Abstract

In vascular plants, three elements, carbon (C), hydrogen, and oxygen, comprise 96% of the plant’s dry weight. Source leaves are the primary sites of C reduction and the main photosynthetic 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 (Fondy and Geiger, 1982; Gordon, 1986; Kalt-Torres et al., 1987; Wardlaw, 1990; Geiger and Servaites, 1994; Weise et al., 2011). Our overall knowledge of translocation processes has been derived from diverse experimental approaches (Canny, 1973; Geiger, 1980; Milburn and Kallarackal, 1989; Olrich and Komor, 1989; Farrar, 1993a,b; Heldt, 1997; Gamalei, 2002). For example, imaging techniques that include light, electron, and fluorescent microscopy using dyes or proteins provide valuable qualitative data on intercellular connections and export (Peterson and Currier, 1969; Gamalei, 1985; Knoblauch and van Bel, 1998; Côté et al., 1992a; Fricker and Oparka, 1999). 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 also be employed and be both quantitative and nondestructive (Geiger, 1980; Minchin, 1986; Jiao and Grodzinski, 1996; Verscht et al., 1998; Minchin and Thorpe, 2003). Phloem sap exudation from cut sieve tubes or from aphid stylectomy has provided a practical means of sampling mobile assimilates (Weibull et al., 1990). Collection of apoplastic fluids (Ntsika and Delrot, 1986; Tetlow and Farrar, 1993a,b) and measurements of pH and membrane potential (Delrot, 1981) further demonstrate the physiological and biochemical interactions that operate intercellularly as sugars are loaded or unloaded from the phloem (Gamalei, 2002). More recently, molecular techniques have led to the characterization of sugar transporters (Kuhn et al., 1999; Bräutigam and Weber, 2011) and the engineering of transgenic plants that can be designed to provide important information regarding the role of specific export processes in the leaves (Frommer and Sonnewald, 1995; Heldt, 1997).