Phloem loading in the tulip tree. Mechanisms and evolutionary implications.

Minor vein ultrastructure and phloem loading were studied in leaves of the tulip tree (Liriodendron tulipifera; Magnoliaceae). Plasmodesmatal frequencies leading into minor vein companion cells are higher than in species known to load via the apoplast. However, these companion cells are not specialized as "intermediary cells" as they are in species in which the best evidence for symplastic phloem loading has been documented. Mesophyll cells plasmolyzed in 600 mM sorbitol, whereas sieve elements and companion cells did not plasmolyze even in 1.2 M sorbitol, indicating that solute accumulates in the phloem against a steep concentration gradient. Both [(14)C]sucrose and (14)C-labeled photo-assimilate accumulated in the minor vein network, as demonstrated by autoradiography. [(14)C]sucrose accumulation was prevented by p-chloromercuribenzenesulfonic acid, an inhibitor of sucrose-proton cotransport from the apoplast. p-Chloromercuribenzenesulfonic acid largely, but not entirely, inhibited exudation of radiolabeled photoassimilate. The evidence is most consistent with the presence of an apoplastic component to phloem loading in this species, contrary to speculation that the more basal members of the angiosperms load by an entirely symplastic mechanism.