ABSTRACT
Phloem loading provides the driving force for long-distance translocation of photoassimilates. The most obvious distinguishing feature of the intermediary cell is the presence of very numerous plasmodesmata at the interface where loading presumably occurs, that is between the intermediary cell and the bundle-sheath Minor veins are typically composed of four cell types: vascular parenchyma, companion cells, sieve- tube members, and xylem elements. An apoplastic loading mechanism requires that transport sugars be actively absorbed from the apoplast, and that these transport sugars are selectively loaded into the sieve element-companion cell complex of the minor veins. Autoradiographic images are useful in localizing sucrose uptake events, but they are difficult to quantitate and they do not provide direct evidence for uptake against a concentration gradient. The most convincing experiments to date are those that employ the inhibitor β-chloromereuribenzenesulfonic acid to eliminate saturable uptake of sucrose, phloem loading, and translocation.
