ABSTRACT
This chapter reviews the main physiological data which could provide an explanation or a basis for the response observed at the plant and crop level. It considers the source properties of the maize (corn) leaf and presents carbon export and allocation patterns and some of the mechanisms of regulation through source-sink manipulations. The carbon fixation in maize, as in any C4 plant, is a two step process involving two different carboxylases located in the mesophyll and bundle-sheath leaf tissues, respectively. Sucrose is synthesized only in the cytosol of green cells and never in the chloroplast. The synthesis of sucrose and starch, the two main stable products of CO2 assimilation in maize, is also compartmentalized. The main regulatory enzyme on the starch synthesis pathway is adenosine diphosphate-glucose pyrophosphorylase, which provides the adenosine diphosphate-glucose for the starch polymerizing enzyme. The first level of source-sink regulation operates between the carbon fixation sites and the primary sinks represented by leaf carbohydrate pools.
