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Nanomaterials have recently been used as growth stimulants to promote the production of crops in saline-alkali through root application. However, if applied through leaves, little is known about the effect of Fe₃O₄ nanoparticles (NPs) on the root growth and yield, especially for medicinal crops. To fill this gap, a single factor experiment was conducted to explore the effects of Fe₃O₄ NPs on growth, yield, the dry matter distribution, chlorophyll content, photosynthetic characteristics, chlorophyll fluorescence parameters, and polysaccharide content of Pseudostellaria heterophylla by foliar spraying under field conditions. Fe₃O₄ NPs (20–50 mg·L^–1) significantly promoted growth, the dry matter distribution of root and root tuber yield per unit area. Fe₃O₄ NPs enhanced net photosynthetic rate (P _n) by increasing chlorophyll content. And Fe₃O₄ NPs increased the daily mean and peak value of P _n, and alleviated the phenomenon of “midday depression” by improving nonstomatal limitation. Chlorophyll fluorescence parameters indicating that Fe₃O₄ NPs promoted the photochemical activity of PSII and alleviated photoinhibition by enhancing the photochemical use of excess excitation energy. Gray correlation analysis showed that Fe₃O₄ NPs enhanced the adaptability of P. heterophylla photosynthesis to high temperatures and strong light. Of note, Fe₃O₄ NPs enhanced the polysaccharide content of the root tuber. Phytotoxic effect was recorded at high NPs (100 mg·L^–1) doses. Collectively, Fe₃O₄ NPs could promote performance of P. heterophylla by improving photosynthetic performance, enhancing its adaptability to the environment, and increasing the distribution ratio of photosynthates to the underground part.

In the actual cultivation process, blind fertilizer application was widespread, resulting in a serious decline in the yield of Pseudostellaria heterophylla. We used the 3414 fertilizer experiment design to study the effects of combined Boron (B), Molybdenum (Mo), and Copper (Cu) on the growth indexes, diurnal changes of photosynthesis, and rapid fluorescence induction dynamics in P. heterophylla. Our results show that the optimal combination of B, Mo, and Cu simultaneously promoted the growth of underground and aboveground parts, and significantly improved the quality of single root tuber and yield per unit area. The best combination was treatment 9 (T9 = B, 1 g/L; Mo, 0.08 g/L; Cu, 0.05 g/L), and resulted in a 35.1% increase in yield per unit area compared with the control group (T1). Although the optimal combined application of microfertilizers did not change the bimodal trend of diurnal variation of photosynthesis, it effectively increased the daily average, peak, and valley values of the photosynthetic rate by alleviating the nonstomatal limitation and the photosynthetic midday depression. Pseudostellaria heterophylla leaves showed greater photochemical activity and less photoinhibition of photosystem II in T9. Major effects were that it helped protect the activity of the oxygen-evolving complex to reduce the oxidative damage of chloroplasts and prevent the dissociation of thylakoid. The microfertilizer application also enhanced the electron receiving ability of the Q_B and plastoquinone (PQ) electronic pools, thereby increasing the ability of electron transfer from Q_A to Q_B. The number of reaction centers per unit area was promoted notably by the fertilization treatment.