The Function of Phloem Connections in Regenerating In Vitro‐Grafts*

Abstract

AbstractIn order to answer the question whether functioning phloem connections exist between graft partners, phloem transport has been studied in cultured explant‐grafts after application of 14C‐sucrose and carboxyfluorescein (CF) to the scion. Autografts of Lycopersicon esculentum and Helianthus annuus were investigated at various regeneration periods. Ungrafted internodes served as controls.A segmental analysis was used to determine the tissue distribution of 14C‐sucrose in a graft. The 14C‐profiles obtained show that sucrose translocation across the graft interface started 4 days after grafting and increased later. The observed translocation appears to occur via wound phloem, since at this time the first complete wound‐phloem bridges (shown as files of aniline‐blue‐positive sieve plates) traverse the graft interface. In 7‐d‐old autografts, sucrose transport across the graft interface returned to normal again, as indicated by the distribution of the label. In addition, 14C‐profiles reveal accumulation of label in sink tissues. Here the basal callus of the stock, and temporarily the graft union itself, represent the main sinks for labelled sucrose.Translocation of CF was analyzed in hand sections of the grafts. The beginning of translocation into the stock was confirmed with the dye. Moreover, effective phloem translocation across the graft interface, visualized with CF, could undoubtedly be assigned to wound‐phloem bridges reconnecting the cut vascular bundles of scion and stock. Thus, the function of phloem connections in regenerated in vitro‐grafts is directly shown.