THE ECOPHYSIOLOGY OF XYLEM HYDRAULIC CONSTRAINTS BY BASAL VESSELS IN CANELLA WINTERANA (CANELLACEAE)

Early angiosperms are hypothesized as constrained to wet environments where many of their defining characteristics evolved. A functional capability potentially enforcing the wet habitation of early angiosperms was possession of xylem with low hydraulic capability and drought intolerance. Recent studies indicate that a structural hallmark of these performances is long, steeply angled scalariform perforation plates in extant basal angiosperms. However, Canella winterana (Canellaceae), a magnoliid, appears to have radiated into much drier tropical dry forest habitats despite possessing a wood vasculature structurally similar to hypothesized ancestral wet-adapted system for angiosperms as a whole. Based on a field study of Canella wood hydraulic function, we present ecophysiological data that basal vessels do not necessarily mark low resistance to drought. We found that Canella wood was resistant to drought-induced cavitation. Leaf hydraulic capacity as well as leaf pressure-volume relations and leaf water potential minima found for Canella were comparable to some of the most drought-tolerant tropical dry forest angiosperms. Thus, our results suggest that vessels of an ancestral design can be exported to dry environments. We found that scalariform-plated vessels in Canella were associated with low hydraulic capacities at the xylem and whole shoot-scales as well as limited plasticity of the xylem to varying hydrodynamic demand. Consequently, our results support the hypothesis that evolution of simple perforation plates, not vessels per se, represented a critical adaptation for producing large hydraulic gains during angiosperm evolution across a broader range of environments.