Plant hydraulic safety and efficiency change with tree cambial age in current year growth
Plant structure changes over the course of development. There are two hypotheses that describe how these changes may be related to plant hydraulic function. The vulnerability segmentation hypothesis suggests that younger stems will be more vulnerable to water stress than older stems. Conversely, data from gymnosperms suggests that younger stems may be more resistant to water stress than older stems, perhaps due to tissue biomechanical requirements or the lower water potentials experienced by branch tips. We examined whether tree hydraulic function, including specific conductivity (Ks) and water stress induced xylem cavitation (P50), changed over the course of cambial development in the stems of 13 tree species. This included both evergreen and deciduous species. We compared current year growth of young (“A”), intermediate (“B”), and older (“C”) stems occurring in series along branches and examined 4-6 individuals per species. In B and C aged segments, the inner growth rings were sealed using cyanoacrylate resin so that only current year growth was measured. Stems were flushed to obtain their maximum Ks and a centrifuge-based vulnerability to cavitation curve was constructed. Vulnerability curves were confirmed for a subset of species using single vessel air injection.
There was a significant interaction between age and species (P=0.001), with hydraulic efficiency increasing with age in some species, but not changing with age in others. As cambial age increased, P50 decreased (i.e. became more negative) indicating that xylem produced by older cambium was more resistant to cavitation than xylem produced by younger cambium (P < 0.001). For those species that experienced a change in Ks and also a decrease in P50, this result was surprising, because it contrasts with the pattern predicted based on a safety-efficiency tradeoff. Leaf habit did not significantly impact Ks (P = 0.263) or P50 (P = 0.882). Our results suggest that plant hydraulic function varies with cambial age in tree stems and that these changes are consistent with the vulnerability segmentation hypothesis. The presence of primary xylem in the youngest “A” stem sections could have contributed to their greater vulnerability. In addition, pits in xylem produced by older cambium could contain thicker pit membranes, which could confer greater safety from cavitation. Our intra-organismal study design, using different aged materials, may be informative in examining functional tradeoffs and increasing our understanding of plant hydraulic function and architecture.