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

To the best of our knowledge, this is the first soft X-ray radiographic stu dy 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 ra diographs revealed primary growth and differentiation, secondary growth, an d primordium development. The younger (8-day-old) part of the root system a ppeared fully opaque. Afterwards, the stele and the cortex could be disting uished from each other progressively in a second zone where most of the pri mary differentiation had already occurred. Measuring early cambial growth w as made possible by spacing the radiographs at 10-day intervals. Time-cours e developments of primordia and their histolysis cavities were traced for 8 -15 days, depending on the sample. The acropetal sequences of primordia alo ng the same adventitious root were shown. Primordia had different growth cu rves, most of which were nearly logistic. These meristems were produced by unequal primordia, which generated axes of unequal diameters. Our technique s provide new opportunities for understanding the differentiation of axes a nd defining potential root growth during primordium development.