氮素形态对黄檗幼苗生长及氮代谢相关酶类的影响

通过改变水培溶液中NH4^＋-N和NO3^--N的比例,研究了不同氮素形态对黄檗（Phellodendron amurense）幼苗生长及氮代谢相关酶类的影响。结果表明,硝态氮比例较高的营养供给比铵态氮比例较高的营养供给有利于黄檗幼苗的生长,叶片叶绿素含量和可溶性蛋白含量也高。在NH4^＋-N／NO3^--N为25／75时黄檗幼苗具有最大生物量。在铵态氮比例大的营养供给下,黄檗幼苗的谷氨酰胺合成酶（GS）活性增强,而在硝态氮比例大的营养供给下幼苗的硝酸还原酶（NR）活性则较高,叶片中的硝态氮较低。营养液的氮素形态及其组成通过影响GS与NR的活性而调控黄檗幼苗的氮素代谢。     

氮素形态对黄檗幼苗生长及氮代谢相关酶类的影响

通过改变水培溶液中NH4+-N和NO3--N的比例, 研究了不同氮素形态对黄檗(Phellodendron amurense)幼苗生长及氮代谢相关酶类的影响。结果表明, 硝态氮比例较高的营养供给比铵态氮比例较高的营养供给有利于黄檗幼苗的生长, 叶片叶绿素含量和可溶性蛋白含量也高。在NH4+-N/NO3--N为25/75 时黄檗幼苗具有最大生物量。在铵态氮比例大的营养供给下, 黄檗幼苗的谷氨酰胺合成酶(GS)活性增强,而在硝态氮比例大的营养供给下幼苗的硝酸还原酶(NR)活性则较高, 叶片中的硝态氮较低。营养液的氮素形态及其组成通过影响GS与NR的活性而调控黄檗幼苗的氮素代谢。     

不同氮素形态比例对五味子幼苗生长特性的影响

以2年生五味子苗木为试验材料,在田间条件下,施以铵态氮(NH4+-N)和硝态氮(NO3--N)不同比例,分析了叶片可溶性蛋白、叶绿素含量、根系及茎叶中全氮含量、生物量等季节的动态变化规律,探讨了不同氮素形态比例对五味子苗木生长的影响。结果表明,五味子苗木在不同生长时期对不同氮素形态的吸收和利用存在明显差异,NH4+-N和NO3--N对五味子幼苗生长有显著的联合效应。在五味子生长前期,五味子主要以吸收和同化NH4+-N为主,并以铵态氮和硝态氮比例为75∶25时地上部生物量积累较多;而在五味子生长的中后期,五味子主要以NO3--N吸收和同化为主,并以铵态氮和硝态氮比例为25∶75时地上部生物量积累较多。     

淹水条件下籼稻与粳稻苗期根际土壤硝化作用的时空变异

由于硝态氮（NO3^--N）对于水稻的生长起到非常重要的作用,所以发生在水稻根际的硝化作用越来越受到人们的重视.试验采用根盒（3室）--速冻切片技术研究了常规籼稻（扬稻6号）和常规粳稻（农垦57）苗期根际土壤矿质态氮、硝化作用和氨氧化细菌数量的时空变异.结果表明,在淹水条件下,土壤矿质态氮主要为铵态氮（NH4^＋-N）,NH4^＋含量随水稻生育期的推进变化不大,但随着距根区的距离增加其含量随之增加,两个水稻品种之间差异不显著;而NO3-的变化趋势与NH4^＋不一致,NO3^-含量随水稻生育期的延长而显著下降,在培养58d时其平均含量约为0.05 mg kg^-1,同时在整个土体内呈均匀分布,两个水稻品种之间差异显著.土壤的硝化强度随水稻的生长而增强,且两种水稻的硝化强度均为根际土壤最高,然后依次为土体土壤和根区土壤.扬稻6号和农垦57硝化强度最大值分别出现在距根6 mm和2 mm处,最大值分别为0.88 mg kg^-1h^-1和0.73 mg kg^-1h^-1.土壤氨氧化细菌（AOB）数量随水稻生长时间的增加而增加,且其水平变异趋势与土壤的硝化强度一致,根际土壤AOB数量最多,土体土壤次之,根区土壤最少.相关分析结果表明,硝化强度和AOB数量呈显著正相关关系（r=0.86,p＜0.01）.种植扬稻6号的土壤NO3^-浓度、硝化强度以及AOB数量总是高于农垦57.     

不同烤烟品种衰老叶片的无机氮积累及其生理调控

以不同烤烟品种‘红花大金元’和‘中烟100’为试验对象,研究同一生育期不同部位叶片的无机氮积累及其与氮素代谢和氨挥发的关系。结果表明,烟草植株下部衰老叶片（第5片叶）NO3^-和NH4^＋的含量要高于中部叶（第10和第15片叶）和上部叶（第20片叶）,并且‘红花大金元’下部叶NO3^-和NH4^＋的含量比‘中烟100’显著偏高;‘中烟100’植株各部位叶片的氨气补偿点比‘红花大金元’高。两个品种在第5到第20叶位间的谷氨酰胺合成酶（GS）、硝酸还原酶（NR）和谷氨酸脱氢酶（GDH）活性大小及其变化不一致,是叶片无机氮积累存在品种间差异的生理基础。     

不同氮素形态及水分胁迫对水稻苗期水分吸收、光合作用及生长的影响

采用室内营养液培养,聚乙二醇（PEG6000）模拟水分胁迫处理、HgCl2抑制水通道蛋白活性的方法,在3种供氮形态下（NH4^＋-N／NO36-N为100／0、50／50和0／100）,研究了水稻苗期水分吸收、光合及生长的状况。结果表明,在非水分胁迫下,水稻单位干重吸水量以单一供NO3^--N处理最高,加HgCl2抑制水通道蛋白活性后,单一供NO3^--N、NH4^＋-N和NH4^＋-N／NO3^--N为50,50处理的水稻水分吸收分别下降了9．6％、20．7％和16．0％;但在水分胁迫下,单一供N03^--N的处理水分吸收量显著降低,低于其它2个处理,加HgCl2抑制水通道蛋白活性后,水分吸收量分别降低了1．0％、18．8％和23．5％。在2种水分条件（水分胁迫与非水分胁迫）下,净光合速率、气孔导度、蒸腾速率和细胞间隙CO2浓度等指标均以单一供NH4^＋-N处理最大,NH4^＋-N／NO3^--N为50,50处理次之,单一供NO3^--N处理最小。HgCl2处理结果表明,不同形态氮素营养能够影响水稻幼苗根系水通道蛋白活性。在2种水分条件下,NH4^＋-N／N03^--N为50,50处理的生物量（干重）均最大。本研究为水稻苗期合理施肥以壮苗提供了理论依据。     

不同氮效率水稻全生育期内对增硝营养的响应及其生理机制

通过添加硝化抑制剂（二氰胺,DCD）来控制硝化作用的水培试验方法,研究了氮高效水稻品种南光和氮低效水稻品种ELIO的籽粒产量对增硝营养（NH4＋∶NO3-比例为100∶0和75∶25）的响应,同时从产量构成、不同生育时期水稻生长、氮素吸收和同化4个方面研究了造成其产量差异的生理机制。结果表明：增NO3-营养可以显著促进氮高效水稻品种南光的生长,从而使其籽粒产量水平提高21%,而对氮低效水稻品种ELIO的籽粒产量没有显著影响。进一步分析表明：在增NO3-营养条件下,南光的穗粒数增加了25%,结实率增加了16%,而氮低效水稻品种ELIO的结实率和穗粒数在两种营养条件下没有显著变化;增NO3-营养可以促进南光对氮素的吸收,使其在苗期、分蘖盛期、齐穗期和成熟期对氮素的吸收量平均增加了36%,进而促进了其生长,干物质积累量在四个生育时期平均增加了30%;南光叶片硝酸还原酶和根系谷氨酰胺合成酶的活力在增硝营养条件下分别增加了100%和95%,说明增硝营养促进了南光对NH4＋和NO3-的同化利用。与氮低效水稻品种（ELIO）相比,氮高效水稻品种（南光）对增硝营养表现出较强的生理响应。     

香蕉转化中的抗褐化及再生研究

香蕉在进行遗传转化过程中,外植体容易褐化,从而降低了再生频率,影响到遗传转化的效率。为降低香蕉转化过程中外植体的褐化率,采用香（Musa AAA group cv．Brazilian）未成熟雄花作为外植体,以农杆菌介导法进行遗传转化。结果表明,当改良的MS培养基中铵态氮与硝态氮（NH4^＋／NO3^-）的摩尔比为20．6：67．6时,具有较强的抗褐化能力。当6-BA浓度为1．0mg／L时,外植体易于诱导出胚状体。     

施N对不同水稻品种N肥利用率及根际硝化作用和硝化微生物的影响

发生在水稻根际的硝化作用对水稻的氮素（N）营养受到人们越来越大的关注。在田间条件下研究了不同N效率粳稻品种（4007、武运粳7号和Elio）在无肥（0kgN/hm^2）、中肥（180kgN/hm^2）和高肥（300kgN/hm^2）水平下籽粒产量、吸N量、N肥利用率、根际土壤铵态氮（NH4^＋-N）和硝态氮（NO3^--N）含量、硝化强度和氨氧化细菌（AOB）数量。结果表明不同水稻品种的籽粒产量在3个N处理中差异极显著,4007在中肥处理中获得最高产量11117kg/hm^2,而Eilo在所有处理中籽粒产量均最低。各品种地上部吸N量随施N量增加而增加,但各品种之间差异不显著。不同水稻品种N肥利用率差异显著,4007显著高于武运粳7号和Elio。本试验根据不同品种水稻在不施N肥水平下的籽粒产量与N肥利用率的大小,将3个品种分为N肥高效敏感型（4007）、N肥高效不敏感型（武运粳7号）和N肥低效不敏感型（Elio）。在水稻中后期干湿交替的水分管理条件下,无肥和中肥区的水稻根际土壤以NO3^--N为主;而在高肥区则以NH4^＋-N为主。随着施N量增加,水稻根际土壤铵、硝态N含量也随之增加。NH4^＋-N含量在无肥、中肥和高肥水平下分别为0.88、0.94mg/kg和13.5mg/kg,而NO3^--N含量分别为1.61、1.73mg/kg和2.33mg/kg。不同水稻品种根际土壤硝化强度之间差异极显著,在3个施N水平下均表现为4007〉武运粳7号〉Elio。其平均值分别为6.94、5.46μg/（kg·h）和2.42μg/（kg·h）。在3个施N水平下,Elio根际土壤AOB数量均显著低于4007和武运粳7号。4007根际土壤AOB数量在高肥水平下达最大值2.02×106个/g土,而最小值为中肥水平下Elio的根际土壤（1.89×105个/g土）。相关性分析表明,水稻根际土壤硝化强度在无肥、中肥和高肥条件下与产量呈极显著正相关关系（r=0.799,0.877,0.934）,而且在中肥条件下与水稻N肥利用率显著相关（r=0.735）。水稻根际土壤AOB数量分别和硝化强度以及水稻籽粒产量呈极显著正相关关系。试验结果表明,水稻根际的硝化作用较大程度上决定着水稻籽粒产量或水稻N肥利用率。     

短期内^15N标记的铵盐和硝酸盐在青藏高原高寒草甸中的运移规律

运用^15N稳定性同位素示踪技术,对高寒草甸植物和土壤微生物固持沉降氮的能力及沉降氮在小嵩草（Kobresia pygaea)草甸中的运移规律进行了研究。施肥2周后,NO3^-－^15N和NH4^ －^15N的总恢复率分别为73．5％和78％。无论是NO3^-－^15N,还是NH4^ －^15N植物所固持的^15N总是比土壤有机质或者是土壤微生物固持的多。4周后,70．6％的NO3^-－^15N和57．4％的NH4^ －^15N被固持在土壤和植物中。其中,土壤微生物所固持。在施肥6周和8周后,NO3^-－^15N的总恢复率分别为58．4％和67％,而NH4^ －^15N的总恢复率分别为43．1％和49％。植物和土壤微生物所固持的NO3^-－^15N比NH4^ －^15N多。在整个实验期间,植物固持的NO3^-N较多,而且比土壤微生物固持了较多^15N。由于无机氮的含量一直很低,无机氮库所固持的^15N一般不超过1％。上述结果意味着短期内植物在高寒草甸中对沉降氮的去向起着决定作用。     

The regulation of ammonium translocation in plants

Much controversy exists about whether or not NH(+)(4) is translocated in the xylem from roots to shoots. In this paper it is shown that such translocation can indeed take place, but that interference from other metabolites such as amino acids and amines may give rise to large uncertainties about the magnitude of xylem NH(+)(4) concentrations. Elimination of interference requires sample stabilization by, for instance, formic acid or methanol. Subsequent quantification of NH(+)(4) should be done by the OPA-fluorometric method at neutral pH with 2-mercaptoethanol as the reducing agent since this method is sensitive and reliable. Colorimetric methods based on the Berthelot reaction should never be used, as they are prone to give erroneous results. Significant concentrations of NH(+)(4), exceeding 1 mM, were measured in both xylem sap and leaf apoplastic solution of oilseed rape and tomato plants growing with NO(-)(3) as the sole N source. When NO(-)(3) was replaced by NH(+)(4), xylem sap NH(+)(4) concentrations increased with increasing external concentrations and with time of exposure to NH(+)(4). Up to 11% of the translocated N was constituted by NH(+)(4). Glutamine synthetase (GS) incorporates NH(+)(4) into glutamine, but root GS activity and expression were repressed when high levels of NH(+)(4) were supplied. Ammonium concentrations measured in xylem sap sampled just above the stem base were highly correlated with NH(+)(4) concentrations in apoplastic solution from the leaves. Young leaves tended to have higher apoplastic NH(+)(4) concentrations than older non-senescing leaves. The flux of NH(+)(4) (concentration multiplied by transpirational water flow) increased with temperature despite a decline in xylem NH(+)(4) concentration. Retrieval of leaf apoplastic NH(+)(4) involves both high and low affinity transporters in the plasma membrane of mesophyll cells. Current knowledge about these transporters and their regulation is discussed.     

Growth, nitrogen uptake, and metabolism in two semiarid shrubs grown at ambient and elevated atmospheric CO2 concentrations: effects of nitrogen supply and source

The effect of differences in nitrogen (N) availability and source on growth and nitrogen metabolism at different atmospheric CO(2) concentrations in Prosopis glandulosa and Prosopis flexuosa (native to semiarid regions of North and South America, respectively) was examined. Total biomass, allocation, N uptake, and metabolites (e.g., free NO(3)(-), soluble proteins, organic acids) were measured in seedlings grown in controlled environment chambers for 48 d at ambient (350 ppm) and elevated (650 ppm) CO(2) and fertilized with high (8.0 mmol/L) or low (0.8 mmol/L) N (N(level)), supplied at either 1 : 1 or 3 : 1 NO(3)(-) : NH(4)(+) ratios (N(source)). Responses to elevated CO(2) depended on both N(level) and N(source), with the largest effects evident at high N(level). A high NO(3)(-) : NH(4)(+) ratio stimulated growth responses to elevated CO(2) in both species when N was limiting and increased the responses of P. flexuosa at high N(level). Significant differences in N uptake and metabolites were found between species. Seedlings of both species are highly responsive to N availability and will benefit from increases in CO(2), provided that a high proportion of NO(3)- to NH(4)-N is present in the soil solution. This enhancement, in combination with responses that increase N acquisition and increases in water use efficiency typically found at elevated CO(2), may indicate that these semiarid species will be better able to cope with both nutrient and water deficits as CO(2) levels rise.     

Regulation of the high-affinity NO3- uptake system by NRT1.1-mediated NO3- demand signaling in Arabidopsis

The NRT2.1 gene of Arabidopsis thaliana encodes a major component of the root high-affinity NO(3)(-) transport system (HATS) that plays a crucial role in NO(3)(-) uptake by the plant. Although NRT2.1 was known to be induced by NO(3)(-) and feedback repressed by reduced nitrogen (N) metabolites, NRT2.1 is surprisingly up-regulated when NO(3)(-) concentration decreases to a low level (<0.5 mm) in media containing a high concentration of NH(4)(+) or Gln (>or=1 mm). The NRT3.1 gene, encoding another key component of the HATS, displays the same response pattern. This revealed that both NRT2.1 and NRT3.1 are coordinately down-regulated by high external NO(3)(-) availability through a mechanism independent from that involving N metabolites. We show here that repression of both genes by high NO(3)(-) is specifically mediated by the NRT1.1 NO(3)(-) transporter. This mechanism warrants that either NRT1.1 or NRT2.1 is active in taking up NO(3)(-) in the presence of a reduced N source. Under low NO(3)(-)/high NH(4)(+) provision, NRT1.1-mediated repression of NRT2.1/NRT3.1 is relieved, which allows reactivation of the HATS. Analysis of atnrt2.1 mutants showed that this constitutes a crucial adaptive response against NH(4)(+) toxicity because NO(3)(-) taken up by the HATS in this situation prevents the detrimental effects of pure NH(4)(+) nutrition. It is thus hypothesized that NRT1.1-mediated regulation of NRT2.1/NRT3.1 is a mechanism aiming to satisfy a specific NO(3)(-) demand of the plant in relation to the various specific roles that NO(3)(-) plays, in addition to being a N source. A new model is proposed for regulation of the HATS, involving both feedback repression by N metabolites and NRT1.1-mediated repression by high NO(3)(-).     

Partial substitution of NO(3)(-) by NH(4)(+) fertilization increases ammonium assimilating enzyme activities and reduces the deleterious effects of salinity on the growth of barley

Productivity of cereal crops is restricted in saline soils but may be improved by nitrogen nutrition. In this study, the effect of ionic nitrogen form on growth, mineral content, protein content and ammonium assimilation enzyme activities of barley (Hordeum vulgare cv. Alexis L.) irrigated with saline water, was determined. Leaf and tiller number as well as plant fresh and dry weights declined under salinity (120 mM NaCl). In non-saline conditions, growth parameters were increased by application of NH(4)(+)/NO(3)(-) (25:75) compared to NO(3)(-) alone. Under saline conditions, application of NH(4)(+)/NO(3)(-) led to a reduction of the detrimental effects of salt on growth. Differences in growth between the two nitrogen regimes were not due to differences in photosynthesis. The NH(4)(+)/NO(3)(-) regime led to an increase in total N in control and saline treatments, but did not cause a large decrease in plant Na(+) content under salinity. Activities of GS (EC 6.3.1.2), GOGAT (EC 1.4.1.14), PEPC (EC 4.1.1.31) and AAT (EC 2.6.1.1) increased with salinity in roots, whereas there was decreased activity of the alternative ammonium assimilation enzyme GDH (EC 1.4.1.2). The most striking effect of nitrogen regime was observed on GDH whose salinity-induced decrease in activity was reduced from 34% with NO(3)(-) alone to only 14% with the mixed regime. The results suggest that the detrimental effects of salinity can be reduced by partial substitution of NO(3)(-) with NH(4)(+) and that this is due to the lower energy cost of N assimilation with NH(4)(+) as opposed to NO(3)(-) nutrition.     

Effects of ammonia on the structure of the oxygen-evolving complex in photosystem II as revealed by light-induced FTIR difference spectroscopy

NH(3) is a structural analogue of substrate H(2)O and an inhibitor to the water oxidation reaction in photosystem II. To test whether or not NH(3) is able to replace substrate water molecules on the oxygen-evolving complex in photosystem II, we studied the effects of NH(3) on the high-frequency region (3750-3550 cm(-1)) of the S(2)Q(A)(-)/S(1)Q(A) FTIR difference spectra (pH 7.5 at 250 K), where OH stretch modes of weak hydrogen-bonded active water molecules occur. Our results showed that NH(3) did not replace the active water molecule on the oxygen-evolving complex that gave rise to the S(1) mode at ~3586 cm(-1) and the S(2) mode at ~3613 cm(-1) in the S(2)Q(A)(-)/S(1)Q(A) FTIR difference spectrum of PSII. In addition, our mid-frequency FTIR results showed a clear difference between pH 6.5 and 7.5 on the concentration dependence of the NH(4)Cl-induced upshift of the S(2) state carboxylate mode at 1365 cm(-1) in the S(2)Q(A)(-)/S(1)Q(A) spectra of NH(4)Cl-treated PSII samples. Our results provided strong evidence that NH(3) induced this upshift in the spectra of NH(4)Cl-treated PSII samples at 250 K. Moreover, our low-frequency FTIR results showed that the Mn-O-Mn cluster vibrational mode at 606 cm(-1) in the S(2)Q(A)(-)/S(1)Q(A) spectrum of the NaCl control PSII sample was diminished in those samples treated with NH(4)Cl. Our results suggest that NH(3) induced a significant alteration on the core structure of the Mn(4)CaO(5) cluster in PSII. The implication of our findings on the structure of the NH(3)-binding site on the OEC in PSII will be discussed.     

Ecophysiological adjustment of two Sphagnum species in response to anthropogenic nitrogen deposition

Here, it was investigated whether Sphagnum species have adjusted their nitrogen (N) uptake in response to the anthropogenic N deposition that has drastically altered N-limited ecosystems, including peatlands, worldwide. A lawn species, Sphagnum balticum, and a hummock species, Sphagnum fuscum, were collected from three peatlands along a gradient of N deposition (2, 8 and 12 kg N ha(-1) yr(-1)). The mosses were subjected to solutions containing a mixture of four N forms. In each solution one of these N forms was labeled with (15)N (namely (15)NH(+)(4), (15)NO(-)(3) and the amino acids [(15)N]alanine (Ala) and [(15)N]glutamic acid (Glu)). It was found that for both species most of the N taken up was from , followed by Ala, Glu, and very small amounts from NO(-)(3). At the highest N deposition site N uptake was reduced, but this did not prevent N accumulation as free amino acids in the Sphagnum tissues. The reduced N uptake may have been genetically selected for under the relatively short period with elevated N exposure from anthropogenic sources, or may have been the result of plasticity in the Sphagnum physiological response. The negligible Sphagnum NO(-)(3) uptake may make any NO(-)(3) deposited readily available to co-occurring vascular plants.     

Root-zone acidity and nitrogen source affects Typha latifolia L. growth and uptake kinetics of ammonium and nitrate

The NH(4)(+) and NO(3)(-) uptake kinetics by Typha latifolia L. were studied after prolonged hydroponics growth at constant pH 3.5, 5.0, 6.5 or 7.0 and with NH(4)(+) or NO(3)(-) as the sole N-source. In addition, the effects of pH and N source on H(+) extrusion and adenine nucleotide content were examined. Typha latifolia was able to grow with both N sources at near neutral pH levels, but the plants had higher relative growth rates, higher tissue concentrations of the major nutrients, higher contents of adenine nucleotides, and higher affinity for uptake of inorganic nitrogen when grown on NH(4)(+). Growth almost completely stopped at pH 3.5, irrespective of N source, probably as a consequence of pH effects on plasma membrane integrity and H(+) influx into the root cells. Tissue concentrations of the major nutrients and adenine nucleotides were severely reduced at low pH, and the uptake capacity for inorganic nitrogen was low, and more so for NO(3)(-)-fed than for NH(4)(+)-fed plants. The maximum uptake rate, V(max), was highest for NH(4)(+) at pH 6.5 (30.9 micro mol h(-1) g(-1) root dry weight) and for NO(3)(-) at pH 5.0 (31.7 micro mol h(-1) g(-1) root dry weight), and less than 10% of these values at pH 3.5. The affinity for uptake as estimated by the half saturation constant, K((1/2)), was lowest at low pH for NH(4)(+) and at high pH for NO(3)(-). The changes in V(max) and K((1/2)) were thus consistent with the theory of increasing competition between cations and H(+) at low pH and between anions and OH(-) at high pH. C(min) was independent of pH, but slightly higher for NO(3)(-) than for NH(4)(+) (C(min)(NH(4)(+)) approximately 0.8 mmol m(-3); C(min)(NO(3)(-)) approximately 2.8 mmol m(-3)). The growth inhibition at low pH was probably due to a reduced nutrient uptake and a consequential limitation of growth by nutrient stress. Typha latifolia seems to be well adapted to growth in wetland soils where NH(4)(+) is the prevailing nitrogen compound, but very low pH levels around the roots are very stressful for the plant. The common occurrence of T. latifolia in very acidic areas is probably only possible because of the plant's ability to modify pH-conditions in the rhizosphere.     

Integrative physiology of splanchnic glutamine and ammonium metabolism

The substrates for hepatic ureagenesis are equimolar amounts of ammonium and aspartate. The study design mimics conditions in which the liver receives more NH(+)(4) than aspartate precursors (very low-protein diet). Fasted dogs, fitted acutely with transhepatic catheters, were infused with a tracer amount of (15)NH(4)Cl. From arteriovenous differences, the major NH(+)(4) precursor for hepatic ureagenesis was via deamidation of glutamine in the portal drainage system (rather than in the liver), because there was a 1:1 stoichiometry between glutamine disappearance and NH(+)(4) appearance, and the amide (but not the amine) nitrogen of glutamine supplied the (15)N added to the portal venous NH(+)(4) pool. The liver extracted all this NH(+)(4) from glutamine deamidation plus an additional amount in a single pass, suggesting that there was an activator of hepatic ureagenesis. The other major source of nitrogen extracted by the liver was [(14)N]alanine. Because alanine was not produced in the portal venous system, we speculate that it was derived ultimately from proteins in peripheral tissues.     

Effects of nitrogen source on the synthesis of the UV-screening compound, scytonemin, in the cyanobacterium Nostoc punctiforme PCC 73102

The effects of nitrogen source (N(2), NO(3)(-) and NH(4)(+)) on scytonemin synthesis were investigated in the heterocystous cyanobacterium Nostoc punctiforme PCC 73102. With the required UVA radiation included, Nostoc synthesized three to seven times more scytonemin while fixing nitrogen than when utilizing nitrate or ammonium. A similar increase in scytonemin synthesis occurred when nitrate or ammonium became depleted by growth and Nostoc switched to diazotrophic metabolism with the differentiation of heterocysts. In addition, UVA-exposed cultures grown in medium with both NO(3)(-) and NH(4)(+) synthesized some scytonemin but synthesis increased when NH(4)(+) was depleted and growth had become dependent on NO(3)(-) reduction. Although the mechanism is unclear, these results suggest that the greater the restriction in nitrogen accessibility, the greater the production of scytonemin. Perhaps the entire response may be an interaction between this restriction and a resultant sensitivity to UV radiation that acts as a cue for determining the level of scytonemin synthesis. Scytonemin is a stable UVR screening compound and appears to be synthesized by cyanobacteria as a long-term solution for reducing UVR exposure and damage, but mainly or solely, when metabolic activity is absent. It is likely that during metabolic resurgence, the presence of a dense scytonemin sheath would facilitate the recovery process without the need for active defenses against UV radiation.