Xylem vessel structure of current year growth varies with cambial age

Background/Question/Methods

The structure of vessels within the xylem of flowering plants is related to xylem function, which includes plant hydraulic transport efficiency and resistance to water stress-induced hydraulic failure. Due to the importance of plant hydraulic architecture and development, we examined whether vessel diameter and length changed with cambial age. We hypothesized that vessel diameter and length would both increase with increased cambial age. Furthermore, we predicted that the ratio between the diameters of early and late wood vessels (i.e. vessels occurring in the initial compared to the latter half of the annual growth increment; E:L) would also increase with cambial age in ring porous species. Xylem vessel structure was examined in young (1 to 4 yr), intermediate (2 to 7 yr), and older (3 to 10 yr) samples that were collected in series. A total of 12 tree species and four to six individuals per species were studied. Branches where sectioned using a sledge microtome and an average of 100 vessels total per sample where measured for diameter. Only the outer-most growth ring was sampled (i.e. current year growth) in order to control for inter-annual variation and time since xylem production. Maximum vessel length was determined using air-injection.

Results/Conclusions

For all species, vessel length and diameter increased with cambial age (P < 0.001 for both).  This included an increase in both the mean and maximum vessel diameter of vessels occurring in early wood xylem (P < 0.001 for both) and an increase in the mean and maximum vessel diameter of late wood vessels (P < 0.001 for both); however, there was an interaction between species and age for late wood vessel diameters (P < 0.001 and 0.004 for the interaction term for mean and maximum, respectively). For the four species that have been reported as being semi-ring to ring porous, there was a significant increase in E:L with cambial age when maximum vessel diameter was analyzed (P = 0.031), indicating that the degree of their ring-porosity was increasing in the xylem produced by an older cambium.  Our data indicate that vessel structure varies with cambial age, even between samples from current year growth in segments that are collected in series. This implies that a complete understanding of tree structure and function requires the examination of samples of many ages.