Leaf NO3 Research Articles

Effect of different day and night nutrient solution concentrations on growth, photosynthesis, and leaf NO3- content of aeroponically grown lettuce

Nitrate content in leafy green vegetables has raised concerns among consumers and policy makers worldwide. Several cultural practices have been evaluated to manipulate NO3- content in fresh leaves with varying degrees of success. The present study was conducted to evaluate different concentrations of the nutrient solution applied during the day (D) and night (N) to aeroponically grown lettuce (Lactuca sativa L.) in Davis, California, USA, in the spring of 2012 with the objective of assessing the effect on growth, leaf photosynthesis, and nitrate accumulation in leaves. Two different treatments in the nighttime solution concentration (D25/N75, EC: 1.8 dS m-1; and D25/N50, EC: 1.2 dS m-1), a day nutrient solution of EC 0.6 dS m-1, plus a day and night treatment with constant EC (D50/N50, EC: 1.2 dS m-1) were applied. Plant growth, leaf photosynthesis, and leaf nutrient content were evaluated after 3 wk of growth. Mean shoot weight was 106.3 g with no differences among treatments. Root biomass was lower with D25/N75 (0.14 vs. 0.85 g in the other treatments). The maximum rate of leaf photosynthesis was 66% lower with D25/N75 than in the other treatments. Nitrogen, P, K, Ca, and Mg were lower in leaf tissue in the treatments with different solution concentrations where leaf NO3- content was reduced by approximately 75%. Switching nutrient solution concentration between day and night is a viable practice to reduce NO3- in lettuce leaves with no detriment to leaf production.

Alternative strategies for nitrogen fertilization of overwinter processing spinach (Spinacia oleracea L.) in Southern Italy

To identify the best practice for nitrogen (N) fertilization of overwinter processing spinach, two field experiments were carried out in the Foggia plain (Southern Italy), one of the most vocated area for leafy vegetables production. The field trials were aimed to define and suggest the proper fertilizer dose, typology and the right time of application. Experiment 1 evaluated four N fertilizer doses (0, 150, 225, 300kgha−1) in a two-year field trial. Experiment 2 was aimed to assess the effect of the split distribution of prilled urea fertilizer in comparison with the application of nitrification inhibitor (DMPP) containing urea fertilizer, broadcasted at sowing.Spinach yield, yield quality (nitrate – NO3− – and carotenoids content), N-use efficiency and risk of soil nitrate (NO3−-N) leaching were evaluated. The processing spinach yielded 37.8 and 3.6tha−1 of fresh and dry yield, respectively (average of the two experiments). Fresh and dry yield among the fertilizing treatments were similar. Also the β-carotene and the lutein content of spinach leaves (19.5 and 38.1mgkg−1, respectively) were not affected by the N fertilizer dose. Conversely, the N dose strongly influenced the NO3− content of the leafy vegetable tissues (1286mgkg−1 on average, 58% lower than the limits imposed by the EC regulation). As expected, the different rainfall pattern influenced both the leaf NO3− content and the risk of soil NO3−-N leaching. The results achieved demonstrated that, in order to get a favorable trade-off, among yield, yield quality, N-use efficiency and environmental impact, the processing spinach growers of the Foggia plain area should be encouraged to apply 225kgNha−1 as maximum fertilization rate. Also, the split urea fertilizer application appeared as the more effective strategy for N fertilization of overwinter spinach in comparison with the use of the nitrification inhibitor containing urea fertilizer, being the last strategy not able to adequately match the N crop demand.

Regulation of hormonal responses of sweet pepper as affected by salinity and elevated CO2 concentration

This study examines the extent to which the predicted CO2 -protective effects on the inhibition of growth, impairment of photosynthesis and nutrient imbalance caused by saline stress are mediated by an effective adaptation of the endogenous plant hormonal balance. Therefore, sweet pepper plants (Capsicum annuum, cv. Ciclón) were grown at ambient or elevated [CO2] (400 or 800 µmol mol(-1)) with a nutrient solution containing 0 or 80 mM NaCl. The results show that, under saline conditions, elevated [CO2] increased plant dry weight, leaf area, leaf relative water content and net photosynthesis compared with ambient [CO2], whilst the maximum potential quantum efficiency of photosystem II was not modified. In salt-stressed plants, elevated [CO2 ] increased leaf NO3(-) concentration and reduced Cl(-) concentration. Salinity stress induced ABA accumulation in the leaves but it was reduced in the roots at high [CO2], being correlated with the stomatal response. Under non-stressed conditions, IAA was dramatically reduced in the roots when high [CO2] was applied, which resulted in greater root DW and root respiration. Additionally, the observed high CK concentration in the roots (especially tZR) could prevent downregulation of photosynthesis at high [CO2], as the N level in the leaves was increased compared with the ambient [CO2], under salt-stress conditions. These results demonstrate that the hormonal balance was altered by the [CO2], which resulted in significant changes at the growth, gas exchange and nutritional levels.

Role of Grafting in Resistance to Water Stress in Tomato Plants: Ammonia Production and Assimilation

In general, drought depresses nutrient uptake by the root and transport to the shoot due to a restricted transpiration rate, which may contribute to growth limitation under water deprivation. Moreover, water stress may also restrict the ability of plants to reduce and assimilate nitrogen through the inhibition of enzymes implicated in nitrogen metabolism. The assimilation of nitrogen has marked effects on plant productivity, biomass, and crop yield, and nitrogen deficiency leads to a decrease in structural components. Plants produce significant quantities of NH4 + through the reduction of NO3 − and photorespiration, which must be rapidly assimilated into nontoxic organic nitrogen compounds. The aim of the present work was to determine the response of reciprocal grafts made between one tomato tolerant cultivar (Lycopersicon esculentum), Zarina, and a more sensitive cultivar, Josefina, to nitrogen reduction and ammonium assimilation under water stress conditions. Our results show that when cv. Zarina (tolerant cultivar) was used as rootstock grafted with cv. Josefina (ZarxJos), these plants showed an improved N uptake and NO3 − assimilation, triggering a favorable physiological and growth response to water stress. On the other hand, when Zarina was used as the scion (JosxZar), these grafted plants showed an increase in the photorespiration cycle, which may generate amino acids and proteins and could explain their better growth under stress conditions. In conclusion, grafting improves N uptake or photorespiration, and increases leaf NO3 − photoassimilation in water stress experiments in tomato plants.

Induced leaf intercellular CO2, photosynthesis, potassium and nitrate retention and strawberry early fruit formation under macronutrient limitation

Relationships between induced high leaf intercellular CO₂ concentrations, leaf K⁺ and NO₃⁻ ion movement and early fruit formation under macronutrient limitation are not well understood. We examined the effects and interactions of reduced K/N input treatments on leaf intercellular CO₂, photosynthesis rate, carboxylation and water use efficiency, berry formation as well as leaf/fruit K⁺, NO₃⁻ and photosynthate retention of strawberry (Fragaria × ananassa Duch.) to enhance low-input agriculture. The field study was conducted in Nova Scotia, eastern Canada during 2009-2010. The experimental treatments consisted of five K₂O rates (0, 6, 12, 18, and 24 kg ha(-1)) and five N rates (0, 5, 10, 15, and 20 kg ha(-1)), representing respectively, 0, 25, 50, 75, and 100 % of regular macronutrient recommendations based on the soil testing. The treatments were arranged in a split-plot design with three blocks in the field. The cultivar was 'Mira', a June-bearing crop. The results showed that strawberry plants treated with 25 %-reduced inputs could induce significantly higher leaf intercellular CO₂ concentrations to improve plant photosynthesis, carboxylation and water use efficiency and translocation of leaf/fruit K⁺ and dissolved solids, which could advance berry formation by 6 days and produce significantly higher marketable yields (P < 0.05). Higher leaf intercellular CO₂ inhibited leaf/fruit NO₃⁻ ion retention, but this inhibition did not occur in leaf/fruit K⁺ retention. Linear interactions of the K/N treatments were significant on fruit marketable yields, intercellular CO₂, net photosynthesis, leaf transpiration rates, and leaf temperatures (P < 0.05). It was concluded that higher leaf CO₂ could enhance plant photosynthesis, promote plant carboxylation and water use efficiency, and advance berry formation, but it could inhibit leaf NO₃⁻ retention. This inhibition did not find in leaf K⁺ ion and dissolved solid retention. Overlay co-limitation of leaf intercellular CO₂ and translocation of leaf/fruit K(+)/NO₃ (-) and total dissolved solids could constrain more fruit formation attributes under full macronutrient supply than reduced inputs. It was suggested that low input would be an optimal and sustainable option for improving small fruit crop physiological development and dealing with macronutrient deficiency challenge.

Effect of salinity on growth, ion accumulation and the roles of ions in osmotic adjustment of two populations of Suaeda salsa

The effect of salinity on growth, ion accumulation and the roles of ions in osmotic adjustment of two populations of Suaeda salsa were investigated. Seeds were collected from an intertidal zone or a saline inland zone in the Yellow River Delta in Shandong province, China. Seedlings were exposed to 10, 100, 200, 400 or 600 mM NaCl for 18 days in a greenhouse. NO3 − concentration in the soil where S. salsa grows in an intertidal zone was much lower than that for the second population, but leaf NO3 − concentration was the same in the two populations under field conditions. When plants were cultured in a greenhouse under natural light conditions, S. salsa from the intertidal zone showed fewer main stem branches and lower relative shoot growth compared to S. salsa from saline inland. Leaf Cl− concentration of saline inland S. salsa was significantly higher than that of S. salsa from the intertidal zone, while the opposite was true for the concentration of NO3 − in leaves of plants. For S. salsa from the intertidal zone NO3 − contributed more than Cl− to the osmotic potential, whereas S. salsa from the saline inland exhibited a reverse relationship under saline conditions, indicating that NO3 − plays an important osmotic role in S. salsa from the intertidal zone in high salinity. In conclusion, S. salsa from the intertidal zone may employ superior control of ion uptake and content than S. salsa from the saline inland zone. The two populations of Suaeda salsa presented different ability in chloride exclusion and nitrate accumulation. These characteristics may affect the distributions of S. salsa in natural highly saline environments.

Vegetative growth, superoxide dismutase activity and ion concentration of salt-stressed watermelon as influenced by rootstock

SUMMARYWatermelon is a crop with a high water demand and is frequently grown under conditions of higher than normal root-zone salinity. In the present study, seedlings of watermelon (cv. Fantasy, Citrullus lanatus (Thunb.) Matsum &amp; Nakai) were grown either ungrafted or grafted on three rootstocks: Strong Tosa, S1 (both Cucurbita maxima×Cucurbita moschata), or Emphasis (Lagenaria siceraria). All the plants were exposed to an NaCl-induced salinity stress (electrical conductivity, EC=2·2, 4·0, or 6·0 dS/m). The vegetative growth of all the plants substantially reduced after 2 weeks of exposure to 6·0 dS/m; however, growth of the plants grafted on Strong Tosa reduced less than that of the others. The leaf water content and specific leaf area (SLA, m2/g) decreased with an increasing salinity in grafted plants, but not in ungrafted plants. Salinity induced an increase of superoxide dismutase (SOD) activity in grafted plants up to two-fold depending on the rootstock, whereas it had no effect on this enzyme activity in ungrafted plants. Leaf Na+ concentration increased with increasing salinity in ungrafted and S1 grafted plants, whereas there was no significant leaf Na+ accumulation in Emphasis and Strong Tosa grafted plants. Leaf K+ concentration was affected by the rootstock but not by salinity, thus, the ability to keep a high K+/Na+ ratio was achieved mainly by limiting leaf Na+ concentration. The rootstock determined the leaf Cl− accumulation, with lower overall concentrations found if plants were grafted on the S1 rootstock than on Emphasis or ungrafted plants. Salinity significantly decreased the leaf NO3− concentration on Emphasis grafted plants only, while the NO3−/Cl− ratio was reduced in all the rootstocks. The capacity of Strong Tosa to withstand salt stress better than other tested rootstocks was probably due to the ability to induce anatomical adaptation (SLA) and SOD activity in response to salt stress, and also to the efficiency of Na+ exclusion from the shoot.

Chronic Atmospheric NO 3 − Deposition Does Not Induce NO 3 − Use by Acer saccharum Marsh.

The ability of an ecosystem to retain anthropogenic nitrogen (N) deposition is dependent upon plant and soil sinks for N, the strengths of which may be altered by chronic atmospheric N deposition. Sugar maple (Acer saccharum Marsh.), the dominant overstory tree in northern hardwood forests of the Lake States region, has a limited capacity to take up and assimilate NO 3 − . However, it is uncertain whether long-term exposure to NO 3 − deposition might induce NO 3 − uptake by this ecologically important overstory tree. Here, we investigate whether 10 years of experimental NO 3 − deposition (30 kg N ha−1 y−1) could induce NO 3 − uptake and assimilation in overstory sugar maple (approximately 90 years old), which would enable this species to function as a direct sink for atmospheric NO 3 − deposition. Kinetic parameters for NH 4 + and NO 3 − uptake in fine roots, as well as leaf and root NO 3 − reductase activity, were measured under conditions of ambient and experimental NO 3 − deposition in four sugar maple-dominated stands spanning the geographic distribution of northern hardwood forests in the Upper Lake States. Chronic NO 3 − deposition did not alter the V max or K m for NO 3 − and NH 4 + uptake nor did it influence NO 3 − reductase activity in leaves and fine roots. Moreover, the mean V max for NH 4 + uptake (5.15 μmol 15N g−1 h−1) was eight times greater than the V max for NO 3 − uptake (0.63 μmol 15N g−1 h−1), indicating a much greater physiological capacity for NH 4 + uptake in this species. Additionally, NO 3 − reductase activity was lower than most values for woody plants previously reported in the literature, further indicating a low physiological potential for NO 3 − assimilation in sugar maple. Our results demonstrate that chronic NO 3 − deposition has not induced the physiological capacity for NO 3 − uptake and assimilation by sugar maple, making this dominant species an unlikely direct sink for anthropogenic NO 3 − deposition.

A Role for Shoot Protein in Shoot–Root Dry Matter Allocation in Higher Plants

It is stated in many recent publications that nitrate (NO3-) acts as a signal to regulate dry matter partitioning between the shoot and root of higher plants. Here we challenge this hypothesis and present evidence for the viewpoint that NO3- and other environmental effects on the shoot:root dry weight ratio (S:R) of higher plants are often related mechanistically to changes in shoot protein concentration. The literature on environmental effects on S:R is reviewed, focusing on relationships between S:R, growth and leaf NO3- and protein concentrations. A series of experiments carried out to test the proposal that S:R is dependent on shoot protein concentration is highlighted and new data are presented for tobacco (Nicotiana tabacum). KEY RESULTS/EVIDENCE: Results from the literature and new data for tobacco show that S:R and leaf NO3- concentration are not significantly correlated over a range of environmental conditions. A mechanism involving the relative availability of C and N substrates for growth in shoots can explain how shoot protein concentration can influence shoot growth and hence root growth and S:R. Generally, results in the literature are compatible with the hypothesis that macronutrients, water, irradiance and CO2 affect S:R through changes in shoot protein concentration. In detailed studies on several species, including tobacco, a linear regression model incorporating leaf soluble protein concentration and plant dry weight could explain the greater proportion of the variation in S:R within and between treatments over a wide range of conditions. It is concluded that if NO3- can influence the S:R of higher plants, it does so only over a narrow range of conditions. Evidence is strong that environmental effects on S:R are often related mechanistically to their effects on shoot protein concentration.

Nitrate-dependent control of root architecture and N nutrition are altered by a plant growth-promoting Phyllobacterium sp

Both root architecture and plant N nutrition are altered by inoculation with the plant growth-promoting rhizobacteria (PGPR) Phyllobacterium strain STM196. It is known that NO3- and N metabolites can act as regulatory signals on root development and N transporters. In this study, we investigate the possible interrelated effects on root development and N transport. We show that the inhibition of Arabidopsis lateral root growth by high external NO3- is overridden by Phyllobacterium inoculation. However, the leaf NO3- pool remained unchanged in inoculated plants. By contrast, the Gln root pool was reduced in inoculated plants. Unexpectedly, NO3- influx and the expression levels of AtNRT1.1 and AtNRT2.1 genes coding for root NO3- transporters were also decreased after 8 days of Phyllobacterium inoculation. Although the mechanisms by which PGPR exert their positive effects remain unknown, our data show that they can optimize plant development independently from N supply, thus alleviating the regulatory mechanisms that operate in axenic conditions. In addition, we found that Phyllobacterium sp. elicited a very strong induction of AtNRT2.5 and AtNRT2.6, both genes preferentially expressed in the shoots whose functions are unknown.

Effects of NO3- availability on NO3- use in seedlings of three woody shrub species

We determined the effects of short-term cultivation with various amounts of available nitrate nitrogen (NO3-) on NO3- use by woody shrub species. Nitrate concentration ([NO3-]) and nitrate reductase activity (NRA) were measured in leaves and roots of seedlings of Hydrangea hirta (Thunb.) Siebold, Lindera triloba (Sieb. et Zucc.) Blume and Pieris japonica (Thunb.) D. Don. Root [NO3-] increased with increasing NO3- supply in all species, whereas leaf [NO3-] remained low. There were significant correlations between [NO3-] in roots and leaves in all species, but no correlation was found between root NRA and leaf NRA. The low proportion of leaf NO3- assimilation to total NO3- assimilation in all species can be ascribed to the lack of NO3- transport from roots to leaves. In all species, root NRA increased with increasing NO3- supply until reaching a plateau. Species ranking based on maximum root NRA was H. hirta > L. triloba > P. japonica. Root NRA in P. japonica was low, even though root [NO3-] increased with NO3- supply, indicating that NO3- was not an effective N source for this species. The ranking also suggested that H. hirta depended more on NO3- as an N source than L. triloba. The increase in root NRA with increasing NO3- supply was greater in H. hirta than in L. triloba, possibly indicating that a change in NO3- availability has a stronger influence on NO3- use in H. hirta than in L. triloba.

Concentração foliar de molibdênio e exportação de nutrientes pelo feijoeiro "Ouro Negro" em resposta à adubação foliar com molibdênio

Objetivando avaliar os efeitos da aplicação foliar de molibdênio (Mo) na concentração de Mo, N total, N orgânico e nitrato nas folhas e nos grãos e a exportação dos nutrientes pelos grãos do feijoeiro cv. Ouro Negro, realizou-se o experimento, em condições de lavoura, na Universidade Federal de Viçosa. Os tratamentos constituíram-se de doses crescentes de Mo (0, 40, 80 e 120 g ha-1 de Mo) na forma de molibdato de amônio, aplicado em adubação foliar 25 dias após a emergência. Usou-se o delineamento de blocos ao acaso, com quatro repetições. Pelos teores de Mo, de N total e de N orgânico nas folhas e nos grãos, observou-se uma resposta quadrática à aplicação de doses crescentes de Mo. A aplicação foliar de Mo aumentou a concentração de Mo nas folhas, de 0,49 mg kg-1 (testemunha) para 0,95 mg kg-1 (para a máxima eficiência técnica - MET), o que proporcionou incremento na utilização do N, com maiores teores de N total e de N orgânico nas folhas e nos grãos e plantas com crescimento satisfatório e com folhas de coloração verde-escura. As plantas que não receberam Mo apresentaram teor médio de 26,6 g kg-1 de N e sintomas de deficiência de N. A aplicação de Mo não exerceu efeito na concentração de NO3-, tanto nas folhas (0,39 g kg-1 NO3-) como nos grãos (0,03 g kg-1 NO3-). Considerando a dose de 80,3 g ha-1 de Mo, que correspondeu à MET para a produtividade estimada de 1.893 kg ha-1 de grãos, as quantidades dos nutrientes exportados nos grãos do feijoeiro por hectare foram de 72 kg de N; 15,7 kg de Ca; 15,3 kg de K; 9,4 kg de P; 4 kg de Mg; 102 g de Fe; 63 g de Zn; 25 g de Cu; 23 g de Mn e 3,2 g de Mo.

Inorganic nitrogen acquisition in the seagrass Thalassia testudinum: Development of a whole‐plant nitrogen budget

Whole‐plant nitrogen (N) uptake experiments were used to quantify the N budget of Thalassia testudinum growing under different sediment nutrient regimes at two locations in the western Gulf of Mexico. At both sites, Corpus Christi Bay (CCB) and lower Laguna Madre (LLM), Texas, concurrent measurements of plant biomass and levels of dissolved inorganic nitrogen (DIN) in the water column and sediments were made over a 12–month period (October 1996–October 1997). Water‐column NH4+ and NO3− + NO2‐ concentrations were not significantly different between study sites (ca. 1.2 µM [NH4+] and 0.7 µM [NO3− + NO ] at both sites), but sediment NH4+ concentrations in CCB (87 µM) were significantly higher than in LLM (26 µM). The higher sediment NH4+ levels at CCB correlated with significantly higher leaf biomass at CCB, but there was no difference in root biomass between study sites. Leaf NH4+ uptake showed clear seasonal variation: Vmax was highest in summer and fall, but Km was highest in winter. Vmax of leaf NO3‐ uptake did not change with season, but Km decreased with increasing incubation temperature. There were no clear differences in leaf NH4+ and NO3− uptake rates between study sites, although leaf NH4+ uptake affinity was higher than that of NO3−. Root NH4+ uptake was variable with season and did not saturate at the experimental NH4+ concentrations at either site (0–300 µM). Based on these measurements, N acquisition was highest during summer and fall and lowest during winter and spring. Roots and leaves contributed nearly equally to total plant N acquisition (root NH4+ = 52%; leaf NH4+ = 38%; and leaf NO3− = 10%) at both sites. Annual N acquisition in CCB was double that of LLM (97.03 and 53.49 g N m−2 yr−1, respectively), but &gt;50% of N uptake was not incorporated into biomass at either site. DIN turnover time ranged from 0.21 to 0.91 d in the water column and from 0.95 to 1.75 d in sediment pore water, indicating the importance of DIN regeneration processes for supporting seagrass production. The similarity in the relative tissue contributions between plants at both sites, despite a significant difference in sediment NH4+ pool sizes, results from the higher fraction of biomass allocated to below‐ground tissues in plants living under low‐sediment N conditions (LLM). In N‐sufficient sediments, overall plant productivity is greater as T. testudinum is able to allocate a greater proportion of its biomass into photosynthetic aboveground tissues.

Comparison between nitrate and ammonium nutrition in fennel, celery, and Swiss chard

To evaluate the chance to reduce leaf NO3 content and to increase capability to use NH4‐N even in the absence of NO3‐N in the nutrient solution, plants of two Apiaceae species, fennel (Foeniculum vulgare Miller var. azoricum Mill. Thell.) and celery (Apium graveolens L. var. dulce Mill. Pers.), and of one species of Chenopodiaceae, Swiss chard (Beta vulgaris L. var. vulgaris), were hydroponically grown in a growth chamber with three different NH4‐N: NO3‐N (NH4: NO3) ratios (100: 0,50: 50, and 0: 100), but with the same total N level (4 mM) for 14 days. Swiss chard growth was inhibited by NH4 nutrition and reached the highest values with the NH4: NO3 ratio 0: 100. For all the morphological and yield features analyzed, fennel and celery resulted to be quite unresponsive to nitrogen (N) chemical form. Water use efficiency increased in Swiss chard and decreased in fennel and celery with the increase of NO3‐N percentage in the nutrient solution. The dependency of N uptake rate on shoot increment per unit root was more conspicuous for Swiss chard than fennel and celery. All species took more NO3‐N than NH4‐N when N was administered in mixed form. In the best conditions of N nutrition, Swiss chard accumulated NO3 in leaves in high concentration (3,809 mg kg"1 fresh mass). On average, fennel and celery accumulated 564 mg NO3 kg−1 fresh mass with the ratio NH4: NO3100: 0 and showed that by using NH4 produce having very low NO3 content can be obtained. By increasing NO3‐N percentage in the nutrient solution; NO3 leaf content of fennel and celery increased remarkably (7,802 mg kg−1 fresh mass with the ratio N H4: NO3 0: 100).

Nitrogen relations of natural and disturbed plant communities in tropical Australia.

Nitrogen relations of natural and disturbed tropical plant communities in northern Australia (Kakadu National Park) were studied. Plant and soil N characteristics suggested that differences in N source utilisation occur at community and species level. Leaf and xylem sap N concentrations of plants in different communities were correlated with the availability of inorganic soil N (NH+4 and NO-3). In general, rates of leaf NO-3 assimilation were low. Even in communities with a higher N status, including deciduous monsoon forest, disturbed wetland, and a revegetated mine waste rock dump, levels of leaf nitrate reductase, xylem and leaf NO-3 levels were considerably lower than those that have been reported for eutrophic communities. Although NO-3 assimilation in escarpment and eucalypt woodlands, and wetland, was generally low, within these communities there was a suite of species that exhibited a greater capacity for NO-3 assimilation. These "high- NO-3 species" were mainly annuals, resprouting herbs or deciduous trees that had leaves with high N contents. Ficus, a high-NO-3 species, was associated with soil exhibiting higher rates of net mineralisation and net nitrification. "Low-NO-3 species" were evergreen perennials with low leaf N concentrations. A third group of plants, which assimilated NO-3 (albeit at lower rates than the high-NO-3 species), and had high-N leaves, were leguminous species. Acacia species, common in woodlands, had the highest leaf N contents of all woody species. Acacia species appeared to have the greatest potential to utilise the entire spectrum of available N sources. This versatility in N source utilisation may be important in relation to their high tissue N status and comparatively short life cycle. Differences in N utilisation are discussed in the context of species life strategies and mycorrhizal associations.

Nitrate and ammonium nutrition in chicory and rocket salad plants

To evaluate chicory (Cichorium intybus L.) and rocket salad [Eruca vesicaria (L.) Cav.subsp. sativa (Mill.)] capability to use ammonium‐nitrogen (NH4‐N) even in the absence of nitrate‐nitrogen (NO3‐N) in the nutrient solution, and the chances they offer to reduce leaf NO3 content, cultivated rocket and two cultivars of chicory ('Frastagliata’, whose edible parts are leaves and stems, and ‘Clio’, a leaf hybrid) were hydroponically grown in a growth chamber. Three nutrient solutions with the same nitrogen (N) level (4 mM) but a different NH4‐N:NO3‐N (NH4:NO3) ratio (100:0, 50:50, and 0:100) were used. Rocket growth was inhibited by NH4 nutrition, while it reached the highest values with the NH4:NO3 ratio 50:50. Water and N‐use efficiencies increased in rocket with the increase of NO3‐N percentage in the nutrient solution. In the best conditions of N nutrition, however, rocket accumulated NO3 in leaves in a very high concentration (about 6,300 mg kg‐1 fresh mass). For all the morphological and yield features analyzed, chicory resulted to be quite unresponsive to N chemical forms, despite it took more NO3‐N than NH4‐N when N was administered in mixed form. By increasing NO3‐N percentage in the nutrient solution, NO3 leaf content increased (5,466 mg kg‐1 fresh mass with the ratio NH4:NO3 0:100). On average, both chicory cultivars accumulated 213 mg NO3 kg‐1 fresh mass with the ratio NH4:NO3 100:0 and, differently from rocket, they showed that by using NH4 produce can be obtained very low in NO3 content.

Abolition of Posttranscriptional Regulation of Nitrate Reductase Partially Prevents the Decrease in Leaf NO3- Reduction when Photosynthesis Is Inhibited by CO2 Deprivation, but Not in Darkness.

The activity of nitrate reductase (NR) in leaves is regulated by light and photosynthesis at transcriptional and posttranscriptional levels. To understand the physiological role of these controls, we have investigated the effects of light and CO2 on in vivo NO3- reduction in transgenic plants of Nicotiana plumbaginifolia lacking either transcriptional regulation alone or transcriptional and posttranscriptional regulation of NR. The abolition of both levels of NR regulation did not modify the light/dark changes in exogenous 15NO3- reduction in either intact plants or detached leaves. The same result was obtained for 15N incorporation into free amino acids in leaves after 15NO3- was supplied to the roots, and for reduction of endogenous NO3- after transfer of the plants to an N-deprived solution. In the light, however, deregulation of NR at the posttranscriptional level partially prevented the inhibition of leaf 15NO3- reduction resulting from the removal of CO2 from the atmosphere We concluded from these observations that in our conditions deregulation of NR in the transformants investigated had little impact on the adverse effect of darkness on leaf NO3- reduction, and that posttranscriptional regulation of NR is one of the mechanisms responsible for the short-term coupling between photosynthesis and leaf NO3- reduction in the light.

Changes in nitrate accumulation and growth of endive plants during light period as affected by nitrogen level and form1

Endive plants (Cichorium endivia L. var. crispum Hegi) were hydroponically grown in a growth chamber to assess, 7 and 14 days after beginning of treatments (DAT), the variation of leaf nitrate (NO3) accumulation and growth during the light period in relation to both nitrogen (N) availability (8 and 16 mM) and form percentage ratio [50:50 and 0:100 ammonium (NH4):NO3] in the nutrient solution. Both 7 and 14 DAT, fresh weight and leaf area linearly increased during the light period without influence of N treatments. Dry matter (d.m.) percentage increased over the 12 hours of light. Leaf NO3 concentration decreased during the light period more with 50:50 than with 0:100 NH4:NO3 ratio: 7 DAT NO3 decreased by 25% with the former N treatment and only by 4% with the latter. After 7 days more, the difference between the two N form ratios was more evident: NO3 decreased by 12% with the mixed N form, while increased by 10% with 100% NO3‐N in the nutrient solution. Nitrate leaf concentration was higher with 16 mM compared to 8 mM of NO3‐N (11.3 and 8.5 g/100 g of d.m., respectively, 14 DAT). Total N leaf content was higher with 16 than with 8 mM of NO3‐N in the nutrient solution and was not influenced by the two N form ratios. Chloride (Cl) leaf concentration was higher with 8 that with 16 mM of N and more with the 50:50 than 0:100 NH4:NO3 ratio.

In situnitrate reductase activity in leaves of adult peach trees

SummaryThe inducibility of nitrate reductase (NR) in the leaves of five year old orchard-grown peach trees was investigated using the in situ method for determination of NR activity (NRA) (Robin et al., 1983) The exogenous supply of NO3− to cut young shoots resulted in the induction of leaf in situ NRA. Full induction of the enzyme activity (i.e. increase to values of 2 µmoles NO2− h−1 g−1 FW) was obtained after 6 h on 50 mM exogenous NO3−. Under these conditions, leaf NO3− accumulation reached 30 to 50 µmoles g−1 FW. In leaves harvested from the adult trees in the orchard, and not subjected to the induction treatment, leaf NO3− accumulation was no longer detected, but the addition of 100 mM NO3− in the assay medium for in situ NRA (+NO3−NRA) led to an activity far from negligible (up to 1 µmol NO2− h−1 g−1 FW). These data reveal that although not accumulated in the tissues. NO3− was supplied through the xylem to the leaves of the adult trees, where it induced a noticeable level of NRA. This is interprete...

Labeled‐Nitrate Assimilation and Nitrogen‐15 Export from Leaves of Burley Tobacco

At harvest, burley tobacco (Nicotiana tabacum L.) leaves frequently contain undesirably high levels of NO3, which are usually greater in older leaves lower in the canopy than in the younger leaves of the upper canopy. Substantial NO3 uptake occurs during the 4‐ wk period between topping (apical bud removal) and harvest. To evaluate N use characteristics of leaves differing in stalk position during this period, the 4th or 10th leaf from the top of field‐grown plants received 15N‐NO3 solution (99 atom % 15N). The 15N‐NO3 was fed to one half of the leaf through a lateral vein in the middle of the leaf. Plants were sampled 1, 3, and 7 d later and separated by leaf position for N analysis. Xylem exudate was collected from the base of the plant at 3 and 7 d after labeling. The 15N‐NO3 was readily reduced at both leaf positions. Reduced 15N accumulation by the uppermost three leaves and stalk was greater in plants fed 15N‐NO3 at the 10th leaf than at the fourth leaf position; however, the amount accumulated was small relative to the amount of reduced 15N retained in labeled leaves. Reduced N in the xylem exudate of plants fed 15N at either leaf position was enriched to 0.16 and 0.078 atom % 15N excess at 3 and 7 d, respectively. These results indicate that, after topping, both upper‐ and mid‐canopy leaves had similar NO3‐reduction capacities and a portion of the newly reduced N was cycled through the root system. Differences in leaf NO3 accumulation may be related to greater N supply and uptake during early plant growth rather than to differences in NO3 assimilation potential of leaves late in the season.