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The western corn rootworm ( Diabrotica virgifera virgifera LeConte (WCR)) is a major pest of corn in USA. The pest arrived in Croatia in 1995, and today over 250.000 ha are infested. Insecticides are regularly used to control WCR, but the cost is high, can pose environmental risks, and may become ineffective due to resistance. Growing corn that is resistant to corn rootworms would be a valuable alternative to insecticides. Nine Croatian (Institute of Agriculture, Osijek) and two Pioneer Hi-Bred Int. Inc. (Johnston, Iowa, USA) commercial corn hybrids were evaluated for WCR resistance at two locations using complete randomized block design with four replications. One location was in east Croatia (Osijek) and one was in the USA (Iowa). The hybrids’ tolerance to larval feeding was rated using root injury, root regrowth and root size. Root injury was rated using the Node-Injury Scale 0–3 and evaluation of root size and root regrowth were done by scale 1–6. In an analysis of variance combined across locations, root injury didn’t show any significantly differences among hybrids, but root regrowth and root size were significantly different.

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A new method is introduced to agricultural practice for measuring the living active root of the plants. The measured root capacitance is interpreted in electro-chemical principles. In addition to the electrochemical interpretation of the measurements we aimed to find a non-wounding electrode instead of the needle plant electrode. Another reason for dealing with the tweezer plant electrode was to decrease the relatively high standard deviation of the root capacitance readings due to the relatively high uncertainty of hitting the xylem with the needle plant electrode. To improve and standardize the contact between the tweezer plant electrode and the stem a high electrical conductivity gel (UNIGEL) was applied on the stem before clipping the tweezers.   Experiments for the root capacitance measurements were made in temperature and light controlled climate chambers (Conviron, Canada) in 2 litre plastic pots filled with 4:1 soil:sand mixture and water culture. Comparison of the root capacitances of five-week old sunflower plants measured with the needle and the tweezer plant electrodes proved identical in water culture and capillary water saturated soil. However, the applicability of the tweezer plant electrode needs further study for other plants and environmental conditions. The effect of measurement frequency on root capacitance and resistance with the HP4284A impedance bridge was also studied to see the effectiveness of polarization (Figure 1).  From Figure 1 it can be seen that root capacitance decreased at frequencies above 1 kHz, while it increased up to the dielectric constant of water at lower frequencies. An interpretation of measurable root capacitance in the soil-plant system is given using separate measured plant tissue and soil capacitances. We established that root capacitance in the soil-plant system approximates the capacitance of the root tissue.  Good correlation was found between root capacitance and the calculated root surface area (RA) for sunflower plants (Figure 2).   The GW LCR-814 was found suitable for making root capacitance measurements.  Finally, further experimental work is needed to collect information for the more general and extended applicability of the method before it becomes a routine tool in ecological and agricultural practice. 

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Rajkai, K., Végh, K. R., Nacsa, T. (2002): Electrical capacitance as the indicator of root size and activity. Agrokémia és Talajtan, 51, 8998. Electrical capacitance as the indicator of

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Chloupek, O., Skácel, M., Ehrenbergova, J. (1999): Effect of divergent selection for root size in field-grown alfalfa. Can. J. Plant Sci. , 79 , 93-95. Effect of divergent selection for root size in field-grown alfalfa

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size and distribution of field grown wheat genotypes. Crop Sci. 34 :810–812. Teyker R.H. Root size and distribution of field grown wheat genotypes

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. , Larsen , E. & Nørbæk , R. ( 2004 ): Effects of genotype, root size, storage, and processing on bioactive compounds in organically grown carrots ( Daucus carota L.) . J. Food Sci. , 69 , S388 – S394

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pepper ( Capsicum annuun L.) . Exp Syst Appl 36 : 8975 – 8979 10.1016/j.eswa.2008.11.038 . 5. Kulkarni , M , Phalke , S 2009 Evaluating variability of root size system and its

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Tollefson , J.J . ( 1989 ). Relationship of root ratings, root size, and root regrowth to yield of corn injured by western corn rootworm (Coleoptera, Chrysomelidae) . Journal of Economic Entomology , 82 : 1760 – 1763 https://doi.org/10.1093/jee/82

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