Intraspecific variation in photosynthetic thermal acclimation in Fagus crenata seedlings from two populations growing at different elevations in northern Japan.

Plants can acclimate their photosynthesis to growth temperature, but the contribution of local adaptation to intraspecific variation in thermal acclimation of photosynthesis is not fully understood. Here, we experimentally investigated the photosynthetic thermal acclimation in Fagus crenata seedlings from two populations growing at different elevations and temperature regimes (low- and high-elevation sites) in northern Japan. We acclimated seedlings for 14-23 days at 22 °C (control) or 27 °C (warm treatment) daytime temperature and obtained photosynthetic temperature-response curves in the range of 19 to 32 °C. The optimum temperature of photosynthesis (Topt) was ca. 0.6 °C higher in seedlings acclimated at 27 °C than in those acclimated at 22 °C, and it was significantly lower in seedlings with higher stomatal sensitivity to leaf-to-air vapor pressure deficit (VPD) than in those with lower sensitivity. The effects of warm treatment, population, and treatment-population interaction on Topt were not significant in the two-way analysis of variance, but the effect of treatment became significant when stomatal sensitivity to leaf-to-air VPD was included as a covariate in the model. Structural equation modelling (SEM) indicated that seedlings with lower root biomass had lower Topt because of the high stomatal sensitivity to leaf-to-air VPD. SEM also indicated that the way of shifting the Topt differed between the two populations: seedlings from a high-elevation site depended on decreasing photosynthetic rates at low temperatures for the increase in Topt but seedlings from a low-elevation site did not. We suggest that difference in thermal acclimation of photosynthesis between the two populations may reflect adaptation to different climate regimes and that belowground traits should be considered when investigating thermal acclimation capacity, especially in seedlings.