White Clover Residues Inhibit the Promoting Effect of <i>Funneliformis mosseae</i> on the Growth and Photosynthetic Physiology of Trifoliate Orange

In citrus orchards, the impact of incorporating cover crop residues into soil on mycorrhizal symbiosis remains unclear. This study aimed to analyze the effects of applying dried white clover residues to the rhizosphere of trifoliate orange (Poncirus trifoliata) plants inoculated with Funneliformis mosseae by evaluating plant growth, leaf gas exchange, chlorophyll fluorescence variables, chlorophyll synthesis intermediates, sugar contents, and the expression of genes related to chlorophyll synthesis. The results showed that white clover residues significantly inhibited the colonization rate of F. mosseae in the roots. Single inoculation with F. mosseae significantly promoted above-ground growth (height, thickness, and leaf number), leaf gas exchange (net photosynthetic rate, transpiration rate, and stomatal conductance), sugar (glucose, fructose, and sucrose) contents, chlorophyll synthesis intermediates (δ-aminolevulinic acid, porphobilinogen, protoporphyrin IX, Mg-protoporphyrin IX, and protochlorophyllide), and chlorophyll fluorescence response (maximum quantum efficiency of photosystem II, steady-state quantum efficiency, and steady-state non-photochemical quenching), along with the up-regulated expression of two chlorophyll synthesis-related genes (PtHEMA1 and PtHEME2). However, the addition of white clover residues to F. mosseae-inoculated plants, regardless of the application form, significantly diminished the beneficial effects of F. mosseae on growth, gas exchange, chlorophyll fluorescence, sugars, and chlorophyll intermediates. It concludes that F. mosseae exerted significantly promoting effects on the growth and photosynthetic performance of trifoliate orange, while white clover residue incorporation substantially weakened these mycorrhizal benefits.