Interacting drought and salinity reshape maize isohydric behavior through soil–plant hydraulic constraints

Plant water regulation plays a critical role in land–atmosphere coupling and ecosystem responses to climate extremes. Isohydricity is widely used to characterize how plants regulate water loss under water stress, yet its behavior under interacting drought and salinity remains poorly understood. Here, we investigated maize (Zea mays L.) water-use strategies under combined water and salinity constraints using a controlled pot experiment. Maize plants were exposed to two water availability regimes (well-watered and drought conditions) and irrigated with either fresh or saline water. Isohydric behavior was assessed using three complementary hydraulic relationships: (i) transpiration rate (normalized by leaf area) versus soil water potential, (ii) leaf water potential versus soil water potential, and (iii) stomatal conductance versus leaf water potential. In addition, the vulnerability of soil–plant hydraulic conductance was also examined.Under drought or salinity applied separately, maize tended to exhibit more anisohydric behavior, characterized by relatively weak reductions in transpiration and stomatal conductance with declining water potential and a broader range of leaf water potential variation. In contrast, when drought and salinity occurred simultaneously, maize shifted toward a more isohydric mode of regulation, clearly differing from responses under single stress conditions. Moreover, under drought conditions, isohydricity inferred from the leaf–soil water potential relationship tended toward a more isohydric behavior under saline treatment, whereas isohydricity inferred from transpiration- and stomatal conductance–based relationships under salinity indicated a more anisohydric behavior. This discrepancy highlights the influence of evaluation methods on isohydricity characterization. Furthermore, we conclude that maize isohydricity is closely linked to the vulnerability of soil–plant hydraulic conductance. Under drought or salinity conditions, maize tends to exhibit more anisohydric behavior, which is associated with enhanced resistance of the soil–plant hydraulic system to the loss of hydraulic conductance. These findings advance our understanding of crop water relations under combined water and salinity stress and support integrated irrigation and salinity management strategies to improve water use efficiency and sustain yields in salt-affected regions.