Fractal analysis for the evaluation of complexity of plant root system under heavy metal stress

As a critical factor for the resources acquisition of water and nutrient, the root system responding the in-situ environment includes root length, root weight, root branching density, root hair density etc. Fractal geometry has been used to describe the complexity of many objects in nature such as leaves, trees, sea shores and so on. The aim of this study is to use fractal geometry in examining the developmental responses of the root system under a heavy metal stress, namely, Cd stress. Based on fractal geometry, fractal dimension (FD) was evaluated and comparison with conventional measurements was done. Wheat plants that were grown in hydroculture system for three weeks under different concentrations, 0, 0.001, 0.01, and 0.05mM of Cd were used as samples. Photographs of the entire root systems of the plants were taken weekly with a digital camera for three consecutive weeks, and then, they were used for analysis. The analysis of the images involved segmentation and skeletonization followed by application of box-counting method to calculate the FD. The changes of FD in response to different Cd stresses were examined. The results indicate that FDs under Cd stress were lower than those for the control condition. The results demonstrate that FD can be an effective measure to describe the changes in the structural development of the root system under heavy metal stress. It was also found that FD was more sensitive to changes under the influence of the heavy metals than those of the conventional measurements.