Vascular Differentiation in the Graft Union of in-vitro Grafts with Different Compatibility. — Structural and Functional Aspects

Abstract

Summary Various in-vitro grafts of explanted internodes - the heterografts Nicandra physaloides on Lycopersicon esculentum and Vicia faba on Helianthus tuberosus as well as autografts of Nicandra physaloides and Vicia faba - were established in sterile culture. The histology of the graft union was studied by light and electron microscopy at different stages of development following the grafting procedure. Callus proliferation at the graft union started mainly in close association with the cut vascular bundles, while in the pith region an isolation layer was formed. Wound vascular elements differentiated either directly from parenchyma or, as a rule, from callus cells 3–5 days after grafting. Continuous wound xylem and wound phloem reconnecting the cut vascular bundles of both partners across the graft interface were formed in Nicandra as well as in Vicia autografts after 5 days. In the interfamiliar graft Nicandra/Lycopersicon continuous phloem connections were found 7 days after grafting. Wound cambium giving rise to secondary phloem and xylem elements was established in Nicandra autografts 7–10 days following grafting. Functioning of the wound phloem across the graft union has been examined at different developmental stages of in-vitro grafts using 14C-sucrose. One to 3 days after grafting there was only a small import of labelled sucrose into the stock, likely due to symplasmic or apoplasmic diffusion across the callus of the graft union. The translocation rate increased with the onset of phloem regeneration about 5-7 days after grafting. In the less compatible Nicandra/Lycopersicon graft about 4 weeks after grafting the import into the stock decreased again and abnormal vascular complexes were found in the scion at the same time; finally the whole graft withered away. Incompatibility of the interfamiliar graft Vicia faba on Helianthus tuberoses was characterised by a complete loss of vascular regeneration between the scion and stock and - consequently - by an extremely low rate of 14C-translocation into the stock.