ABSTRACT
The carbohydrate source available to plants fluctuates as a result of the impact of variable light and environmental conditions on photosynthesis. Carbohydrates are polyhydroxy-aldehydes or ketones and contain carbon, hydrogen, and oxygen in the ratio 1:2:1. Three carbon or triose sugars are the simplest carbohydrates and include glyceraldehyde and dihydroxyacetone. Carbohydrates with free aldehyde or ketone groups under alkaline conditions can reduce a cupric ion to a cuprous ion and are therefore also called reducing sugars. Because hydrolysis of glyceraldehyde and dihydroxyacetone yields products that are not carbohydrates, they are also described as monosaccharides. The photoassimilates stored as starch or other carbohydrates must subsequently be broken down to their component monosaccharide units to be available to the plant to meet their respiratory needs. The carbohydrate biosynthesis and breakdown in plants are reviewed. Fructans are probably the most abundant storage carbohydrates in plants after starch and sucrose.
