Polyploid plants often display functional trait values distinct from those of diploids, influencing their stress tolerance and adaptive capacity. These differences shape how polyploids interact with their environment, a factor that is crucial to their evolutionary success. Here, we investigated the species complex Dianthus broteri, where ploidy level is known to correlate with water availability, as a model system to understand the possible link between ploidy and whole-plant water relations. We quantified allocation between leaves, xylem, and roots in 4 different ploidies of D. broteri (2��, 4��, 6��, 12��), and examined its relationship with hydraulic efficiency (Kr-s), water potential regulation, and stomatal conductance (gc) in response to varying leaf-to-air vapor pressure deficits (VPDL). A gradient in tissue allocation with increasing ploidy led to contrasting water-use strategies within D. broteri. Higher ploidy was associated with greater allocation to roots and xylem, resulting in higher Kr-s and gc and lower water potential gradients. Despite these differences, gc responses to VPDL were largely consistent across ploidies. In D. broteri 12��, the significant investment in water uptake and transport without a proportional increase in leaf area appeared suboptimal, incurring high xylem costs per unit water transport. However, this trade-off also led to increased water uptake and transport efficiency, potentially advantageous under water-limited conditions. Overall, our results indicate that multiple rounds of genome duplication cause substantial changes in whole-plant water relations, likely impacting water stress exposure in the field.
