Abstract
To improve the performance of pinewood nematode-resistance of seedlings obtained from the nematode-resistant seed orchards, it is important to examine the relative implications of several factors affecting the performance that previous studies have highlighted. We evaluated the resistance of seedlings collected from 64 mother trees of 22 nematode-resistant Japanese red pine (Pinus densiflora) clones planted at six nematode-resistant seed orchards, which are different ages and consisted of different clones. After the nematode-inoculation tests, the soundness rate of two-year-old seedlings exhibited highly positive correlation with the resistance ranking of the mother clone in most of orchards in two times of the trial. Paternity analyses for the seedlings based on eight nuclear microsatellite markers showed that seedlings collected in the orchards younger than ten years old had higher percentages of external paternity and lower sound tree rates. Generalized linear mixed models showed that the resistance rankings of both the mother clone and the assigned paternal parent had large positive effects on the soundness rate of seedlings, and the proportion of internal paternity, which increased with the orchard age, also had significant positive effects. These results indicate that, to improve the level of the resistance of seedlings, it is effective to (1) change the clones into those with genetically higher resistance and (2) collect seeds at more than fifteen years old or regenerate orchards partially.
