The functional significance of phloem anastomoses in stems of Dahlia pinnata Cav.

The role of phloem anastomoses in translocation was studied experimentally in intact and wounded internodes of Dahlia pinnata Cav. Translocation was visualized with fluorescein, a fluorescent dye capable of movement in the phloem. Translocation was analyzed in large areas of living phloem tissue which were peeled from the xylem at the cambium region. Under normal conditions, fluorescein was observed in sieve tubes of the longitudinal phloem strands but very rarely in the sieve tubes of the anastomoses. However, when a few longitudinal strands were severed, fluorescein was translocated through the anastomoses located around the wound within 24 h. It is suggested that the phloem anastomoses in mature internodes of Dahlia serve mainly as an emergency system which enable a fast response to damage by providing alternative pathways for assimilates around the stem. A possible regulatory mechanism based on differences in resistance to flow in longitudinal versus lateral sieve tubes is discussed.