Fractal Dimensions of Root X-ray Computed Tomography Images – a Measure of their Volume-Filling Capacity

Fractal dimensions (FDs) were suggested as a measure of the capacity of an irregular or branched natural object to fill a certain volume. Such volume filling capacity is an important feature of plant root systems, which are expected to utilize the rooting volume to a maximum. However, commonly used parameters of real or modelled root systems do not describe the characteristic of filling of the rooting volume.

In order to determine the FDs of an object it must be rendered throughout the occupied volume with uniform high resolution. In this study we used X-ray computed tomography (CT) that has been shown to be an effective tool for such imaging of plant root systems. Tomato seedlings were grown on two soil types – clay loam and loamy sand, which was either compacted (1.6 Mg m^-3) or non-compacted (1.2 Mg m^-3). Plants were scanned 10 days after germination. Calculating FDs of CT images of roots was done using the "box-count" method.

Judging by the total number of intercepted pixels, the root systems of the plants grown on non-compact soil was much [2-3 times] larger than those of the plants grown on compacted soils. However, FDs of the roots from the compacted soils were larger. This indicates that the 3D-FD can be used as a measure of a root system property which is different from what can be gathered from conventional root system size parameters. These data indicate that FDs of CT images can be used as a measure of the volume-filling capacity of the root systems.