Histological study of micrografting in passionfruit

Abstract

Micrografting, used to eliminate viruses, involves the utilization of very small grafts, and detailed structural analyses of the micrografting region in different phases are presented here. Shoot tips with 2–3 leaf primordia, and 600–800 µm in length, were grafted to the hypocotyl of 21–28 day-old rootstock seedlings, and their development was followed for 30 days using scanning electron and visible light microscopy. The success of micrografting was found to depend on the preservation of the vascular tissue of the rootstock and the placement of the scion adjacent to the rootstock phloem. Callus formation, which initiates approximately 3 days after micrografting (DAM) through the proliferation of parenchymatous cells of the rootstock cortex, fills the space left by the incision and guarantees adherence and nutrition for the scion during the initial phases of development. At seven DAM, connective cells that develop at the base of the scion produce a junction with the callus. At 10 DAM, differentiation of procambial strands and parenchymatous cells initiates in the callus. At 15 DAM, parenchymatous cells derived from the callus give rise to procambial strands and initiate the differentiation of tracheal elements and the epidermis in the junction region. Vascular connections are established at 20 DAM, promoting the accelerated development of the scion, which, at 30 DAM, shows shoot development. The developmental phases of micrografting in passionfruit plants therefore include: placement of the scion; callus formation by the rootstock; cellular connections; differentiation of the callus; vascular connection; and shoot development.