LAUSR

Abstract In order to explore the mitigatory mechanism of rare earths on the midday depression of photosynthesis, this study used fast-phase chlorophyll fluorescence technology combined with an antioxidant enzyme system to analyze the effects of lanthanum and cerium on the photosystem II (PSII) function of Pseudostellaria heterophylla leaves at noon. The results show that the maximum photochemical efficiency (Fv/Fm) and the photosynthetic performance index (PIABS) of P. heterophylla leaves during the photosynthetic noon-break are only 0.73 and 0.86, respectively. The leaves of P. heterophylla show obvious photoinhibition. The application of two rare earth fertilizers promote the light energy absorption, capture, conversion, and electron transfer efficiencies of PSII in the P. heterophylla leaves and alleviate the photoinhibition caused by excess excitation energy. Additionally, the effect of lanthanum is better than that of cerium. In the fertilization groups, P. heterophylla leaves show higher PSII photochemical activity. Fertilization protects the oxygen-evolving complex (OEC) activity of P. heterophylla, thus reducing oxidative damage to the chloroplasts and preventing the dissociation of the thylakoids to maintain the normal physiological functioning of PSII, thereby reducing the degree of photoinhibition. Additionally, fertilization increases the electron transfer ability from QA to QB on the PSII electron receptor side of P. heterophylla. Correlation analysis shows that the fluorescence parameters VL, VK, and VJ are significantly negatively correlated with antioxidant enzyme activity and are significantly positively correlated with malondialdehyde content, relative electric conductivity and reactive oxygen species (ROS), which further indicates that rare earths can enhance the ROS scavenging system in plants and reduce the degree of membrane lipid peroxidation and plasma membrane damage, thereby maintaining the structure and function of the OEC on the donor side of PSII, increasing the electron receptor pool, and contributing to the defense against photoinhibition.