Phloem translocation in regenerating in vitro-heterografts of different compatibility

Abstract

Phloem translocation of [14C]-sucrose and 5/6-carboxyfluorescein (CF) from scion into the stock was studied in in vitro-heterografts of Lycopersicon on Solanum (L/S) and Vicia on Helianthus (V/H) at various stages of regeneration. Autografts of all partners served as controls. Corresponding with the translocation experiments newly formed sieve-tube connections between the graft partners were counted. 14C-translocation experiments with [14C]-sucrose revealed an age-dependent increase of radioactivity in the stock of all combinations. In L/S and all autografts the major increase of 14C-label in the stock occurred 5-10 d after grafting. In V/H, however, import of label into the stock remained low throughout the regeneration period. In L/S grafts, increases in the numbers of sieve-tube connections parallel the increasing rate of 14C-transport, indicating functioning sieve-tube connections in the graft union. In contrast, V/H grafts did not show this strong correlation between structure and function of wound repair phloem. This suggested the existence of non-transporting sieve-tubes between the graft partners. Similar results were obtained with CF-transport, showing that effective phloem translocation across the graft interface occurred in L/S, but not in V/H grafts. The observed differences in phloem translocation are discussed with regard to compatibility/incompatibility phenomena in heterografts.