Visualization of the 3D structure of the graft union of grapevine using X-ray tomography

Abstract

Successful grafting in plants requires the development of a functional vascular system between the scion and the rootstock. Understanding the spatial organization of the graft interface is important to the evaluation of new rootstock genotypes and to the development of new grafting technologies. Until now the graft interface has only been studied using 2D classical histology and low resolution 3D magnetic resonance imaging. Here we investigate the ability of X-ray tomography to examine the graft interface of Vitis vinifera in high resolution and in 3D.Data were collected using a Skyscan 1076, scanning parameters, such as, X-ray energy, filter selection, pixel size and rotation angles, were optimized to study the particularities of the graft interface. The X-ray tomography technique was then used to evaluate graft quality. Two young vines were compared; one graft was classified as of ‘good’ quality, whereas the other was classified as of ‘bad’ quality. We were able to distinguish the “omega cut”, the pith, the phloem and the xylem vessels in the images. The analysis shows several differences between the two vines. In the good graft, tissues appear well-connected in the wood and phloem, and had a regular structure; the wood appears homogenous with a lot of vessels that form a compact mass. By contrast, in the bad graft, the structures appear disorganized and not completely connected.Numerous new vessels, continuous between the scion and the rootstock, are visible in the “good graft” whereas only few ones are visible in the “bad one”. It is the first time, to our knowledge, that 3D imaging of the graft interface and the vascular connections across it have been reported, opening new avenues for graft quality assessment in woody plants.