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Accelerated Nitrate Uptake in Wheat Seedlings: Effects of Ammonium and Nitrite Pretreatments and of 6-Methylpurine and Puromycin 总被引:1,自引:0,他引:1
The experiments reported herein had two objectives. One was to determine if the slow rate of nitrate uptake which occurs upon initial exposure of nitrogen-depleted wheat (Triticum vulgare cv. Knox) plants to nitrate was the result of insufficient reduced nitrogen. The second was to determine the impact of restrictions in ribonucleic acid or protein synthesis on both nitrate uptake and nitrate reduction. Pretreatments of 14-day-old seedlings for a few hours in ammonium or nitrite did not result in an enhancement of the initial slow rate of nitrate uptake. Growth for two additional weeks in ammonium also failed to eliminate the induction period. The evidence indicates that the presence of nitrate, rather than a product of its reduction, was required to initiate development of the accelerated rate of nitrate uptake. Puromycin (400 μg ml?1) and 6-methylpurine (0.5 mM) prevented development of the accelerated phase of nitrate uptake. With both compounds, the relative restriction of nitrate uptake was greater than that of nitrate reduction as revealed by incorporation of 15N from labeled nitrate into reduced forms. The proportion of reduction which occurred in the root system under the imposed treatments could not be delineated precisely, preventing an unequivocal determination of the extent to which the two processes are coupled in the root system. The data nevertheless indicate nitrate reduction was closely associated with nitrate uptake. Accumulation of nitrate in the shoots was markedly restricted in presence of 6 methylpurine. This effect most likely was a result of a severe restriction in the translocation of nitrate into the xylem, rather than an increase in the reduction rate in the shoots. 相似文献
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MATTHIAS VOLK PATRICK BUNGENER FRANÇOIS CONTAT MYRTA MONTANI JÜRG FUHRER 《Global Change Biology》2006,12(1):74-83
The sensitivity of seminatural grasslands to ozone (O3) pollution is not well known, in spite of the important function of these common ecosystems for agriculture and nature conservation. A 5‐year field experiment was carried out at a rural, mid‐elevation site at Le Mouret (Switzerland) to investigate the effect of elevated O3 on yield and species composition of a permanent, extensively managed, species‐rich old pasture. Using a free‐air fumigation system, circular plots of 7 m diameter were exposed to either ambient air (control plots) or to air containing O3 of approximately 1.5 × ambient concentrations. The resulting accumulated O3 exposure over the threshold of 40 ppb for one season ranged from 13.3 to 59.5 ppm h in the elevated O3 plots, and from 1.0 to 20.7 ppm h in the control plots. Subplots in each ring were harvested three times each year, and harvested biomass was separated into functional groups (FGs) (grasses, legumes, forbs). Productivity in both treatments decreased over time, but the yield of O3‐exposed plots decreased faster than that of the control plots, with the reduction being twice as large by the end of the fifth season. Compared with the ambient air control, loss in annual dry matter yield was 23% after 5 years. The change in annual biomass production because of O3 stress was accompanied by a change in the fractions of FGs, with the legume fraction showing a strong negative response. Such long‐term effects of O3 stress could have negative implications for the maintenance of biological diversity in rural landscapes across large areas of Europe. The results from this first long‐term experiment show that a moderately elevated O3 level reduces the productivity of intact grasslands during a 5‐year exposure under real field conditions. 相似文献
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Endogenous Ammonium Generation in Maize Roots and its Relationship to other Ammonium Fluxes 总被引:4,自引:0,他引:4
JACKSON WILLIAM A.; CHAILLOU SYLVIAN; MOROT-GAUDRY JEAN-FRANCOIS; VOLK RICHARD J. 《Journal of experimental botany》1993,44(4):731-739
An investigation to determine the magnitude of the back reactionswhich occur during net ammonium uptake by roots and during netammonium assimilation within roots was undertaken with maize(Zea mays L.). Ten-day-old seedlings, which had been grown on250 mmol m3 ammonium at pH 4 or 6, were pretreated for3 h in the absence or presence of 500 mmol m 3 MSX (methionine-DL-sulphoximine),an inhibitor of the glutamine synthetase-catalysed pathway ofammonium assimilation. They were then exposed for 2 h to 99A% 15N-ammonium ± MSX. Substantial ammonium cycling occurredduring net ammonium uptake. Efflux was enhanced by MSX treatment,reflecting a 2- to 3-fold accumulation of ammonium in the roottissue. Influx of ammonium was also increased by treatment withMSX, indicating that influx was enhanced when products of ammoniumassimilation were dissipated. The decline in root 14N-ammoniumaccounted for only a small fraction of the 14N-ammonium recoveredin the ambient 15N-ammonium solution, revealing a substantialgeneration of endogenous 14N-ammonium during the 2 h exposure.The net quantity of ammonium generated was increased appreciablywhen assimilation of ammonium was restricted by MSX and it wasestimated to occur at least 50% faster than net ammonium uptake.Presence of MSX severely decreased translocation of 15N to shootsbut had a smaller influence on incorporation of 15N into macromoleculesof the root tissue. The various ammonium flux rates were notgreatly affected by growth at pH 4.0, implying a considerableresistance of ammonium assimilation processes in these maizeroots to the high ambient acidity commonly induced by exposureto ammonium Key words: Ammonium generation, uptake, assimilation 相似文献
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