Figure 1.

Seasonal series of Populus tremuloides (A, C, E, and G) and Populus balsamifera (B, D, F, and H) stems representative of three biological replicates (n = 3) for each time period and species. In late May, fluorescence was detected in the rows of sieve elements closest to the cambial zone (A and B). More rows of sieve elements had matured when sampled in July (C and D). In the samples from early November (E and F), fluorescence was only detected in some sieve elements within the row closest to the vascular cambium. In early December (G and H), fluorescence was only detected in portions of a small number of cell walls within the phloem. Representative sieve elements within the phloem tissue (P) are indicated by arrowheads in all panels. The cambial zone (CZ) and xylem tissue (X) are also labeled in each panel. Freeze-substitution (C) reduced autofluorescence in the fluorescein channel from the xylem, compared with staining of fresh tissue (A, B, D, E, F, G, and H). Micrographs were captured using a Photometrics Coolsnap Dyno charge-coupled device (CCD) camera attached to a Nikon Eclipse Ts2-2-FL inverted microscope. All scale bars = 30 µm.