Understanding the mechanisms of graft union formation in Solanaceae plants using in vitro techniques

The failure of graft union formation in incompatible combinations of tomato/capsicum (scion/rootstock) and eggplant/capsicum grafts were investigated using in vitro techniques. Tomato, eggplant and capsicum shoots were grafted onto capsicum calli. There was no effect of scion species on capsicum callus growth. Vascular bundles formed in the calli that were produced from 'scion' shoots of all species. In the 'rootstock' calli, no vascular tissue formed in the tomato/capsicum combination and limited vascular tissue was produced in the eggplant/capsicum combination. Bundles of vascular tissues were evident in the 'rootstock' calli of capsicum/capsicum combination. This suggests that the failure of graft union formation in some solanaceous plants may be due to difficulties in the differentiation of vascular bundles in the rootstock callus at the graft union. To enhance vascular bundle differentiation in the graft incompatible combinations, IBA, 0.5 mM, was applied to the scion cut surface before grafting onto rootstock stems. However, this treatment failed to promote grafting success. Although auxin may not be the reason for the graft incompatibility, further trials will be required.