Pattern of secondary growth in monocot roots: unveiling longitudinal and cross-sectional variability

Monocot cambium activity varies along the root axis and circumference, resulting in eccentric secondary growth. Variation in secondary growth structure along the root diameter indicates functional specialization but without reaction wood characteristics. Secondary growth in roots is one of the most important adaptive features, providing mechanical support to stabilize the aboveground part of an arborescent plant. Our knowledge of this phenomenon in arborescent monocots is limited: it occurs exclusively in Dracaena species, it has a bundled structure and it is formed by the monocot cambium. To add to our understanding, we investigated the pattern of secondary thickening along the axis and along the diameter of the stem-borne roots of a dragon tree Dracaena draco L. by analyzing the direction of eccentricity vector and examining root anatomy. We hypothesized that the distribution of secondary growth changes along the root and that there are anatomic differences between concentric secondary growth (uniform around the root) and eccentric secondary growth (asymmetric), that may shed light on its adaptive significance. We found that roots show irregular eccentricity, with the direction of the eccentricity vector changing from up to sideways, counterclockwise or clockwise. Vascular bundle density was lower in eccentric secondary growth and these bundles differed in size, shape, and components (tracheid lumen fraction, tracheid wall fraction) compared to concentric secondary growth. Distinct arcs in eccentric secondary growth were the result of varying thickness of the ground parenchyma cell walls, variation in bundle size, or a combination of both. Our study was a pioneering effort to investigate the variability of secondary growth along roots in monocots, and suggests a spatial separation of the mechanical and transport functions in the root, but without the contribution of features characteristic of reaction wood.