Figure 1.

Schematic diagrams of putative phloem-loading strategies at the minor vein (A–C) and whole leaf (D–F) levels. In A and D, Suc diffuses through plasmodesmata (gaps in walls) into the minor vein companion cells (CC) and sieve elements (SE), a passive process. The Suc concentration in mesophyll cells (M), and therefore the entire leaf, is high; the Suc concentration in the veins is slightly lower. In B and E, Suc diffuses through plasmodesmata into the minor vein CCs (intermediary cells) and is converted to raffinose and stachyose, thus actively elevating the transport sugar concentration in the phloem by polymer trapping; this allows the mesophyll cells to maintain low Suc levels. In C and F, Suc is pumped from the apoplast into the minor vein phloem by transporters (yellow circle), which also enables the leaf to minimize the overall Suc concentration in the leaf blade. Phloem parenchyma cells, which may constitute part of the transport pathway in A and C, are not shown. For a diagram of sugar alcohol loading strategies see Rennie and Turgeon (2009).