Tracing root development using the soft X-ray radiographic method, as applied to young cuttings of western red cedar (Thuja plicata)

Abstract

To the best of our knowledge, this is the first soft X-ray radiographic study of the development of an entire young root system of western red cedar (Thuja plicata D. Don). The experimental device was adapted to radiographic constraints and allowed the plant to grow for several months. Successive radiographs revealed primary growth and differentiation, secondary growth, and primordium development. The younger (8-day-old) part of the root system appeared fully opaque. Afterwards, the stele and the cortex could be distinguished from each other progressively in a second zone where most of the primary differentiation had already occurred. Measuring early cambial growth was made possible by spacing the radiographs at 10-day intervals. Time-course developments of primordia and their histolysis cavities were traced for 8-15 days, depending on the sample. The acropetal sequences of primordia along the same adventitious root were shown. Primordia had different growth curves, most of which were nearly logistic. These meristems were produced by unequal primordia, which generated axes of unequal diameters. Our techniques provide new opportunities for understanding the differentiation of axes and defining potential root growth during primordium development.Key words: soft X-ray radiography, root development, primordia, root differentiation, radial growth, morphogenesis.