Ion supply capacity of roots in relation to rejuvenation of primary leaves in vivo

Removal of the shoot above the primary node (detopping) of 3-week-old bean plants ( Phaseolus vulgaris L. cv. Contender) altered the metabolism and development of the remaining leaves. An increase in levels of chlorophyll, protein, stomatal opening, photosynthesis and growth, i.e. rejuvenation of primary leaves, was established within 7 days of detopping. These levels were maintained while the primary leaves of equivalent intact plants senesced.
The flux of xylem solution (mineral ions, cytokinins and water) into leaves is related to the leaf area to be supplied and root supply capacity; it has been suggested that detopping leads to an increased availability of root-supplied solutes and hence rejuvenation of the remaining leaves. This assumes however that root output of solutes is not decreased by the defoliation treatment.
We found that root output of ions (electrical conductivity of passive xylem exudate) in detopped plants was 30% lower than in intact plants after 24 h and 60% lower after 7 days. The output of Ca²⁺, Mg²⁺ and K⁺ were similarly reduced 7 and 14 days after detopping as were fresh and dry weights of roots. Furthermore, neither the calculated xylem flux of ions nor directly measured levels of Ca²⁺, Mg²⁺ and K⁺ were significantly increased in leaves of detopped plants during their rejuvenation. We therefore conclude that root output is tightly coupled to shoot demand and that the apparent rejuvenation of primary leaves caused by detopping bean plants is not a consequence of increased xylem flux of mineral ions into the leaves.     

Effect of light/dark cycles on expression of nitrate assimilatory genes in maize shoots and roots

The level of nitrate reductase (NR) and nitrite reductase (NiR) varied in both shoot and root tissue from nitrate-fed Zea mays L. grown under a 16-hour light/8-hour dark regime over a 10-day period postgermination, with peak activity occurring in days 5 to 6. To study the effect of different light regimes on NR and NiR enzyme activity and mRNA levels, 6-day-old plants were grown in the presence of continuous KNO₃ (10 millimolar). Both shoot NRA and mRNA varied considerably, peaking 4 to 8 hours into the light period. Upon transferring plants to continuous light, the amplitude of the peaks increased, and the peaks moved closer together. In continuous darkness, no NR mRNA or NR enzyme activity could be detected by 8 hours and 12 hours, respectively. In either a light/dark or continuous light regime, root NRA and mRNA did not vary substantially. However, when plants were placed in continuous darkness, both declined steadily in the roots, although some remained after 48 hours. Although there was no obvious cycling of NiR enzyme activity in shoot tissue, changes in mRNA mimicked those seen for NR mRNA. The expression of NR and NiR genes is affected by the light regime adopted, but light does not have a direct effect on the expression of these genes.     

Seedling Nitrate Reductase Activity and Nitrate Partitioning in Maize (Zea mays L.) Strains Divergently Selected For Post-anthesis Leaf-Lamina Nitrate Reductase Activity

Two experiments were conducted to evaluate the effects of phenotypicrecurrent selection for high and low post-anthesis leaf-laminain vivo NRA on nitrate uptake, nitrate partitioning and in vitroNRA of seedling roots and leaves. In Experiment 1, intact plantsof cycle 0, 4, and 6 of the high and low NRA strains were grownon NH₄-N for 11 d, then exposed to 1.0 mol m^–3 KNO₃, andcultures sampled at 6 h and 28 h (induction and post-inductionperiods). Nitrate uptake, tissue nitrate concentration and invitro NRA were determined. The pattern of response to selectionin seedling leaf NRA was similar to that observed for in vivoNRA of field grown plants. Leaf NRA increased between 6 h and28 h. Root NRA was not affected by selection or sampling time.Treatments differed in total fresh weight but not in reductionor uptake of nitrate per unit weight, indicating a lack of correspondencebetween NRA and reduction and supporting the idea that concomitantreduction by NR is not obligatorily linked to nitrate influxin the intact plant. In Experiment 2, dark-grown plants of cycle 0, and 6 of thehigh and low NRA strains were cultured without N, detopped onday 6, transferred the following day to 0-75 mol m^–3 KNO₃and sampled at 6 h and 28 h. In contrast to Experiment 1, selectionpopulations differed in nitrate reduction and root NRA, whichby 28 h reached higher average levels than root NRA of intactplants. Translocation and reduction were inversely related amongstrains within each sampling time. The high level of translocationin detopped plants of the low NRA strain was difficult to reconcilewith its low leaf NRA level of Experiment 1. It is suggestedthat nitrate transport in detopped roots is altered relativeto the intact system in a way which permits greater NRA inductionand nitrate reduction. The results indicate that nitrate partitioningby detopped root systems should be interpreted with caution. Key words: Zea, nitrate reductase activity, nitrate uptake, nitrate reduction, nitrate partitioning, selection     

RNA Synthesis in Phaseolus Chloroplasts: II. RIBONUCLEIC ACID SYNTHESIS IN CHLOROPLASTS FROM DEVELOPING AND SENESCING LEAVES

The rate of RNA synthesis in chloroplasts from the primary leavesof Phaseolus vulgaris L. cv. Canadian Wonder was measured invitro as plant age increased. The rate per leaf began to fallbefore the leaf was 70% expanded. At full expansion, activityhad fallen by 70%. Chloroplast RNA synthesis per unit chlorophyllwas falling before the leaf was 25% expanded. When all parts of the plant above the mature primary leaveswere removed (detopping) chloroplast RNA synthesis in theseleaves rose within 36 h. The rate increased to a maximum 3–4d after detopping, when it was 5–10 times control values;thereafter it fell again. The chlorophyll content began to increaseabout 4 d after detopping, eventually rising by 100%. Detoppingcaused a 3-fold increase in the Triton X-100-soluble DNA contentof chloroplast preparations, measured after 3.5 d. At that timethe rate of RNA synthesis per unit Triton-soluble DNA was thesame in chloroplasts from the primary leaves of intact and detoppedplants. Detopping also resulted in an increase in the depthof the leaf palisade layer. The effects of detopping on chloroplasts were prevented by darknessand reduced by shading. Increased chloroplast RNA polymerase activity was also inducedin the primary leaves by placing a polythene bag over intactplants, enclosing everything above these leaves. Removal ofthe roots from detopped plants prevented the rise in the rateof chloroplast RNA synthesis.     

Effect of calcium on transport of cations to the xylem exudate of tobacco

Summary Roots of detopped tobacco plants (Nicotiana tabacum var. Virginia Gold) were exposed to Na, K, and Ca salts or to water, and cation transfer to xylem vessels was measured. In some cases plants had been exposed to Na in addition to regular nutrient solutions before detopping. Calcium in the external medium greatly depressed the transport of Na from the external medium to the xylem vessels and it often stimulated the transfer of K from the external medium to the xylem vessels. The K/Na ratio in the exudate thus was dependent upon the Ca content of the external medium under these conditions. In contrast, externally applied Ca or Ca deficiency had very little effect on the transfer of preaccumulated K and Na from compartments within roots to the xylem vessels. The K/Na ratio in the exudate under these conditions was not related to Ca levels nor to mild Ca deficiency. The ratios decreased with time after detopping regardless of Ca level. Intact plants accumulated more Na than did root systems of detopped plants in a 6-day period.Riverside University of CaliforniaSoil Science and Agricultural Engineering     

Glycolytic Flux Is Adjusted to Nitrogenase Activity in Nodules of Detopped and Argon-Treated Alfalfa Plants

To investigate the short-term (30–240 min) interactions among nitrogenase activity, NH₄⁺ assimilation, and plant glycolysis, we measured the concentrations of selected C and N metabolites in alfalfa (Medicago sativa L.) root nodules after detopping and during continuous exposure of the nodulated roots to Ar:O₂ (80:20, v/v). Both treatments caused an increase in the ratios of glucose-6-phosphate to fructose-1,6-bisphosphate, fructose-6-phosphate to fructose-1,6-bisphosphate, phosphoenolpyruvate (PEP) to pyruvate, and PEP to malate. This suggested that glycolytic flux was inhibited at the steps catalyzed by phosphofructokinase, pyruvate kinase, and PEP carboxylase. In the Ar:O₂-treated plants the apparent inhibition of glycolytic flux was reversible, whereas in the detopped plants it was not. In both groups of plants the apparent inhibition of glycolytic flux was delayed relative to the decline in nitrogenase activity. The decline in nitrogenase activity was followed by a dramatic increase in the nodular glutamate to glutamine ratio. In the detopped plants this was coincident with the apparent inhibition of glycolytic flux, whereas in the Ar:O₂-treated plants it preceded the apparent inhibition of glycolytic flux. We propose that the increase in the nodular glutamate to glutamine ratio, which occurs as a result of the decline in nitrogenase activity, may act as a signal to decrease plant glycolytic flux in legume root nodules.     

Nitrogen pretreatmentsvs nitrate treatments after detopping on xylem exudation in tobacco

Summary Nitrate salts resulted in a large stimulation of exudation from detopped tobacco and also increased cation transport to the exudate, but only if nutrient solutions applied previously to detopping had become depleted in salts. Such plants were not necessarily nitrogen deficient. Pregrowth in ammonium nitrogen resulted in the same effect as pregrowing with low amount of salts. Pregrowth with high levels of salt other than nitrate, however had the same effect as pregrowing with a high level of nitrate. There was no evidence that synthesis of nitrate reductase was necessary for the stimulation of exudation by nitrate. The nitrate response was usually apparent within 1 h. Glucose added with nitrate at 10°C resulted in prolonged and increased exudation for 32 days following detopping. Mg (NO₃)₂ as a single salt was toxic in terms of exudation production. Only one cycle of response to nitrate on exudation could be obtained from detopped plants. A cycle could be obtained when nitrate was added many days after detopping if nitrate had not been added previously since detopping.     

Further Errors in the Acetylene Reduction Assay: Effects of Plant Disturbance

A flow-through gas system was used to study the effects of disturbanceon nitrogenase (acetylene reduction) activity of nodulated rootsystems of soyabean (Glycine max) and white clover (Trifoliumrepens). Detopping plus removal of the rooting medium (by shaking)produced a substantial decrease in maximum nitrogenase activity.This response is due to a reduction in oxygen flux to the bacteroidscaused by an increase in the oxygen diffusion resistance ofthe nodule. The decrease in maximum nitrogenase activity wasmuch smaller for roots subjected to detopping only. Thus, theeffect of root shaking is more important than that of shootremoval. The effect of detopping plus root shaking on nitrogenase activityoccurred whether the plants were equilibrated and assayed at25°C or 15°C. However, the effect of disturbance onthe oxygen diffusion resistance of the nodules, and thus onnitrogenase activity, was greater at the higher temperature.At the lower temperature the oxygen diffusion resistance ofthe nodules had already been increased in response to the reducedrequirement for oxygen. These nodules were less susceptibleto the effects of disturbance. Thus, comparisons of the effectsof equilibration temperature on nitrogenase activity produceddifferent results depending on whether intact or disturbed systemswere used. With intact systems activity was lower at the lowertemperature but with detopped/shaken roots the lowest activityoccurred at the higher temperature. It is concluded that the use of detopped/shaken roots can producesubstantial errors in the acetylene reduction assay, which makesthe assay invalid even when used for comparative purposes. However,comparisons with rates of ¹⁵N₂ fixation and H₂ production showthat accurate measurements of nitrogenase activity can be obtainedfrom maximum rates of acetylene reduction by intact plants ina flow-through gas system. The continued use of assay proceduresin which cumulated ethylene production from disturbed systemsis measured in closed vessels must be questioned. Key words: Nodules, acetylene, nitrogenase activity     

Rapid modulation of nitrate reductase in pea roots

The regulatory properties of nitrate reductase (NR; EC 1.6.6.1) in root extracts from hydroponically grown pea (Pisum sativum L. cv. Kleine Rheinländerin) plants were examined and compared with known properties of NR from spinach and pea leaves. Nitrate-reductase activity (NRA) extracted from pea roots decreased slowly when plants were kept in the dark, or when illuminated plants were detopped, with a half-time of about 4 h (= slow modulation in vivo). In contrast, the half-time for the dark-inactivation of NR from pea leaves was only 10 min. However, when root tip segments were transferred from aerobic to anaerobic conditions or vice versa, changes in NRA were as rapid as in leaves (= rapid modulation in vivo). Nitrate-reductase activity was low when extracted from roots kept in solutions flushed with air or pure oxygen, and high in nitrogen. Okadaic acid, a specific inhibitor of type-1 and type-2A protein phosphatases, totally prevented the in vivo activation by anaerobiosis of NR, indicating that rapid activation of root NR involved protein dephosphorylation. Under aerobic conditions, the low NRA in roots was also rapidly increased by incubating the roots with either uncouplers or mannose. Under these conditions, and also under anaerobiosis, ATP levels in roots were much lower than in aerated control roots. Thus, whenever ATP levels in roots were artificially decreased, NRA increased rapidly. The highly active NR extracted from anaerobic roots could be partially inactivated in vitro by preincubation of desalted root extracts with MgATP (2 mM), with a half-time of about 20 min. It was reactivated by subsequently incubating the extracts with excess AMP (2 mM). Thus, pea root NR shares many of the previously described properties of NR from spinach leaves, suggesting that the root enzyme, like the leaf enzyme, can be rapidly modulated, probably by reversible protein phosphorylation/ dephosphorylation.     

Short-term leaf elongation kinetics of maize in response to salinity are independent of the root

The essentiality of roots to the short-term responses of leaf elongation to salinity was tested by removing the roots of maize (Zea mays L.) from the shoots and comparing the initial short-term response of leaf elongation to that with intact plants. Eightday-old seedlings growing in solution culture were treated with 80 millimolar NaCl and their leaf elongation rate (LER) was monitored with a linear variable differential transformer connected to a computerized data aquisition system. Initially, LER of intact plants was sharply reduced by salinity, then rose rapidly to reach a new steady-state rate about 1.5 hours after salinization. The new steady-state rate of salinized intact plants was about 80% of the control rate. When the roots of nonsalinized plants were excised under the surface of the nutrient solution, excision did not disturb the steady-state LER. When these shoots were salinized, they responded in a manner nearly identical to that of intact plants, indicating that roots are not essential for the modulation of short-term LER of salt-stressed plants.     

The effect of differential root and shoot temperature on the nitrate reductase activity,assayed in vivo and in vitro in roots ofHordeum vulgare (barley)

There was a large increase in nitrate reductase activity (NAR) assayed both in vivo and in vitro in roots of barley plants (cv. Midas_ grown with roots at 10°C and shoots at 20°C, compared with whole plants grown at 20°C. There were diurnal fluctuations in NRA in roots from both treatments, but they were much greater in roots grown at 20°C, where NRA fell to a very low value in the dark period. The diurnal fluctuations in the malate content of the roots were also related to the root growth temperature. Plants with roots grown at the lower temperature had a higher malate content, especially in the dark period where it was 20 times greater than in plants with roots at 20°C. At all times there was a three-fold increase in soluble carbohydrate in cooled roots and diurnal fluctuations were much less pronounced than those of malate. Growth at low temperatures increased the total flux of amino N into the xylem sap and increased the proportion of reduced N in the total N flux. At certain times of day both 10°C- and 20°C-grown roots responded to exogeneous malate by increasing the flux of amino acid into the xylem sap, although this effect was always more pronounced in 20°C-grown roots.     

Influence of Applied NaCl on Crassulacean Acid Metabolism and Ionic Levels in a Cactus, Cereus validus

To determine possible physiological responses to salinity, seedlings of Cereus validus Haworth, a cactus from Salinas Grandes, Argentina, were treated with up to 600 millimolar NaCl for up to 16 days when they were about 9 months old and 100 millimeters tall. Salt stress decreased stem biomass, e.g. it was 19.7 grams for controls and 11.4 grams for plants treated with 400 millimolar NaCl for 14 days. Nocturnal CO₂ uptake in these obligate Crassulacean acid metabolism (CAM) plants was inhibited 67% upon treatment with 400 millimolar NaCl for 14 days (controls, 181 millimoles CO₂ per square meter), while nocturnal accumulation of malate was inhibited 49% (controls, 230 millimoles malate per square meter). The larger accumulation of malate as compared to uptake of atmospheric CO₂ suggests that internal CO₂ recycling occurred during the dark period. Such recycling was lower in the controls (~20%) than in the NaCl-treated plants (~50%). The nocturnal increase in malate and titratable acidity depended on the total daily photosynthetically active radiation available; measurements suggest a quantum requirment of 26 photons per malate. As NaCl in the medium was increased to 600 millimolar in daily increments of 50 millimolar, Na and Cl concentrations in the roots increased from about 7 to 100 millimolar, but K concentration in the cell sap remained near 26 millimolar. Concomitantly, concentrations of Na and Cl in the shoots increased from 8 to 17 millimolar and from 1 to 7 millimolar, respectively, while the K concentration increased about 16 to 60 millimolar. In plants maintained for 14 days at 500 millimolar NaCl, the root levels of Na and Cl increased to 260 millimolar, the shoot levels were about 60 millimolar, and the stem bases began to become necrotic. Such Na retention in the roots together with the special possibilities of carbon reutilization given by CAM are apparently survival mechanisms for the temporarily saline conditions experienced in its natural habitat.     

Transport of sodium into the xylem exudate of tobacco

When tobacco (Nicotiana tabacum L. var. Virginia Gold) plants were pretreated with Na (²²Na) several days before detopping, from 2.3 to 4.9% of Na previously accumulated in roots appeared in the xylem exudate in 7 days after detopping. Na from the external medium, however, was readily transported to the exudate. Moreover, the amount of the pretreatment Na that was transported to the exudate was not influenced by the presence of Na in the external medium. When Na was present in the external medium after detopping, about 4% (with an NaNO₃ post treatment) to 10% (with an NaCl post treatment) of the Na transported to the xylem in the 7 days following detopping originated in the vacuoles. Nitrate salts of K or Na in the external medium after detopping resulted in transport of large quantities of the respective cation to the exudate, but not in increased transport of the pretreatment Na. A much larger percentage of the K that was accumulated after detopping than of the Na similarly accumulated was transferred to the xylem exudate.     

Respiration Rate of Barley Roots: its Relation to Growth, Substrate Supply and the Illumination of the Shoot

The respiration rate of roots on intact barley plants grownin 16 h light 8 h dark cycles shows an exponential decay inthe dark, rises on re-illumination and there is a transientfall 12–14 h into the photoperiod Roots of plants placedin the dark for up to 48 h show a continued exponential decay,and a rather small fall in soluble carbohydrate levels The respirationof roots excised from predarkened plants does not rise on additionof sucrose to the medium bathing them Respiration rate, measured10 h into the photoperiod, shows a constant relation to rootweight in plants 8–24 days old, during which time rootcarbohydrate content first falls and later rises It is concludedthat root respiration rate is not a simple function of carbohydratesupply from the shoot The importance of root respiration inthe carbon budget of barley plants is evaluated and the levelsof control operating on root respiration rate are briefly discussed Hordeum distichum (L ) Lam, barley, respiration rate, light, carbohydrate     

The Involvement of Aspartate and Glutamate in the Decarboxylation of Malate by Isolated Bundle Sheath Chloroplasts from Zea mays

Aspartate or glutamate stimulated the rate of light-dependent malate decarboxylation by isolated Zea mays bundle sheath chloroplasts. Stimulation involved a decrease in the apparent K_m (malate) and an increased maximum velocity of decarboxylation. In the presence of glutamate other dicarboxylates (succinate, fumarate) competitively inhibited malate decarboxylation by intact chloroplasts with respect to malate with an apparent K_i of about 6 millimolar. For comparison the K_i for inhibition of nicotinamide adenine dinucleotide phosphate-malic enzyme from freshly lysed chloroplasts by these dicarboxylates was 15 millimolar. A range of compounds structurally related to aspartate stimulated malate decarboxylation by intact chloroplasts. K_a values for stimulation at 5 millimolar malate were 1.7, 5, and 10 millimolar for l-glutamate, l-aspartate, and β-methyl-dl-aspartate, respectively. Certain compounds, notably cysteic acid, which stimulated malate decarboxylation by intact chloroplasts inhibited malate decarboxylation by nicotinamide adenine dinucleotide phosphate-malic enzyme obtained from lysed chloroplasts and assayed under comparable conditions. It was concluded that aspartate, glutamate, and related compounds affect the transport of malate into the intact chloroplasts and that malate translocation does not take place on the general dicarboxylate translocator previously reported for higher plant chloroplasts.     

Day/Night Changes in the Sensitivity of Phosphoenolpyruvate Carboxylase to Malate during Crassulacean Acid Metabolism

Phosphoenolpyruvate carboxylase (PEPC) was extracted from Mesembryanthemum crystallinum L. performing Crassulacean acid metabolism, at frequent intervals during a 12-hour light/12-hour dark cycle. Inhibition of PEPC by malate was followed at pH 8.0 and 7.5, 1 minute after homogenization of leaves. PEPC was more sensitive to malate during the light than during the dark periods and inhibition by malate was more pronounced at pH 7.5 than 8.0. For example, PEPC was not or only slightly inhibited by 0.5 millimolar malate during the dark period at both pH values and the rates per milligram chlorophyll were about the same. During the light period, 0.5 millimolar malate resulted in a 20 to 30% reduction of PEPC activity at pH 8.0 and a 80 to 90% reduction at pH 7.5. These and other experiments, in which plants were kept in prolonged dark periods, indicate that the increase in sensitivity of PEPC to malate is correlated with the change from acidification to deacidification in the tissue. These interactions account for apparent changes in pH response of PEPC in crude extracts assayed at different times of the day/night cycle.     

The Role of Oxygen in the Regulation of Nitrogenase Activity in Drought-Stressed Soybean Nodules

The aim of this study was to investigate the mechanism of nitrogenase inhibition in drought-stressed soybean (Glycine max L.) nodules to determine whether this stress was similar to other inhibitory treatments (e.g. detopping) known to cause an O2 limitation of nodule metabolism. Nodulated soybean plants were either detopped or subjected to mild, moderate, or severe drought stress by growth in different media and by withholding water for different periods. All treatments caused a decline in nitrogenase activity, and in the drought-stressed nodules, the decline was correlated with more negative nodule water potentials. Increases in rhizosphere O2 concentration stimulated nitrogenase activity much more in detopped plants than in drought-stressed plants, reflecting a greater degree of O2 limitation with the detopped treatment than with the drought-stressed treatment. These results indicated that drought stress differs from many other inhibitory treatments, such as detopping, in that its primary cause is not a decrease in nodule permeability and a greater O2 limitation of nodule metabolism. Rather, drought stress seems to cause a decrease in the maximum O2-sufficient rate of nodule respiration or nitrogenase activity, and the changes in nodule permeability reported to occur in drought-stressed nodules may be a response to elevated O2 concentrations in the infected cell that may occur as nodule respiration declines.     

Drought Stress,Permeability to O2 Diffusion,and the Respiratory Kinetics of Soybean Root Nodules

In legume nodules, treatments such as detopping or nitrate fertilization inhibit nodule metabolism and N2 fixation by decreasing the nodule's permeability to O2 diffusion, thereby decreasing the infected cell O2 concentration (Oi) and increasing the degree to which nodule metabolism is limited by O2 availability. In the present study we used nodule oximetry to assess and compare the role of O2 limitation in soybean (Glycine max L. Merr) nodules inhibited by either drought or detopping. Compared to detopping, drought caused only minor decreases in Oi, and when the external O2 concentration was increased to raise Oi, the infected cell respiration rate in the drought-stressed plants was not stimulated as much as it was in the nodules of the detopped plants. Unlike those in detopped plants, nodules exposed to moderate drought stress displayed an O2-sufficient respiration rate that was significantly lower than that in control nodules. Despite possible side effects of oximetry in altering nodule metabolism, these results provided direct evidence that, compared to detopping, O2 limitation plays a minor role in the inhibition of nodule metabolism during drought stress and changes in nodule permeability are the effect, not the cause, of a drought-induced inhibition of nodule metabolism and the O2-suffiecient rate of respiration.     

Role of sugars in nitrate utilization by roots of dwarf bean

Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen-depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium. The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m^?3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m^?3). When plants were decapitated after an 18 h NO₃^? pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG^?₃ uptake. Presumably due to a difference in NO₃^? due to a difference in NO₃^? uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar-stimulation of, oxygen consumption as well as the release of ¹⁴CO₂ from freshly absorbed (U-¹⁴C) sugar was the same for glucose and fructose. Therefore, we propose a glucose-specific effect on NO₃^? uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose-fructose effect on nitrate reductase activity independent of the effect on NO₃^? uptake was not indicated. A constant level of NRA occurred in roots of NO₃^? induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half-life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots.     

Effect of Zn2+ on water and K+ fluxes in detopped maize plants

Water and K⁺ fluxes were examined in detopped plants ofZea mays L. (cv. White Horse Tooth), which were grown and exuded on half-strength Long Ashton nutrient solution containing the appropriate concentration of Zn²⁺ at 20 °C. In light-grown plants, 100 and 500 μM Zn²⁺ increased both water and K⁺ fluxes in detopped maize plants whereas 1 000 μM Zn²⁺ inhibited both fluxes. In the dark-pretreated plants, 1 000 μM Zn²⁺ in the medium stimulated K⁺ flux. The fluxes of K⁺, Zn²⁺, Ca²⁺ and Mg²⁺ were usually higher in detopped plants than in intact ones. At 1 000 μM Zn²⁺ in the exudation medium, Zn²⁺ concentration was higher in the xylem exudate of dark-pretreated plants than in roots of plants maintained in light. The results are discussed in relation to the influence of Zn²⁺ on the membrane permeability and transport in plants.