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 Blume seedlings from two populations growing at different elevations and temperature regimes (low- and high-elevation sites) in northern Japan. We acclimated seedlings for 14 to 23 days at daytime temperatures of either 22 degrees C (control) or 27 degrees C (warm treatment) and obtained photosynthetic temperature-response curves in the range of 19 to 32 degrees C. The optimum temperature of photosynthesis (T-opt) was similar to 0.6 degrees C higher in seedlings acclimated at 27 degrees C than in those acclimated at 22 degrees C, and it was significantly lower in seedlings with higher stomatal sensitivity to leaf-to-air vapor pressure deficit than in those with lower sensitivity. The effects of warm treatment, population and treatment-population interaction on T-opt were not significant in the two-way analysis of variance, but the effect of treatment became significant when stomatal sensitivity to leaf-to-air vapor pressure deficit was included as a covariate in the model. Structural equation modeling indicated that seedlings with lower root biomass had lower T-opt because of the high stomatal sensitivity to leaf-to-air vapor pressure deficit. Structural equation modelling also indicated that the way of shifting the T-opt differed between the two populations: seedlings from a high-elevation site depended on decreasing photosynthetic rates at low temperatures for the increase in T-opt but seedlings from a low-elevation site did not. We suggest that the 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.