Extreme Polyploidy Reveals Functional Trade-Offs in Plant Drought Responses

Polyploidy shapes plant physiology and anatomy, yet the extent to which different ploidy levels influence drought responses remains unclear. Here, we investigated how ploidy affects cell-level coordination and functional traits associated with water transport and retention. We used glasshouse-grown plants of the carnation Dianthus broteri, a species complex composed of naturally distinct populations differing in cytotype: diploid (2×), low-polyploid (4×), and high-polyploid (6× and 12×). Leaf anatomical traits scaled with ploidy: higher-ploidy cytotypes exhibited larger stomata and pavement cells, with coordinated reductions in stomatal and vein densities. However, the 12× cytotype deviated from this trend and displayed higher g_min and thinner epicuticular wax. Stem xylem anatomical traits and hydraulic vulnerability were largely conserved, except for thicker intervessel pit membranes in 6× and 12× individuals, which correlated with delayed embolism spread (P₂₅). Plant structure-function trait combinations indicate that cytotypes differ in water-use syndromes rather than aligning along a single dimension of drought response. Under drought conditions, lower ploidies maintained functional canopies through reduced water loss, whereas 12× showed a less conservative water-use strategy and seasonal canopy loss. Altogether, we provide new insights into how polyploidy can influence drought adaptation and survival strategies via anatomical and functional trait shifts.