AbstractAbiotic stresses such as drought and heat constitute one of the major constraints to global crop production, and climate change will worsen the scenario. There is growing interest in investigating the application of biostimulants to palliate yield losses due to the impact of drought and heat stress. This study focuses on assessing the impact of coating seeds with either thyme essential oil or Paraburkholderia phytofirmans PsJN strain on agronomic performance of durum wheat subjected to contrasting levels of irrigation. For this aim, the Tunisian durum wheat genotype Karim was cultivated in a field experiment in a semi‐arid climate, with two supplemental irrigation levels (SI): moderate drought stress (100% SI) and severe drought stress (50% SI). The impact of the seed coating technique was assessed on grain yield (GY) and its agronomic components (spikes/m2, straw yield [SY], 1,000 kernel weight [TKW] and the harvest index [HI]) across two consecutive crop seasons; 2015–16 and 2016–17. In addition, the effect on leaf nitrogen and carbon isotope composition (δ15N, δ13C) and nitrogen and carbon contents of leaves at grain filling was also assessed in the 2016–17 season. The two seed coating treatments (SCTs) enhanced GY, SY, spikes/m2, TKW and HI under both moderate stress (100% SI) and severe stress (50% SI). The positive effect of the SCTs on GY and agronomic yield components was more marked under the combined heat–water stress conditions of the first season (2015–16). In general, thyme oil enhanced vegetative growth, tillering and tiller fertility, while, PsJN had a positive effect, more centred on the grain filling stage. Under moderate stress, both SCTs decreased δ15N and δ13C at grain filling and straw N content at harvest, illustrating a better carbon and nitrogen uptake and mobilisation. Under severe water stress, both SCTs induced higher leaf total N, and lower δ13C, and leaf total C at grain filling. The effect on physiological traits and carbon and nitrogen metabolism is discussed. The findings emphasise a practical implication of these biostimulants.