Xylem parenchyma and the mitigation of abiotic stress across gradients of temperature, moisture, and salinity

Background/Question/Methods: 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 woody perennials, there is surprisingly little information available on the phenotypic plasticity of their abundance in response to abiotic stress. We will illustrate their phenotypic plasticity in response to two distinct stressors in the context of two separate projects. The first project links RAPCs and their role in carbohydrate storage with the drought and temperature stresses experienced by a given individual or species along an elevation transect in California's Sierra Nevada. We collected wood cores from 15 species, mostly gymnosperms, at 72 randomly distributed south-facing sites (six sites every 250m from 500 to 3250m). We determined non-structural carbohydrate content using hydrolytic enzyme reactions coupled with a sulfuric-anthrone acid assay and the fraction of xylem taken up by living cells with anatomical analysis. The second project considers the role of RAPCs in the storage of minimally essential but environmentally pervasive nutrients like sodium and chloride and observes their plasticity in response to gradations of salinity in several representatives of the salt tolerant Pistachia genus. We determined ion concentrations with ICPMS and parenchyma, again, with anatomical analysis.

Results/Conclusions: Generally, we are finding that RAPC fraction increases at either end of a species' elevational range—not at either end of the elevation transect’s entirety and that stem storage is indeed critical in reducing leaf accumulation of toxic ions.