Root tolerance characteristics of mining ecotype Polygonum hydropiper under high P application

  Objectives  Phosphorus (P) accumulation tolerance is fundamental to the phytoremediation of excessive P in the environment. Previous studies have reported mining ecotype Polygonum hydropiper as a hyperaccumulator of P.

  Methods  We conducted a hydroponic experiment using mining ecotype Polygonum hydropiper (ME) and non-mining ecotype Polygonum hydropiper (NME) as test materials. The treatments were high inorganic P concentrations in nutrient solution at 2, 4, 8, and 16 mmol/L, taking normal P concentration at 0.5 mmol/L as the control. We investigated the tolerance of ME and NME roots to high P.

  Results  Compared to normal P control, the shoot and root biomass of ME increased when exposed to P concentrations 2 and 4 mmol/L but decreased when exposed to P concentrations 8 and 16 mmol/L. The shoot and root biomass of ME was 1.35–2.56 and 1.18–1.86 times those of NME under high P treatments, showing higher tolerance to high P than NME. The shoot and root P accumulation of ME increased when exposed to P concentrations 2 and 4 mmol/L but decreased when exposed to P concentrations8 and 16 mmol/L. The shoot and root P accumulation of ME was 1.35–2.58 and 1.36–1.96 higher than NME, indicating the former's stronger P accumulation potential. The ME and NME were stained when exposed to 8 and 4 mmol/L P concentration respectively. This showed that the plasma membranes of the root cells were (P<0.05) damaged at P concentrations 8 mmol/L for ME and P concentrations 4 mmol/L for NME. With increasing P concentration, SOD, POD, and CAT activities in ME roots increased and then decreased. The concentrations of H₂O₂ and MDA were stable at first and then increased. However, the concentration of H₂O₂ exhibited a constant increase in NME roots. The P in the roots of both ME and NME was mostly distributed in the soluble fraction (containing the vacuole) and cell walls. The P content in the subcellular component (P<0.05) increased in both ME and NME roots with increasing P concentrations. The soluble fraction (containing the vacuole) P (P<0.05) increased at P concentrations 2 and 4 mmol/L than the control, indicating the importance of vacuole for P storage.

  Conclusions  The tolerance and accumulation of P in ME were stronger than NME. Under high P, ME showed root free radicals and protective enzyme dynamic balance, cell wall retention, and vacuole compartments, contributing to its root tolerance characteristics. Our findings indicate that ME shows higher antioxidant enzyme activities and lower lipid membrane peroxidation than NME.