Co-Application of Trichoderma harzianum and the Strigolactone Analog GR24 Enhances Wheat Tolerance to Cadmium Stress: Effects on Growth, Photosynthesis, Antioxidant Defense, and Cd Accumulation

Cadmium (Cd) contamination threatens wheat productivity by impairing growth, redox homeostasis, and metal partitioning. Beneficial fungi and phytohormones improve plant stress tolerance, but the combined role of Trichoderma harzianum and the synthetic strigolactone analog GR24 in wheat Cd tolerance remains unclear. This study evaluated whether co-application of T. harzianum and GR24 enhances Cd tolerance by improving growth, photosynthetic activity, and antioxidant defense, and by reducing Cd accumulation. A greenhouse pot experiment was conducted with two Cd levels (Cd0 and Cd1), two GR24 treatments (H0, without GR24; H1, 100 mg L−1 GR24), and two T. harzianum treatments (TH0 and TH1; 2 g kg−1 soil), using five biological replications. Cd stress markedly inhibited wheat growth, photosynthetic performance, and antioxidant defense while increasing oxidative injury and Cd accumulation in plant tissues. Under Cd stress, the combined GR24-T. harzianum treatment increased shoot growth and biomass by 53.04–80.43%, reduced EL, MDA, and H2O2 by 18.70–30.36%, enhanced SOD, POD, and CAT activities by 24.01–44.50%, and lowered Cd concentrations in roots, shoots, and leaves by 41.09–56.19%. These responses strengthen the treatment’s practical relevance. Collectively, these findings indicate that T. harzianum-GR24 co-application offers a promising biological approach to improve wheat resilience and reduce Cd accumulation in wheat tissues.