Mycorrhizal Inoculation Enhances Heat Resistance in Trifoliate Orange (Poncirus trifoliata L. Raf.) via Photosynthesis, Osmoregulatory Substances, the Antioxidant System, and the Expression of Related Genes

High-temperature stress can negatively influence citrus growth and fruit quality. Arbuscular mycorrhizal fungi (AMF) can enhance the absorption of water and nutrients, as well as improve heat resistance. In this experiment, we investigated the influence of heat stress (42 °C) on trifoliate orange (Poncirus trifoliata L. Raf.) with Funneliformis mosseae, (F.m, an symbiotic AMF associated with citrus). The results show that AMF markedly promoted seedling growth and enhanced photosynthesis. Compared with the 42 °C, 0 h conditions, the 42 °C, 6 h conditions markedly improved the REC and concentration of MDA, H2O2, soluble protein, soluble sugar, and Pro and significantly increased the enzyme activities of SOD (superoxide dismutase), CAT (catalase), and POD (Peroxidase), but they dramatically reduced the relative water content and photosynthetic parameters. Under high temperature (42 °C, 6 h), in contrast to the non-AMF groups, AMF treatment also observably improved the chlorophyll fluorescence and photosynthetic parameters, significantly decreased the REC by 26.52%, and markedly reduced MDA and H2O2 content by 25.61% and 35.66%. Moreover, AMF improved the concentrations of soluble protein, soluble sugar, and Pro markedly at 42 °C, 0 h (by 36.12%, 39.93%, and 20.81%), but these increased only by 13.31%, 3.11%, and 19.73% at 42 °C, 6 h. Furthermore, AMF increased the activities of SOD, CAT, and POD by 2.05%, 23.49%, and 2.02% at 42 °C, 0 h and by 4.74%, 20.85%, and 34.09% at 42 °C, 6 h compared with the non-AMF treatment. In addition, we found that the PtHSP7, PtHSP9, PtGOLS1, and PtHSFA1 genes all responded to high-temperature stress. Compared with the non-AMF treatment, after administering it at a high temperature (42 °C) for 6 h, AMF inoculation significantly up-regulated PtHSP7 but significantly down-regulated PtHSP9. In conclusion, AMF promotes the growth of trifoliate orange and enhances heat resistance via photosynthesis, osmoregulatory substances, the antioxidant system, and related genes.