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Biomedical subjects

W A Jackson

Publications and source records attributed to W A Jackson.

At least 37 records · Page 2Linked to original sources
PubMed1992

Differential Inhibition by Ferulic Acid of Nitrate and Ammonium Uptake in Zea mays L.

The influence of the allelopathic compound ferulic acid (FA) on nitrogen uptake from solutions containing both NO(3) (-) and NH(4) (+) was examined in 8-day-old nitrogen-depleted corn (Zea mays L.) seedlings. Concurrent effects on uptake of Cl(-) and K(+) also were assessed. The presence of 250 micromolar FA inhibited the initial (0-1 hours) rate of NO(3) (-) uptake and also prevented development of the NO(3) (-)-inducible accelerated rate. The pattern of recovery when FA was removed was interpreted as indicating a rapid relief of FA-restricted NO(3) (-) uptake activity, followed by a reinitiation of the induction of that activity. No inhibition of NO(3) (-) reduction was detected. Ammonium uptake was less sensitive than NO(3) (-) uptake to inhibition by FA. An inhibition of Cl(-) uptake occurred as induction of the NO(3) (-) transport system developed in the absence of FA. Alterations of Cl(-) uptake in the presence of FA were, therefore, a result of a beneficial effect, because NO(3) (-) uptake was restricted, and a direct inhibitory effect. The presence of FA increased the initial net K(+) loss from the roots during exposure to the low K, ammonium nitrate uptake solution and delayed the recovery to positive net uptake, but it did not alter the general pattern of the response. The implications of the observations are discussed for growth of plants under natural conditions and cultural practices that foster periodic accumulation of allelopathic substances.

Journal Article↗
PubMed1992

Population ageing and intergenerational conflict: a post-Keynesian view.

The author reexamines the relationships among demographic aging, the dependency burden, and intergenerational conflict from a post-Keynesian perspective, in which unemployment and excess capacity are normal to the functioning of capitalist economies, and resources are not generally fully utilized. He "argues that the Keynesian process of national income determination precludes any immediate relationship between population ageing and the 'burden' imposed on income recipients. Below full employment, a rising dependency ratio is not guaranteed to reduce the expenditure share of income recipients or raise their tax rates. An exclusive emphasis on intergenerational conflict can give a misleading impression of the consequences of population ageing." The focus is on developed countries.

Demography↗
PubMed1990

Nitrate Reduction in Response to CO(2)-Limited Photosynthesis : Relationship to Carbohydrate Supply and Nitrate Reductase Activity in Maize Seedlings.

The effects of CO(2)-limited photosynthesis on (15)NO(3) (-) uptake and reduction by maize (Zea mays, DeKalb XL-45) seedlings were examined in relation to concurrent effects of CO(2) stress on carbohydrate levels and in vitro nitrate reductase activities. During a 10-hour period in CO(2)-depleted air (30 microliters of CO(2)/ per liter), cumulative (15)NO(3) (-) uptake and reduction were restricted 22 and 82%, respectively, relative to control seedlings exposed to ambient air containing 450 microliters of CO(2) per liter. The comparable values for roots of decapitated maize seedlings, the shoots of which had previously been subjected to CO(2) stress, were 30 and 42%. The results demonstrate that reduction of entering nitrate by roots as well as shoots was regulated by concurrent photosynthesis. Although in vitro nitrate reductase activity of both tissues declined by 60% during a 10-hour period of CO(2) stress, the remaining activity was greatly in excess of that required to catalyze the measured rate of (15)NO(3) (-) reduction. Root respiration and soluble carbohydrate levels in root tissue were also decreased by CO(2) stress. Collectively, the results indicate that nitrate uptake and reduction were regulated by the supply of energy and carbon skeletons required to support these processes, rather than by the potential enzymatic capacity to catalyze nitrate reduction, as measured by in vitro nitrate reductase activity.

Journal Article↗
PubMed1989

Utilitarian pension and retirement policies under population ageing.

The author analyzes population aging and its impact on pension and retirement policies by utilizing a simple utilitarian model for alternative types of pension finance. Findings indicate that "when specific adjustments to population ageing are necessary, changes in the retirement age are preferred to changes in pensions or contributions." A geographical focus on developed countries is implied.

Age Factors↗
PubMed1989

Baunscheidtism.

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Complementary Therapies↗
PubMed1989

Pneumocephalus associated with a frontoethmoidal osteoma.

A 51-year-old man complained of a headache of 2-months duration. Computerized tomography revealed pneumocephalus and an osteoma at the confluence of the frontal and ethmoid sinuses on the right. Craniotomy confirmed an osteoma with a spur as the cause of the pneumocephalus.

Bone Neoplasms↗
PubMed1988

Exogenous NO(3) Influx and Endogenous NO(3) Efflux by Two Maize (Zea mays L.) Inbreds during Nitrogen Deprivation.

The influence of nitrogen stress on net nitrate uptake resulting from concomitant (15)NO(3) (-) influx and (14)NO(3) (-) efflux was examined in two 12-day-old inbred lines of maize. Plants grown on (14)NO(3) (-) were deprived of nitrogen for up to 72 hours prior to the 12th day and then exposed for 0.5 hour to 0.15 millimolar nitrate containing 98.7 atom% (15)N. The nitrate concentration of the roots declined from approximately 100 to 5 micromolar per gram fresh weight during deprivation, and (14)NO(3) (-) efflux was linearly related to root nitrate concentration. Influx of (15)NO(3) (-) was suppressed in nitrogen-replete plants and increased with nitrogen deprivation up to 24 hours, indicating a dissipation of factors suppressing influx. Longer periods of nitrogen-deprivation resulted in a decline in (15)NO(3) (-) influx from its maximal rate. The two inbreds differed significantly in the onset and extent of this decline, although their patterns during initial release from influx suppression were similar. Except for plants of high endogenous nitrogen status, net nitrate uptake was largely attributable to influx, and genetic variation in the regulation of this process is implied.

Journal Article↗
PubMed1987

Nitrogen Enhancement of Phosphate Transport in Roots of Zea mays L. : I. Effects of Ammonium and Nitrate Pretreatment.

The effect of nitrogen status on phosphorous uptake and translocation was examined in 6-day-old dark-grown decapitated maize seedlings exposed to 25 micromolar phosphorous. Transfer to complete solutions containing 1 millimolar ammonium resulted in an increase in phosphorous uptake rate after 6 to 8 hours. The stimulus remained effective for at least 5.5 hours upon subsequent transfer to nitrogen-free solutions. Pretreatments for 16 hours with either nitrate or ammonium resulted in enhanced rates of subsequent phosphorous uptake and in enhanced translocation to the xylem of the exogenously supplied phosphorous. Both processes reached a plateau following pretreatment with 0.1 to 1.0 millimolar concentrations of either nitrogen ion. Further enhancement occurred with 10 millimolar nitrate, but not with 10 millimolar ammonium pretreatment. Although nitrogen pretreatments slightly increased the quantity of exogenous phosphorous retained in the root tissue, most of the extra phosphorous taken up by the nitrogen-pretreated seedlings was translocated to the xylem. The enhanced translocation, however, did not totally account for the increase in uptake implying a specific stimulation of the uptake process.

Journal Article↗
PubMed1987

Nitrogen Enhancement of Phosphate Transport in Roots of Zea mays L: II. Kinetic and Inhibitor Studies.

Exposure to 1 millimolar ammonium pretreatments increased V(max) for phosphorous uptake in dark-grown decapitated maize seedlings without a statistically measurable change in K(m). Sulfate uptake also was stimulated. The stimulation in phosphorous uptake due to ammonium pretreatment was greater in seedlings grown without phosphorous than in those grown with 25 micromolar phosphorous. The stimulus was not expressed unless the entire root system was pretreated with ammonium, and pretreatment of a part of the root system inhibited phosphorous uptake by the remaining part unless it also had been pretreated. Pretreatment with the amino acid analogs p-fluoro-dl-phenylalanine and l-azetidine-2-carboxylic acid (AZ) restricted phosphorous uptake in seedlings that were pretreated with ammonium and in those that were not, but the effect of ammonium pretreatment was not completely eliminated by the analogs. In general, translocation of the entering phosphorous was affected similarly to uptake by experimental treatments. Enhanced translocation, however, was not sufficient to account quantitatively for the increase in uptake, and an increased uptake was still evident when translocation was completely prevented by 50 micromolar AZ pretreatment.

Journal Article↗
PubMed1987

Potassium influx into maize root systems : influence of root potassium concentration and ambient ammonium.

Potassium influx into roots of dark-grown decapitated maize seedling (Zea mays L., cv Pioneer 3369A) was examined in presence and absence of ambient ammonium and at various root potassium concentrations. Six-day old seedlings which were dependent on the endosperm reserves for their energy source were exposed to KCl (labeled with (86)Rb) ranging from 5 to 200 micromolar. At both low (13 micromoles per gram fresh weight) and high (100 micromoles per gram fresh weight) root potassium concentration, isotherms indicated two potassium influx systems, one approaching saturation at 50 to 100 micromolar potassium and an additional one tentatively considered to be linear. A mixed-type inhibition by ammonium for the low-concentration saturable system was indicated by a concomitant decrease in V(max) and increase in K(m). High root potassium concentration decreased V(max) but had little effect on K(m) of this system. The rate constant for the second quasilinear system was decreased by ambient ammonium and by high root potassium status. Transfer of high potassium roots to potassium-free solutions resulted in an increase in influx within 2 hours; by 24 hours influx significantly exceeded that of roots not previously exposed to potassium. In roots of both low and high root potassium concentrations, potassium influx was restricted progressively as ambient ammonium increased to about 100 micromolar, but there was little further inhibition as ammonium concentrations increased beyond that to 500 micromolar. The data imply that potassium influx has two components, one subject to inhibition by ambient ammonium and one relatively resistant.

Journal Article↗
PubMed1987

Brockedon's press.

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History, Modern 1601-↗
PubMed1986

Utilization of Previously Accumulated and Concurrently Absorbed Nitrogen during Reproductive Growth in Maize : Influence of Prolificacy and Nitrogen Source.

A prolific maize (Zea mays L.) genotype was grown to physiological maturity under greenhouse conditions to examine the effects of reproductive sink demand on (a) the remobilization of N accumulated during vegetative growth, and (b) the partitioning of N accumulated concurrent with ear development. One- and two-eared plants were treated with either a NO(3) (-) or NH(4) (+) source of (15)N-labeled N during reproductive growth. Plants with two ears enhanced grain production, N remobilization from the stalk and roots, and N translocation to the grain from concurrently assimilated N. But, remobilization of leaf-N was unaffected by ear number. In addition, N uptake and total dry matter accumulation during the reproductive period were also unaffected, although P uptake was greater in the two-eared plants. Less than 15% of the total K(+) uptake was accumulated after silking while during this time more than 40% of the total N and more than 50% of the total P were absorbed. The data also indicate that with NO(3) (-) nutrition, internal recirculation of K(+) between shoots and roots may play a prominent role in the transport of nitrogenous solutes during grain development. N source had no effect on dry matter production and N uptake of both one- and two-eared plants. However, slightly greater partitioning of labeled-N from the NH(4) (+) source to the grain was observed in the two-eared plants.

Journal Article↗
PubMed1985

Nitrate Uptake and Partitioning by Corn Root Systems : Differential Effects of Ammonium among Genotypes and Stages of Root Development.

The relative effects of ammonium on nitrate uptake and partitioning during induction were compared among decapitated seedlings of three corn (Zea mays L.) genotypes at two developmental stages. This study tested the hypothesis that root systems efficient at translocating products of ammonium assimilation away from sites of nitrate uptake or reduction would exhibit less inhibition of nitrate uptake by ammonium compared to root systems with inefficient N translocation efficiency. Inhibition of nitrate uptake by ammonium was relatively slight at day 5 ranging from 0% to 20% among the three genotypes, as compared to greater inhibition, from 20% to 37%, at day 8. Five-day-old roots exhibited negligible xylem translocation capacity in comparison with those grown for 8 days. Thus, although the capability to translocate ammonium assimilates out of the root increased between days 5 and 8, inhibitory effects of ammonium also increased. In the absence of ammonium, nitrate uptake per unit root mass increased between days 5 and 8. This increased activity of the uptake system was proportionally more sensitive to ammonium.Partitioning of entering nitrate into the reduction process was positively correlated with lateral root development of the inbred root systems at 5 and 8 days. This is supportive of a localization of a major portion of nitrate reduction occurring in root apical regions. Nitrate reduction was the partitioning process most severely inhibited by ammonium in all cases, ranging from 39% to 55% inhibition. In contrast, ammonium-inhibition of nitrate accumulation in the root tissue and translocation via xylem vessels varied with genotype and root age.Two mechanisms of ammonium-inhibition of nitrate are implicated, one which directly affects nitrate reduction and the uptake system associated with it, and another which may involve potassium as an intermediate regulator of nitrate accumulation in the root tissue and nitrate translocation out of the root tissue.

Journal Article↗
PubMed1985

p-Fluorophenylalanine-Induced Restriction of Ion Uptake and Assimilation by Maize Roots.

Roots of decapitated maize seedlings (Zea mays L.) were exposed for 12 hours to 1.0 millimolar KNO(3) (98.5 atom per cent (15)N) in the presence and absence (control) of 0.1 millimolar p-fluorophenylalanine (FPA), an analog of the amino acid phenylalanine. FPA decreased nitrate uptake but had little effect on potassium uptake. In contrast, accumulation of both ions in the xylem exudate was greatly restricted. The proportion of reduced (15)N-nitrogen that was translocated at each time was also restricted by FPA. These observations are interpreted as indicating that synthesis of functional protein(s) is required for nitrate uptake and for transport of potassium, nitrate, and reduced-(15)N from xylem parenchyma cells into xylem elements. The effect of FPA on nitrate reduction is less clear. Initially, FPA limited nitrate reduction more than nitrate uptake, but by 8 hours the cumulative reduction of entering nitrate was similar ( approximately 35%) in both control and FPA-treated roots. A relationship between nitrate uptake and nitrate reduction is implied. It is suggested that nitrate influx regulates the proportion of nitrate reductase in the active state, and thereby regulates concurrent nitrate reduction in decapitated maize seedlings.

Journal Article↗