Heterophyllous plants reorganize plant trait coordination between floating and emergent habitats

Plant structures function as integrated modules, reflecting coordinated development and function across traits. In terrestrial plants, stomatal traits that regulate carbon uptake are tightly coordinated with xylem traits supplying water, maintaining trade-offs between photosynthetic demand and hydraulic capacity. In aquatic plants, however, contrasting environments experienced by emergent and floating leaves may alter these coordination patterns. Whether heterophylly modifies fundamental scaling relationships among traits remains unclear. Here, we examined 15 heterophyllous aquatic species that produce both floating and emergent leaves within the same individual, allowing isolated effects from phylogeny. We found that emergent leaves exhibited greater leaf area, total stomatal area, and petiole thickness, indicating increased hydraulic and mechanical investment. Both leaf types followed hypoallometric scaling between leaf and petiole traits, but coordination regimes diverged. Emergent leaves showed tighter scaling between total stomatal area and petiole xylem area, reflecting strengthened coupling between transpirational demand and hydraulic supply. In contrast, floating leaves exhibited steeper scaling between leaf area and petiole transverse area and a more centralized trait network structure. These divergences persisted after accounting for phylogeny. Together, our results showed that heterophyllous plants could maintain core developmental proportionality while reorganizing trait coordination in response to different habitats. • Using two leaf types on one plant separates habitat effects from ancestry • Floating and emergent leaves invest differently in petiole and leaf scaling • Floating leaves show stronger coordination among their traits • Scaling differences come from habitat differentiation, not evolutionary history