Seasonal changes in leaf water relations in regards to leaf drought tolerance in mature Cryptomeria japonica canopy trees

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 (psi(tlp)), predawn (psi(pre)), and midday (psi(mid)); osmotic potential (psi(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 psi(tlp), and the turgor safety margin (psi(mid) - psi(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. psi(tlp) varied significantly throughout the year and was particularly high during the shoot growth period. Increasing LMA and decreasing psi(s) caused a reduction in psi(tlp) from autumn to winter, and psi(mid) dropped to approximately the level of psi(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 psi(tlp) during the shoot growth period.