New insight into secondary growth in roots of monocotyledons using the example of the dragon tree, Dracaena draco L.

The species of the genus Dracaena are the only monocotyledonous plants that have a secondary growth in the stem as well as in the roots. Our knowledge of its structure is based mainly on the study of the stem, and little is known about the structure of root secondary growth. We aimed to characterize the radial variation in the anatomy of the secondary growth of the roots of a tree-like monocot representative Dracaena draco L. and compared it with that of the stem, expecting to find some differences since root and stem grow under different environmental conditions. Based on the anatomical analysis of large-area cross sections of secondary growth we show that roots have clearly eccentric growth with more secondary tissue on the upper root side. Following bundle density across the root enables to distinguish between the concentric and eccentric phases of secondary growth. The total cross-sectional area of roots and the stem is almost identical, but they differ in the contribution of secondary growth, its distribution and the lumen area of tracheids, which is almost twice larger in roots. This study shows that the differences in root and stem secondary growth structure are quantitative, and that vascular bundle density is a promising trait to further study the eccentricity of monocot secondary growth.