lSN.Nuclea r magnetic resonance spectroscopy was used to follow nitrogen assimilation and amino-acid production in Wolffia arrhiza (L.) Hork. ex. Wimmer, clone Golan exposed to 4.0 mM 15NH4C1 solutions for 24 h. The main 15N_labelled metabolites were asparagine and glutamine, as well as substantial amounts of unreacted, intracellular NH~. These results were compared with those of a previous study on Lemna 9ibba L. clone Hurfeish (Monselise et al., 1987, New Phytol. 10, 341-345) with regard to NH2 uptake, assimilation and detoxification efficiencies. Both species, grown under continuous white light, were capable of preferential uptake of NH2 in the presence of nitrate. Relative growth rates indicate that both species tolerate increased levels of NH +, up to 10 -2 mol" 1-1, with L. 9ibba showing a slightly greater tolerance. No 15N-labelled free NH2 was detectable in L. 9ibba, while in W. arrhiza excess NH2 was found within the cells. This fact indicates that L. 9ib -ba is more efficient in detoxification than W. arrhiza, presumably because of inability of W. arrhiza to regenerate the "NH + traps", glutamate and aspartate, rapidly enough. This is also evident from the observation that addition of a-ketoglutarate to the medium caused nearly complete assimilation of intracellular NH + in W. arrhiza. In both plants, addition of a-ketoglutarate increased both NH2 uptake and assimilation. Addition of L-methionine DL-sulfoximine, an inhibitor of glutamine synthetase inhibited NH + assimilation, while addition of azaserine, an inhibitor of glutamate synthase, resulted in 15N incorporation into the glutamine-amide position only. These results are consistent with the glutamine synthetase-glutamate synthase pathway being the * This article is dedicated to Professor Bernhard Schrader on the occasion of his 60th birthday Abbreviations: AZA = azaserine (O-diazoacetyl-L-serine) ; GOGAT=glutamine oxoglutarate amine transferase=glutamate synthase E.C. 1.4.7. and E.C. 1.4.1.13. ; GS = glutamine synthetase E.C. 6.3.1.2. ; a-KG = c~-ketoglutarate = 2-oxoglutarate; MSO = Lmethionine DL-sulphoximine; NMR -nuclear magnetic resonance; RGR = relative growth rate major route of NH2 assimilation in the two plants under the conditions used.
SUMMARŶ *N Nuclear magnetic resonance (NMR) spectroscopy was used to follow nitrogen assimilation and amino acid production in Lemna gibba L. exposed to 0 4 mM ^^NH^Cl solution for 24 h. NMR analysis of the "N content of the treated plant tissues (aqueous extract) revealed "N incorporation into glutamine (N-amide plus amino-N or amide-N only) and glutamic acid and no detectable free ammonium ion.Methionine sulphoximine (MSO), an inhibitor of glutamine synthetase, at 10 mM inhibited completely the incorporation of ^*N. In the presence of 05 mM azaserine (AZA), a glutamate synthase inhibitor, the incorporation of ^*N was detected only in the amide group of glutamine. The results confirm the involvement of the glutamine synthetase/glutamate synthase (GS/ GOGAT) pathway in the assimilation of ammonium ions.
The 24 h exposure of water plants (etiolated duckweed) to RF-EMF between 7.8 V m(-1) and 1.8 V m(-1), generated by AM 1.287 MHz transmitting antennas, resulted in alanine accumulation in the plant cells, a phenomenon we have previously shown to be a universal stress signal. The magnitude of the effect corresponds qualitatively to the level of RF-EMF exposure. In the presence of 10 mM vitamin C, alanine accumulation is completely suppressed, suggesting the involvement of free radicals in the process. A unique biological connection has thus been made between exposure to RF-EMF and cell stress, in the vicinity of RF transmitting antennas. This simple test, which lasts only 24 h, constitutes a useful bioassay for the quick detection of biological cell stress caused in the vicinity of RF irradiating antennas.
In previous publications, we reported that sinusoidal varying magnetic fields (SVMF) modify the activity and dynamics of the malignancy marker adenosine deaminase, and enhance the proliferation of chick embryo fibroblasts (CEF). While the SVMF examined by us (50, 60 & 100 Hz / 0.06-0.7 mT) were all below kT, they may have the potential of altering chemical processes in which excited radicals are involved.We tested this hypothesis in two experimental systems: CEF in culture and Spirodela oligorrhiza (Lemnaceae) (a small aquatic plant, commonly known as Duckweed). CEF were exposed to SVMF of 100 Hz/0.7 mT for 24 h. The addition of the exogenous radical scavengers catalase, superoxide dismutase or vitamin E to the cells during exposure significantly suppressed enhancement of cell proliferation caused by the field (by 79, 67 and 82%, respectively, as evaluated by the MTT colorimetric assay). 15 N NMR analysis of Duckweed plants fed by 15 N-labeled ammonium chloride and exposed to SVMF at 60 and 100 Hz/0.7 mT for 24 h, revealed augmented alanine production. Alanine did not accumulate in the absence of SVMF. The addition of vitamin C, a radical scavenger, reduced alanine production by 82%.Exposure to SVMF resulted in specific metabolic stress effects in Duckweed plants and enhanced proliferation of CEF. In both cases, it is suggested that free radicals are involved.Abbreviations: SVMF = sinusoidal varying magnetic fields; CEF = chick embryo fibroblasts; ADA = adenosine deaminase; SOD = superodxide dismutase.
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