Development of internal graft incompatibility symptoms in Pinus radiata D. Don.

Detailed anatomical study of P. radiata grafts revealed that phloem tissues of incompatible grafts are abnormally thick. Grafts with thick phloem often displayed "pitted " stems. Thickened bark resulted jointly from overproduction of sieve cells and underproduction of tracheids. Normal pattern of phloem differ-entiation was altered and phloem areas formed which lacked axial parenchyma. Other internal symptoms in union zones of some incompatible grafts were suberisation and necrosis of cortex cells, excessive tannin accumulation indicated by abnormally darkly-stained bark tissues, formation of abnormal parenchyma in the xylem, and atypical dilation of horizontal ray cells and axial parenchyma. Increasingly more grafts developed symptoms of incompatibility as graft age increased: 10%, 24%, 35%, 54%, and 72 % of 1-, 3-, 5-, 8-, and 13- to 18-year-old grafts, respectively. Possible interactions between occult viruses and true stock-scion incompatibility may be responsible for the graft survival and vigour problems.