Anatomical characteristics including tracheid lumen area, tracheid thickness-to-span ratios, and torus-aperture overlap have been shown to correlate with drought tolerance in saplings and mature conifers. However, quantification of these critical anatomical features for first-year conifer seedlings is non-existent. This is surprising considering conifers experience the highest mortality rates during their first year of growth, frequently attributed to water stress. Here we combine high resolution confocal laser scanning microscopy (CLSM) with x-ray computed microtomography (μCT) to assess development and hydraulic functioning of xylem in Pseudotsuga menziesii and Pinus ponderosa first-year seedlings. Mid-stem segments were imaged using CLSM at 2, 3, 4, 6, and 10 weeks after planting (AP) for anatomical measurements and xylem development. Hydraulic functioning was assessed using μCT imaging of intact and excised stems during desiccation as well as vulnerability curves on 1-2 month old seedlings. We hypothesized that P. ponderosaseedlings would exhibit characteristics indicative of better drought tolerance because it is commonly considered to be the more drought tolerant species.
Results/Conclusions
In both species, mature primary xylem with torus-margo pit membranes were present in discrete clusters by 2 weeks AP, indicating that even very young germinants have the physiological ability to isolate embolized tracheids. Differentiation of primary xylem continued adjacent to the pith and then centrifugally, transitioning to secondary xylem differentiation between 2 and 3 weeks AP. In terms of desiccation, the pith and intercellular spaces within the cortex were the first to embolize in both species, followed closely by the initial primary xylem clusters. Sustained desiccation resulted in embolism of the remaining primary xylem and severe tissue damage to the cortex, epidermal tissues, and resin canals (P. ponderosa only). Secondary xylem tracheids embolized only at the most severe levels of water stress. Based on stem vulnerability curves, quantitative image analysis, and a qualitative greenhouse common garden experiment, we conclude that, contrary to our hypothesis, neither P. ponderosa nor P. menziesii clearly exhibit characteristics indicative of better drought tolerance. Overall, our results suggest that (1) there are other factors besides xylem anatomy that facilitate P. ponderosa’s drought tolerance in natural ecosystems and (2) drought tolerance of saplings and adults cannot be extrapolated to predict drought tolerance at the earliest stages of seedling establishment.