We grafted scions of Dahurian larch (Larix gmelinii var. japonica) onto Japanese larch (L. kaempferi) and F1 hybrid larch (L. gmeli­nii var. japonica × L. kaempferi) rootstocks and examined root­stock-scion compatibility by assessing the survival rate (SR) in two independent experiments. Scion overgrowth on the root­stock was not observed. SR was not significantly different among rootstocks due to large interquartile ranges (IQR) among clones within a rootstock type. Results suggested that the SR was more dependent on the clonal characteristics of the scion than on the growth vigor of the rootstock. Shoot elonga­tion of grafts on F1 hybrid rootstock was superior to that of grafts on Japanese larch rootstock. Selection of an appropriate combination of scion and rootstock may improve the SR of grafted Dahurian larch and shorten the cultivation period.

A previous study revealed low survival rates for Japanese larch (Larix kaempferi) summer-planted seedlings grown in Hiko-V-120 containers. This study examines nursery practices that could potentially prevent deterioration of the seedling water balance after planting to improve the survival rate of this species, which has a low drought tolerance. During summer planting, we tested (1) drought hardening or high-potassium fertilization for two months before planting, (2) antitranspirant or topping treatment at planting, and (3) the use of the JFA-150 container with a larger capacity and lower growing density than the Hiko-V-120 container. Drought hardening increased seedling drought tolerance because of the low leaf:root ratio, due to lower leaf mass production, resulting in increased survival from 74% to 93% in Hiko-V-120 containers. When JFA-150 containers were used, the leaf:root ratio was lower because of higher root mass, resulting in an increase in survival to 87%, with the highest survival of 97% when combined with drought hardening. The application of antitranspirant increased survival to over 90%, whereas topping did not, probably because of severer competition from weeds. High-potassium fertilization did not affect seedling traits or survival. For better survival of summer-planted container-grown Japanese larch seedlings, it is recommended that they be grown in containers providing sufficient cell volume and density for root growth while the seedlings are in the nursery and that irrigation be withheld for two months before planting. In addition, to obtain higher survival, an antitranspirant can be applied at planting at a cost.

The hybrid between Kuril larch (Larix gmelinii var. japonica) and Japanese larch (L. kaempferi) is an important afforestation tree species in Hokkaido, Japan, because of its rapid juvenile growth, straight stem, and resistance to bark gnawing by voles. To produce desirable hybrid seedlings, precise seedling discrimination is essential. However, continuous variations in morphological and phenological traits occur across L. gmelinii var. japonica × L. kaempferi and L. gmelinii var. japonica × L. gmelinii var. japonica seedlings. Therefore, we used DNA markers to verify and improve the morphological and phenological discrimination method. We collected seeds from an interspecific seed orchard and, using chloroplast DNA analysis, we showed that the hybridization rates of 1-year-old seedlings were different between years (2004, 23.2 %; 2005, 53.6 %) and between mother trees (2004, 5.8–39.4 %; 2005, 20.0–81.0 %). Discriminant analyses revealed that the root collar diameter of 2-year-old seedlings, number of sylleptic branches of 2-year-old seedlings, and day of terminal bud set in 1- and 2-year-old seedlings are traits that aid in discriminating hybrid seedlings. The proportions of correctly discriminated individuals were found to be 81.7–88.2 % when using improved phenotypic discrimination methods. These methods adopted traits with high discrimination abilities during discriminant analyses and determined the selection intensity according to the hybridization rate estimated from the DNA markers. In contrast, the proportions of correctly discriminated individuals obtained using the current method were lower, 72.7–78.5 %, because this method was based on seedling height and adopted a constant selection intensity.

Demand for nursery stock of Japanese larch (Larix kaempferi) is growing. Containerized seedlings are expected to meet this demand. However, the conventional methods are labor intensive and time consuming because one-year-old bare-root seedlings are transplanted to containers. In this study, we selected seeds using a spectroscope and examined two propagation methods, direct sowing on nursery container (direct sowing) and sowing on a small plug tray followed by transplantation to a container (plug transplant), to shorten the nursery period of 2 years. Germination rate increased in the selected seed when compared with the unselected seed (χ² test). The days to germinate (DtG) shortened with the use of heavier seeds, and the one-year-old seedlings grew taller as the DtG decreased. However, transplantation from plug trays had negative effects (generalized linear model (GLM) and analysis of variance (ANOVA). On the other hand, large volume cell had positive effects on root collar diameter (GLM and ANOVA). The results revealed that the nursery periods were shortened by using large volume cell, irrespective of the seedling type (direct sowing or plug transplant). Graphical abstract

炭素固定能が高く，野鼠害抵抗性，材強度に優れるグイマツ（Larix gmelinii var. japonica）×カラマツ（L. kaempferi）のF1雑種「クリーンラーチ」は，挿し木で苗が生産されているが，挿し付け時期が限られること，育苗ハウスの環境制御が難しく得苗率が低いことから需要を満たしていない。そこで筆者らは閉鎖型育苗施設で挿し木苗の通年生産技術を開発している。自然環境下では休眠導入できない9月に挿し付けた苗を対象に最適な休眠導入条件を検討するため，短日条件下（8時間）で温度を23℃，10℃，5℃，暗黒条件下で－5℃に段階的に下げる試験を実施した。23℃の工程を省略し10℃で短日（8時間）処理を開始すると頂芽が形成されず，最初に高温（23℃）短日処理が必要であった。23℃短日処理を行わない，もしくは次のステップの10℃短日処理が短い（1週間）と休眠導入中に苗が枯死し，休眠解除後の成長に負の影響があった。23℃短日3週間-＞10℃短日4週間-＞5℃短日2週間の休眠導入条件の場合，暗黒条件下での－5℃4週間の低温暴露で休眠が解除され，その後の生存，成長も良好であった。