Higher Flower Hydraulic Safety, Drought Tolerance and Structural Resource Allocation Provide Drought Adaptation to Low Mean Annual Precipitation in <i>Caragana</i> Species

Determining the differences in flower hydraulic traits and structural resource allocation among closely related species adapted to low mean annual precipitation (MAP) can provide insight into plant adaptation to arid environments. Here, we measured the maximum flower hydraulic conductance (K_max-flower), water potential at induction 50% loss of K_max-flower (P_50-flower), flower pressure-volume parameters, dry mass of individual flowers and structural components (vexillum, wings, keels, stamens and sepals) of six Caragana species growing in regions ranging from 110 to 1400 mm MAP. Compared with species from high-MAP environments, those from low-MAP environments presented lower K_max-flower, more negative P_50-flower, osmotic potential at full turgor (π_o) and turgor loss points (π_tlp), and a greater bulk modulus of elasticity (ε). Consequently, a negative correlation between K_max-flower (hydraulic efficiency) and P_50-flower (hydraulic safety) was observed across Caragana species. Furthermore, the dry masses of individual flowers and structural components (vexillum, wings, keels, stamens and sepals) were greater in the species from the low-MAP environment than in those from the high-MAP environment. These findings suggest that greater flower hydraulic safety and drought tolerance combined with greater structural resource allocation promote drought adaptation in Caragana species to low-MAP environments.