The stimulatory effects and associated mechanism of lanthanum nitrate on the root tuber yield of the traditional Chinese medicinal plant Pseudostellaria heterophylla, by means of improved photosynthetic characteristics, were investigated. Field experiments were conducted and the foliage was sprayed with varying concentrations of lanthanum nitrate. The results indicated that growth indexes, chlorophyll and carotenoid contents initially increased, and then decreased in a dose-dependent manner with increasing concentrations of lanthanum nitrate. The fresh and dry weights of root tuber yield per unit area significantly increased by 58.34% and 56.87% with a lanthanum nitrate concentration of 100 mg/L, respectively, compared with the control. Appropriate concentrations of lanthanum nitrate were found to alleviate photosynthetic depression at midday, as well as improve the mean values of photosynthetic rate (P-n), stomatal conductance (G(s)) and transpiration rate (T-r) associated with the diurnal variation in photosynthetic parameters, while reducing intercellular CO2 concentration (C-i). During the experiment, the chlorophyll fluorescence parameters Fv/Fm, Fv'/Fm', phi(PSII) and qP increased initially and then decreased along with the increasing concentration of lanthanum nitrate, while NPQ followed the opposite trend. Correlation analysis revealed that P-n, had a significantly positive relationship with seedling height, ground diameter, chlorophyll and carotenoid contents, Gs and phi(PSII), while a negative relationship was observed with Ci and NPQ. Grey relational analysis (GRA) indicated that photosynthetic pigments, phi(PSII) and qP were the primary factors impacting photosynthesis. These results suggested that the application of an appropriate concentration of lanthanum nitrate (100 mg/L) was effective in improving growth, root tuber yield and photosynthetic characteristics of Psendoste llaria (P) heterophylla. The regulatory effect of lanthanum nitrate on photosynthesis was related to the promotion of light energy absorption and conversion, improvement in photochemical efficiency and the alleviation of photoinhibition. The improvement of the non-stomatal limitation factor was the primary basis for the mitigation of midday photosynthetic depression.
