Not all 'pine cones' flex: functional trade-offs and the evolution of seed release mechanisms.

Seed dispersal is critical for plants, but the evolution of mechanisms that actually release seeds from their parents is not well understood. We use the reproductive cones of conifers, specifically the Pinaceae clade, to explore the factors driving the evolution of different release mechanisms in plants. We combine comparative anatomical and phylogenetic analyses to test whether fundamental trade-offs in the mechanical and hydraulic properties of vasculature underlie the evolution of two seed release mechanisms: cone scale flexion and cone scale shedding. We then test whether these mechanisms are linked with differences in seed size, dispersal syndrome and reproductive allocation. Cone scale xylem in flexing species is tough, but poorly conductive. Xylem in shedding species is less extensive, fragile and highly conductive; its thin-walled tracheids allow scales to easily fracture at maturity. Shedding is also consistently associated with large, densely packed seeds. Pinaceae cones exploit a well-known trade-off in xylem mechanical strength vs hydraulic efficiency to generate release mechanisms that allow seeds of various sizes to leave the protecting cone. The linkage among release mechanisms, vascular anatomy and seed traits illustrates how a wide variety of selective pressures may influence the function and physiology of reproductive structures.