Abstract: The responses of anatomical structure and photosynthesis characteristics of Carpinus tientaiensis seedlings to three simulative irradiance regimes (viz. full irradiance, FI, moderate irradiance, MI, and low irradiance, LI) were examined in this present study. The results showed that plants under FI exhibited the thickest leaves with the significant greater leaf mass per area (LMA), greater chlorophyll a/b ratio, maximal net photosynthetic rate (Pmax), respiration rate (R_d), carboxylation efficiency (CE), and higher nitrogen content per unit leaf area (N_area), among the three irradiance regimes. The chloroplasts of FI leaves were tightly packed in the mesophyll cells and had no spaces along the mesophyll cell walls, the chloroplasts in MI leaves were randomly dispersed throughout the cytoplasm of mesophyll cells, and the chloroplasts of LI leaves had a relatively sparse distribution along the mesophyll cell wall, and left the wide open spaces. The thickened mesophyll in FI leaves increased the availability of space along the mesophyll cell wall for chloroplasts and the surface area of both mesophyll cells (S_mes) and chloroplasts (S_c) facing intercellular space, and there thus was higher Pmax. The higher capacity of photosynthetic quantum conversion in FI leaves was also evidenced by the higher values of the chlorophyll fluorescence parameters of F_v/F_m and F_v/F_o. Thus, C. tientaiensis can acclimate to full sunlight by altering leaf anatomy and hence increase the photosynthetic capacity, as a result it can grow under the high light intensity such as forest gaps.

Key words: Carpinus tientaiensis, irradiance, leaf anatomical structure, surface area of chloroplasts facing intercellular space, photosynthesis characteristics