d by HPLC. The m/z 191 (Figure 2), m/z 179, 225, 277, 311, 339, 431, 471, 540, and 702 (Figure 3), and m/z 327, 481, and 655 (Figure 4) in fragmentation patterns for water extract of lettuces cultivated in EPCS was significantly different from those in CCS, indicating that production of specific metabolites may change with variation of soil conditions induced by electric pulse or directly influenced by the electric pulse.
Biogeochemical condition of culture soil for growth of each lettuce may not be very different but may be changed by electric pulse based on the weight difference of lettuces cultivated in CCS and EPCS. Twenty percent increase of crop weight in EPCS may be caused by improvement of nutritional soil condition for lettuce growth or physiological activation of lettuces by the electric pulse. Ammonium and nitrate are commonly absorbable nutrient ions by plants but nitrite is not [21,22]. However, nitrite can be chemically and biochemically oxidized to nitrate and biochemically reduced to ammonium [23-25]. Electrochemical redox reactions induced by the electric pulse may activate oxidation of nitrite to nitrate . Total nitrogen contents may thus be useful to estimate nutritional soil condition for lettuce growth. The total nitrogen content in EPCS was more than double that in CCS, reciprocally proportional to weight of lettuces. Inorganic nitrogen may be more effectively generated from soils composed of organic and inorganic nitrogen compounds by biochemical and chemical reaction in EPCS than CCS. Phosphate has a strong tendency to be adsorbed onto soil particles and readily becomes a waterinsoluble salt. However, phosphate may be desorbed by
Figure 3. Fragmentation patterns for water-extracted compounds of lettuce detected at retention time from 8.77 to 15.97 min (Figure 1, box II) in HPLC.
Figure 4. Fragmentation patterns for water-extracted compounds of lettuce detected at retention time from 20.87 to 23.61 min (Figure 1, box III) in HPLC.
biological weathering induced by bacterial metabolites and exudate secreted by roots of plants [27-29]. Higher phosphate in EPCS than CCS may be caused by additional weathering induced by the electric pulse . Minerals also may be dissolved in watered soil by the biochemical weathering induced by microbial metabolism, roots of plants  and chemical weathering induced by electric pulse . Nutrient salts and minerals are essential factors for plant growth, which may be more effectively balanced or increased for lettuce growth in EPCS than CCS .
Nutritional soil conditions were improved by the electric pulse. However, the improvement of nutritional soil condition may not have increased lettuce growth based on the difference of HPLC and mass spectrometry pattern for water extract of lettuces cultivated in CCS and EPCS. Difference of peak patterns in HPLC and fragmentation patterns in mass spectrometry for lettuces cultivated in CCS and EPCS are an indicator that some organic compounds produced by lettuces are directly influenced by the electric pulse or indirectly by variation of nutritional soil condition induced by the electric pulse. Metabolites produced by growing lettuces may be sugars, organic acids, amino acids, nucleic acids, and fatty acids. Practically, most of the metabolites produced in cells of lettuces grown for 21 days may be converted to structural polymers. Metabolite concentrations may be difficult to measure without separate purification. However, the differences of metabolites of hormones produced by lettuces grown in CCS and EPCS can be compared using patterns of HPLC and mass spectrometry. Hormones produced by lettuces can be identified using the fragmentation (m/z) pattern of precursor ions and product ions generated by electrospray ionization of standard hormones. The m/z patterns of organic compounds contained in lettuce extract were compared with the mass spectrometric database obtained using the standard hormones [19,20]. The m/z patterns obtained in mass spectrometry for the lettuce extract were not identified with the m/z pattern for the standard hormone (Table 4 and Figures 2-4). Variation and difference of m/z pattern in the mass spectrometry for lettuce extract can be an indicator to clearly show difference of metabolites produced by lettuces grown in CCS and EPCS. The difference of peak height and m/z pattern in the mass spectrometry for the lettuce extract does not predict whether the electric pulse charged to lettuce culture soil directly or indirectly activates lettuce growth by improvement of nutritional soil condition. Conclusively, the low intensity electric pulse charged to the culture soil for lettuce cultivation influenced improvement of nutritional soil condition for the water-soluble nutrient salts and minerals and increase of some organic compounds contained in lettuce extract. The relatively higher content of water-soluble organic compounds, of which molecular weight is possible to be minimally 133 (Figure 2) and maximally 711 (Figure 3) based on the m/z measured by the mass spectrometry, may be a clue that building blocks for biosynthesis of structural compounds may be more actively produced.
This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea governmental Ministry of Knowledge Economy (2010T1001100334).
- Yi, J.Y., Choi, J.W., Jeon, B.Y. and Park, D.H. (2012) Effect of a low-voltage electric pulse charged to culture soil on plant growth and variation of the bacterial community. Agricultural Sciences, 3, 339-346. doi:10.4236/as.2012.33038
- Liu, S.Q., Wu, N.J. and Ignatiev, A. (2000) Electricpulse-induced reversible resistance change effect in magnetoresitive films. Applied Physics Letters, 76, 2749- 2751. doi:10.1063/1.126464
- Meilhoc, E., Masson, J.M. and Teissié, J. (1990) High efficiency transformation of intact yeast cells by electric field pulse. Nature Biotechnology, 8, 223-227. doi:10.1038/nbt0390-223
- Glick, B.R., Karaturovic, D.M. and Newell, P.C. (1995) A novel procedure for rapid isolation of plant growth promoting Pseudomonads. Canadian Journal of Microbiology, 41, 533-536. doi:10.1139/m95-070
- Kennedy, I.R., Perg-Gerk, L.L., Wood, C., Deaker, R., Gilchrist, K. and Katupitiya, S. (1997). Biological nitrogen fixation in non-leguminous field crop: Facilitating the evolution of an effective association between Azospirillum and wheat. Plant Soil, 194, 65-79. doi:10.1023/A:1004260222528
- Kleeberger, A., Castroph, H. and Klingmuller, W. (1983) The rhizosphere microflora of wheat and barley with special reference to gram-negative bacteria. Archives of Microbiology, 136, 306-311. doi:10.1007/BF00425222
- Sakthivel, N. and Gnanamanikam, SS. (1987) Evaluation of Pseudomonas fluorescens for suppression of sheath rot disease and for enhances in rice (Oryza sativa L.). Applied and Environmental Microbiology, 53, 2056-2059.
- Ochs, M., Brunner, I., Stumm, W. and Ćosović, B. (1993) Effects of root exudates and humic substances on weathering kinetics. Water, Air and Soil Pollution, 68, 213-229. doi:10.1007/BF00479404
- House, K.Z., House, C.H., Schrag, D.P. and Aziz, M.J. (2007) Electrochemical accelaeation of chemical weathering as an energetically feasible approach to mitigating anthropogenic climate change. Environmental Science & Technology, 41, 8864-8870. doi:10.1021/es0701816
- Jenny, H. and Overstreet, R. (1939) Surface migration of ions and contact exchange. Journal of Physical Chemistry, 43, 1185-1196. doi:10.1021/j150396a010
- Unwin, P.R. and Bard, A.J. (1992) Scanning electrochemical microscopy. 14. Scanning electrochemical microscope induced desorption: A new technique for the measurement of adsorption/desorption kinetics and surface diffusion rates at the solid/liquid interface. The Journal of Physics Chemistry, 96, 5035-5045.
- Zhou, W., Inoue, S., Iwahashi, t., Kanai, K., Seki, K., Miyamae, T., Kim D., Katayama, Y. and Ouchi, Y. (2010) Double layer structure and adsorption/desorption hysteresis of neat inonic on Pt electrode surface-an in-situ IRvisible sum-frequency generation spectroscopic study. Elec-trochemistry Communication, 12, 672-675. doi:10.1016/j.elecom.2010.03.003
- Yeung, A.T. Hsu, C. and Menon, R.M. (1997) Physicochemical soil-contaminant interactions during electrokinetic extraction. Journal of Hazardous Materials, 55, 221-237. doi:10.1016/S0304-3894(97)00017-4
- Palaniappan, S., Sastry, S.K. and Richter, E.R. (1990) Effects of electricity on microorganisms: A review. Journal of Food Processing & Preservation, 14, 393-414. doi:10.1111/j.1745-4549.1990.tb00142.x
- Zhang, Q., Qin, B.L., Barbosa-Cánovas, G.V. and Swanson, B.G. (1995) Inactivation of E. coli for food pasteurization by high-strength pulsed electric fields. Journal of Food Processing & Preservation, 19, 103-118. doi:10.1111/j.1745-4549.1995.tb00281.x
- Grahl, T. and Maerkl, H. (1996) Killing of microorganisms by pulsed electric fields. Applied Microbiology and Biotechnology, 45, 148-157. doi:10.1007/s002530050663
- Hulsheger, H., Potel, J. and Niemann, E.G. (1983) Electric field effects on bacteria and yeast cells. Radiation and Environmental Biophysics, 22, 149-162. doi:10.1007/BF01338893
- Marquez, V.O., mittal, G.S. and Griffiths, M.W. (1997) Destruction and inhibition of bacterial spores by high voltage pulsed electric field. Food Science, 62, 399-401. doi:10.1111/j.1365-2621.1997.tb04010.x
- Chiwacha, S.D.S., Abrams, S.R., Amberose, S.J., Cutler, A.J., Loewen, M., Ross, A.R.S. and Kermode, A.R. (2003) A method for profiling classes of plant hormones and their metabolites using liquid chromatography-electrospray ionization tandem mass spectrometry: An analysis of hormone regulation of thermodormancy of lettuce (Lactuca sativa L.) seeds. The Plant Journal, 35, 405- 471. doi:10.1046/j.1365-313X.2003.01800.x
- Kojima, M., Kamada-Nobusada, T., Komatsu, H., Takei, K., Kuroha, T., Mizutani, M., Ashikari, M., Ueguchi-Tanaka, M., Matsuoke, M., Suzuki, K. and Sakakibara, H. (2009) Highly sensitive and high-throughput analysis of plant hormones using ms-probe modification and liquid chromatography-tandem mass spectrometry: An application for hormone profiling in Oryza sativa. Plant Cell Physiology, 50, 1201-1214. doi:10.1093/pcp/pcp057
- Kronzucker, H.J., Siddiqi, M.Y., Glass A.D.J. and Kirk G.J.D. (1999) Nitrate-ammonium synergism in rice. A subcellular flux analysis. Plant Physiology, 119, 1041- 1046. doi:10.1104/pp.119.3.1041
- Cao, W. and Tibbits, T.W. (1993) Study of various / mixtures for enhanced growth of potatoes. Journal of Plant Nutrition, 16, 1691-1704. doi:10.1080/01904169309364643
- Lees, H. and Simpson, J.R. (1957) The biochemistry of the nitrifying organisms. Nitrite oxidation by Nitrobacter. Biochemical Journal, 65, 297-305.
- Belser, L.W. and Mays, E.L. (1980) Specific inhibition of nitrite oxidation by chlorate and its use in assessing nitrification in soil and sediments. Applied and Environmental Microbiology, 39, 505-510.
- Rice, C.W. and Tiedje, J.M. (1989) Regulation of nitrate assimilation by ammonium in soils and in isolated soil microorganisms, Soil Biology and Biochemistry, 21, 597- 602. doi:10.1016/0038-0717(89)90135-1
- Jiang, L., Wang R., Li, X., Jiang, L. and Lu, G. (2005) Electrochemical oxidation behavior of nitrite on a chitosan-carboxylated multiwall carbon modified electrode. Electrochemistry Communication, 7, 597-601. doi:10.1016/j.elecom.2005.04.009
- Freitas, J.R., Banerjee, M.R. and Germida, J.J. (1997) Phosphate-solubilizing rhizobacteria enhance the growth and yield but not phosphorus uptake of canola (Brassica napus L.). Biology and Fertility of Soil, 24, 358-364. doi:10.1007/s003740050258
- Narsian, V. and Patel, H.H. (2000) Aspertgillus aculeatus as a rock phosphate solubilizer. Soil Biology& Biochemistry, 32, 559-565. doi:10.1016/S0038-0717(99)00184-4
- Hilda, R. and Reynaldo, F. (1999) Phosphate solubilizing bacteria and their role in plant growth promotion. Biotechnology Advances, 17, 319-339. doi:10.1016/S0734-9750(99)00014-2
- Shenker, M., Seitelbach, S., Brand, S., Haim, A. and Litaor, M.I. (2005) Redox reactions and phosphorus release in re-flooded soils of an altered wetland. European Journal of Soil Science, 56, 515-525. doi:10.1111/j.1365-2389.2004.00692.x
- Uroz, S., Calvaruso, C., Turpault, M.P. and Frey-Klett, P. (2009) Mineral weathering by bacteria: Ecology, actors and mechanisms. Trends in Microbiology, 17, 378-387. doi:10.1016/j.tim.2009.05.004
- Ayllon, E.S., Granese, S.L. and Rosales, B.M. (1990) Electrochemical response of weathering and plain c steels in different environments. Corrosion Reviews, 9, 246- 269. doi:10.1515/CORRREV.1990.9.3-4.245
- Yamaguchi, K.E. (2001) Evolution of the geochemical cycles of redox-sensitive elements. Frontier Research on Earth Evolution, 1, 249-252.