Objectives Exploring the effects of exogenous application of GR24 and N and P supply on the N uptake and tanshinones accumulation in the roots of Salvia miltiorrhiza-AMF symbionts.

Methods The isotope experiment was conducted using a three chamber system, with three P levels: 0 μmol/L (P0), 35 μmol/L (P35), 700 μmol/L (P700), and three ¹⁵N labeled N sources (KNO₃, (NH₄)₂SO₄, urea, all with a ¹⁵N labeled concentration of 4 mmol/L). The ¹⁵N abundance in Salvia miltiorrhiza mycorrhiza were analyzed. The GR24 experiment was conducted using sand culture method, KNO₃, (NH₄)₂SO₄, and urea were used as N sources and applied in the same N rate (4mmol/L) and three AMF treatments were set up under each N source, as: no inoculation of AMF (NM), inoculation of AMF (AM), and inoculation of AMF+0.1 mmol/L GR24 (AM+GR24). The N and P contents, N/P ratio, and tanshinone substances in plants were measured.

Results The results of isotope experiments showed that P supply level and N source jointly affected the absorption and transportation of N by Salvia miltiorrhiza mycorrhiza. Under KNO₃ treatment, the P supply level did no significantly affect the abundance of ¹⁵N in Salvia miltiorrhiza mycorrhiza; under (NH₄)₂SO₄ treatment, P supply significantly reduced the ¹⁵N abundance of Salvia miltiorrhiza mycorrhiza, and the decrease in P700 was much greater than that in P35; under urea treatment, P35 increased but P700 sharply decreased the abundance of ¹⁵N in Salvia miltiorrhiza mycorrhiza, the decrease in P700 was much greater than the increase in P35. At the P0 and P35 levels, the ¹⁵N abundance of Salvia miltiorrhiza mycorrhizal fungi treated with urea was significantly higher than that with (NH₄)₂SO₄ and KNO₃. The effect of exogenous GR24 on improving AMF colonization rate was limited within 50 days of Salvia miltiorrhiza growth . Under the three N source treatments, AM inoculation significantly increased the aboveground and underground biomass of Salvia miltiorrhiza. AM+GR24 did not further increase the aboveground biomass, but further significantly increased the root biomass under KNO₃ and urea. Compared to NM, the promotion of AM on N and P accumulation in the aboveground and root parts was unstable, while AM+GR24 treatment showed significant promotion effect. Under AM+GR24 treatment, (NH₄)₂SO₄ and urea were favorable for the accumulation of N and P in the aboveground parts, and KNO₃ was favorable for root N and P accumulation, followed by urea. The harvested mycorrhizal Salvia miltiorrhiza contained the highest tanshinone IIA and cryptotanshinone content when supplied with (NH₄)₂SO₄, which were significantly higher than those supplied with KNO₃ and urea.

Conclusions The P level and N form affect the growth, N and P absorption, and quality of mycorrhizal Salvia miltiorrhiza. The absorption pathway of NH₄⁺ by AMF is influenced by P levels, which may be a coupled transport relationship with P, while NO₃⁻ may have a transport pathway that is independent of P transport channels. Exogenous GR24 has no significant effect on the infection of AM fungi, but can stabilize its growth promoting effect on Salvia miltiorrhiza, especially on roots. Under AM+GB24 condition, supplying KNO₃ and urea is conducive to the growth and nutrient accumulation of Salvia miltiorrhiza in the early growing stage, and supplying (NH₄)₂SO₄ is conducive to the accumulation of tanshinones in Salvia miltiorrhiza in the late growing stage.