Abstract: To elucidate the adaption mechanism for the growth and photosynthetic capacity of Michelia chapensis under different light intensities, two-year-old seedlings were treated with five shading levels of 100% (CK), 70% (T1), 50% (T2), 30% (T3) and 10% (T4) of natural light intensity for one year, after that the growth and photosynthetic characteristics were measured. The results showed that survival rate, plant height and ground diameter were significantly higher under the 70% and 100% light intensity treatments compared with other treatments. The highest value (8.553 μmol·m^-2·s^-1) of net photosynthetic rate (P_n) was recorded in the 70% light intensity treatment. With increasing shading intensity, P_n values decreased gradually. The decrease of P_n in the 50% natural light intensity treatment was mainly caused by stomatal limitation, but by non-stomatal limitation under the 30% and 10% natural light intensity treatments. Compared to other shading treatments, leaves under the 100% and 70% natural light intensity treatments had higher maximum net photosynthetic rate (8.166 and 8.735 μmol·m^-2·s^-1), light saturation point (1215.956 and 1145.328 μmol·m^-2·s^-1), and light compensation point (16.28 and 13.572 μmol·m^-2·s^-1). In particular, with increasing shading intensity, the actual photochemical quantum yield of PSII (Φ_PSII), photochemical quenching coefficient (q_p) and proportion of photochemical reaction energy (P) increased gradually, while non-photochemical quenching coefficient (NPQ) decreased gradually. All the treatments did not affect maximum photochemical quantum yield (F_v/F_m) and potential activity (F_v/F₀). The results indicated that M. chapensis seedlings could reduce light compensation point, light saturation point, net photosynthetic rate and dark respiration rate, while increase the openness of PSII reaction center, electron transfer activity and light use efficiency, thus enhancing growth ability under moderate and serious shading conditions. Our results suggested that 70% of natural light intensity was the most optimal for the growth of M. chapensis seedlings.

Key words: Michelia chapensis, light intensity, growth, photosynthesis characteristics, chlorophyll fluorescence