Root Dimorphism in Ludwigia peploides (Onagraceae): Development of Two Root Types from Similar Primordia

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Abstract
The marsh plant L. peploides produces downward-growing roots (DGR) and upward-growing roots (UGR) from submerged nodes. The 2 root forms differ from each other in structure and behavior. Development was studied to determine if these diverse roots have similar origins or if they differ from each other at inception. At each submerged node, root formation is restricted to a crescent-shaped region extending from 1 stipule, over the axillary buds and down to the remaining stipule. A given position along the crescent may be occupied by either DGR or UGR. The 1st roots to emerge from any node are DGR, but once UGR appear, 94% of the roots which later emerge from that side of the node are UGR. Most roots growing from the ventral (lower) side of nodes are DGR. The type of root which will emerge from a given site on the node can be predicted. Nodes sectioned before all their roots have emerged reveal DGR, UGR and primordia. Prior to emergence from the node, both root types are structurally indistinguishable from one another. Unlike other shoot-borne or adventitious roots, nodal roots of Ludwigia develop a provascular cylinder, cortex and root cap while the primordium is small. As roots emerge from the node, DGR and UGR can be distinguished from each other by their structure. The axis of UGR swells as cells beneath the outer layer of the root cap produce strutlike arms. The axis of DGR remains compact and smooth and the cap stays intact. Since the early developmental stages of DGR and UGR are similar and primordia in the node may grow into either root type, both root forms in Ludwigia develop from uncommitted primordia which assume their distinctive structure only after emerging from the node.