Ray and axial parenchyma cells (RAPCs) of the secondary xylem are often discussed in terms of the supportive roles they play in the storage and release of carbohydrates and/ or water and ions. The energy stored and mobilized by these cells is credited with frost protection and post dormancy growth. RAPCs likely play additional roles during or succeeding periods of stress when they are providing energy for xylem refilling, and as water capacitors for the temporally dynamic relief of sudden changes in evaporative demand. In spite of the critical role that RAPCs play in the physiology of perennial plants, there is surprisingly little information available on the phenotypic plasticity of their abundance in response to abiotic stress. This project aims to link xylem RAPC fractions with maximum non-structural carbohydrate (NSC) concentration and then to relate both values to the length of temperature-imposed or drought-imposed dormancy experienced by a given individual or species along an elevation transect in California’s Sierra Nevada. Hypothesizing that RAPC fraction is phenotypically plastic (within species) but also that a genetic signal among species may be buried among a multitude of other species-specific traits which impart stress resilience, we collected wood cores at several physiologically relevant time points of all tree species present at 72 randomly distributed south-facing sites (6 sites every 250 m from 500 to 3250m). With these cores we determined RAPC volume with anatomical analysis and non-structural carbohydrate content using hydrolytic enzyme reactions coupled with a sulfuric-anthrone acid assay.
Results/Conclusions:
A relatively strong correlation between stored starch and RAPC fraction provides evidence for the conclusion that RAPCs deliver the stored energy required to survive periods of reduced photosynthesis imposed by cold (high elevation) and/or drought (low precipitation and/or high vapor pressure deficit). We also found that while RAPC fractions and starch concentrations are relatively low in evergreen species, these low concentrations are diffuse within sapwood. In deciduous angiosperms, starch concentration declines rapidly to zero as you move away from the cambium. Conifers, in contrast, show relatively constant values, indicating dilute, but deep reserves.