Biomedical subjects
W A Jackson
Publications and source records attributed to W A Jackson.
Ion balance, uptake, and transport processes in n(2)-fixing and nitrate- and urea-dependent soybean plants.
The objective of this study was to examine the influence of N(2) fixation and NO(3) (-)-N and urea-N assimilation on ion balance, uptake, and transport processes in soybean (Glycine max L. Merr.).Inoculated plants were grown in Perlite supplied daily with nutrient solutions which contained zero-N, 10 and 20 millimolar NO(3) (-)-N, and 10 and 20 millimolar urea-N, and they were sampled 41, 76, and 151 days after transplanting. Total uptake of inorganic cations and anions was determined by analysis of tissue for K(+), Ca(2+), Mg(2+), Na(+), total N from NO(3) (-), total S, H(2)PO(4) (-), and Cl(-). Differences in total inorganic cations (C) and inorganic anions (A) in plant tissue were used to estimate total carboxylate content.Internal OH(-) generation resulting from excess cation uptake (net H(+) excretion) by the roots accounted for more than 89% of the carboxylate accumulation in N(2)- and urea-fed plants, while OH(-) generation resulting from SO(4) (2-) reduction accounted for less than 11%. Shoots contained over 89% of the total plant carboxylate content. Malate balanced about 75% of the excess inorganic cationic charge of the xylem sap; allantoate and aspartate balanced most of the remaining charge. These results indicate that carboxylates (primarily malate) are synthesized in roots of N(2)- and urea-fed plants and transported to the shoots in the xylem to maintain charge balance. The high malate concentration resulted in the C/N weight ratio of xylem sap from N(2)-fed plants being >2.0, even though 83% of the N was transported as allantoin and allantoic acid which have a C/N ratio of 1.0. The data emphasize that C and N content of N compounds should not be the sole basis for calculating the C/N weight ratio of xylem sap.The C-to-A uptake ratio for plants supplied 10 millimolar NO(3) (-) ranged from 1.24 to 1.57 during development, indicating that internal OH(-) was generated both by excess cation uptake and by NO(3) (-) and SO(4) (2-) reduction. The C-to-A uptake ratio for 20 millimolar NO(3) (-) -fed plants ranged from 0.86 to 0.96 during development, indicating a small net OH(-) efflux from the roots for support of excess anion uptake. On a seasonal basis, only 15% of the OH(-) generated during NO(3) (-) and SO(4) (2-) reduction was associated with OH(-) efflux (excess anion uptake), while 85% was associated with carboxylate accumulation. The malate concentration in xylem sap from plants supplied 20 millimolar NO(3) (-) was only one-third that of N(2)- and urea-fed plants; however, it did balance 75% of the excess inorganic cationic charge. Potassium, recycling to accommodate excess anion uptake by 20 millimolar NO(3)-fed plants, was calculated to involve at most 17% of the total K(+) absorbed during the 41- to 76-day growth interval.
Restricted nitrate influx and reduction in corn seedlings exposed to ammonium.
The effect of ambient ammonium (0.5 millimolar [(14)NH(4)](2)SO(4)) added to a nutrient solution containing 1.0 millimolar K(15)NO(3), 99 atom per cent (15)N, upon [(15)N]nitrate assimilation and utilization of previously accumulated [(14)N]nitrate was investigated. Corn seedlings, 5-day-old dark-grown decapitated (experiment I) and 10-day-old light-grown intact (experiment II), which had previously been grown on K(14)NO(3) nutrient solution, were used. In both experiments, the presence of ambient ammonium decreased [(15)N]nitrate influx (20% after 6 hours) without significantly affecting the efflux of previously accumulated [(14)N]nitrate. In experiment I, relative reduction of [(15)N]nitrate (reduction as a percentage of influx) was inhibited more than was [(15)N]nitrate influx. Nevertheless, in experiment I, where all reduction could be assigned to the root system, the absolute inhibition of reduction during the 12 hours (13 micromoles/root) was less than the absolute inhibition in influx (24 micromoles/root). The data suggest that the influence of ammonium on [(15)N]nitrate influx could not be totally accounted for by the decrease in the potential driving force which resulted from restricted reduction; an additional impact on the influx process is indicated. Reduction of [(15)N]nitrate in experiment II after 6 hours accounted for 30 and 18% of the tissue excess (15)N in the control and ammonium treatments, respectively. Relative distribution of (15)N between roots and exudate (experiment I), or between roots and shoots (experiment II) was not affected by ammonium. On the other hand, the accumulation of [(15)N]nitrate in roots, shoots, and xylem exudate was enhanced by ammonium treatment compared to the control, whereas the accumulation of reduced (15)N was inhibited.
Nitrate Accumulation, Assimilation, and Transport by Decapitated Corn Roots : EFFECTS OF PRIOR NITRATE NUTRITION.
The effects of accumulated [(14)N]nitrate and its utilization in decapitated, 5-day-old dark-grown corn roots on influx, accumulation, xylem deposition, and reduction of concurrently absorbed nitrate during an 18-hour exposure to 0.5 millimolar K(15)NO(3) nutrient solution were examined. A 20-hour pretreatment in 15.0 millimolar K(14)NO(3) high nitrate (HN) resulted in a 2-fold greater tissue nitrate level than pretreatment in 0.5 millimolar K(14)NO(3) low nitrate (LN). Upon transfer to the 0.5 millimolar K(15)NO(3) solution, the net nitrate uptake rate in HN roots after 2 hours was 52% of the LN rate, but increased to 93% at the end of the uptake period. Despite an enhanced [(14)N]nitrate efflux from HN roots to the uptake solution, the efflux differences between the two pretreatments did not compensate for the decrease in net nitrate uptake. The [(15)N]nitrate influx rate was initially restricted by 33% in the HN roots compared to LN roots, but it had decreased to 7% by the end of the 18-hour uptake period. At this time, the total tissue nitrate levels were similar for both pretreatments. The rate of accumulation of [(15)N]nitrate in the tissue was relatively constant for both pretreatments, but was 25% less in HN roots. Of the previously accumulated [(14)N]nitrate, 52 and 46% remained after 18 hours in the LN and HN roots, respectively. The [(14)N]nitrate decline for HN roots was initially more rapid than in the LN roots which was linear over time. Xylem transport and efflux more than accounted for the decline in [(14)N]nitrate of LN roots and all but 4% in the HN roots which was attributed to reduction. Compartmentation of the previously accumulated nitrate was evident from the higher atom per cent (15)N of xylem nitrate compared to that of the tissue nitrate of both LN and HN roots. During the first 2 hours, xylem transport of [(14)N]nitrate by the HN roots was 49% greater than for LN roots, while [(15)N]nitrate transport was 9% less in HN roots compared to LN roots. Even though the reduction of [(15)N]nitrate in HN roots was 31% less than LN roots during the first 2 hours, [(15)N]nitrate was reduced more rapidly than the previously accumulated [(14)N]nitrate. After the first 4 hours, the relative partitioning of absorbed [(15)N]nitrate between accumulation, reduction, and translocation was similar regardless of pretreatment.
Nitrate Reduction in Roots as Affected by the Presence of Potassium and by Flux of Nitrate through the Roots.
Dark-grown, detopped corn seedlings (cv. Pioneer 3369A) were exposed to treatment solutions containing Ca(NO(3))(2), NaNO(3), or KNO(3); KNO(3) plus 50 or 100 millimolar sorbitol; and KNO(3) at root temperatures of 30, 22, or 16 C. In all experiments, the accelerated phase of NO(3) (-) transport had previously been induced by prior exposure to NO(3) (-) for 10 hours. The experimental system allowed direct measurements of net NO(3) (-) uptake and translocation, and calculation of NO(3) (-) reduction in the root. The presence of K(+) resulted in small increases in NO(3) (-) uptake, but appreciably stimulated NO(3) (-) translocation out of the root. Enhanced translocation was associated with a marked decrease in the proportion of absorbed NO(3) (-) that was reduced in the root. When translocation was slowed by osmoticum or by low root temperatures, a greater proportion of absorbed NO(3) (-) was reduced in the presence of K(+). Results support the proposition that NO(3) (-) reduction in the root is reciprocally related to the rate of NO(3) (-) transport through the root symplasm.
Reduction of plant tissue nitrate to nitric oxide for mass spectrometric 15N analysis.
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Antiques of Mander, Weaver and Co., Wolverhampton.
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Lupton of Levenshulme.
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Farr's Patent Ampulla.
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Nitrate Uptake and Assimilation by Wheat Seedlings during Initial Exposure to Nitrate.
Nitrate uptake, reduction, and translocation were examined in intact, 14-day-old, nitrogen-depleted wheat (Triticum vulgare var. Knox) seedlings during a 9-hour exposure to 0.2 mm Ca (NO(3))(2). The nitrate uptake rate was low during the initial 3-hour period, increased during the 3- to 6-hour period, and then declined. By the 3rd hour, 14% of the absorbed nitrate had been reduced, and this increased to 36% by the 9th hour. Shoots accumulated reduced (15)N more rapidly than roots and the ratio of reduced (15)N to (15)N-nitrate was higher in the shoots. A significant proportion of the total reduction occurred in the root system under these experimental conditions. Accumulation of (15)N in ethanol-insoluble forms was evident in both roots and shoots by the 3rd hour and, after 4.5 hours, increased more rapidly in shoots than in roots.An experiment in which a 3-hour exposure to 0.2 mm Ca ((15)NO(3))(2) was followed by a 12-hour exposure to 0.2 mm Ca ((14)NO(3))(2) revealed a half-time of depletion of root nitrate of about 2.5 hours. A large proportion of this depletion, however, was due to loss of (15)N-nitrate to the ambient (14)N-nitrate solution. The remaining pool of (15)N-nitrate was only slowly available for reduction. Total (15)N translocation to the shoot was relatively efficient during the first 3 hours after transfer to Ca ((14)NO(3))(2) but it essentially ceased after that time in spite of significant pools of (15)N-nitrate and alpha-amino-(15)N remaining in the root tissue.
Nitrate translocation by detopped corn seedlings.
Five-day-old seedlings of corn (Zealpha mays L.) grown without nitrate were decapitated and exposed to 0.5 mm KNO(3) or 0.5 mm KCl in aerated solutions at 30 C. Uptake of nitrate, chloride, and potassium was determined by replacing solutions hourly and measuring their depletion. Translocation of these ions and of organic nitrogen was determined by hourly analysis of the vascular exudate. Nitrate reduction was estimated by the difference between nitrate uptake and nitrate recovered in the tissue and exudate. Nitrate uptake exhibited its usual pattern of apparent induction resulting in the development of an accelerated uptake phase. Chloride uptake remained fairly constant throughout the experimental period. Translocation of nitrate increased progressively for at least 7 hours whereas chloride translocation reached a maximum about the 3d hour and then declined to a lower rate than nitrate translocation. Nitrate uptake and translocation were restricted by anaerobiosis, by 20 and 40 C relative to 30 C, and by 0.05 mm 6-methylpurine, an RNA-synthesis inhibitor. Accumulation, reduction and translocation of nitrate had different sensitivities to all these factors. The effect of 0.05 mm 6-methylpurine was more detrimental to nitrate translocation and nitrate reduction than to nitrate uptake.Ambient nitrate, relative to chloride, enhanced the exudation volume and the translocation of organic nitrogen within 4 hours from initiation of the experiments. Translocation of nitrate and organic nitrogen decreased shortly after removal of external nitrate. The higher rates of organic nitrogen translocation which occurred during nitrate uptake indicates either (a) rapid translocation of amino acids synthesized from the entering nitrate, or (b) an accelerated rate of protein turnover and a resulting enhancement in translocation of endogenous amino acids.
Nitrate Uptake by Dark-grown Corn Seedlings: Some Characteristics of Apparent Induction.
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.
Simultaneous Influx and Efflux of Nitrate during Uptake by Perennial Ryegrass.
Experiments with intact plants of Lolium perenne previously grown with (14)NO(3) (-) revealed significant efflux of this isotopic species when the plants were transferred to solutions of highly enriched (15)NO(3) (-). The exuded (14)NO(3) (-) was subsequently reabsorbed when the ambient solutions were not replaced. When they were frequently replaced, continual efflux of the (14)NO(3) (-) was observed. Influx of (15)NO(3) (-) was significantly greater than influx of (14)NO(3) (-) from solutions of identical NO(3) (-) concentration. Transferring plants to (14)NO(3) (-) solutions after a six-hour period in (15)NO(3) (-) resulted in efflux of the latter. Presence of Mg(2+), rather than Ca(2+), in the ambient (15)NO(3) (-) solution resulted in a decidedly increased rate of (14)NO(3) (-) efflux and a slight but significant increase in (15)NO(3) (-) influx. Accordingly, net NO(3) (-) influx was slightly depressed. A model in accordance with these observations is presented; its essential features include a passive bidirectional pathway, an active uptake mechanism, and a pathway for recycling of endogenous NO(3) (-) within unstirred layers from the passive pathway to the active uptake site.
Photorespiratory phenomena in maize: oxygen uptake, isotope discrimination, and carbon dioxide efflux.
Concurrent O(2) evolution, O(2) uptake, and CO(2) uptake by illuminated maize (Zea mays) leaves were measured using (13)CO(2) and (18)O(2). Considerable O(2) uptake occurred during active photosynthesis. At CO(2) compensation, O(2) uptake increased. Associated with this increase was a decrease in O(2) release such that a stoichiometric exchange of O(2) occurred. The rate of O(2) exchange at CO(2) compensation was directly related to O(2) concentration in the atmosphere at least up to 8% (v/v).When illuminated maize leaves were exposed to saturating CO(2) concentrations containing approximately equal amounts of (12)CO(2) and (13)CO(2), the latter was taken up more rapidly, thus depressing the atom% (13)C in the atmosphere. Moreover, upon exposure to CO(2) containing 96 atom% (13)C, there occurred a directly measurable efflux of (12)CO(2) from the leaves for at least 15 minutes. During this period an equimolar evolution of (16)O(2) and uptake of (13)CO(2) was observed. Thereafter, although the rate of (16)O(2) evolution remained unchanged, the rate of (13)CO(2) uptake declined markedly, suggesting continual (13)C enrichment of the photorespiratory substrate.It is concluded that a finite photorespiratory process occurs in maize and that the CO(2) generated thereby is efficiently recycled. Recycling maintains the internal CO(2) concentration at a level difficult to detect by most photorespiratory assays.
Nitrate absorption and assimilation in ryegrass as influenced by calcium and magnesium.
The absorption and assimilation patterns of (15)NO(3) (-) supplied as the Ca(2+) and Mg(2+) salts to intact ryegrass (Lolium perenne) seedlings were compared. No statistically significant effect of ambient cation on the amounts of (15)NO(3) (-) absorbed was observed in the initial six hours, but during the subsequent six hours, absorption from Ca((15)NO(3))(2) exceeded that from Mg ((15)NO(3))(2).Lower rates of (15)NO(3) (-) assimilation were found in roots exposed to Mg((15)NO(3))(2) than in those exposed to Ca((15)NO(3))(2). It was proposed that Mg(2+) initiated a restriction in (15)NO(3) (-) reduction in roots, probably as a consequence of a Mg(2+)-induced physiological Ca(2+) deficiency. Lower (15)N translocation rates were also observed from Mg((15)NO(3))(2). These effects of Mg(2+) in depressing (15)NO(3) (-) assimilation and translocation occurred prior to an effect on (15)NO(3) (-) uptake.In shoots, larger amounts of reduced (15)N products occurred with Ca((15)NO(3))(2) than with Mg((15)NO(3))(2). It was concluded that this was due to enhanced translocation of (15)NO(3) (-) (and possibly its reduced products) in presence of Ca(2+) rather than to specific cation effects on (15)NO(3) (-) assimilation in the shoots.
Oxygen uptake by illuminated maize leaves.
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Root environment acidity as a regulatory factor in ammonium assimilation by the bean plant.
Previous experiments have revealed a much greater efficiency of ammonium utilization by bean plants (Phaseolus vulgaris L.) when the acidity of the ambient medium was maintained at near-neutral conditions with carbonates or hydroxides. The present investigation, in which (15)N-labeled ammonium was used, permitted an assessment of the origin of nitrogen in tissue nitrogen pools with and without acidity control (CaCO(3) treated and untreated, respectively) in the root environment. Control of acidity resulted in greater ammonium uptake and greater incorporation into the amino fraction, amide, and ethanol-insoluble nitrogen by the root tissue. These differences were clearly evident by the fifth day after ammonium nitrogen had been applied.Shoots of the untreated plants rapidly accumulated free ammonium and amino nitrogen. A substantial portion of both fractions came from pre-existing nitrogen in the plants, indicating significant protein degradation. No evidence was found for such degradation in the roots of the untreated plants or in either roots or shoots of CaCO(3) treated plants. The data indicate that control of ambient acidity in the root environment during ammonium absorption enhanced the conversion of entering ammonium to organic nitrogen compounds in the root tissue thereby restricting movement of free ammonium to shoots. Consequently, the detrimental effects of high ammonium concentrations in the leaves were largely prevented.