Weighty issues in respiratory metabolism: intriguing carbon isotope signals from roots and leaves.

Nonphotosynthetic cells of higher plants depend on the supply of a carbon substrate, usually sucrose, for cellular metabolism. Typically this sucrose is synthesized in leaves from the products of photosynthesis. Because of the lack of light below ground, roots do not generally photosynthesize (we note the exception of certain mangrove species and members of the Orchidaceae), although many roots from genetically distinct taxa, such as Asteraceae, Solanaceae and Cucurbitaceae, have the ability to turn green when grown in the light (Flores et al., 1993). Nonetheless, plant roots generally grow underground as heterotrophic organs, depending on leaves for their energy source.