Leaf Nitrate Reductase Activity Research Articles

Partitioning of nitrate assimilation among leaves, stems and roots of poplar.

Plants differ in tissue localization of nitrate reduction and assimilation. Some species reduce nitrate primarily in the leaves, whereas other species localize nitrate reduction and assimilation in the roots. We determined how nitrate assimilation is partitioned among leaves, stems and roots of poplar (Populus tremula L. x P. alba L.) by comparing tissue differences in in vivo nitrate reductase activity (NRA), nitrate reductase abundance and tissue nitrate concentration. Compared with stems or roots, NRA was greater in leaves, and the highest leaf NRA was found in young leaves. Leaf and root NRA increased with increasing nitrate supply, whereas stem NRA remained constant. Leaf NRA was at least 10-fold greater than root NRA at all external nitrate concentrations. Nitrate reductase abundance increased in all tissues with increasing nitrate availability, and nitrate reductase abundance was at least 10-fold greater in leaves than in stems or roots at all nitrate availabilities. Tissue nitrate concentration increased with increasing nitrate supply and was greater in roots than in stems and leaves. Photoperiod influenced NRA, with leaf NRA declining in nitrate-fertilized plants with short daily photoperiods (8-h). We conclude that different tissues of poplar vary in nitrate assimilation with little nitrate assimilation occurring in roots and the most nitrate assimilation taking place in leaves.

An Analysis of Seasonal Effects on Leaf Nitrate Reductase Activity and Nitrogen Accumulation in Maize (Zea mays L.)

The important enzyme in nitrogen (N) assimilation, nitrate reductase (NR), is an inducible enzyme influenced by many external (light, temperature, etc.) and internal (genotype) factors. The efficiency of the N assimilation system may vary with genotype and season. In the present study, the effects of season on NR activity in relation to N accumulation in maize plants were investigated. Six different cultivars of maize, namely Ganga‐11, Deccan‐103, Hi‐starch (hybrids), Arun, Manjari and Vijay (composites), were sown during the monsoon (88‐day crop duration) and in winter (150‐day crop duration). In vivo NR activity in the last fully expanded leaf (LFEL), and the N contents of the whole plant and the LFEL were estimated at seven phenological growth stages. Three different states of N metabolism in maize, namely (i) low NR activity per unit leaf area per unit time coinciding with high accumulation of N, (ii) high NR activity coinciding with low N accumulation, and (iii) low NR activity coinciding with low N accumulation, were noted. These results clearly demonstrate that the relationships between N uptake and accumulation parameters change with the season and crop growth stage and are subject to a genotypic influence. Thus it is necessary to evaluate genotypes under similar environments to select a genotype with high N use efficiency. As these relationships are growth dependent, care must be taken to evaluate them at a particular phenological stage rather than on the basis of days after sowing.

DROUGHT-INDUCED EFFECTS AND RECOVERY OF NITRATE ASSIMILATION AND NODULE ACTIVITY IN COWPEA PLANTS INOCULATED WITH BRADYRHIZOBIUM SPP. UNDER MODERATE NITRATE LEVEL

This study was carried out to establish comparative effects of drought and recovery on the nitrate assimilation and nodule activity related to N2 fixation in cowpea plants [Vigna unguiculata L. (Walp.)] previously inoculated with Bradyrhizobium spp. BR-3256 (CB-756) strain in the presence of 5 mol m -3 NO - 3. Twenty-eight-day-old nodulated plants were submitted to water deprivation during 4 consecutive days and afterwards resupplied with nutrient solution during 2 days. The water deprivation caused a rapid increase in the nitrate content in root and a marked reduction in leaf nitrate reductase (NR) activity. In contrast nodule NR activity was slightly increased by water deprivation. Concomitantly, in nodules of water stressed plants, leghemoglobin and glutamine synthetase (GS) activity declined and a progressive reduction in ureide-N concentration in xylem sap was observed. Leaf-NR activity increased rapidly after rehydration while leaf nitrate content declined. In contrast both GS activity and soluble protein content in the nodule continued to decline in rewatered plants. In addition the concentration of leghemoglobin recovered well, while the xylem ureide-N content experienced a slight increase after rehydration. Despite the nitrate assimilation in leaves and the nodule activity had been both severely affected by water stress, the rapid recovery of nitrate reductase activity suggests that the nitrate assimilation process is less sensitive to drought/rehydration cycle when cowpea plants are nodulated in presence of moderate nitrate level.

Effect of salt stress on some chemical components and yield of potato

To obtain more information about the effect of salt stress on potato production, the effects on emergence, growth and yield, carbohydrate and mineral contents, and nitrate reductase (NR) activity of potato plants (Solanum tuberosum L.) were investigated using two cultivars (May Queen and Dejima). Salt stress was achieved by the application of NaCl (0, 10, 20, and 30 g pot-1 and the corresponding ECe values were 0.20, 0.77, 1.37, and 1.95 dS m-1 respectively). Emergence was delayed, growth and dry matter production, especially in tubers, were depressed with the increase in the salt level. The salt stress decreased the total and marketable tuber yield due to the decrease in the tuber number per plant and average tuber weight. The contents of water-soluble carbohydrates (WSC) and starch of leaves increased and that of total nonstructural carbohydrates (TNC) also increased by salt stress. The WSC and starch contents of stems increased at the early stage of growth, while at the later stages, both the WSC and starch contents decreased and also the TNC content decreased by salt stress. Sodium content of leaves, stems, and tubers markedly increased with the increase of the salt level. Potassium content of leaves decreased but in stems and tubers it increased by salt stress. Calcium content of leaves and stems decreased but in tubers it increased due to the increase of the salt level. Tuber N content increased by salt stress. The leaf NR activity was suppressed by salt stress, and the increase of the salt level decreased the NR activity throughout the season. It was concluded that the yield decrease caused by salt stress was due to nutritional imbalance, resulting in the inactivation of enzymes such as NR.

Nitrite Concentration Effects on NO3-N Uptake and Reduction, Growth, and Fruit Yield in Strawberry

Strawberries (Fragaria xananassa Duch. 'Osogrande') were grown hydroponically with three NO3-N concentrations (3.75, 7.5, or 15.0 mM) to determine effects of varying concentration on NO3-N uptake and reduction rates, and to relate these processes to growth and fruit yield. Plants were grown for 32 weeks, and NO3-N uptake and nitrate reductase (NR) activities in roots and shoots were measured during vegetative and reproductive growth. In general, NO3-N uptake rates increased as NO3-N concentration in the hydroponics system increased. Tissue NO3- concentration also increased as external NO3-N concentration increased, reflecting the differences in uptake rates. There was no effect of external NO3-N concentration on NR activities in leaves or roots during either stage of development. Leaf NR activity averaged approximately 360 nmol NO2 formed/g fresh weight (FW)/h over both developmental stages, while NR activity in roots was much lower, averaging approximately 115 nmol NO2 formed/g FW/h. Vegetative organ FW, dry weight (DW), and total fruit yield were unaffected by NO3-N concentration. These data suggest that the inability of strawberry to increase growth and fruit yield in response to increasing NO3-N concentrations is not due to limitations in NO3-N uptake rates, but rather to limitations in NO3- reduction and/or assimilation in both roots and leaves.

Salinity and nitrogen fertilization and nitrogen metabolism in rose plants

Rose production is limited by salinity and highly affected by the nitrogen source present in the nutrient solution. The influence of sodium on several aspects of nutrition has been investigated in ‘Lambada' rose plants using different sources of nitrogen in the fertilization treatment. Experiments using a previously defined mono-shoot model plant and a simplified hydroponic culture allowed us to study the effects of salinity v. nitrogen on NPK uptake during the culture period. Mineral concentrations, nitrate reductase (NR) and glutamine synthetase (GS) activities were also analysed. This study showed that rose plants were more sensitive to saline conditions under NH4+ fertilization without detectable effects on growth or in NPK mineral contents in shoots. Parameters affected most were enzymatic activities analysed such as leaf nitrate reductase activity which was reduced under NH4+ nutrition. Leaf glutamine synthetase was also enhanced by saline conditions. The Na/K ratio showed that under NH4+ nutrition, the highest sodium accumulation occurred in roots. Nitrate uptake did not show a clear pattern related to nitrogen source, however, ammonium uptake was affected by salinity when NH4+ was the sole nitrogen source in the nutrient solution. Potassium and phosphate uptake were always lower when NH4+ was present in the nutrient solution.

Physiological and morphological adaptations of the fruit tree Ziziphus rotundifolia in response to progressive drought stress.

The physiological basis of drought resistance in Ziziphus rotundifolia Lamk., which is an important, multipurpose fruit tree of the northwest Indian arid zone, was investigated in a greenhouse experiment. Three irrigation regimes were imposed over a 34-day period: an irrigation treatment, a gradual drought stress treatment (50% of water supplied in the irrigation treatment) and a rapid drought stress treatment (no irrigation). Changes in gas exchange, water relations, carbon isotope composition and solute concentrations of leaves, stems and roots were determined. The differential rate of stress development in the two drought treatments did not result in markedly different physiological responses, but merely affected the time at which they were expressed. The initial response to decreasing soil water content was reduced stomatal conductance, effectively maintaining predawn leaf water potential (Psi(leaf)), controlling water loss and increasing intrinsic water-use efficiency, while optimizing carbon gain during drought. Carbon isotope composition (delta13C) of leaf tissue sap provided a more sensitive indicator of changes in short-term water-use efficiency than delta13C of bulk leaf tissue. As drought developed, osmotic potential at full turgor decreased and total solute concentrations increased in leaves, indicating osmotic adjustment. Decreases in leaf starch concentrations and concomitant increases in hexose sugars and sucrose suggested a shift in carbon partitioning in favor of soluble carbohydrates. In severely drought-stressed leaves, high leaf nitrate reductase activities were paralleled by increases in proline concentration, suggesting an osmoprotective role for proline. As water deficit increased, carbon was remobilized from leaves and preferentially redistributed to stems and roots, and leaves were shed, resulting in reduced whole-plant transpiration and enforced dormancy. Thus, Z. rotundifolia showed a range of responses to different drought intensities indicating a high degree of plasticity in response to water deficits.

Phytophthora Resistance of Organically-Fertilized Tomato Plants

Summary Tomato plants (Lycopersicon esculentum L. cv. Momotaro T96) grown with organic fertilizer possessed higher resistance against phytophthora infection (Phytophthora infestans) in comparison with plants given chemical fertilization. However, the photosynthetic activity was not low in leaves of chemically-fertilized plants before phytoph-thora infection. There were no differences in leaf proteins between the two fertilization treatments, as revealed by electrophoresis. This suggested that high phytophthora infection was not related to physiological activity as indicated by photosynthetic rate and protein profile. Concentrations of leaf nitrogenous compounds, nitrate and amino acids were higher in chemically-fertilized plants. Nitrate concentration in the soil was also higher in chemical plots. On the other hand, leaf nitrate reductase activity and soil hydrogenase activity were lower in chemical plots. The integrated results suggested that nitrogen metabolism in organically-fertilized tomato plants ac...

Diurnal Fluctuations of Nitrate Uptake and In Vivo Nitrate Reductase Activity in Pima and Acala Cotton

This study was conducted to determine whether diurnal fluctuations of nitrate reductase activity in two cotton species, Pima (Gossypium barbadense L.) and Acala (Gossypium hirsutum L.), are regulated by NO−3 uptake or leaf NO−3 concentration. The two species differ greatly in their N use efficiency. The seedlings were grown for 25 d in nutrient solutions containing 0.05, 0.10 or 1.0 mM NO−3 under a 14‐h light/10‐h dark cycle at 30°C/20°C. Uptake rates were measured by following NO−3 depletion from the uptake solutions at 20°C or 30°C. Nitrate reductase activity (NRA) in fully expanded first true leaves was determined by an anaerobic method in vivo. Upon illumination uptake rates for both species, measured at 30°C, increased until they plateaued in about 5 h and were maintained at that level for the remainder of the light period and the following dark cycle. When measured at 20°C in the dark, however, the uptake rates decreased 25 to 30% for the first 3 h and then remained constant. Leaf NRA increased rapidly during the first hour of illumination, reached a plateau and then decreased after 4 to 6 h of illumination. The decline was more rapid when NRA was assayed in the absence of NO−3 In darkness, NRA levels were low and did not fluctuate. Upon illumination, leaf NO−3 concentration also increased for about 7 h and then decreased gradually. The results indicate that (i) the diurnal rhythm of NO−3 uptake is modulated by temperature rather than by light/dark transitions, and (ii) while the increase in NRA upon illumination may be regulated by NO−3 flux from roots to shoots, the decrease in NRA under prolonged illumination is independent of that flux.

Influence of elevated CO2 and mycorrhizae on nitrogen acquisition: contrasting responses in Pinus taeda and Liquidambar styraciflua.

An understanding of root system capacity to acquire nitrogen (N) is critical in assessing the long-term growth impact of rising atmospheric CO2 concentration ([CO2]) on trees and forest ecosystems. We examined the effects of mycorrhizal inoculation and elevated [CO2] on root ammonium (NH4+) and nitrate (NO3-) uptake capacity in sweetgum (Liquidambar styraciflua L.) and loblolly pine (Pinus taeda L.). Mycorrhizal treatments included inoculation of seedlings with the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith in sweetgum and the ectomycorrhizal (EM) fungus Laccaria bicolor (Maire) Orton in loblolly pine. These plants were then equally divided between ambient and elevated [CO2] treatments. After 6 months of treatment, root systems of both species exhibited a greater uptake capacity for NH4+ than for NO3-. In both species, mycorrhizal inoculation significantly increased uptake capacity for NO3-, but not for NH4+. In sweetgum, the mycorrhizal effect on NO3- and NH4+ uptake capacity depended on growth [C02]. Similarly, in loblolly pine, the mycorrhizal effect on NO3- uptake capacity depended on growth [CO2], but the effect on NH4+ uptake capacity did not. Mycorrhizal inoculation significantly enhanced root nitrate reductase activity (NRA) in both species, but elevated [CO2] increased root NRA only in sweetgum. Leaf NRA in sweetgum did not change significantly with mycorrhizal inoculation, but increased in response to [CO2]. Leaf NRA in loblolly pine was unaffected by either treatment. The results indicate that the mycorrhizal effect on specific root N uptake in these species depends on both the form of inorganic N and the mycorrhizal type. However, our data show that in addressing N status of plants under high [CO2], reliable prediction is possible only when information about other root system adjustments (e.g., biomass allocation to fine roots) is simultaneously considered.

Effects of fruiting on the growth of Arabica coffee trees as related to carbohydrate and nitrogen status and to nitrate reductase activity

Active vegetative and reproductive growth in field-grown trees of Coffea arabica L. in Viçosa (20º45’S, 650 m altitude), south-eastern Brazil, occur concurrently. The overall patterns of branch growth and leaf area gain were to a certain extent altered by fruit removal, with growth rates being remarkably greater in de-fruited trees. The content of N-NO3 was not affected by fruiting, whilst that of amino-N was greater in de-fruited than fruiting trees most of the time, but the differences were not large enough to have significantly contributed to the increased growth rates in de-fruited trees. Leaf nitrate reductase activity was greater in plants bearing fruit than in non-bearing ones most of the time; activity was roughly inversely associated with growth. Although the roots contained much more nitrate than the leaves, the root nitrate reductase activity was much lower and not affected by fruiting. Much of the restrictive effects of fruiting on vegetative growth appeared to be associated to starch exhaustion, in addition to the outstanding effect of supra-optimum temperatures per se.

The potential of NO3--N utilization by a woody shrub species Lindera triloba: a cultivation test to estimate the saturation point of soil NO3--N for plants.

Responses of seedlings of a shrub species, Lindera triloba, grown in perlite culture medium, to nitrate (NO3-N) supply were investigated to estimate the saturating point of available NO3-N for plant utilization. NO3-N concentration and nitrate reductase activity (NRA) in leaves and roots were used as indicators of NO3-N uptake and assimilation by L. triloba. Root NRA increased with NO3-N supply when concentrations were low and reached a plateau at high NO3-N concentrations. On the other hand, root NO3-N concentration increased linearly with NO3-N supply; therefore, it is suggested that NO3-N uptake did not limit NO3-N assimilation by L. triloba. In contrast, leaf NRA and leaf NO3-N concentration were low and were not influenced by NO3-N supply. This may be caused by the lack of transport of NO3-N from roots to leaves. The NO3-N retained in perlite was compared with NO3-N pool sizes in soils from a forest where L. triloba occurs naturally to estimate the level of NO3-N availability to plants in the forest soil. The maximum NO3-N pool size in the forest soil was comparable to concentrations at which root NRA reached a plateau in perlite cultures. These results indicate that soil NO3-N availability is below the saturation point for NO3-N uptake by L. triloba, and it is the limiting factor of NO3-N utilization by L. trilobaunder field conditions in which this species naturally occurs.

Diurnal patterns of nitrate assimilation in Kentucky bluegrass

Kentucky bluegrass (Poapratensis L.) is a major C3‐type forage and turfgrass, but it is less efficient than many grasses in utilizing nitrogen(N). To determine how this grass can accommodate its greater N need, diurnal patterns of nitrate reductase activity (NRA) and nitrite reductase activity (NiRA) in its leaves and roots were examined and compared with those in barley (Hordeum vulgare L.). Plants were grown under greenhouse or growth room conditions and assayed for NRA and NiRA by optimized in vivo methods. The diurnal patterns of NRA and NiRA indicated that Kentucky bluegrass could assimilate nitrate during the night at rates greater than or similar to those during the day. Leaf NRA of Kentucky bluegrass was minimal approximately 4 and 10 h after illumination commenced and increased at night. The diurnal pattern of leaf NRA among Kentucky bluegrass cultivars did not differ significantly. In roots, NRA of Kentucky bluegrass was high in the morning and decreased sharply during the afternoon and evening, but increased again late at night. Unlike Kentucky bluegrass, barley exhibited greater leaf NRA during the day than during the night and exhibited the greatest activity 6 or 10 h after illumination commenced. In both species, the equilibrium leaf nitrate pool was 20 to 30 times larger than the ammonium pool and 3, 000 to 13, 000 times larger than the nitrite pool. Leaf nitrate pool size showed a diurnal pattern complementary to that of leaf NRA. Our results suggest that a nighttime N use strategy might exist in Kentucky bluegrass.

Effects of Temperature at Different Growth Stages on Nonstructural Carbohydrate, Nitrate Reductase Activity and Yield of Potato.

The effect of temperature (15, 25, 30°C and glasshouse as control) at different growth stages (stage-I: vegetative to tuber initiation, stage-II: tuber initiation to initial tuber bulking and stage-III: tuber bulking to maturity) on leaf nitrate reductase (NR) activity, carbohydrate contents and tuber yield were investigated in potatoes (Solanum tuberosum L. cvs. May Queen and Normn 1) as pot experiment in naturally lit glasshouse and phytotron. The high temperature treatment decreased the total dry matter production as well as tuber yield and degraded the tuber quality by reducing the specific gravity. NR activity of leaves was also decreased by high temperature treatment, whereas low temperature increased the activity. Water soluble carbohydrate (WSC) and starch content in leaves were sequentially decreased and as a result total nonstructural carbohydrates (TNC) were also decreased by high temperature. The inhibition of tuber yield by high temperature was due to the limited translocation of carbohydrates from leaves to the tubers, following the reduction of NR activity and the expense of carbohydrates for dark respiration. The maximum yield reduction was observed at high temperature treatment at stage-I which was most critical for tuber yield, and low temperature at stage-II produced the highest yield which was considered as more advantageous step towards yield improvement.

Inhibition of wheat leaf nitrate reductase activity by phenolic compounds.

Phenolic acids inhibited the activities of wheat leaf nitrate reductase depending on phenolic structure and concentration. Possible conformational change(s) in the enzyme induced by hydrogen bonding and/or hydrophobic interactions might be the cause of the enzyme inhibition. NADH:cytochrome C reductase partial activity was unaffected, which indicates that terminal nitrate-reducing domain of NR may be the site of polyphenol binding.

High Irradiance Impairments on Photosynthetic Electron Transport, Ribulose-1,5-bisphosphate Carboxylase/ oxygenase and N Assimilation as a Function of N Availability in Coffea arabica L. Plants

Summary Young coffee (Coffea arabica L.) plants grown under low irradiance (PPFD up to ca. 150μmol m−2s−1) and high (2N), medium (IN) and low (ON) nitrogen availability conditions were exposed to natural sunlight (noon PPFD up to ca. 1,700 (¼mol m−2s−1) for 14–15 days, during which parameters related to photosynthesis and nitrogen assimilation were monitored. In the plants with lower N availability, a donor and an acceptor side photoinhibition may have affected PS II functioning. During the high irradiance stress the plants with higher N availability maintained the highest contents of cytochromes (cyt) b559HPJ, b559LP, b563 and f, and in plastoquinone-9 (PQ-9). Those plants also presented significant increases in rubisco content and activity, and a preferential investment in rubisco rather than in light harvesting components by the end of the high irradiance stress. Despite the effects observed on PSII and rubisco, the PS I activity and cyt b6/f complex were much more affected in all N treatments. Leaf nitrate reductase activity decreased whereas nitrate and amino acid contents increased during the high irradiance in 0N and 1N plants. Our data shows that high irradiance affected both the photochemical and enzymatic reactions of photosynthesis (especially in ON and 1N plants), that PS I was a preferential photoinhibitory target and that the higher N availability promoted the recovery of important physiological parameters, such as electron transport rates, PQ-9 and rubisco activity and content.

Light modulation and in vitro effects of adenine nucleotides on leaf nitrate reductase activity in cucumber (Cucumis sativus)

Nitrate reductase (NR, EC 1.6.6.1) activity in attached cucumber (Cucumis sativus L. cv. Ashley) leaves changed rapidly and reversibly during light/dark transitions, especially when assayed in the presence of free Mg2+. Light decreased and darkness increased the sensitivity of the enzyme to inhibition by Mg2+. The NR activation state, i.e. activity in the presence of Mg2+ relative to activity in the absence of Mg2+, increased with light intensity up to 400 μmol m−2 s−1 PAR (photosynthetically active radiation). When a desalted crude extract from illuminated leaves was preincubated with ATP, NR was gradually inactivated. Inactivation was only observed when activity was assayed in the presence of Mg2+. The ATP‐inactivated NR remained inactive after removing the excess of ATP by gel filtration and it did not occur in partially purified NR preparations. NR extracted from darkened attached leaves was markedly activated when preincubated with 5′‐AMP. These results support the view that inactivation/activation of cucumber‐leaf NR in response to light/dark signals most likely involves phosphorylation/dephosphorylation of the enzyme catalysed by endogenous proteins. A substantial activation of NR by preincubation with 5′‐AMP was also observed when activity was assayed in the absence of Mg2+, thus indicating that 5′‐AMP can directly activate NR. Irradiation of an extract from darkened leaves containing FAD promoted a partial activation of NR. This effect was observed both in the +Mg2+ and in the −Mg2+ assay, indicating that activation was caused by photoexcited flavin and did not involve dephosphorylation of the enzyme.

Changes in Leaf Nitrate Reductase Activity In Vivo and In Vitro During Light-Dark Transitions

Nitrate reductase activity in the leaves of a number of plants after transfer from light to dark was assayed both by in vivo and in vitro methods. The initial activity persisted during the dark phase for a considerable length of time and declined gradually. After exposure to light again, the NR activity increased rapidly. The possibility of nitrate assimilation in complete darkness is discussed.

Induction of nitrate uptake and nitrate reductase activity in trembling aspen and lodgepole pine

13NO3– influx into the roots and in vivo nitrate reductase activity (NRA) in the roots and leaves have been measured in trembling aspen (Populus tremuloides Michx.) and lodgepole pine (Pinus contorta Dougl.) seedlings after exposure to either 0·1 or 1·5 mol m–3 NO3– for varying periods up to 20 d. Both NO3– influx and NRA were inducible in these species and, in trembling aspen, peak induction of nitrate influx and NRA were achieved within 12 h, compared to 2–4 d for influx and 4–12 d for NRA in lodgepole pine. In trembling aspen, ≈ 30% of the total 13N absorbed during a 10 min influx period followed by 2 min of desorption was translocated to the shoot. In lodgepole pine, by contrast, translocation of 13N to the shoot was undetectable during the same time period. Root NRA as well as NO3– influx from 0·1 mol m–3 NO3– were substantially higher in trembling aspen than in lodgepole pine at all stages of NO3– exposure, i.e. during the uninduced, the peak induction, and steady‐state stages. In order to examine whether the lower rates of NO3– influx and NRA were related to proportionately fewer young (unsuberized) roots in lodgepole pine, we determined these parameters in young and old (suberized) roots of this species separately. Induction of influx and NRA were initially greater in young roots but at steady‐state there were only minor differences between the young and the old roots. However, even the elevated initial rates in the young roots of lodgepole pine were substantially lower than those of aspen. In pine, influx at 1·5 mol m–3 NO3– was ~ 6‐fold higher than at 0·1 mol m–3 NO3– and appeared to be mostly via a constitutive system. By contrast, in aspen, steady‐state influxes at 0·1 and 1·5 mol m–3 were not significantly different, being similar to the rate attained by pine at only the higher [NO3–]. In aspen, leaf NRA was ~ 2‐fold higher than that of roots. In lodgepole pine NRA of the needles was below the detection limit. These results show that trembling aspen seedlings are better adapted for NO3– acquisition and utilization than lodgepole pine seedlings.

Effects of Sucrose and Oxygen on Nitrate Reductase Activity and Growth of Kentucky Bluegrass

Nitrate reductase activity (NRA) in turfgrass roots should play an important role in the N metabolism of the whole plant because a major part of the shoots is removed by mowing. However, preliminary experiments demonstrated a lack of in vivo NRA in roots of Kentucky bluegrass grown in Hoagland's nutrient solutions, which were constantly aerated. We hypothesized that O2 inhibited NRA by oxidizing carbohydrate available for NO3– reduction in roots. To test this hypothesis, Kentucky bluegrass (Poa pratensis L.), cultivar Merit, was grown in nutrient solutions containing 0.1 mM NO3–. Grass cultures were treated with +O2 (solution aerated), –O2 (not aerated), +Suc (sucrose added to solution, final concentration = 25 mM) or –Suc (not added) for 12 days. Plants were harvested at 5:00 pm, separated into shoots and roots, and analyzed for in vivo NRA. The results partly supported the above hypothesis because the –Suc–O2 roots showed significantly higher NRA than –Suc+O2 roots. However, +Suc roots did not exhibit greater NRA than –Suc roots, possibly because of a decreased pH in the solutions. Oxygen increased root growth and hence the growth of the whole plant, while sucrose decreased leaf N content and leaf NRA but did not improve growth.