Pinus parviflora var. parviflora is naturally distributed and forms isolated small populations over the University of Tokyo Chiba Forest in the Boso hills, Chiba Prefecture. These populations have recently declined probably due to pine wilt disease. Production of P. parviflora var. parviflora seedlings which is resistant to pine-wood nematode has been currently planned. However, open-pollinated seedlings derived from natural mother trees are often resulted from selfing, which might make them more susceptible to pine wood nematode. In this study, we inoculated virulent nematode isolate (Ka-4) into open-pollinated seedlings and crossed-pollinated (outcrossed) seedlings, and then compared their survival rates. The survival rate of open-pollinated seedlings from one natural tree Arakashizawa-13 was 0 %, whereas that of cross-pollinated seedlings of which one parent was the same tree (Maesawa-1 × Arakashizawa-13) was significantly high (65%). The height and root collar diameter for the open-pollinated seedlings were significantly smaller than those for the cross-pollinated seedlings. In contrast, the survival rate of open-pollinated seedlings from the other natural tree (Arakashizawa-3) was not significantly different from that of cross-pollinated seedlings of which one parent was the same tree (Maesawa-5 × Arakashizawa-3). Either of the height and root collar diameter did not show significant difference between these seedlings. Thus, it was assumed that there should be natural trees which can produce seedlings without inbreeding depression by aritificial pollination and increase resistance to pine-wood nematode.

The long-term preservation of germplasm (seeds or pollen) is one of the most important methods for conserving forest genetic resources. In particular, the long-term storage of seeds has several advantages over pollen for the efficient conservation of genetic resources as most tree seeds have ‘orthodox’ characteristics and can usually be readily dried down for long-term storage. However, oak seeds (acorns) are ‘recalcitrant’ , which means that they do not tolerate moisture loss without adversely affecting viability, making it dif.cult to store them for a useful period. From previous studies, it has been determined that to maintain viability of oak seeds storage at low temperatures and humidity and protection from biotic damage (ex. insect, fungus) is vital. Additionally, new storage techniques involving freezing-should be trialled for Japanese oak species.