Delayed Incompatibility of Siberian and Swiss Pine Grafts on Scots Pine in Krasnoyarsk Forest Steppe

Heteroplastic grafts of conifers are characterised by the phenomenon of delayed incompatibility, when scion trees that have been developing well and forming cones for decades begin to show signs of incompatibility and die. Studies of adult trees are mainly limited to comparing the xylem increments of the scion and rootstock; the phloem component and the fusion zone have hardly been studied, so the aim of the work was to study the processes occurring in adult homo- and heteroplastic grafts of conifers in the contact zone of the scion and rootstock. We studied the fusion zone’s anatomical structure of Pinus sibirica Du Tour and P. cembra L. scions grafted onto P. sylvestris L. rootstocks. To isolate phenomena directly related to incompatibility, the obtained data were divided according to the width of the xylem increments and compared with the data for successful homoplastic grafts of Scots pine and Siberian pine, as well as against non-grafted trees. In addition to the xylem study, special attention was paid to the structure of the bark in general and the conducting phloem in particular, including albuminous cells and accumulation of callose on sieve fields. At the first stages of delayed incompatibility development, in addition to the mismatch of xylem growth of the scion and rootstock and reduction of growth rates in general, new periderms formed closer to cambium in the fusion zone than these above and below the trunk, there was also an increase in parenchymatisation in the fusion zone and reduced cell specialization. In the studied successful grafts, the callose content in the fusion zone was higher than in ungrafted trees; a reduction in callose reserves in the fusion zone, especially on the rootstock side, may indicate a difficulty in transporting assimilates and be one of the symptoms of incompatibility.