Tree hydraulic function changes with cambial age

Background/Question/Methods

Plant structure is known to change over the course of plant development.  We examined whether tree hydraulic function also changes over the course of cambial development by comparing current year’s growth of one, two, and three year old shoots harvested from mature (>30 yr old) trees.  We hypothesized that xylem hydraulic function would change as the cambium ages. Furthermore, we predicted that specific conductivity (K_s) would increase with age and that water stress induced xylem cavitation (P₅₀) would increase (i. e. becomes less negative) with age. This was examined in several tree species growing on-campus at California State University, Bakersfield. One, two, and three year old branches were collected in series from 6 individuals per species.  In two and three year old segments, the inner growth rings were sealed using cyanoacrylate resin so that only the current year’s growth was measured.  Stems were then flushed to obtain their maximum K_s and subsequently subjected to successive centrifuge spins to generate a vulnerability to cavitation curve and to estimate P₅₀. 

 Results/Conclusions

Plant hydraulic function varied between the xylem of different aged stems. Our results, however, suggest that this variation was species-specific and patterns for K_s and P₅₀ change with age was variable. Most of the examined species showed no variation in xylem specific conductivity (K_s) with cambial age.  Xylem specific conductivity (K_s) variation, however, was observed within Ceratonia siliqua L. (P<0.001), with greater conductivities in two and three year stems when compared to one year old stems. Water stress induced xylem cavitation (P₅₀) did not change with cambial age in Laurus nobilis (P=0.206); however, other species such as Pyrus sp. (P<0.001) and Magnolia grandiflora L. (P< 0.001), did show a change in P₅₀ with cambial age. In these species, and contrary to our prediction, P₅₀ decreased with stem age. Our results suggest that plant hydraulic function may vary with tissue age and that these variations depend on the species in question. Increased understanding of plant hydraulic function and architecture likely requires the sampling of multiple aged tissues.