A STUDY OF THE GRAFT UNION IN IN-VITRO MICROGRAFTED APPLE

Light microscopy was used to study graft union formation in in vitro m icrografts of tissue cultured apple (Malus domestica. Borkh). Microgra fts were constructed using horizontal incisions to form the grafting s urface, and placing the cut ends of rootstock and scion into sterile s ilicon tubing to permit graft formation to occur. The outer morphologi cal and histological development was similar for different stock-scion micrograft combinations but graft union formation was slower in heter ografts than in autografts. Initial leaf expansion at the scion shoot apex occurred in all micrografted plantlets within 1-4 days and was no t indicative of graft success. Progressive scion growth and developmen t could be used as an indication of graft success by ten to fourteen d ays after grafting and probably was related to establishment of cell t o cell contact at the graft interface. Microscopy showed initiation of callus proliferation in the vascular cambium and the pith ten days af ter grafting. Differentiation was observed subsequently and this was r eflected in scion development. Longitudinally orientated cambial cells began to differentiate between twenty and forty days after grafting, and formed a bridge between the vascular cylinders of scion and rootst ock. The scions at this stage had as many as eight newly expanded leav es and micrografts were strong enough to permit silicon sleeve removal without damage. Continuity of new vascular elements in rootstock and scion was established around forty days. New vascular elements curved slightly in towards the pith to form a 'c' shaped bridge across the gr aft union. Vascular development continued until it reached completion after six months.