Figure 5.

Different TE morphotypes in woody plants depend on specific postmortem accumulated lignins for their resistance against collapse. A, Schematic diagram of the three TE types in the xylem of poplar stems, oriented on the pith–cambium axis: primary vessels (PV) in yellow, old secondary vessels (old SV) in purple, and young secondary vessels (young SV) in blue. B, Relative proportion of adjacent TEs for each TE type. Note that the proportion of TEs neighboring other TEs is independent of TE type and very similar to the proportions in Arabidopsis. C, Lumen area of each TE type determined from cross-sections. D, Relative lignin to cellulose ratio measured by Raman microspectroscopy. E, Relative S to G ratio measured by Raman microspectroscopy. F, Relative G_CHO to G_CHOH ratio measured by Raman microspectroscopy. Because range and intercept of Raman band ratios differ from other biochemical analyses but still maintain a linear relationship (Agarwal, 2019; Blaschek et al., 2020b), references are presented for each lignin parameter using poplar fibers or the interfascicular fibers (IF) of Arabidopsis WT and relevant mutants. Different lowercase letters in panels B–F indicate significant differences according to a Tukey-HSD test (per panel; $\alpha$ = 0.05); n = 56–72 individual cells from five plants per TE type. G, Representation of TE perimeter for each TE type in transverse cross-sections from stems of Populus tremula×tremuloides RNAi plants altering lignin biosynthesis. TE outline color indicates the circularity of each respective TE. H–J, Structural equation models of the factors influencing TE convexity and circularity in the PV (H), old SV (I), and young SV (J). Blue arrows and positive standardized coefficients indicate significant positive effects, red arrows and negative standardized coefficients indicate significant negative effects. Dashed arrows indicated predictors that were included and improved the model, but whose specific effects were not statistically significant. Grayed out variables had no significant effect on TE convexity.