be carried out inside a plant. They proposed that information about the size of the starch store and the time until dawn is encoded in the concentrations of two kinds of molecules (called S for starch and T for time). If the S molecules stimulate starch consumption, while the T molecules prevent this from happening, then the rate of starch consumption is set by the ratio of S molecules to T molecules, in other words S divided by T.