Photosynthetic responses to ozone of upper and lower canopy leaves of Fagus crenata Blume seedlings grown under different soil nutrient conditions

Abstract

We aimed to clarify the effects of ozone (O₃) on photosynthetic ability of upper and lower canopy leaves of Fagus crenata Blume seedlings grown under different soil nutrient conditions. To accomplish this objective, we analyzed the response of photosynthetic parameters such as maximum carboxylation rate (V_cmax) to cumulative stomatal O₃ uptake (ΣF_st) and reduction rate of V_cmax per unit ΣF_st as an index of detoxification capacity for O₃. The seedlings of Fagus crenata were grown for two growing seasons (2014-2015) in nine treatments comprised of a combination of three levels of gas treatments (charcoal-filtered air or 1.0- or 1.5-times ambient O₃ concentration) and three levels of soil nutrient treatments (non-fertilized or a supply of relatively low or high concentrations of compound fertilizer). The nutrient supply significantly increased the degree of O₃-induced reduction in V_cmax in September. However, nutrient supply did not significantly increase ΣF_st and reduce the detoxification capacity for O₃. On the other hand, the degree of O₃-induced reduction in V_cmax of upper canopy leaves was higher as compared with that of lower canopy leaves in August due to the higher ΣF_st. However, the reduction rate of V_cmax per unit ΣF_st in lower canopy leaves was higher than that in upper canopy leaves, indicating lower detoxification capacity for O₃ in lower canopy leaves. Reduction rate of V_cmax per unit ΣF_st over the threshold, which is assumed to be proportional to gross photosynthetic rate, was similar between upper and lower canopy leaves. Therefore, capacity of photosynthetic CO₂ assimilation is likely to be associated with detoxification capacity for O₃ in upper and lower canopy leaves of F. crenata seedlings grown under different soil nutrient conditions.

Keywords: Canopy leaf position; Detoxification capacity; Fagus crenata Blume; Soil nutrient conditions; Stomatal ozone uptake.