Phloem unloading and sink loading

• Whereas phloem loading at the source is an active process (requiring energy), unloading of the phloem at the sink is generally a passive process (i.e., does not require energy).
• In general, levels of sucrose in the cytosol of the sink tissue are kept low by enzymatic hydrolysis to glucose and fructose, and by the incorporation of the resulting sugars into polysaccharides (e.g., starch).
• It has been shown, however, that the uptake of assimilates by wheat endosperm cells and legume embryos is facilitated by an energy-dependent membrane process similar to that responsible for phloem loading from the apoplast, although the movement is down a concentration gradient.
• In tomatoes, there is evidence that suggests that the rate of assimilate import is inversely related to the sucrose content of the fruit, implying that the rate of import depends on the concentration gradient.
• Differences in the rate of fruit growth have been associated with sucrose hydrolysis by acid invertase. Sucrose is also converted into glucose and fructose before entering the maize endosperm.
• The rate of assimilate uptake is also influenced by sink metabolism and physical restrictions for the movement of assimilate. In wheat and maize, the rate of kernel growth is related to several endosperm cells and the number of starch granules per cell. The position of a sink relative to other competing sinks may influence rates of assimilate import in terms of proximity (to assimilate supply) and resistance of the pathway (i.e., kernels and the distal tip of the maize ear tend to abort first or are frequently smaller than kernels close to the base of an ear).