ABSTRACT
Carbon, hydrogen, and oxygen constitute 96% of a plant’s dry weight. Source leaves are the primary sites of C reduction and the main organs exporting reduced C to growing sinks. It is well known that in almost all species, sugars, starch, and amino acids accumulate in leaves during the daytime and export of assimilates derived from these reserves occurs both concurrently with photosynthesis and subsequently during night periods [1-5]. Our overall knowledge of translocation processes has been derived from diverse experimental approaches [6-11]. For example, imaging techniques, which include light, electron, and fluorescence microscopy using dyes or proteins, provide valuable qualitative data on intercellular connections and export [12-16]. Generally, these imaging techniques are destructive. However, procedures using isotopes of carbon (e.g., mass isotopes, 13C, and radioisotopes, 11C and 14C) to study export can be both quantitative and noninvasive [7,17-19]. Phloem sap exudation from cut sieve tubes or from aphid stylectomy has provided a practical means of sampling mobile assimilates [20]. Collection of apoplastic fluids [21-22] and measurements of pH and membrane potential [23] further demonstrate the physiological and biochemical interactions that operate intercellularly as sugars are loaded or unloaded from the phloem. More recently, molecular techniques have led to characterization of sugar transporters [24] and the engineering of transgenic plants that can be designed to provide important information regarding the role of specific export processes in the leaves [11,25].
