Seasonal changes in temperature response of photosynthesis and its contribution to annual carbon gain in D aphniphyllum humile , an evergreen understorey shrub

We evaluated seasonal variation in photosynthetic temperature dependence and its contribution to annual carbon gain in an evergreen understorey shrub, D aphniphyllum humile M axim, growing at the forest border and in the understorey of a deciduous forest. Plants at both sites exhibited similar optimal temperatures for photosynthesis ( T opt ). The activation energy for ribulose‐1,5‐bisphosphate (Ru BP ) carboxylation ( H aV ) at both sites tended to be higher in summer than in spring or autumn, suggesting that H aV may be the controlling factor in the T opt shift in D . humile . In contrast to the seasonal changes in T opt , the maximum photosynthetic rate at the optimal temperature ( P opt ) differed between the two sites: it was lower in autumn than in summer at the forest border, but was the same in summer and autumn in the understorey. In the understorey plants, nitrogen content ( N area ) increased in autumn, but this was not the case for forest border plants. In addition, R ubisco content increased significantly in autumn in the understorey leaves but decreased distinctly in forest border leaves. Increased N area and R ubisco in understorey leaves resulted in increased in photosynthesis in autumn. Annual carbon gain was 30.8 mol·m −2 in forest border leaves and 5.8 mol·m −2 in understorey leaves. Carbon gain in understorey leaves during the short period after overstorey leaf fall and before snow accumulation was approximately 49% of annual carbon gain. Furthermore, autumn carbon gain calculated using activation energy of summer with autumn photosynthetic parameters underestimated the autumn carbon gain by as much as 31%. In conclusion, photosynthetic temperature acclimation may be a key factor in increasing annual carbon gain in understorey D . humile .