Polyploidy drives changes in tissue allocation modifying whole-plant water relations.

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.