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1.
The heterosporous water-fern genusAzolla is one of the few symbioses with a cyanobacterium in the genusAnabaena. TheAzolla-Anabaena association includes six extant speciesof Azolla, which are widely distributed in relatively placid tropical and/or temperate freshwater environments. The earliest mention of the plant seems to be in an ancient Chinese dictionary that appeared about 2000 years ago.Azolla was used in about the 11th century in Vietnam. By 1980 renewed interest in this symbiotic association was shown by the demand for a less fossil energy-dependent agricultural technology. The importation of a variety ofA. filiculoides may have been a most significant breakthrough for the improvementof Azolla cultivation in China. The history of research may be divided into three periods and a new biotechnological stageof Azolla research has recently begun. Each mature dorsal leaf lobe has an ellipsoid cavity which containsAnabaena azollae throughout its development. HeterocystousA. azollae from sixAzolla species share identical and highly specific antigens.Azolla and its endophyte exhibit a coordinated pattern of differentiation and development. Epidermal hair cells of the host are probably interactive with the symbiont. The interior surface of a mature leaf cavity is lined with an envelope and covered by a mucilaginous layer.A. azollae shares the cavity with small populations of the bacteriaPseudomonas andAzotobacter. Endophyte-freeAzolla may rarely occur in nature and can be generated by aseptic techniques.Anabaena azollae can be isolated fromAzolla fronds by gentle pressure and by enzymatic digestion. The free living cultures derived from theAnabaena so obtained differ in some respects, however, from the freshly extracted symbiont, and might better be called the presumptive isolate. BothAzolla andAnabaena contain specific photosynthetic pigments. The optimum conditions for photosynthesis have been measured.Azolla is a C3 plant and has high net photosynthesis. PSII activity in the symbiont is low. Nitrogenase is localized in the heterocysts of the symbiont and has some advantages compared with free-living cyanobacteria. SymbioticA. azollae has a high frequency of heterocysts. Unidirectional hydrogenase occurs in the symbiont and recycles electrons and ATP. Simultaneous measurements of N2 fixation and photosynthesis show the dependence of nitrogenase on photosynthetically captured radiation for energy by an indirect dependence on CO2 fixation. The host contains most of the total GS and GDH activities, and the symbiont excretes a substantial portion of its newly fixed nitrogen as ammonium. The two partners in the association exhibit a comparable developmental gradient and a mechanism of cooperative integration for their energy metabolism, thus improving the efficiency of solar energy conversion and presenting a unique model for biotechnology. 相似文献
2.
Antigenic similarity among Anabaena azollae separated from different species of Azolla 总被引:5,自引:0,他引:5
Direct fluorescent antibody (FA) reaction results of 5 FAs against symbiotic Anabaena azollae indicated that all the A. azollae freshly separated from 32 specimens of Azolla collected worldwide (belonging to 6 different species) shared identical and highly specific antigens. None of these FAs exhibited cross-reaction with any of the free-living blue-green algae tested. FA absorption results confirmed these results and also indicate the existence of cross-reactive antigens between Azolla leaves and the surfaces of A. azollae. Antibodies made against free-living A. azollae did not cross-react with any of the symbiotic A. azollae indicating either: (i) these isolates are not true isolates, or (ii) their antigenic properties were altered during isolation and culturing. Such possibilities and their implications are discussed. 相似文献
3.
Alfalfa (Medicago sativa L.) is a deeply rooted perennial legume which, under field conditions, may be exposed to varying NO3? concentrations with depth. Our objective was to characterize the effect of localized (deep vs shallow) exposure of alfalfa root systems to NO3? on symbiotic N2 fixation and NO3?-N uptake. Cuttings of a single alfalfa plant were grown in vertical split root systems in a controlled environment chamber. The split root system was a rigid acrylic tube (5 cm diam. by 60 cm long) filled with silica sand and divided into upper and lower sections at the 30-cm depth by a 5-mm-thick wax layer. Roots penetrated the wax layer, but mixing of nutrient solutions between the sections was prevented. Nodulation was restricted to the upper section. The plants were subjected for 10 days to the following treatments: both sections of the split root system received nutrient solution containing either 0.5, 5.25, or 10 mM NO3?; the upper section received 0.5 mM NO3? while the lower section received 10 mM NO3?; or the upper section received 10 mM NO3? while the lower section received 0.5 mM NO3?. Increasing supply of NO3? in the nutrient solution to both sections resulted in higher NO3?-N uptake, lower nodule mass and lower specific nitrogenase activity. Although NO3?-N uptake did not differ, plants exposed to 10 mM NO3? for 10 days in the upper, nodulated section of the root system had a 20% lower nodule mass than plants exposed to the same NO3? concentration in the lower, non-nodulated section of the root system. Specific nitrogenase activity was not different between these two treatments. Therefore, we conclude that: (1) nodule mass was dependent on two factors, the amount of NO3?-N taken up and the concentration of NO3? within the nodulated root zone; and (2) specific nitrogenase activity was little affected by the concentration of NO3? surrounding the nodules, but was strongly inhibited by NO3?-N taken up. 相似文献
4.
The effect of nitrogen starvation on the NO3-dependent induction of nitrate reductase (NR) and nitrite reductases (NIR) has been investigated in the halophilic alga Dunaliella salina. When D. salina cells previously grown in a medium with NH 4 + as the only nitrogen source (NH 4 + -cells) were transferred into NO 3 ? medium, NR was induced in the light. In contrast, when cells previously grown in N-free medium were transferred into a medium containing NO 3 ? , NR was induced in light or in darkness. Nitrate-dependent NR induction, in darkness, in D. salina cells previously grown at a photon flux density of 500 umol · m?2 s?1 was observed after 4 h preculture in N-free medium, whilst in cells grown at 100 umol · m?2 s?1 NR induction was observed after 7–8 h. An inhibitor of mRNA synthesis (6-methylpurine) did not inhibit NO 3 ? -induced NR synthesis when the cells, previously grown in NH 4 + medium, were transferred into NO 3 ? medium (at time 0 h) after 4-h-N starvation. However, when 6-methylpurine was added simultaneously with the transfer of the cells from NH 4 + to NO 3 ? medium (at time 0 h), NO 3 ? induced NR synthesis was completely inhibited. The activity of NIR decreased in N-starved cells and the addition of NO 3 ? to those cells greatly stimulated NIR activity in the light. The ability to induce NR in darkness was observed when glutamine synthetase activity reached its maximal level during N starvation. Although cells grown in NO 3 ? medium exhibited high NR activity, only 0.33% of the total NR was found in intact chloroplasts. We suggest that the ability, to induce NR in darkness is dependent on the level of N starvation, and that NR in D. salina is located in the cytosol. Light seems to play an indirect regulatory role on NO 3 ? uptake and NR induction due to the expression of NR and NO 3 ? -transporter mRNAs. 相似文献
5.
The effect of a range of concentrations of nitrate (NO?3) on the growth rate and nitrate reductase (NR) activity of both young and mature sporophytes of Laminaria digitata (Huds.) Lamour has been studied by means of laboratory batch culture experiments. The growth rate of young sporophytes was found to increase in a hyperbolic fashion with increasing NO?3 availability, with a ks value of 19 μmol·dm?3. The potential in vivo NR activity of these plants (obtained under optimum assay conditions) remained constant over the range of NO?3 concentrations used, while the actual in vivo NR activity (sustained by the internal NO?3 pool within the cell) increased in a similar hyperbolic manner to that shown by the growth rate (ks 20 μmol·dm?3). The changes in the actual in vivo NR activity were consistent with those of the internal NO?3 content of these plants, which also increased with increasing external NO?3 concentration.The NR activity in the blade meristem of the mature sporophytes behaved in a similar manner to that of the entire young plants. In contrast, the potential in vivo NR activity of the old, non-meristematic region of the blades of mature plants (where the maximum NR activities were located) did respond to the external availability of NO?3, being greater in those plants grown in high concentrations of NO?3 than in those in which growth was nitrogen-limited. In addition to this trend, a similar dependence of the ratio of actual : potential NR activity on the degree of nitrogen limitation to that found in the young sporophytes occurred in this region of the blade of mature plants.Pronounced diurnal variations in NR activity, with maximum values in the light period and minimum in the dark, were observed in both field and laboratory populations of L. digitata. The amplitude of these fluctuations appeared to be controlled by the degree of nitrogen limitation experienced, being much greater when growth was light- rather than nitrogen-limited (minimum values 44 and 74% of maximum, respectively).Overall the data indicate that the ratio between the actual : potential in vivo NR activity in L. digitata provides an unambiguous indicator of the state of the nitrogen metabolism within the cells, the interpretation of which, unlike growth rate, is not affected by differences in other culture or environmental conditions. This finding is believed to have important implications for the commercial cultivation of this and other species of macroalgae. 相似文献
6.
7.
I. Prášil 《Biologia Plantarum》1975,17(3):223-225
The absorption of nitrate and the activity of nitrate reductase were much lower in Ca-deficient plants ofCururbita pepo L., cv. ‘Kveta’ than in normal plants grown in complete nutrient solution for a period of 8 days. After the addition of nitrate to the nutrient medium, nitrate reductase activity in the roots of NO3-deficient plants sharply rose during the first 6 h and then remained constant during the following 6 h; the content of endogenous NO3 ? rose slowly and continuously. These processes were depressed in (Ca, NO3)-deficient plants independently of the addition of Ca2+ to the medium in the variant with NO3 ?. Thus it seems that the whole nitrogen metabolism,i.e. both NO3 ? absorption and the synthesis of nitrate reductase, is impaired in Ca-deficient plants. 相似文献
8.
We examined the energetic dependency of the biochemical and physiological responses of Thalassiosira pseudonana Hasle and Heimdal. Chaetoceros gracilis Schütt, Dunaliella tertiolecta Butcher, and Gymnodinium sanguineum Hirasaka to NH4+, NO3?, and urea by growing them at subsaturating and saturating photon flux (PF). At subsaturating PF, when energy was limiting, NO3? and NH4+ grown cells had similar growth rates and C and X quotas. Therefore, NO3? grown cells used up to 48% more energy than NH4+ grown cells to assimilate carbon and nitrogen. Based on our measurements of pigments, chlorophyll-a-specific in vivo absorption cross-section, and fluorescence-chlorophyll a?1, we suggest that NO3?, grown cells do not compensate for the greater energy requirements of NO3? reduction by trapping more light energy. At saturating PF, when energy is not limiting, the utilization of NO3?, compared to NH4+ resulted in lower growth rates and N quotas in Thalassiosira pseudonana and lower N quotas in Chaetoceros gracilis, suggesting enzymatic rather than energetic limitations to growth. The utilization of urea compared to Nh4+ resulted in lower growth rates in Chaetoceros gracilis and Gymnodinium sanguineum (saturating PF) and in lower N quotas in all species tested at both subsaturating and saturating PF. The high C:N ratios observed in all urea-grown species suggest that nitrogen assimilation may be limited by urea uptake or deamination and that symptoms of N limitation in microalgae may be induced by the nature of the N source in addition to the N supply rate. Our results provide new eridence that the maximum growth rates of microalgae may be limited by enzymatic processes associated with the assimilation of NO3?, or urea. 相似文献
9.
Effect of Previous Nitrate Deprivation on 15N-nitrate Absorption and Assimilation by Wheat Seedlings
A. Talouizte G. Guiraud A. Moyse C. Marol M.L. Champigny 《Journal of plant physiology》1984,116(2):113-122
Nitrate uptake and assimilation were examined in intact 18 days old wheat (Triticum aestivum, cv Capitole) seedlings either permanently grown on nitrate (high-N seedlings) or N-stressed by transfer to an 0 N-solution for the final 7 days (low-N seedlings). The N-stressed seedlings were characterized by a lower organic N content (2.5 mg instead of 4.9 mg per seedling) and an increased root dry weight.The seedlings received 15NO3K for 7 h in the light. Nitrate uptake was 2.8 times higher in low-N than in high-N seedlings. The assimilation rate was 35 and 16 μmol NO3?·h?1· g?1 dry weight respectively. Partitioning of NO3? to reduction and assimilation was the very same in both kinds of seedlings. The results support the view that 50 % of the nitrate reduction in Triticum aestivum, cv Capitole could be achieved in the roots.The present observations are interpreted as evidence that factors closely associated with the seedling N-status may have a major role in regulating NO3? uptake and assimilation. In low-N seedlings, the high amount of carbohydrates in roots may add its stimulus to the specific inducing effect of nitrate whereas in high-N seedlings, excess of nitrate or amino-acids may set the pace by negative feedback control. 相似文献
10.
Aquaspirillum magnetotacticum strain MS-1 and two nonmagnetic mutants derived from it reduced C2H2 microaerobically but not anaerobically even with NO3 ?. This organism apparently is not capable of NO3 ?-dependent nitrogen fixation. Cells ofA. magnetotacticum reduced C2H2 at rates comparable to those ofAzospirillum lipoferum grown under similar conditions, but much lower than that ofAzotobacter vinelandii grown aerobically. Cells ofA. magnetotacticum in anaerobic cultures lacking NO3 ? did not reduce C2H2 until O2 was introduced. Optimum rates of C2H2 reduction byA. magnetotacticum were obtained at 200 Pa O2. C2H2 reduction was inhibited by more than 1 kPa O2 or 0.2 mM NO3 ? or NH4 +. These results suggest thatA. magnetotacticum fixes N2 only under microaerobic, N-limited conditions. 相似文献
11.
Ángeles Calatayud Elisa Gorbe Dolors Roca Pedro F. Martínez 《Environmental and Experimental Botany》2008,64(1):65-74
The effect of two nutrient solution temperatures (cold (10 °C) and warm (22 °C)) during two flowering events of rose plants (Rosa × hybrida cv. Grand Gala) were examined by measuring chlorophyll (Chl) a fluorescence, ammonium (NH4+) content and nitrate reductase (NR) activity in four different leaf types, that is, external and internal leaves of bent shoots and lower and upper leaves of flowering stems. Besides, nitrate (NO3?) uptake and water absorption, total nitrogen (N) concentration in the plant, dry biomass, and the ratios of shoot/root and thin-white roots/suberized-brown roots were determined. Generally, cold solution increased NO3? uptake and thin-white roots production but decreased water uptake, so plants grown at cold solution had to improve their NO3? uptake mechanisms to obtain a higher amount of nutrient with less water absorption than plants grown at warm solution. The higher NO3? uptake can be related to an increase in NR activity, NH4+ content and total N concentration at cold solution. Nutrient solution temperature also had an effect on the photosynthetic apparatus. In general terms, the effective quantum yield (?PSII) and the fraction of open PSII reaction centres (qL) were higher in rose plants grown at cold solution. These effects can be associated to a higher NO3? uptake and total N concentration in the plants and were modulated by irradiance throughout all the experiment. Plants could adapt to cold solution by enhancing their metabolism without a decrease in total dry biomass. Nevertheless, the effect of nutrient solution temperature is not simple and also affected by climatic factors. 相似文献
12.
The role of nitrate and ammonium ions and light on the induction of nitrate reductase in maize leaves 总被引:2,自引:1,他引:1
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Corn seedlings (Zea mays cv W64A × W182E) were grown hydroponically, in the presence or absence of NO3−, with or without light and with NH4Cl as the only N source. In agreement with earlier results nitrate reductase (NR) activity was found only in plants treated with both light and NO3−. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by transfer of the proteins to nitrocellulose paper and reaction with antibodies prepared against a pure NR showed that crude extracts prepared from light-grown plants had a polypeptide of approximately 116 kilodaltons (the subunit size for NR) when NO3− was present in the growth medium. Crude extracts from plants grown in the dark did not have the 116 kilodalton polypeptide, although smaller polypeptides, which reacted with NR-immunoglobulin G, were sometimes found at the gel front. When seedlings were grown on Kimpack paper or well washed sand, NR activity was again found only when the seedlings were exposed to light and NO3−. Under these conditions, however, a protein of about 116 kilodaltons, which reacted with the NR antibody was present in light-grown plants whether NO3− was added to the system or not. The NR antibody cross-reacting protein was also seen in hydroponically grown plants when NH4Cl− was the only added form of nitrogen. These results indicate that the induction of an inactive NR-protein precursor in corn is mediated either by extremely low levels of NO3− or by some other unidentified factor, and that higher levels of NO3− are necessary for converting the inactive NR cross-reacting protein to a form of the enzyme capable of reducing NO3− to NO2−. 相似文献
13.
Seven species ofAzolla (A. caroliniana, A. microphylla, A. nilotica, A. filiculoides, A. mexicana, A. rubra, A. pinnata the last from both Malaysia and India) grown in pots of flooded soil were subjected to three different treatments with respect to P: none, single application, split application. The experiments were carried out under greenhouse conditions. Heterocyst frequency inAnabaena azollae and acetylene reducing activity (ARA) were studied in successiveAzolla leaves. Both variables increased from the first leaf (shoot apex) to the last one (before branch) in all species in the presence or absence of P. However, heterocyst frequency, ARA andAzolla biomass were all less in the treatment lacking P. Heterocyst frequency inA. azollae, ARA and biomass ofAzolla were higher when P was applied in split doses than in the other treatments.Azolla plants exhibited more ARA than the isolated leaves. 相似文献
14.
J. Sahulka 《Biologia Plantarum》1972,14(5):330-336
Nitrate reductase (NO3R) activity, nitrite reductase (NO2R) activity and NADH2 dependent glutamate dehydrogenase (GDH) activity were followed in extracts from excised pea roots incubated under aseptic conditions for 9 and 24 h in nitrate containing nutrient medium to which IAA was added in concentrations promoting lateral root formation (1 × 10?5; 3 × 10?5; 5 × 10?5 M) and kinetin in concentrations which reduce lateral root formation (0.1; 1; 5 mg 1?1, that is 4.65 × 10?7;4.65 × 10?6 and 2.3 × 10?5 M). NO3R activity was not influenced by IAA, NO2R activity was slightly depressed by IAA after 24 h incubation and GDH activity was slightly increased after 24 h incubation in the presence of IAA. Kinetin decreased NO3R activity significantly both after 9 h and 24 h incubation, slightly increased NO2R activity after 9 h incubation but slightly decreased it after 24 h incubation, and did not affect GDH activity after 24 h incubation. However, when applied together with IAA, kinetin abolished the promoting effect of IAA on GDH activity. IAA neither reversed nor accentuated the effect of kinetin on NO2R activity. Nevertheless the depressing effect of kinetin on NO3R activity was emphasized by the presence of IAA after 9 h incubation. The results obtained indicate that reduced nitrate assimilation due to the depression of nitrate reductase activity caused by kinetin probably contributes to the negative growth effect of kinetin in pea root segments grown in nitrate medium. 相似文献
15.
Rafael M. Arag?o Evandro N. Silva Cinthya F. Vieira Joaquim A. G. Silveira 《Acta Physiologiae Plantarum》2012,34(6):2135-2143
This study was performed to determine if a high supply of N-NO3 ? is capable of mitigating negative salinity effects on photosynthesis and growth through the stimulation of nitrate assimilation, which could act as an sink from photosynthetic electron transport chain and restrict the over reduction in thylakoid membrane in Jatropha curcas leaves. The experiment was arranged in a factorial design with two nitrate concentrations (1 and 10?mM) and two NaCl levels (0 and 100?mM). Salt-stressed plants supplied with high NO3 ? demonstrated a higher nitrate uptake rate, nitrate reductase activity and soluble-protein content when compared with plants that presented low nitrate uptake. High nitrate assimilation was associated with higher leaf growth, CO2 assimilation and lower membrane damage in salt-stressed plants. The superior performance of salt-stressed plants grown with high NO3 ? was indicated by a higher effective quantum yield of PSII and electron transport rate and lower energy excess at the PSII level and non-photochemical quenching. Interestingly, a high NO3 ? level in the absence of NaCl did not alter the leaf growth, photochemical activity and gas exchange parameters when compared with plants supplied with low nitrate. The proline and glycinebetaine contents were similarly increased in both low- and high-NO3 ? salt-stressed plants. Our data suggest that the favorable effects induced by high nitrate supply were possibly associated with stimulation in the nitrate assimilatory pathway. This process might have acted as a sink of electrons from the thylakoid membranes minimizing photo-damage and stimulating CO2 assimilation under salinity in J. curcas. 相似文献
16.
Oxygen stress in Salvinia natans: Interactive effects of oxygen availability and nitrogen source 总被引:1,自引:0,他引:1
The ability of Salvinia natans (L.) All. to tolerate growth in oxic, hypoxic and anoxic nutrient solutions when supplied with either NH4+ or NO3? were studied in the laboratory to test the hypothesis that inorganic N-source affects the response of the plants to O2 deprivation. The relative growth rate (RGR) was significantly reduced in the anoxic treatment, but in the hypoxic treatment RGR was only slightly affected. The NH4+ fed plants generally had a higher shoot to root ratio than the NO3? fed plants, and highest in the anoxic treatment. Plants had more roots and larger leaves when supplied with NH4+ as compared with NO3?, particularly in the oxic treatment, and root length was most affected by O2 deprivation for NO3? fed plants. Cell walls in the endodermis, the bundle sheath and the cortex adjacent to endodermis developed thickened sclerenchymatous walls when deprived of O2, and more so in plants supplied with NO3?. Plants lost chlorophylls, had lower rates of photosynthetic electron transport (ETRmax) and lower quantum yields (Fv/Fm ratios) when grown in anoxic solutions, and the negative effects were mildest for NO3? fed plants suggesting that NO3? may be used as an alternative e?-acceptor in non-cyclic electron transport in the chloroplasts. Overall S. natans grew best on NH4+, but it also grew well on NO3?, and the O2 stress symptoms differed somewhat between NH4+ fed and NO3? fed plants. However, because N-form itself significantly influenced morphology and cell metabolism, it was impossible to conclusively identify the role of N-form for the O2 stress reactions. S. natans is not well-adapted to grow in O2 deficient waters and will not tolerate completely anoxic conditions as will prevail in waters receiving high loadings of organic pollutants such as livestock wastewater. 相似文献
17.
Jose M. Barbosa Narendra K. Singh Joe H. Cherry Robert D. Locy 《Plant Physiology and Biochemistry》2010,48(6):443-450
Exogenously applied GABA modulates root growth by inhibition of root elongation when seedlings were grown in vitro on full-strength Murashige and Skoog (MS) salts, but root elongation was stimulated when seedlings were grown on 1/8 strength MS salts. When the concentration of single ions in MS salts was individually varied, the control of growth between inhibition and stimulation was found to be related to the level of nitrate (NO3?) in the growth medium. At NO3? concentrations below 40 mM (full-strength MS salts level), root growth was stimulated by the addition of GABA to the growth medium; whereas at concentrations above 40 mM NO3?, the addition of GABA to the growth medium inhibited root elongation. GABA promoted NO3? uptake at low NO3?, while GABA inhibited NO3? uptake at high NO3?. Activities of several enzymes involved in nitrogen and carbon metabolism including nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (NADH-GOGAT), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and phosphoenol pyruvate carboxylase (PEPCase) were regulated by GABA in the growth medium. Supplementing 1/8 strength MS medium with 50 mM GABA enhanced the activities of all of the above enzymes except ICDH activities in root tissues. However, at full-strength MS, GABA showed no inhibitory effect on the activities of these enzymes, except on GS in both root and shoot tissues, and PEPCase activity in shoot tissues. Exogenous GABA increased the amount of NR protein rather than its activation status in the tissues. This study shows that GABA affects the growth of Arabidopsis, possibly by acting as a signaling molecule, modulating the activity of enzymes involved in primary nitrogen metabolism and nitrate uptake. 相似文献
18.
A. W. STIENSTRA 《Plant, cell & environment》1986,9(4):307-313
Abstract The ‘tidal salt marsh’ ecotype of the halophyte Aster tripolium L. was grown in a nutrient solution with either a continuous or an intermittent NO3? supply with either Cl? or SO42? as the alternative anion. With increasing periods of NO3? supply per week the rate of the dry weight increment increased. When NO3? was supplied for longer than 48 h per week, the dry weight and the organic-N content in the shoots hardly increased, whereas the NO3? content in shoots and roots increased further. With alternated supply of a nutrient solution containing NO3? with one containing Cl?, the internal NO3? content in the shoot was lower than in shoots grown in solutions in which NO3? alternated with SO42?. It is concluded, that NO3? does not have a specific function in osmoregulation. 相似文献
19.
Xue-Yan Liu Keisuke Koba Yu Takebayashi Cong-Qiang Liu Yun-Ting Fang Muneoki Yoh 《Biogeochemistry》2013,114(1-3):399-411
Nitrate (NO3 ?) is an important form of nitrogen (N) available to plants. The measurements of NO3 ? concentration [NO3 ?] and isotopes (δ15N and δ18O) in plants provide unique insights into ecosystem NO3 ? availability and plant NO3 ? dynamics. This work investigated the variability of these parameters in individuals of a broadleaved (Aucuba japonica) plant and a coniferous (Platycladus orientalis) plant, and explored the applicability of tissue NO3 ? isotopes for deciphering plant NO3 ? utilization mechanisms. The NO3 ? in washed leaves showed concentration and isotopic ratios that were much lower than that in unwashed leaves, indicating a low contribution of atmospheric NO3 ? to NO3 ? in leaves. Current leaves showed higher [NO3 ?] and isotopic ratios than mature leaves. Moreover, higher leaf [NO3 ?] and isotopic enrichments (relative to soil NO3 ?) were found under higher soil NO3 ? availability for A. japonica. In contrast, leaves of P. orientalis showed low [NO3 ?] and negligible isotopic enrichments despite high soil NO3 ?. Higher [NO3 ?] was found in both fine and coarse roots of the P. orientalis plant, but significant isotopic enrichment was found only in coarse roots. These results reflect that the NO3 ? accumulation and isotopic effects decreased with leaf age, but increased with soil NO3 ? supply. Leaves are therefore identified as a location of NO3 ? reduction for A. japonica, while P. orientalis did not assimilate NO3 ? in leaves but in coarse roots. This work provided the first organ-specific information on NO3 ? isotopes in plant individuals, which will stimulate further studies of NO3 ? dynamics in a broader spectrum of plant ecosystems. 相似文献
20.
John Karl Böhlke Ronald C. Antweiler Judson W. Harvey Andrew E. Laursen Lesley K. Smith Richard L. Smith Mary A. Voytek 《Biogeochemistry》2009,93(1-2):117-141
Denitrification is an important net sink for NO3 ? in streams, but direct measurements are limited and in situ controlling factors are not well known. We measured denitrification at multiple scales over a range of flow conditions and NO3 ? concentrations in streams draining agricultural land in the upper Mississippi River basin. Comparisons of reach-scale measurements (in-stream mass transport and tracer tests) with local-scale in situ measurements (pore-water profiles, benthic chambers) and laboratory data (sediment core microcosms) gave evidence for heterogeneity in factors affecting benthic denitrification both temporally (e.g., seasonal variation in NO3 ? concentrations and loads, flood-related disruption and re-growth of benthic communities and organic deposits) and spatially (e.g., local stream morphology and sediment characteristics). When expressed as vertical denitrification flux per unit area of streambed (U denit, in μmol N m?2 h?1), results of different methods for a given set of conditions commonly were in agreement within a factor of 2–3. At approximately constant temperature (~20 ± 4°C) and with minimal benthic disturbance, our aggregated data indicated an overall positive relation between U denit (~0–4,000 μmol N m?2 h?1) and stream NO3 ? concentration (~20–1,100 μmol L?1) representing seasonal variation from spring high flow (high NO3 ?) to late summer low flow (low NO3 ?). The temporal dependence of U denit on NO3 ? was less than first-order and could be described about equally well with power-law or saturation equations (e.g., for the unweighted dataset, U denit ≈26 * [NO3 ?]0.44 or U denit ≈640 * [NO3 ?]/[180 + NO3 ?]; for a partially weighted dataset, U denit ≈14 * [NO3 ?]0.54 or U denit ≈700 * [NO3 ?]/[320 + NO3 ?]). Similar parameters were derived from a recent spatial comparison of stream denitrification extending to lower NO3 ? concentrations (LINX2), and from the combined dataset from both studies over 3 orders of magnitude in NO3 ? concentration. Hypothetical models based on our results illustrate: (1) U denit was inversely related to denitrification rate constant (k1denit, in day?1) and vertical transfer velocity (v f,denit, in m day?1) at seasonal and possibly event time scales; (2) although k1denit was relatively large at low flow (low NO3 ?), its impact on annual loads was relatively small because higher concentrations and loads at high flow were not fully compensated by increases in U denit; and (3) although NO3 ? assimilation and denitrification were linked through production of organic reactants, rates of NO3 ? loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas k1denit and v f,denit are linked implicitly with stream depth, NO3 ? concentration, and(or) NO3 ? load, estimates of U denit may be related more directly to field factors (including NO3 ? concentration) affecting denitrification rates in benthic sediments. Regional regressions and simulations of benthic denitrification in stream networks might be improved by including a non-linear relation between U denit and stream NO3 ? concentration and accounting for temporal variation. 相似文献