| Summary: | Citrus plants are prone to phosphorus (P) deficiency, especially in acidic soil, making them more dependent on root-associated endophytic fungi for growth and development. Beni-Madonna, a hybrid of <i>Citrus nanko</i> × <i>C. amakusa</i>, is a citrus known as “tangor” that is highly popular in China and other parts of the world due to its deep red color and jelly-like flesh. In this study, the inoculation response to two arbuscular mycorrhizal fungi (<i>Diversispora versiformis</i>, <i>Dv</i>; <i>Funneliformis mosseae</i>, <i>Fm</i>) and an endophytic fungus (<i>Serendipita indica</i>, <i>Si</i>) with regard to the growth, leaf gas exchange, light energy efficiency, P levels, acid phosphatase activity, and expression of the purple acid phosphatase (<i>PAP</i>) and phosphate transporter (<i>PT</i>) genes through a potted experiment using Beni-Madonna (tangor) citrus plants grafted on trifoliate orange (<i>Poncirus trifoliata</i>) was studied. Two years following inoculation, the root fungal colonization rates of inoculated plants were significantly increased, accompanied by an increase in plant height and stem diameter, with <i>Si</i> presenting the best effect. Fungal inoculations also significantly increased the leaf chlorophyll index, nitrogen-balance index, photosynthesis rate, stomatal conductance, transpiration rate, photosynthetic efficiency, and quantum yield of PSII, while it reduced photoinhibition and heat dissipation, with <i>Si</i> having the best effect on light energy efficiency. <i>Si</i> significantly increased leaf and root P levels, as well as root <i>CsPAP1‒3</i>, <i>CsPT2</i>, <i>CsPT3</i>, and <i>CsPT7</i> expression and soil acid phosphatase activity; <i>Fm</i> in mycorrhizal fungi significantly increased leaf and root P levels, as well as root <i>CsPT1</i> and <i>CsPT7</i> expression and root/soil acid phosphatase activity. These observations suggested an advantage of <i>Si</i> over the other two AMF in terms of improved plant growth and P acquisition, while <i>Fm</i> displayed prominent effects on increased photosynthetic efficiency.
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