Phosphate-modified biochar synergized with AM mitigate cell death in Malus roots by regulating Cd accumulation and phosphorus availability

The safe disposal of apple pruning waste and mitigation of cadmium (Cd) toxicity in orchards are pressing environmental challenges. This study developed a synergistic strategy using phosphate-modified carbonized apple branches (PCAB) and arbuscular mycorrhizal (AM) fungi to immobilize Cd and reduce its uptake in apple trees. Compared to unmodified carbonized apple branches (CAB), PCAB exhibited a 1.35-fold higher Cd^{2 +} adsorption capacity and was more effective in converting soil acid-soluble Cd into stable residual fractions. The combination of PCAB and AM fungi (PCAB+AM) was most effective, enhancing AM root colonization by over 200 % under Cd stress. This synergy increased rhizosphere available phosphorus by 128 % and the proportion of labile P by 50 %, while reducing bioavailable Cd to less than half of that in CAB+AM treatment. Consequently, PCAB+AM minimized root Cd²⁺ influx to 31 % of the control, reduced leaf Cd concentration by 86 %, and decreased Cd translocation factor by 66 %. This treatment also more effectively alleviated Cd-induced oxidative stress and cell death, downregulating key apoptosis genes (MhVPEγ/MhBAX1) and VPEγ activity. Our findings demonstrate that the PCAB-AM combination creates a protective rhizosphere barrier, offering a sustainable waste-to-wealth solution for Cd-contaminated orchards.