@article{feng_volk_jackson_1998, title={Source and magnitude of ammonium generation in maize roots}, volume={118}, ISSN={["0032-0889"]}, DOI={10.1104/pp.118.3.835}, abstractNote={Studies with 15N indicate that appreciable generation of NH4+ from endogenous sources accompanies the uptake and assimilation of exogenous NH4+ by roots. To identify the source of NH4+ generation, maize (Zea mays L.) seedlings were grown on 14NH4+ and then exposed for 3 d to highly labeled 15NH4+. More of the entering 15NH4+ was incorporated into the protein-N fraction of roots in darkness (approximately 25%) than in the light (approximately 14%). Although the 14NH4+ content of roots declined rapidly to less than 1 &mgr;mol per plant, efflux of 14NH4+ continued throughout the 3-d period at an average daily rate of 14 &mgr;mol per plant. As a consequence, cumulative 14NH4+ efflux during the 3-d period accounted for 25% of the total 14N initially present in the root. Although soluble organic 14N in roots declined during the 3-d period, insoluble 14N remained relatively constant. In shoots both soluble organic 14N and 14NH4+ declined, but a comparable increase in insoluble 14N was noted. Thus, total 14N in shoots remained constant, reflecting little or no net redistribution of 14N between shoots and roots. Collectively, these observations reveal that catabolism of soluble organic N, not protein N, is the primary source of endogenous NH4+ generation in maize roots.}, number={3}, journal={PLANT PHYSIOLOGY}, author={Feng, JN and Volk, RJ and Jackson, WA}, year={1998}, month={Nov}, pages={835–841} }
 @article{jackson_volk_1995, title={ATTRIBUTES OF THE NITROGEN UPTAKE SYSTEMS OF MAIZE (ZEA-MAYS L) - MAXIMAL SUPPRESSION BY EXPOSURE TO BOTH NITRATE AND AMMONIUM}, volume={130}, ISSN={["0028-646X"]}, DOI={10.1111/j.1469-8137.1995.tb01827.x}, abstractNote={summary Higher plants arc commonly exposed to rapid changes in the amounts and proportions of ammonium and nitrate in their root environment. Some of the potential consequences of such changes have been determined by examining the suppressive effects of growing maize ( Zea mays L.) seedlings with a high concentration (3 mM) of nitrogen on the subsequent uptake of nitrogen from a more dilute (0.2 mM) solution. To document the interactive role of nitrate and ammonium supply in suppression, ratios of these ions in the growth solution were maintained at 3:0, 2:1, 1:2 and 0:3. Rates of nitrate and ammonium uptake were then measured during initial (0–1 h) exposure to 0.2 mM KNO 3 or NH 4 NO 3 and hourly during a subsequent 7 h period of adaptation to the dilute solutions. Maximal suppression of nitrate uptake occurred only when both nitrate and ammonium were present (2: 1 and 1:2) during prior growth. Nitrate uptake rates increased two‐ to three‐fold as the seedlings adapted to the dilute KNO 3 solution, but the presence of ammonium with nitrate in the prior growth solution restricted the rate and extent of recovery from suppression. Recovery was further restricted when ambient ammonium was present during the adaptation period. Neither the magnitude of suppression nor the rate and extent of recovery was readily explained by ( a ) the proportion of nitrate and ammonium present during prior growth, ( b ) carbohydrate concentrations in root and shoot tissues, ( c ) concentrations of nitrate and ammonium in the root tissue, or ( d ) indirect effects of ammonium on potassium uptake. Total nitrate reduction by the entire plant in the 8 h adaptation period decreased as the proportion of ammonium in the prior growth solution increased. Ambient ammonium restricted nitrate reduction only in seedlings previously grown entirely with ammonium. Thus the effect of ambient ammonium on nitrate reduction by the whole plant differed substantially from that on nitrate uptake. Since nearly all the ammonium taken up by the roots was assimilated, as well as that generated by nitrate reduction, it appears that carbohydrate supply did not limit the observed uptake of ammonium and nitrate. The accumulation in roots of both nitrate and specific metabolites of ammonium serving as negative electors could account for most of the observations.}, number={3}, journal={NEW PHYTOLOGIST}, author={JACKSON, WA and VOLK, RJ}, year={1995}, month={Jul}, pages={327–335} }
 @article{jackson_chaillou_morotgaudry_volk_1993, title={ENDOGENOUS AMMONIUM GENERATION IN MAIZE ROOTS AND ITS RELATIONSHIP TO OTHER AMMONIUM FLUXES}, volume={44}, ISSN={["0022-0957"]}, DOI={10.1093/jxb/44.4.731}, abstractNote={An investigation to determine the magnitude of the back reactions which occur during net ammonium uptake by roots and during net ammonium assimilation within roots was undertaken with maize (Zea mays L.). Ten-day-old seedlings, which had been grown on 250 mmol m−3 ammonium at pH 4 or 6, were pretreated for 3 h in the absence or presence of 500 mmol m −3 MSX (methionine-DL-sulphoximine), an inhibitor of the glutamine synthetase-catalysed pathway of ammonium assimilation. They were then exposed for 2 h to 99 A% 15N-ammonium ± MSX. Substantial ammonium cycling occurred during net ammonium uptake. Efflux was enhanced by MSX treatment, reflecting a 2- to 3-fold accumulation of ammonium in the root tissue. Influx of ammonium was also increased by treatment with MSX, indicating that influx was enhanced when products of ammonium assimilation were dissipated. The decline in root 14N-ammonium accounted for only a small fraction of the 14N-ammonium recovered in the ambient 15N-ammonium solution, revealing a substantial generation of endogenous 14N-ammonium during the 2 h exposure. The net quantity of ammonium generated was increased appreciably when assimilation of ammonium was restricted by MSX and it was estimated to occur at least 50% faster than net ammonium uptake. Presence of MSX severely decreased translocation of 15N to shoots but had a smaller influence on incorporation of 15N into macromolecules of the root tissue. The various ammonium flux rates were not greatly affected by growth at pH 4.0, implying a considerable resistance of ammonium assimilation processes in these maize roots to the high ambient acidity commonly induced by exposure to ammonium}, number={261}, journal={JOURNAL OF EXPERIMENTAL BOTANY}, author={JACKSON, WA and CHAILLOU, S and MOROTGAUDRY, JF and VOLK, RJ}, year={1993}, month={Apr}, pages={731–739} }
 @article{jackson_kwik_volk_1976, title={NITRATE UPTAKE DURING RECOVERY FROM NITROGEN DEFICIENCY}, volume={36}, ISSN={["0031-9317"]}, DOI={10.1111/j.1399-3054.1976.tb03931.x}, abstractNote={Abstract Two‐week‐old nitrogen‐deficient wheat plants attained a high rate of nitrate uptake on the first day of exposure to nutrient solutions supplemented with KNO 3 . Ammonium uptake from similar solutions supplemented with NH 4 NO 3 was also high during the first day of exposure, but nitrate uptake from this solution was lower than from the KNO 3 treatment. During the next two to three days there was a progressive decrease in uptake of both nitrogen ions. A steady increase in uptake then occurred as the plants fully recovered from the nitrogen‐deficient state. The transient low nitrate uptake after three or four days of exposure to KNO 3 was not due to an excessive accumulation of nitrate in the tissue, nor to a failure in nitrate reduction as indicated by the rate of nitrate accumulation relative to the uptake rate. Nitrogen supplied as 15 N‐nitrite during the low uptake period was effectively incorporated into organic forms and effectively translocated to the shoots. Failure of the root tissue to increase in soluble carbohydrates during illumination was characteristic of the low uptake period. This contrasted with an increase in root soluble carbohydrates in the light during rapid uptake associated with full recovery from the nitrogen‐deficient state. It is concluded that carbohydrate translocation to the root system was insufficient during the intermediate recovery period for optimal nitrate uptake, although it was sufficient for effective reduction and translocation of nitrate and reduced nitrogen. Ammonium uptake from NH 4 NO 3 was restricted during darkness by the third day whereas there was little difference between light and dark periods in nitrate uptake from KNO 3 until about the sixth day of recovery. The extent to which ammonium restricted nitrate uptake increased progressively for two or three days following which a lessening influence seemed evident, and the effects were not directly associated with the rate of ammonium uptake.}, number={2}, journal={PHYSIOLOGIA PLANTARUM}, author={JACKSON, WA and KWIK, KD and VOLK, RJ}, year={1976}, pages={174–181} }
 @article{jackson_johnson_volk_1974, title={NITRITE UPTAKE BY NITROGEN-DEPLETED WHEAT SEEDLINGS}, volume={32}, ISSN={["0031-9317"]}, DOI={10.1111/j.1399-3054.1974.tb03723.x}, abstractNote={Abstract Intact, 14‐day‐old nitrogen‐depleted wheat ( Triticum vulgare cv. Blueboy) seedlings were exposed to solutions of 0.5 m M KNO 2 , 0.05 m M CaSO 4 and 1 m M sodium 2‐[N‐morpholino]‐ethanesulfonate, pH 6.1. Nitrite uptake was determined from depletion of the ambient solution or from incorporation of 15 N in the tissue. An initial nitrite uptake shoulder was followed by a relatively slow uptake rate which subsequently increased to a substantially greater rate. This accelerated phase was maintained through 24 h. Nitrite accumulated to a slight extent in the root tissues during the first few hours but declined to low values when the accelerated rate was fully developed, indicating an increase in nitrite reductase activity paralleling the increase in nitrite uptake capacity. About 50% of the nitrogen absorbed as nitrite was translocated to the shoots by 9–12 h. Development of the accelerated nitrite uptake rate was restricted in excised roots, in intact plants kept in darkness, by 400 μg puromycin ml −1 and by 1 m M L‐ethionine. When puromycin and L‐ethionine were added after the accelerated phase had been initiated, their effects were not as detrimental as when they were added at first exposure to KNO 2 . The two inhibitors restricted translocation more than uptake. The data indicate an involvement of protein synthesis and a requirement for movement of a substance from shoots to roots for maximal development of the accelerated nitrite uptake phase. A requirement for protein synthesis in the transport of soluble organic nitrogen from roots to shoots is also suggested.}, number={1}, journal={PHYSIOLOGIA PLANTARUM}, author={JACKSON, WA and JOHNSON, RE and VOLK, RJ}, year={1974}, pages={37–42} }
 @article{jackson_johnson_volk_1974, title={NITRITE UPTAKE PATTERNS IN WHEAT SEEDLINGS AS INFLUENCED BY NITRATE AND AMMONIUM}, volume={32}, ISSN={["0031-9317"]}, DOI={10.1111/j.1399-3054.1974.tb03736.x}, abstractNote={Abstract Nitrite and nitrate uptake by wheat ( Triticum vulgare ) from 0.5 m M potassium solutions both showed an apparent induction pattern characterized by a slow initial rate followed by an accelerated rate. The accelerated phase was more rapid for nitrate uptake, was initiated earlier, and was seriously restricted by the presence of equimolar nitrite. The accelerated phase of nitrite uptake was restricted by nitrate to a lesser extent. The two anions seem not to be absorbed by identical mechanisms. Ammonium pretreatments or prior growth with ammonium had relatively little influence on the pattern of nitrite uptake. However, prior growth with nitrate eliminated the slow initial phase and induced development of the accelerated phase of nitrite uptake. A beneficial effect was noted after 3 h nitrate pretreatment and full development had occurred by 12 h nitrate pretreatment. The evidence suggests that a small amount of tissue nitrite, which could be supplied either by absorption or by nitrate reduction, was specifically required for induction of the accelerated phase of nitrite uptake. Cycloheximide (2 μg ml −1 ) seriously restricted development of the accelerated phase of nitrite uptake, but its effect was not as severe when it was added after the accelerated phase had been induced by prior exposure to nitrite or nitrate. However, translocation of 15 N from the absorbed nitrite was sharply decreased under the latter conditions, indicating a difference in sensitivity of the uptake and translocation processes to cycloheximide. Potassium uptake was greater from KNO 3 than from KNO 2 and in both instances it was enhanced during the early stages of the accelerated phase of anion uptake. Moreover, addition of NaNO 3 to KNO 2 substantially increased potassium uptake. A coupling between anion and potassium uptake was therefore evident, but the coupling was not obligatory because the accelerated phase of nitrite uptake could occur in absence of rapid potassium uptake.}, number={2}, journal={PHYSIOLOGIA PLANTARUM}, author={JACKSON, WA and JOHNSON, RE and VOLK, RJ}, year={1974}, pages={108–114} }
 @article{jackson_flesher_hageman_1973, title={NITRATE UPTAKE BY DARK-GROWN CORN SEEDLINGS - SOME CHARACTERISTICS OF APPARENT INDUCTION}, volume={51}, ISSN={["1532-2548"]}, DOI={10.1104/pp.51.1.120}, abstractNote={Five-or six-day old seedlings of corn (Zea mays L.) were exposed to 0.25 mm Ca(NO(3))(2), 1.0 mm sodium 2-[N-morpholino]-ethanesulfonate, 5 mug Mo per liter and 50 mug of chloramphenicol per ml at pH 6. Nitrate uptake was determined from depletion of the ambient solution. The pattern of nitrate uptake was characterized, after the first 20 minutes, by a low rate which increased steadily to a maximal rate by 3 to 4 hours. Transfer of nitrate to the xylem did not totally account for the increase. Development of the maximal accelerated rate did not occur at 3 C with excised roots nor with seedlings whose endosperm had been removed. Use of CaCl(2) rather than Ca(NO(3))(2) resulted in a linear rate of chloride uptake during the first 4 hours, and chloride uptake was not as restricted by endosperm removal as was nitrate uptake.Nitrite pretreatments or the addition of cycloheximide (2 mug ml(-1)), puromycin (400 mug ml(-1)) and 6-methylpurine (0.5 mm) restricted maximal development of the accelerated nitrate uptake rate. Actinomycin D (20 mug ml(-1)) inhibited the rate only after about three hours exposure. The RNA and protein synthesis inhibitors also restricted nitrate reductase induction in the apical segments of the root tissue. The data suggest that development of the maximal accelerated rate of nitrate uptake depended upon continuous protein synthesis, and the hypothesis that synthesis of a specific nitrate transport protein must occur is advanced. But the alternative hypothesis, i.e., that induction of nitrate reductase (and/or a consequence of the act of nitrate reduction) provided the required stimulus, remains tenable.}, number={1}, journal={PLANT PHYSIOLOGY}, author={JACKSON, WA and FLESHER, D and HAGEMAN, RH}, year={1973}, pages={120–127} }