Acknowledgments
This work was supported by the Slovak Research and Developmen
Agency under the contract no. APVV-0140-10, by the Slovak Gran
Agency for Science (no. 2/0083/14), and the National Research Founda
tion, Pretoria, South Africa (UID 67869). Authors are grateful to Prof
Eiichi Tanimoto (Nagoya City University, Graduate School of Natural Sci
ences, Japan) for stimulating discussions on root fragility and advice on
the method of root strength measurement. The authors are also thank
ful to Mgr. Marek Kučera, PhD (Faculty of Mathematics, Physics and In
formatics, Comenius University in Bratislava), for the professiona
advice and technical assistance at the realization of experimental roo
strength measurement.