Seedlings, stress, and sugar space: smaller ray parenchyma fractions may reinforce the nonstructural carbohydrate strategy of some young conifers in drought

We ask if drought generates a shift towards lower wood fractions occupied by xylem parenchyma cells or higher wood densities, both changes which might reduce the volume over which limited nonstructural carbohydrates (NSC) are distributed and so concentrate the osmotic power of any available sugars within a smaller space. Using one-year-old Pinus ponderosa and Thuja plicata seedlings, we measured photosynthesis, growth, stem water potential (Ψ stem ), and wood NSC (sugar and starch) six times over ten weeks of drought and once after a five-week recovery period. Within each sampling day, five measurements separated by 6-hour intervals captured the diurnal dynamics of Ψ stem and wood NSC. Measurements of ray parenchyma fraction (RPF) and wood density at the experiment’s end demonstrated that P. ponderosa acclimated to drought by significantly decreasing RPF from a control average of 5.4 % to a drought-treated average of 4.2 %, while the RPF of T. plicata was much lower to start, declining insignificantly from 2.0 % to 1.8%. Wood density differences were not significant between drought treatments, but were significant between species. The anatomical changes observed in droughted P. ponderosa would mean that the sugar concentrations required to balance xylem tensions were not exceeded until approximately -2.0 MPa.