Selenium nanoparticles and wheat straw biochar synergistically alleviate combined drought-heat stress in wheat (Triticum aestivum L.) by modulating antioxidant defense, photosynthetic efficiency, and ion homeostasis.

Combined drought and heat stress (DS+HS) represents one of the most devastating multi-factorial constraints on global wheat (Triticum aestivum L.) production, imposing synergistic physiological and molecular damage that far exceeds the effects of each stress applied individually. Emerging evidence suggests that selenium nanoparticles (Se NPs) and biochar independently confer abiotic stress tolerance in cereal crops; however, their combined soil application under concurrent DS+HS in wheat has not been previously investigated. The present study was conducted at the wire house facility of the Four Brothers Group (FBG), Lahore, Pakistan, using soil collected from Faisalabad, to evaluate the individual and combined effects of soil-applied Se NPs (50 mg kg-1) and wheat straw biochar (2% w/w) on wheat cv. FBG-1800 subjected to drought (30% field capacity) and heat stress (42 °C, 6 h d-1) in a completely randomized design with three replications. Combined DS+HS severely impaired growth, photosynthetic function, and grain yield, while markedly elevating oxidative damage markers and disrupting ionic homeostasis. Notably, combined Se NPs + biochar application produced superior synergistic amelioration relative to either amendment alone, restoring net photosynthetic rate and grain yield to approximately 89%-90% and 88% of control values, respectively, alongside significant reductions in oxidative stress indicators. Gene expression analysis confirmed marked upregulation of key stress-responsive genes (TaSOD1, TaDREB2, TaHSP70, TaNHX1) and coordinated antioxidant enzyme activation. These findings establish that soil co-application of Se NPs and biochar is an effective, agronomically feasible strategy to sustain wheat productivity under the increasingly prevalent combined DS+HS conditions in Pakistan's irrigated cropping systems.