Soil arbuscular mycorrhizal (AM) fungi form a mutualistic symbiosis with plant roots and produce many benefits on host plants under potted conditions, while field inoculation of AM fungi on citrus (a woody plant) has been rarely reported. The present study aimed to analyze the changes in mycorrhizal growth, root vitality, and fruit quality of Citrus reticulata Blanco var. Ponkan mandarin cv. Jinshuigan grafted on Poncirus trifoliata L. after inoculation with a mix of AM fungi (Diversispora versiformis, Funneliformis mosseae, and Rhizophagus intraradices) and single F. mosseae. After the second year of AM fungal inoculations, root mycorrhizal colonization (%), root vitality, hyphal length in soil, and easily extractable glomalin-related soil protein content were significantly increased, while difficult-to-extract glomalin-related soil protein content was decreased. Two mycorrhizal fungal inoculation treatments collectively improved fruit quality parameters such as polar diameter, equatorial diameter, the weight of single fruits, fruit peel, and sarcocarp, coloration value, and soluble solids content. Our study, therefore, suggested that field inoculation with AM fungi improved root physiological activities in terms of mycorrhizal growth and root vitality and thus improved fruit quality. The effect of mixed-AM treatment was more significant than that of F. mosseae alone.

Arbuscular mycorrhizal (AM) fungi can improve plant phosphorus (P) uptake; however, information about how AM fungi affect rhizosphere organic acid and microbial activity to alleviate citrus low P stress is limited. Here, a pot experiment was conducted to evaluate the effect of AM fungi (Rhizophagus intraradices, Ri) inoculation on rhizosphere organic acid content, microbial biomass (MB), and enzyme activity of trifoliate orange (Poncirus trifoliata L. Raf.) seedlings grown under three low P conditions. The results showed that mycorrhizal seedlings all recorded higher P concentrations, plant biomass, and better root morphology with more lateral and fine roots, but lower root mass ratios, irrespective of P conditions. Mycorrhizal P absorption contribution did not differ significantly among three P conditions. Mycorrhizal seedling rhizosphere soil exhibited lower organic acid content, soil organic P content and ratio of MB-carbon (C)/MB-P, but higher MB and enzyme activity. Additionally, the main organic acids showed a negative relationship with mycorrhizal colonization rate and hyphal length, however phosphatase and phytase activity had a significantly positive relationship with MB. Therefore, the results suggest that AM fungi inoculation may help citrus to efficiently utilize organic P source by improving microbial activity under low available P conditions.

Protein hydrolysates (PHs) and arbuscular mycorrhizal fungi (AMF) are environmentally friendly biostimulants that effectively promote crop growth and alleviate the damage from abiotic stress. However, the physiological and molecular regulatory mechanisms are still unclear. This study aimed to explore the effects of PHs and AMF on growth, mineral nutrient absorption, and expression of Aquaporins and SOSs in Goutoucheng (Citrus aurantium) under salt stress. Results showed that PH application and AMF inoculation significantly promoted plant growth and enhanced mineral element absorption and sodium effluxion in citrus under salt stress. The biomass, root activity, leaves mineral nutrition contents in PHs, AMF, and combined (PHs and AMF) treatments were significantly higher than those of control. Leaves sodium content in three treatments was significantly lower than in the control. AMF and combined treatments showed dominant effects than PHs alone. Besides, PHs interacted with AMF on growth, nutrient absorption, and sodium effluxion. Importantly, AMF and PHs induced stress-responsive genes. PIP1, PIP3, SOS1, and SOS3 expression in PHs and AMF treatments was significantly higher than control. Thus, it was concluded that AMF and PHs enhanced the salt tolerance of citrus by promoting nutrient absorption and sodium effluxion via up-regulating the expression of PIPs and SOSs. The mixed application of PHs and AMF had a better effect.