ABSTRACT

Although seasonal differences in drought damage to Japanese cedar (Cryptomeria japonica D. Don) have been well studied, the mechanisms underlying these seasonal differences remain poorly understood. In this study, we measured seasonal changes in current-year shoot growth; leaf mass per area (LMA); water potential at turgor loss point (Ψ_tlp), predawn (Ψ_pre), and midday (Ψ_mid); osmotic potential (Ψ_s); maximum transpiration rates; and maximum stomatal conductance in a 43-year-old Japanese cedar stand, with particular focus on the relationship between the shoot growth period and seasonal variation in Ψ_tlp, and the turgor safety margin (Ψ_mid – Ψ_tlp) to identify the season in which these trees are most vulnerable to drought. Current-year shoot growth began in April, with high shoot growth continuing until August. Ψ_tlp varied significantly throughout the year and was particularly high during the shoot growth period. Increasing LMA and decreasing Ψ_s caused a reduction in Ψ_tlp from autumn to winter, and Ψ_mid dropped to approximately the level of Ψ_tlp during the summer due to high transpiration activity, and increased with decreasing transpiration rates during the winter. Thus, the turgor safety margin was smaller in the summer than in the winter. These results suggest that Japanese cedar is most vulnerable to drought in the summer due to high Ψ_tlp during the shoot growth period.

摘要

尽管对日本雪松（Cryptomeria japonica D. Don）干旱损害的季节差异已进行了深入研究，但这些季节差异背后的机制仍知之甚少。在这项研究中，我们测量了当年枝条生长的季节变化；单位面积叶质量（LMA）；_{膨胀损失点 (Ψ tlp} )、黎明前 (Ψ _pre ) 和中午 (Ψ _mid )的水势；渗透势 (Ψ _s ); 最大蒸腾速率；43 年树龄的日本雪松林中的最大气孔导度和最大气孔导度，特别关注芽生长期与 Ψ tlp 季节变化之间的关系，_以及膨压安全裕度 (Ψ _mid – Ψ _tlp）以确定这些树木最容易遭受干旱的季节。今年的芽生长从四月开始，高芽生长持续到八月。Ψ _tlp全年变化显着，在芽生长期尤其高。从秋季到冬季，增加 LMA 和减少 Ψ _{s会导致 Ψ tlp}减少，并且在夏季，由于蒸腾活动较高， Ψ _{mid下降到大约 Ψ tlp}的水平，而在冬季，随着蒸腾速率的降低而增加。因此，夏季的膨胀安全裕度小于冬季。这些结果表明，由于 Ψ 高，日本雪松在夏季最容易遭受干旱。_tlp在芽生长期。