Translocation of photoassimilates in wound-sieve tubes.

Microautoradiographic examination of wound-sieve tubes, formed after surgical interruption of a vascular bundle in the stem of Impatiens holstii Engl. et Warb, was conducted to investigate their structurefunction relationship in photoassimilate translocation. The irregular shaped wound-sieve tubes form tangential bridges connecting the wounded bundle with the intact neighbouring bundles. The wound-sieve elements differentiate in parenchyma cells which dedifferentiate and resume mitotic activity. Companion cells and various numbers of associated cells accompany the wound-sieve elements. Microautoradiographs of wound-sieve tubes were obtained from both cross and tangential sections after exposing the leaf above the wounded bundle to (14)CO2. Labeled photoassimilates were found in woundsieve tubes, beginning about 6 d after wounding. Adjacent to completed wound-sieve tube bridges there are small complexes of isolated wound-sieve elements and incomplete bridges. Almost all wound-sieve elements contain the nucleus. Functionality of a wound-sieve tube is reached when its members are connected by sieve plates.