Warming enhances the detrimental impacts of drought and salinity on maize growth, water-use efficiency, and nitrogen recovery

Introduction: Climate change is increasing the co-occurrence of heat, drought, and soil salinization in Mediterranean agroecosystems, yet their interactive effects on crop physiology remain poorly understood. We investigated how warming modifies maize growth and resource-use efficiency when combined with drought or salinity during early vegetative development. Methods: A pot experiment was conducted in a semi-controlled wire house. Three air temperature regimes were established: ambient conditions and two warming treatments that resulted in mean temperature increases of +1.1 °C and +1.9 °C relative to ambient, despite target increases of +1.5 °C and +4.0 °C. In addition, three water regimes were imposed: well-watered, water stress, and salt stress. Plant growth, water- and N-use efficiency, 15N fertilizer recovery, and D13C discrimination were measured. Additivity of combined stress effects was evaluated using bootstrap-based effect size analyses. Results: Salinity imposed the strongest individual constraints on growth, water consumption, and N uptake, while drought induced moderate shoot reduction but enhanced root growth. Warming alone had limited effects on biomass but increased water demand. Under combined stress, warming amplified the detrimental impacts of both drought and salinity on plant performance, sometimes resulting in non-additive responses, including synergistic declines in water-use efficiency and N recovery. Discussion: These findings show that maize responses to multifactorial stress cannot be predicted from single-stress behaviors. Furthermore, the results showed a progressive intensification of the negative effects of combined stresses on plant performance with increasing temperature. Therefore, considering that the temperature increases effectively achieved were substantially lower than those projected under the most severe climate change scenarios, it follows that, if such projected temperature increases were fully realized, the resulting negative impacts would be even more pronounced.