Effects of combined inoculation with Rhizophagus intraradices and Paenibacillus mucilaginosus on plant growth, root morphology, and physiological status of trifoliate orange (Poncirus trifoliata L. Raf.) seedlings under different levels of phosphorus

• Inoculation with Ri and Pm had better additive effects on P-deficiency orange seedlings.
• Inoculation with both Ri and Pm could mutually enhance each other’s development.
• The efficacy of inoculation with Ri only was superior to Pm alone.

The effects of combined inoculation with arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) on the growth of citrus seedlings under phosphorus deficient conditions have not been extensively studied. A pot experiment was performed to compare growth, root morphology, and other physiological variables in trifoliate orange (Poncirus trifoliata L. Raf.) seedlings that had been inoculated with AMF (Rhizophagus intraradices, Ri), PGPR (Paenibacillus mucilaginosus, Pm), or both, under three phosphate fertilizer conditions by adding 0.00, 73.41, and 220.23 mg kg−1 Ca3(PO4)2 (P0, P1, and P3, respectively) in a low available phosphorus (Olsen-P 4.96 mg kg−1) culture medium. Plant growth and nitrogen and phosphorus uptake were significantly increased by inoculation with Ri and Pm at three phosphorus levels, and combined inoculation yielded the highest efficacy, particularly at P0 level. Furthermore, combined inoculation at each phosphorus level significantly increased the colonization ratios of root length with mycorrhizal hyphae and arbuscules, rhizospheric hyphal length, and Pm populations. Root morphology traits like projected area, total volume, and total root length were also considerably improved by inoculation with Ri and Pm; however, taproot length was notably reduced by mycorrhizal inoculation. At P0 level, seedlings inoculated with a combination of Ri and Pm yielded the greatest leaf chlorophyll concentrations and fine root activity, in comparison to those had either not been inoculated at all, or inoculated with just one of them. Although malondialdehyde contents and anti-oxidative enzymes activities increased in non-inoculated seedlings at P0 level, they were significantly reduced by inoculation, particularly with Ri. At P2 level, mycorrhizal seedlings exhibited markedly higher levels of anti-oxidative enzyme activity than non-mycorrhizal seedlings. Therefore, our results suggested that the efficacy of Ri inoculation was superior to that of Pm inoculation. Furthermore, combined inoculation yielded additive effects on the growth of trifoliate orange seedlings under conditions of phosphorus deficiency.