Foliar Nitrate Reductase Activity Research Articles

Características adaptativas da associação simbiótica e da fixação biológica do nitrogênio molecular em plantas jovens de Lonchocarpus muehlbergianus Hassl., uma leguminosa arbórea nativa do Cerrado

O nitrogênio mineral afeta negativamente a simbiose e a fixação biológica do nitrogênio em plantas cultivadas. Entretanto, este efeito não é verificado em algumas espécies arbóreas, pouco estudadas até o momento. O objetivo deste trabalho foi avaliar a capacidade de Lonchocarpus muehlbergianus, espécie arbórea nativa do Cerrado, em utilizar o nitrogênio molecular, na presença do nitrogênio mineral (nitrato), característica desejável para plantas fixadoras crescendo em solos com nitrato como principal forma de nitrogênio disponível. Foram determinados o número e a massa seca dos nódulos, da parte aérea e das raízes, bem como o conteúdo de nitrato, aminoácidos e ureídeos, transportados pelo xilema e a atividade da redutase do nitrato. Foi observado que o número e a massa seca dos nódulos não foram negativamente afetados pelo nitrato. Ocorreu pequeno aumento na massa seca da parte aérea e do sistema radicular de plantas noduladas tratadas com nitrato, resultado verificado também para os teores de aminoácidos, ureídeos e atividade da redutase do nitrato. Os resultados obtidos sugerem que a espécie tem capacidade de utilizar tanto nitrogênio mineral quanto molecular, uma vez que o transporte de ureídeos, do sistema radicular para a parte aérea, não foi reduzido em plantas noduladas tratadas com nitrato.

Artificially-induced leaf nitrate accumulation affects photosynthesis in micropropagated apple plants with different water supply

SummaryOne-year-old micropropagated ‘Gala’ apple (Malus domestica Borkh.) plants with split root systems were supplied with tungsten (Na2WO4), an inhibitor of nitrate reductase activity, and NaNO3, separately, or in combination. The chemicals were supplied either through the xylem (40 ml 0.3 mM Na2WO4 or 3 mM NaNO3), or as foliar sprays (30 mM NaNO3 and 20 mM Na2WO4). When applied separately, neither increased the NO3– content of leaves, although Na2WO4 inhibited foliar nitrate reductase activity completely, and NaNO3 supplied additional nitrate to the leaves. However, when both were applied together, either through the xylem or as sprays, NO3– accumulated in the leaves of well-irrigated plants, as well as in those subjected to half- or whole-root drought stress. Such artificially induced NO3– accumulation in leaves significantly decreased the net photosynthetic rate, stomatal conductance, and rate of transpiration at approx. 12.00 h following the application, compared to controls supplied with deionised water. Net photosynthesis, stomatal conductance, and transpiration rate were negatively correlated with the foliar concentration of NO3–. None of the treatments influenced leaf temperatures or sub-stomatal CO2 concentrations, indicating that the decrease in net photosynthesis was not due to stomatal limitation. It is concluded that low foliar NO3– concentrations are a characteristic of the species, and important for efficient photosynthesis in apple.

Water and Nutrient Dynamics in Surface Roots and Soils are not Modified by Short-term Flooding of Phreatophytic Plants in a Hyperarid Desert

Little is known of the mechanisms employed by woody plants to acquire key resources such as water and nutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the water table, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viable shallow roots may be equally necessary for nutrient uptake. We sought to better understand the interaction between water and nutrient uptake from soil horizons differing in the relative abundance of these resources. To this end, we monitored plant water and nutrient status before and after applying flood irrigation to four phreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sap flow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica (Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using the heat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitrate reductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in the leaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on water and nutrient status. Rates of sap flow measured during the day and at night in the absence of transpiration did not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5–25 cm h−1) detected during the day in soils that had a near zero water content at the surface indicated that all species had contact to groundwater. There was no evidence from sap flow data that plants had utilised flood water to increase maximum rates of transpiration under similar climatic conditions, and there was no evidence of a process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. the passive movement of water from moist soil to very dry soil via roots). Measurements of plant water status, xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were also unaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of water and nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newly available nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.

Effects of soil drying and subsequent re-watering on the activity of nitrate reductase in roots and leaves of Helianthus annuus.

The effects of drought on the activity of nitrate reductase (NR) were studied in Helianthus annuus L. plants subjected to soil drying and subsequent re-watering. Drought did not negatively affect the activation state of NR, but resulted in linearly-correlated decreases in the activity of the unphosphorylated active form and the total activity of NR, in both roots and leaves. The concentration of nitrate in roots, xylem and leaves also decreased in water-stressed plants, whereas the concentration of total amino acids was only transiently depressed at the leaf level. In contrast, soluble sugars accumulated both in roots and leaves of water-stressed plants. Drought-induced decreases in root NR activity were correlated with the observed changes in root nitrate concentration. A higher percentage of the decrease in foliar NR activity could be explained by the decline in nitrate flux to the leaves than by leaf nitrate content. Following re-watering, the extent of recovery of NR activity was higher in roots than in leaves. The delay in the recovery of foliar NR activity did not result from the persistence of reduced flux of nitrate through the xylem. Several hypotheses to explain the after-effect of soil drying on foliar NR activity are discussed.

The effect of lead on the phytochemistry of Tithonia diversifolia exposed to roadside automotive pollution or grown in pots of Pb-supplemented soil

Tithonia diversifolia, a ruderal weedy species utilized in agroforestry, is widely distributed in the tropics, often being found in wasteland, margins of polluted city rivers and roadsides. The objective of this study was to compare plants sampled at a light and heavy traffic roadside for metal concentration and foliar compounds which may be related to senescence and/or detoxification of metals. The field survey was followed by an experiment in pots of Pb-acetate-supplemented soil to verify if the same response is obtained under controlled conditions with regard to Pb. The concentration of Pb in leaves and roots of plants sampled at a heavy traffic roadside was higher than in samples from a light traffic site. The content of Cr, Al, Fe, Cu and Ni was also higher in leaves but not of roots in plants from the pollutedsite. No significant differences were found in the concentrations of chlorophyll, carotenoids, UV-absorbing compounds and proteins, nor in the chlorophyll a/b ratio and lipid peroxidation of leaves collected at both sites. On the other hand, Pb-contaminated leaves and roots showed higher acid phosphatase activity while the foliar nitrate reductase activity and specific leaf mass were lower. An increase of leaf phenol concentration, thought to be involved in the detoxification of metals, was observed in the polluted field plants and in roots in the pot experiment, where Pb-acetatewas supplied to the soil, but the changes observed in acid phosphatase and specific leaf mass in the field were not found in the pot experiment.

Natural 15 N Abundance of Plants and Soil N in a Temperate Coniferous Forest

Measurement of nitrogen isotopic composition (δ15N) of plants and soil nitrogen might allow the characteristics of N transformation in an ecosystem to be detected. We tested the measurement of δ15N for its ability to provide a picture of N dynamics at the ecosystem level by doing a simple comparison of δ15N between soil N pools and plants, and by using an existing model. δ15N of plants and soil N was measured together with foliar nitrate reductase activity (NRA) and the foliar NO3 – pool at two sites with different nitrification rates in a temperature forest in Japan. δ15N of plants was similar to that of soil NO3 – in the high-nitrification site. Because of high foliar NRA and the large foliar NO3 – pool at this site, we concluded that plant δ15N indicated a great reliance of plants on soil NO3 – there. However, many δ15N of soil N overlapped each other at the other site, and δ15N could not provide definitive evidence of the N source. The existing model was verified by measured δ15N of soil inorganic N and it explained the variations of plant δ15N between the two sites in the context of relative importance of nitrification, but more information about isotopic fractionations during plant N uptake is required for quantitative discussions about the plant N source. The model applied here can provide a basis to compare δ15N signatures from different ecosystems and to understand N dynamics.

Foliar application of molybdenum in common bean. II. Nitrogenase and nitrate reductase activities in a soil of low fertility

Foliar application of molybdenum (Mo) at 40 g ha‐1 25 days after plant emergence greatly enhanced nitrogenase and nitrate reductase activities of common bean (Phaseolus vulgaris L.), resulting in an increase in total nitrogen (N) accumulation in shoots. Application of 20 kg N ha‐1 as ammonium sulfate [(NH4)2SO4] at sowing decreased nodulation and nitrogenase activity. Rhizobium inoculation did not affect nitrogenase activity which demonstrated that Mo application increased the efficiency of native Rhizobia strains. Nitrogen amendment, either at planting (20 kg N ha‐1) or as a side dressing (30 kg N ha‐1) 25 days after plant emergence, did not affect the foliar nitrate reductase activity. Molybdenum foliar spray as ammonium molybdate [(NH4)6Mo7O242H2O] and N applied as a side dressing increased equally the total amount of N in the pods. A 10% increase in the seed N concentration was obtained with foliar application of Mo, while N applied as a side dressing had no effect on seed N concentration. An average increase of 41% in N export to the seeds was obtained by either Mo or N as side dressing. Nitrogen applied at sowing or Rhizobia inoculation had no effect on the characteristics evaluated 74 days after plant emergence. Plants that received either Mo as foliar spray or as side dressed N had similar yields. This demonstrated that, in certain soils, N fertilization may be replaced by a small amount of Mo as a foliar application.

Soil Chemistry and Plants in Fennoscandian Boreal Forest as Exemplified by a Local Gradient

In Fennoscandian boreal forests, in which productivity in general is N limited, there are regular, topographically related variations in forest productivity and plant community composition. Regional surveys have demonstrated strong correlations among soil pH, N content, and base saturation on the one hand and plant productivity and community composition on the other, but the nature of these relationships is poorly understood. We studied in detail the variation in and controls of soil acidity, availability of N and P, and changes in community composition and plant nutrition along a short (only 90 m long) but extreme forest productivity gradient in northern Sweden, which ranged from a groundwater recharge area with low productivity to a very productive discharge area. The pH in the soil solution of the mor layer ranged from 3.5 in the recharge area to 6.4 in the discharge area, and it was strongly correlated with the base saturation of the exchange complex. Neither the acid strength of organic matter, the ionic strength of the soil solution, nor the quantity of acids could explain more than a minor part of this variation in pH. There were strong correlations between total N in the mor layer and soil solution pH (r = 0.97) and base saturation of the exchange complex (r = 0.88). At the poor end of the transect the concentration of inorganic N was very low in the mor, and plants with either ectomycorrhizae (ECM) or ericoid mycorrhizae (EM) dominated. With increasing pH, there was an increase in NH4 concentrations, while plants that potentially have arbuscular mycorrhizae (AM) became prominent along with ECM and EM species. In the discharge area, which comprised only the last 10 m of the transect, NO3 dominated over NH4 in the soil solution, the soil had a high capacity for net nitrification, and the vegetation was totally dominated by potentially AM or nonmycorrhizal herbs, some of which had high foliar nitrate reductase activity. Foliar and root N concentrations increased steeply towards the discharge area, but foliar P/N ratios declined below critical levels at the end of the transect. Root 32P uptake bioassays also indicated a P deficiency in the discharge area, where the soil total P content was high, while the concentration of PO4 in the soil solution was very low. The high capacity of the mor in the discharge area to adsorb PO4, due to the presence of organically complexed Fe and Fe-oxihydroxides, may explain the low PO4 concentrations. Our data indicate that the underlying factors influencing both productivity and community composition are pH and supply of base cations. Fundamental differences in exchange characteristics of soil and soil water underlie other related nutrient supply features, in particular the amount and availability of N. Our study of a single short topographic transect supported a previous suggestion based on a regional survey in Norway that variability in soil pH and the supply of base cations affects plant productivity and community composition via effects on N supply. Our data also encompass the interrelations between soil pH, soil N turnover, and the mycorrhizal type of dominant plant species, which, according to Read (1991), occur along long latitudinal or altitudinal climatic gradients. Through millennia discharge areas like the one observed by us have probably provided a relatively stable environment for plants demanding high soil pH and N supply, at the same time as surrounding recharge areas have been acidified naturally through podzolization.

SOIL CHEMISTRY AND PLANTS IN FENNOSCANDIAN BOREAL FOREST AS EXEMPLIFIED BY A LOCAL GRADIENT

In Fennoscandian boreal forests, in which productivity in general is N limited, there are regular, topographically related variations in forest productivity and plant community composition. Regional surveys have demonstrated strong correlations among soil pH, N content, and base saturation on the one hand and plant productivity and community composition on the other, but the nature of these relationships is poorly understood. We studied in detail the variation in and controls of soil acidity, availability of N and P, and changes in community composition and plant nutrition along a short (only 90 m long) but extreme forest productivity gradient in northern Sweden, which ranged from a groundwater recharge area with low productivity to a very productive discharge area. The pH in the soil solution of the mor layer ranged from 3.5 in the recharge area to 6.4 in the discharge area, and it was strongly correlated with the base saturation of the exchange complex. Neither the acid strength of organic matter, the ionic strength of the soil solution, nor the quantity of acids could explain more than a minor part of this variation in pH. There were strong correlations between total N in the mor layer and soil solution pH (r = 0.97) and base saturation of the exchange complex (r = 0.88). At the poor end of the transect the concentration of inorganic N was very low in the mor, and plants with either ectomycorrhizae (ECM) or ericoid mycorrhizae (EM) dominated. With increasing pH, there was an increase in NH4 concentrations, while plants that potentially have arbuscular mycorrhizae (AM) became prominent along with ECM and EM species. In the discharge area, which comprised only the last 10 m of the transect, NO3 dominated over NH4 in the soil solution, the soil had a high capacity for net nitrification, and the vegetation was totally dominated by potentially AM or nonmycorrhizal herbs, some of which had high foliar nitrate reductase activity. Foliar and root N concentrations increased steeply towards the discharge area, but foliar P/N ratios declined below critical levels at the end of the transect. Root 32P uptake bioassays also indicated a P deficiency in the discharge area, where the soil total P content was high, while the concentration of PO4 in the soil solution was very low. The high capacity of the mor in the discharge area to adsorb PO4, due to the presence of organically complexed Fe and Fe-oxihydroxides, may explain the low PO4 concentrations. Our data indicate that the underlying factors influencing both productivity and community composition are pH and supply of base cations. Fundamental differences in exchange characteristics of soil and soil water underlie other related nutrient supply features, in particular the amount and availability of N. Our study of a single short topographic transect supported a previous suggestion based on a regional survey in Norway that variability in soil pH and the supply of base cations affects plant productivity and community composition via effects on N supply. Our data also encompass the interrelations between soil pH, soil N turnover, and the mycorrhizal type of dominant plant species, which, according to Read (1991), occur along long latitudinal or altitudinal climatic gradients. Through millennia discharge areas like the one observed by us have probably provided a relatively stable environment for plants demanding high soil pH and N supply, at the same time as surrounding recharge areas have been acidified naturally through podzolization.

Induction of nitrate reductase activity by nitric acid vapor in California black oak (Quercus kelloggii), canyon live oak (Quercus chrysolepis), and ponderosa pine (Pinus ponderosa) seedlings

Gaseous nitric acid (HNO3), one of the most abundant nitrogenous air pollutants in photochemical smog, is a major contributor to airborne N deposited in southern California forests. Mechanisms of HNO3 uptake by plants, including southern California forest trees, are not well established. In a controlled greenhouse study, California black oak (Quercus kelloggii Newberry), canyon live oak (Quercus chrysolepis Liebm.), and ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) seedlings were exposed to HNO3 vapor at concentrations ranging between 65 and 80 ppb under light conditions. Nitrate reductase (NR) activity was measured in foliage and roots of seedlings after 24 h of exposure. Significant increase in foliar NR activity was observed in all species after exposure to HNO3 vapor, but no significant differences were observed between the species. NR activities in the exposed foliage of California black oak, canyon live oak, and ponderosa pine were significantly higher than in the control foliage. Activity of the enzyme in plant roots was low, and no evidence of differences between species or between control and fumigated plants was found. This study indicated that during episodes of high HNO3 concentrations, trees incorporated and metabolized substantial amounts of the pollutant in their foliage. This mechanism, in addition to root uptake of the dry and wet deposited N compounds, may provide plants with an ample supply of N.

Modulation of carbon and nitrogen metabolism, and of nitrate reductase, in untransformed and transformed Nicotiana plumbaginifolia during CO 2 enrichment of plants grown in pots and in hydroponic culture

Transformed plants of Nicotiana plumbaginifolia Viv. constitutively expressing nitrate reductase (35S-NR) or β-glucuronidase (35S-GUS) and untransformed controls were grown for two weeks in a CO2-enriched atmosphere. Whereas CO2 enrichment (1000 μl · l−1) resulted in an increase in the carbon (C) to nitrogen (N) ratio of both the tobacco lines grown in pots with vermiculite, the C/N ratio was only slightly modified when plants were grown in hydroponic culture in high CO2 compared to those grown in air. Constitutive nitrate reductase (NR) expression per se did not change the C/N ratio of the shoots or roots. Biomass accumulation was similar in both types of plant when hydroponic or pot-grown material, grown in air or high CO2, were compared. Shoot dry matter accumulation was primarily related to the presence of stored carbohydrate (starch and sucrose) in the leaves. In the pot-grown tobacco, growth at elevated CO2 levels caused a concomitant decrease in the N content of the leaves involving losses in NO− 3 and amino acid levels. In contrast, the N content and composition were similar in all plants grown in hydroponic culture. The 35S-NR plants grown in air had higher foliar maximum extractable NR activities and increased glutamine levels (on a chlorophyll or protein basis) than the untransformed controls. These increases were maintained following CO2 enrichment when the plants were grown in hydroponic culture, suggesting that an increased flux through nitrogen assimilation was possible in the 35S-NR plants. Under CO2 enrichment the NR activation state in the leaves was similar in all plants. When the 35S-NR plants were grown in pots, however, foliar NR activity and glutamine content fell in the 35S-NR transformants to levels similar to those of the untransformed controls. The differences in NR activity between untransformed and 35S-NR leaves were much less pronounced in the hydroponic than in the pot-grown material but the difference in total extractable NR activity was more marked following CO2 enrichment. Foliar NR message levels were decreased by CO2 enrichment in all growth conditions but this was much more pronounced in pot-grown material than in that grown hydroponically. Since β-glucuronidase (GUS) activity and message levels in 35S-GUS plants grown under the same conditions of CO2 enrichment (to test the effects of CO2 enrichment on the activity of the 35S promoter) were found to be constant, we conclude that NR message turnover was specifically accelerated in the 35S-NR plants as well as in the untransformed controls as a result of CO2 enrichment. The molecular and metabolic signals involved in increased NR message and protein turnover are not known but possible effectors include NO3 −, glutamine and asparagine. We conclude that plants grown in hydroponic culture have greater access to N than those grown in pots. Regardless of the culture method, CO2 enrichment has a direct effect on NR mRNA stability.

Nitrate reduction and nitrate content in ash trees (Fraxinus excelsior L.): distribution between compartments, site comparison and seasonal variation

Nitrate reductase activity (NRA), nitrate content and biomass components of leaflets, leaf stalks, old stem, current-year stem and roots of ash trees (Fraxinus excelsior L.) growing in their natural habitats were investigated. In addition, NRA, total nitrogen and nitrate concentration were analyzed in the leaves and roots of ash trees from four different field sites. The highest NRA per gram biomass and also per total compartment biomass was found in the leaflets, even though root biomass was much higher than total leaflet biomass. The highest nitrate concentrations were found in the leaf stalks. Correlations between nitrate availability in the soil and NRA in leaves were not significant due to high variability of the actual soil nitrate concentrations. The seasonal variation in foliar NRA, nitrate concentration and total nitrogen concentration is much smaller in F. excelsior than reported for herbaceous species and is mainly caused by changes in the actual soil nitrate availability and by senescence of the leaves.

Seasonal variation in nitrate reductase activity in needles of high-elevation red spruce trees

To assess seasonal and site variation in foliar nitrate reductase activity and its utility as a biochemical marker for the uptake of nitrogen oxide pollutants in high-elevation forests, we measured nitrate reductase activity in current-year needles of red spruce (Picearubens Sarg.) saplings at two high-elevation stands (1935 and 1720 m) in the Great Smoky Mountains, North Carolina. Measurements spanned two growing seasons between September 1987 and September 1988. Nitrate reductase activity peaked near 60 nmol•g−1•h−1 at both sites in September and October 1987 and August 1988 and declined 80% in November 1987 and 65% in September 1988. Although nitrate reductase activity was 30% greater in saplings at the higher site relative to the lower site in September and October 1987, activity dropped to approximately 10 nmol•g−1•h−1 at both sites in November 1987. No differences among sites were evident the following year. Comparing deposition of nitric acid vapor at a nearby site to nitrate reductase activity suggests that needle nitrate reductase activity is not an unequivocal marker for foliar uptake of nitrogen oxides during air pollutant episodes. The changes in soil nitrate levels in this system provide preliminary evidence that foliar nitrate assimilation may, in part, include nitrate taken up from the soil, as the highest activity occurred during periods of higher A-horizon nitrate concentrations in 1988. These measurements of nitrate reductase activity suggest that red spruce are capable of assimilating nitrate in foliage in the field and that the nitrate assimilation capacity varies throughout the year.

Nitrogen source effects on Al toxicity in nonmycorrhizal and mycorrhizal pitch pine (Pinus rigida) seedlings. II. Nitrate reduction and NO3− uptake

Nitrogen source utilization, mediated by the ectomycorrhizal symbiont Pisolithus tinctorius, may modulate Al toxicity in pitch pine (Pinus rigida) seedlings. Nitrate reduction, occurring primarily in the roots of pitch pine seedlings, represents a critical metabolic pathway that may be directly sensitive to Al or indirectly affected by changes in NO3− availability associated with Al exposure or mycorrhizal infection. To investigate these possibilities, pitch pine seedlings were grown in sand culture with NO3−, NH4NO3, or NH4+ and exposed to 0 or 200 μM Al for 6 weeks. Foliar N concentrations, root nitrate reductase (NR) activity, and Al inhibition of NR activity were highly dependent on the proportion of NO3− of the nutrient solution. The association of Pisolithus tinctorius with seedling roots reduced both root and foliar NR activity compared with non-inoculated controls, suggesting that NO3− uptake and translocation to foliage was reduced by the symbiont. This was confirmed by using 36ClO3− to measure unidirectional plasma membrane NO3− fluxes. Mycorrhizal root tips absorbed 50% less NO3− than nonmycorrhizal root tips. Preferential use of NH4+ by ectomycorrhizal roots may thus result in reduced movement of Al into root tissue and amelioration of Al toxicity. Key words: Pinus rigida, ectomycorrhizae, aluminum toxicity, nitrogen source.

Occurrence of foliar nitrate reductase activity not induced by nitrate in symbiotic nitrogen-fed black alder (Alnus glutinosa)

Occurrence of foliar nitrate reductase activity not induced by nitrate in symbiotic nitrogen-fed black alder (Alnus glutinosa)

Interrelationship Between Plant Developmental Stage, Plant Growth Rate, Nitrate Utilization and Nitrogen Fixation in Hydroponically Grown Soybean

The growth rate of the indeterminate soybean plant [Glycine max (L.)Merr.] slows as it proceeds from vegetative phase into reproductive growth. Yet, the well-nodulated plant acquires most of its nitrogen during reproductive growth. Thus, the interrelationship between plant developmental stage and nitrogen fixation was examined. It is shown that, regardless of the age of the hydroponically grown soybean plant at the time of its inoculation with Bradyrhizobium japonicum, the highest rate of nitrogen fixation occurs during the pod-filling stage (R5). Nevertheless, maximum total nitrogen fixation is generally achieved when inoculation occurs at the full-bloom stage (R2). It is shown, however, that flower buds and flowering are not responsible for the enhanced nodulation and nitrogen fixation. Rather, the data suggest that the onset of rapid nodulation occurs soon after the initiation of the developmentally programmed drop in foliar nitrate reductase activity. The ensuing increase in nitrogen fixation provides the plant with much of its needed nitrogen and hence stimulates plant mass accumulation during pod-fill. It is suggested that nitrogen fixation enhances growth of the soybean plant by increasing its net photosynthetic efficiency during reproductive growth and by providing the needed nitrogen at the appropriate time for maximum seed growth.

Nitrate metabolism in Scots pine seedlings during their first growing season.

Fluctuations in nitrate reductase activity (NRA), nitrate, nitrite, protein and total nitrogen content of bare-root Scots pine seedlings (Pinus sylvestris L.) raised outdoors were investigated during the first growing season. Nitrate reductase activity was higher in roots than in needles, whereas NO(3) (-) content was higher in needles than in roots and increased in both from June to October. Nitrate reductase activity in roots correlated more closely with NO(3) (-) N in the soil than did NO(3) (-) in the roots. In autumn, there was a closer correlation between foliar NRA and NO(3) (-) in the needles than with NO(3) (-)-N in the soil. Nitrite was not detected in the seedlings during the growing season. Total nitrogen content decreased toward the autumn, whereas protein content initially decreased but increased again in autumn. Acrylic netting placed above the seedlings increased both air and soil temperatures and apparently accelerated the use of nitrate.

Nitrate reductase activity in leaves and roots ofAlnus glutinosa (L) Gaertner

Thein vivo nitrate reductase activity (NRA) was determined inAlnus glutinosa plants grown nonsymbiotically on ammonium, nitrate, a combination of both, or symbiotically with atmospheric nitrogen as the only nitrogen source. Root NRA was absent when ammonium or atmospheric nitrogen was the nitrogen source. With nitrate in the culture solution the roots showed a high NRA. However, the leaf NRA behaved quite differently: with negligible activities on all nitrogen sources except atmospheric nitrogen. The foliar NRA measured, however, is likely not due to the activity of the plant but of microbial origin. Methods commonly used to facilitate produced nitrite to leak out of the tissue, such as addition of propanol and cutting the plant material, did not increase the nitrite release from the leaves. A turbidity developed when testing the samples for nitrite which was positively correlated with the NRA. Populations of microorganisms in the phyllosphere did not differ between the nutritional treatments. Bacteria, able to grow on a low-nitrogen medium, were present on the leaves. Nitrifiers could not be detected. The bacteria on the leaves appear to produce nitrite when incubated with leaf material.

Estudos sobre a nutrição mineral do arroz: XXVI. Avaliação do estado nutricional da variedade Dourado Precoce

A variedade Dourado Precoce foi cultivada em solução nutritiva completa e com deficiência de macronutrientes.A produção de matéria seca total, avaliada antes do fim do ciclo, não foi diminuida pela carência de Ca, Mg e S. De um modo geral os teores encontrados na matéria seca foram maiores no tratamento completo. Através de diagnose foliar feita no perfilhamento estabeleceram-se níveis adequados e deficientes de macronutrientes. Foi verificado que os testes rápidos notecido foliar para N-NO3-, P-H2PO4 e K+ solúvel dão indicações do estado nutricional. A atividade da redutase de nitrato foliar foi reduzida significativamente pela deficiência de todos os elementos.