Necrophylactic periderm formation in the roots of western larch and Douglas‐fir trees infected with Armillaria ostoyae. I. The response to abiotic wounding in non‐infected roots

The sequence of events leading to necrophylactic periderm formation was studied throughout the year following the abiotic wounding of the non‐infected roots of 10‐ and 27‐year‐old western larch (Larix occidentalis) and 11‐ and 25‐year‐old Douglas‐fir (Pseudotsuga menziesii) trees that were infected with Armillaria ostoyae. The sequence was the same for both ages and species of trees. Wound repair was more rapid in the summer compared with the spring and autumn. Following cell hypertrophy, a zone of lignified impervious tissue was in the initial stages of formation within 10 days of wounding in the summer and 14 days in the spring or autumn. The new phellogen produced a layer of phellem three to four rows of cells thick after 20 days in the summer or 40 days in the spring. Modified cells abutting the inner boundary of the impervious zone frequently developed thick lignified abaxial walls and thin suberized adaxial walls. A typical exophylactic periderm in healthy root bark tissue of both western larch and Douglas‐fir consisted of stone phellem one to four rows of cells thick and a layer of thin‐walled phellem three to six rows of cells thick in western larch and two to three rows thick in Douglas‐fir, a single row of phellogen cells and one to three rows of phelloderm cells. Mature thin‐walled phellem cells had pigmented contents, red in western larch and light brown in Douglas‐fir. In response to wounding, 27‐year‐old western larch and 25‐year‐old Douglas‐fir developed necrophylactic periderms with annual bands of phellem. The bands included a layer of phellem that was six to 12 and nine to 15 rows of cells thicker than the layer of phellem observed in the respective naturally developed exophylactic periderms. Fifty days following wounding in the summer, stone phellem, one to three rows of cells thick, was observed in the necrophylactic periderm of 10‐year‐old trees. When fully developed, the necrophylactic periderm in 27‐year‐old western larch also had a layer of stone phellem three to five rows of cells thick in each band. Stone phellem development was only sporadic in 25‐year‐old Douglas‐fir. Wounds in the winter showed no signs of activity associated with repair until dormancy broke in the spring.