CORRELATION OF POINSETTIA GRAFT UNION DEVELOPMENT WITH TRANSMISSION OF THE FREE-BRANCHING CHARACTERISTIC

Euphorbia pulcherrima Willd. ex. Klotzsch cv. 'Eckespoint C-1 Red' (CR ), a restricted-branching poinsettia, and TR, a free-branching poinset tia, were approach grafted. Graft unions were removed for anatomical s tudy at 0, 5, 10, 15, 20, 25 or 30 days after grafting, and the portio n below the graft union was allowed to regrow. CR and TR plants had no t yet adhered at 5 days after grafting. By 10 days after grafting, CR and TR parenchyma cells were actively dividing, producing new parenchy ma cells (callus) on both sides of the necrotic layer. Parenchyma cell s differentiated into nodules for the formation of new cambium by 25 d ays after grafting. TR and CR plants were not connected by new vascula r tissue until 30 days after grafting. However, CR plants showed incre ased branching (axillary shoot development on lowermost nodes) by 10 d ays after grafting, indicating that vascular connections were not need ed for transmission of the free-branching agent, which has been tentat ively identified as a phytoplasma. Callus connecting TR and CR stems a llowed transmission of the phytoplasma through the graft union. (C) 19 97 Elsevier Science B.V.