Effect of nitrogen sources on cellulase biosynthesis by a mutant strain of Trichoderma viride 44

The effect of various nitrogen sources on cellulase biosynthesis by the mutant strain Trichoderma viride 44 was examined. This strain may utilized nitrogen in the nitrate, ammonium of organic form. When cultivating this strain, it appears advantageous to add to the nutrient medium yeast and yeast lyzates as well as their mixture with ammonium sulfate. Cellulase reached its maximum activity of 20.2, 21.5 and 23.2 mu/ml when grown on the medium containing ammonium phosphate, peptone and brewing yeast plus ammonium sulfate, respectively. It is useful to apply nitrogen in its organic forms in small quantities and in combination with mineral forms. The nitrogen presence in the medium is necessary only at the exponential stage of fungal growth. The lack of nitrogen in the stationary stage characterized by the maximum cellulase formation does not inhibit an increase in the enzyme activity.     

Inorganic nitrogen source for autotrophic growth of Euglena mutabilis Schmitz

The effect of inorganic nitrogen source on population growth of Euglena mutabilis, an acidophillic benthic protozoa colonizing on the sediment of acid mine drainage, was investigated. Sodium nitrate, ammonium chloride, and ammonium sulfate were tested as nitrogen sources. The population density of E. mutabilis at equilibrium density cultivated in ammonium chloride‐ and ammonium sulfate‐containing media was 9–11 times higher than that in sodium nitrate‐containing medium at the optimal salt medium concentration. The population growth of E. mutabilis in ammonium sulfate‐containing medium was rapid and reached half of the equilibrium density after ca. 228 h, which was ca. 77 h earlier than that in ammonium chloride‐containing medium. Culture medium with ammonium sulfate as the nitrogen source achieved the highest maximum population density and the fastest growth rate among the three nitrogen salts used as nitrogen sources.     

铁尾矿区油松根际溶磷泛菌D2的筛选鉴定及溶磷特性

从河北省迁安市马兰庄镇铁尾矿植被恢复区油松根际分离出2株溶磷细菌,经过平板初筛和摇瓶复筛得到1株溶磷能力较强的菌株D2.通过菌落形态、生理生化特性及16S rDNA序列分析,确定此菌株D2属于泛菌属.利用液体发酵试验测定不同碳源、氮源对菌株D2溶磷能力的影响,通过高效液相色谱测定D2在不同氮源条件下产生有机酸的种类和浓度.结果表明:菌株D2对磷酸三钙有较强的溶磷能力,培养液有效磷含量最高为392.13 mg·L^-1,菌株D2的溶磷能力在碳源为葡萄糖、氮源为硫酸铵时效果最好;高效液相色谱测定发现,不同氮源条件下,D2分泌有机酸的种类和浓度存在差异,以硫酸铵、氯化铵、硝酸钾、硝酸钠、硝酸铵为氮源,均产生草酸、甲酸、乙酸和柠檬酸,以硫酸铵、氯化铵、硝酸铵为氮源还产生苹果酸.相关性分析表明,乙酸含量与有效磷含量间呈显著正相关(r=0.886,P<0.05),表明溶磷泛菌D2分泌的乙酸对无机磷的溶解有明显的促进作用,这也很可能是该菌株的重要溶磷机制之一.     

Effect of nitrogen source on biosynthesis of rapamycin by Streptomyces hygroscopicus

Six non-amino acid nitrogen compounds were examined as nitrogen source for growth of Streptomyces hygroscopicus and biosynthesis of rapamycin. Of the nitrogen sources studied, ammonium sulfate was the best with respect to formation of rapamycin, and supported cell growth comparable to the organic nitrogen sources used in the control chemically defined medium, ie, aspartate, arginine plus histidine. In the new chemically defined medium, which is buffered with 200 mM 2-(N-morpholino)ethanesulfonic acid to prevent decline of pH during fermentation, an ammonium sulfate concentration of 40 mM was optimal for biosynthesis of rapamycin. Rapamycin production increased by more than 30% on both volumetric and specific bases as compared to the previous medium containing the three amino acids as nitrogen source. Received 08 November 1996/ Accepted in revised form 07 April 1997     

Optimization of nitrogen for enhanced citric acid productivity by a 2-deoxy D-glucose resistant culture of Aspergillus niger NGd-280

The present investigation is concerned with the optimization of nitrogen for enhanced citric acid productivity by a 2-deoxy D-glucose resistant culture of Aspergillus niger NGd-280 in a 15 l stirred tank bioreactor. Nutrients, especially nitrogen source have a marked influence on citrate productivity because it is an essential constituent of basal cell proteins. Citric acid has been known to be produced when the nitrogen source was the limiting factor. Ammonium nitrate was employed as a nitrogen source in the present study and batch culture experiments were carried out under various concentrations of ammonium nitrate. Specific growth rate was decreased and the biosynthesis of citric acid was delayed at higher concentrations of ammonium nitrate. Specific citric acid production rate was the highest when intracellular ammonium ion concentration was between 2.0 and 3.0 mmol g(-1) cells. Citrate production was however, stopped when intracellular ammonium ion concentration decreased below 1.0 mmol g(-1) cell.     

INFLUENCE OF AMMONIUM AND NITRATE ON THE PROTEIN- AND FREE AMINO ACIDS IN SHOOTS OF WHEAT SEEDLINGS

Weissman , Gerard S. (Rutgers U., Camden, N. J.) Influence of ammonium and nitrate on the protein- and amino acids in shoots of wheat seedlings. Amer. Jour. Bot. 46(5): 339–346. 1959.—Total and protein nitrogen per shoot of wheat seedlings grown with endosperm attached increased at a steady rate during a 96-hr. growth period, and protein nitrogen, as a percentage of total nitrogen, remained constant at about 53%. Total and protein nitrogen concentration was greatest for 24-hr. shoots and declined as the shoots became older. Total and protein nitrogen were determined in 96-hr. shoots of seedlings grown with endosperm attached but also supplied with ammonium, nitrate, or both in the culture solution. Total nitrogen was greatest in shoots supplied with ammonium, but only 38% was in the form of protein. Maximum protein synthesis occurred in shoots grown in both ammonium and nitrate and protein nitrogen as a percentage of total nitrogen approximated that achieved in shoots lacking nitrogen in the culture solution. The protein amino acid composition of 48-, 72-, and 96-hr. shoots was very similar but differed from 24-hr. shoots which contained higher percentages of arginine and lysine and lower percentages of alanine and threonine. This may be correlated with the higher proportion of meristematic cells in 24-hr. shoots. The protein amino acids in shoots grown with ammonium resembled that of shoots lacking nitrogen in the culture solution, but nitrate shoot protein contained a higher percentage of arginine and a lower percentage of lysine. Nitrate may stimulate the formation of enzymes, possibly of a nitrate-reducing system, with high arginine- low lysine content. Free asparagine and glutamine were both at a maximum in ammonium shoots and at a minimum in nitrate shoots, but asparagine predominated in shoots supplied with ammonium while glutamine was greatest in nitrate shoots. Aspartic acid, asparagine, and glutamine appeared to have ammonia-storage functions, but glutamic acid appeared to be primarily concerned with protein synthesis. Amino acid accumulation was greatest in shoots supplied with both ammonium and nitrate. Protein synthesis in these appeared to be limited by inadequate concentrations of glutamic acid and proline. A hypothesis is proposed in explanation of the high glutamic acid concentration in shoots provided with ammonium and nitrate.     

Effect of ammonium nutrition on the activity of nitrate reductase in the roots of apple seedlings

Active extracts of nitrate reductase were prepared from theroots of apple seedlings c.v. Granny Smith which were grownin nutrient solution under controlled enviromental conditions.The nutrient solutions contained various ratios of nitrate andammonium ions but all the treatments contained a total of 112ppm nitrogen. Maximum nitrate reductase activity in the roots was obtainedwhen plants were supplied with nitrate as the sole source ofnitrogen. Roots grown in solution containing only ammonium nitrogenhad little or no activity. When plants were supplied with bothforms of nitrogen in the nutrient solution, the presence ofammonium ions markedly lowered the activity of nitrate reductasein the roots. Plants supplied with 98 ppm nitrate nitrogen plus14 ppm ammonium nitrogen had activities only half those of plantsgrown in nitrate alone. Plants supplied with equal amounts ofammonium and nitrate nitrogen had activities less than one sixththose of plants grown in nitrate alone. (Received June 3, 1972; )     

Effects of fertilizer-based culture media on the production of exocellular polysaccharides and cellular superoxide dismutase by <Emphasis Type="Italic">Phaeodactylum tricornutum</Emphasis> (Bohlin)

Microalgae cultures are receiving attention because of increasing biotechnological and biomedical production of active biomolecules. We evaluated various fertilizer-based culture media to scale up production of the marine microalga Phaeodactylum tricornutum for production of exocellular polysaccharides (EPS), soluble proteins, and cellular superoxide dismutase (SOD). The standard source of sodium nitrate was the same as that used in the synthetic f/2 culture medium and ammonium nitrate, urea, ammonium sulfate, and calcium nitrate as alternative sources of nitrogen. The maximum production of EPS was achieved in microalgae cells grown in the culture media containing 63 and 23% nitrogen from ammonium sulfate, and also in microalgae cells grown in the culture media containing 3% nitrogen from ammonium nitrate. The maximum production of cellular SOD was achieved in microalgae cells grown in the culture media containing 35 and 26% nitrogen from ammonium sulfate, and in the culture media containing 17% nitrogen from urea. The results suggest that it is possible to use a source of nitrogen, other than sodium nitrate, to scale up growth of P. tricornutum for production of EPS and SOD at reduced costs.     

Quantum requirements for growth and fatty acid biosynthesis in the marine diatom Phaeodactylum tricornutum (Bacillariophyceae) in nitrogen replete and limited conditions

We determined the quantum requirements for growth (1/?_μ) and fatty acid (FA) biosynthesis (1/?_FA) in the marine diatom, Phaeodactylum tricornutum, grown in nutrient replete conditions with nitrate or ammonium as nitrogen sources, and under nitrogen limitation, achieved by transferring cells into nitrogen free medium or by inhibiting nitrate assimilation with tungstate. A treatment in which tungstate was supplemented to cells grown with ammonium was also included. In nutrient replete conditions, cells grew exponentially and possessed virtually identical 1/?_μ of 40–44 mol photons · mol C^?1. In parallel, 1/?_FA varied between 380 and 409 mol photons · mol C^?1 in the presence of nitrate, but declined to 348 mol photons · mol C^?1 with ammonium and to 250 mol photons · mol C^?1 with ammonium plus tungstate, indicating an increase in the efficiency of FA biosynthesis relative to cells grown on nitrate of 8% and 35%, respectively. While the molecular mechanism for this effect remains poorly understood, the results unambiguously reveal that cells grown on ammonium are able to direct more reductant to lipids. This analysis suggests that when cells are grown with a reduced nitrogen source, fatty acid biosynthesis can effectively become a sink for excess absorbed light, compensating for the absence of energetically demanding nitrate assimilation reactions. Our data further suggest that optimal lipid production efficiency is achieved when cells are in exponential growth, when nitrate assimilation is inhibited, and ammonium is the sole nitrogen source.     

氮硅添加对青藏高原高寒草甸土壤氮矿化的影响

为了解全球气候变化背景下氮沉降对土壤氮矿化的影响及硅添加对土壤氮矿化的促进作用, 该试验设置不同浓度的氮肥单独添加(0、20、40、60 g·m ^-2, 分别为对照CK、N20、N40、N60)以及与硅肥配施(硅酸4 g·m ^-2, Si4), 测定不同处理下0-20、20-40、40-60 cm土层土壤硝态氮含量、铵态氮含量、净硝化速率、净氨化速率以及净矿化速率。结果显示: (1)单独添加氮肥, 各土层土壤硝态氮和铵态氮含量均随处理浓度的增加而增加, 0-20 cm土层N20、N40、N60处理下土壤硝态氮和铵态氮分别较CK增加63.48%、126.04%、247.03%和80.66%、152.52%、244.56%; 随着土层深度增加, 土壤硝态氮、铵态氮含量均有下降, 20-40、40-60 cm土层较0-20 cm土层硝态氮含量分别平均减少53.90%、76.05%, 铵态氮含量分别平均减少48.62%、68.23%。(2)土壤净硝化速率、净氨化速率及净矿化速率随着氮肥浓度增加均呈上升趋势。相同氮肥添加浓度下, 土壤净硝化速率、净氨化速率和净矿化速率随着土层深度增加逐渐下降(除CK外)。(3)与单独添加氮肥比较, 氮硅肥配施, 土壤氮含量有显著提高, 在0-20 cm土层硝态氮和铵态氮较CK分别增加98.78%、192.62%、330.16%和99.96%、195.82%、306.32%, 20-40、40-60 cm土层也有类似趋势。同时, 氮硅配施促进了土壤氮矿化行为, 在0-20 cm土层, N60Si4处理下的土壤净硝化速率、净氨化速率较单独施氮时分别增加35.88%、27.41%。以上结果表明, 与单独氮肥添加相比, 氮硅配施不但能提高土壤氮含量, 而且能促进土壤氮的矿化作用, 对大气氮沉降有一定的缓解作用。     

白葡萄酒活性干酵母对不同氮源利用的研究

选用5种不同的白葡萄酒活性干酵母,以硫酸铵、氯化铵、硝酸铵、尿素、酵母粉等5种物质为氮源,观察其生长量并称量菌体重,以此分析其对氮源利用情况及不同氮源对酵母生长的影响。研究表明:供试菌系在不同氮源中均能生长。不同氮源对酵母的生长速度和生长量有不同影响;不同酵母菌种对不同氮源的利用也有差异。在以酵母粉为氮源的培养基中生长最好。在实验提供的氮源中,酵母粉为供试菌最优氮源,其次是硫酸铵,氯化铵与硫酸铵基本相当,而硝酸铵最差。8#菌种对各种氮源的利用能力相对较强。17#菌种对各种氮源的利用能力最弱。     

NUTRITIONAL REQUIREMENTS FOR OVULE FORMATION IN EXCISED PISTILS OF NIGELLA

The nutritional requirements for ovule formation in Nigella saliva L. were investigated by growing excised pistils on defined media. Pistils grown on a medium containing the minerals of Murashige and Skoog produced significantly more ovules than on a medium containing the minerals of Bilderback. When the nitrogen, sulfate, and phosphate of the Bilderback medium were adjusted to levels comparable to those of the Murashige and Skoog medium, a similar number of ovules was formed. The effect of different forms and concentrations of nitrogen on ovule formation and pistil growth was investigated. High concentrations of nitrate (40 mil) favored pistil growth and ovule formation, but comparable levels of ammonium were toxic. When ammonium at concentrations above 10 mM was added to nitrate media, ovule formation was inhibited. A medium containing low concentrations of ammonium (10 mM) and nitrate (5 mM) supported more ovule formation and pistil growth in young pistils than a low-nitrate (5 mM) medium without ammonium. However, ovule formation on a medium containing 10 mM ammonium and 5 mM nitrate was significantly less than on a medium containing only 15 mM nitrate. Low concentrations of organic nitrogen in the form of α-alanine (1 mM) and γ-aminobutyric acid (5 mM) supported ovule formation and pistil growth similar to a high nitrate medium.     

Preferential Assimilation of Ammonium Ions from Ammonium Nitrate Solutions by Apple Seedlings

Apple seedlings, Pyrus malus L., were grown in complete nutrient solutions containing nitrate, ammonium, or ammonium plus nitrate as the nitrogen source. Uptake of nitrogen was calculated from depletion measurements of the nutrient solutions and by using ¹⁵N labelled nitrate and ammonium salts. If the plants received nitrogen as ammonium only or as nitrate only, the amounts of nitrogen taken up were similar. However, if the seedlings were supplied with ammonium nitrate, the amount of nitrate-nitrogen assimilated was only half that of ammonium. Nevertheless, if ammonium and nitrate were supplied to a plant with a split-root system, with each root half receiving a different ion, the uptakes were similar. The possibility of independent inhibition by ammonium of both nitrate uptake and reduction in the roots is discussed.     

Interactions between nitrate and ammonium during uptake by carob seedlings and the effect of the form of earlier nitrogen nutrition

Seedlings of carob ( Ceratonia siliqua L. cv. Mulata) were used in two sets of experiments in order to evaluate; (1) the reciprocal effects of each nitrogen form on net uptake of nitrate and ammonium, and (2) the effect of earlier nitrogen nutrition on ammonium versus nitrate uptake. In the former group of experiments we studied the kinetics of nitrate and ammonium uptake as well as the interference of each of the two forms with net uptake of ammonium and nitrate by both nitrogen depleted and nitrogen fed carob seedlings. On the whole, nitrogen depletion led to increase in both affinity and V_max of the system for both forms of nitrogen, at the same time as the effects of nitrate on uptake of ammonium and vice versa were concentration dependent. In the second group of experiments the effects of earlier nitrogen nutrition on nitrate and ammonium uptake were characterized, and in this case we observed that: (a) if only one form of N was supplied, ammonium was taken up in greater amounts than nitrate; (b) the presence of ammonium enhanced nitrate uptake; (c) ammonium uptake was inhibited by nitrate; (d) there was a significant effect of the earlier nitrogen nutrition on the response of the plants to a different nitrogen source. The latter was evident mainly as regards ammonium uptake by plants grown in ammonium nitrate. The interactions between nitrate and ammonium uptake systems are discussed on the basis of the adaptation to the nitrogen source during early growth.     

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。     

Studies on aspergilli XVII. Effect of nitrogen sources on growth and fruiting

Summary The effect of ammonium, nitrate, and organic nitrogen on growth and sporulation of 18 Aspergilli was examined in a chemically defined medium in surface culture under controlled conditions. All three forms of nitrogen were metabolized by all the Aspergilli tested. Ammonium nitrogen was not good both for growth and fruiting. This was due to the sharp fall in the pH level which resulted due to the rapid utilization of anions of the ammonium nitrogen than cations. The effect of adding succinic acid in the medium containing ammonium nitrogen has been discussed.Good growth of Aspergilli in media containing nitrate nitrogen with the accompanying rise in the pH of the medium showed that these species are capable of reducing nitrate nitrogen to the level of ammonia. The role of succinic acid in the utilization of nitrate nitrogen was investigated. All fungi accomplished good growth on a medium containing asparagine.     

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/ NO 3--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NO3--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/ NO3--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。     

Ureidosuccinic Acid Uptake in Yeast and Some Aspects of Its Regulation

Ureidosuccinic acid (USA) is an intermediary product in pyrimidine biosynthesis. When proline was the sole nitrogen source, USA uptake occurred; however, when ammonium sulfate or glutamic acid was the nitrogen source, uptake was inhibited. Thus, a ura2 strain which does not synthesize USA would not grow when this substance was supplied on an ammonium sulfate or glutamic acid medium. Mutants are described in which uptake was constitutive on such a medium. Permeaseless mutants for USA have been found, and evidence is presented for permease specificity. It is shown that all constitutive mutants use the same transport system that is missing in the permeaseless mutant. These mutants are constitutive for two permeases: the specific USA permease and the general amino acid permease. The transport system studied here, like the general amino acid transport system, is regulated by nitrogen metabolism. These facts and others suggest that our permease constitutive mutants are impaired in nitrogen metabolism.     

Interaction of sulfur and nitrogen nutrition in tobacco (Nicotiana tabacum) plants: significance of nitrogen source and root nitrate reductase

Abstract: The significance of root nitrate reductase for sulfur assimilation was studied in tobacco (Nicotiana tabacum) plants. For this purpose, uptake, assimilation, and long-distance transport of sulfur were compared between wild-type tobacco and transformants lacking root nitrate reductase, cultivated either with nitrate or with ammonium nitrate. A recently developed empirical model of plant internal nitrogen cycling was adapted to sulfur and applied to characterise whole plant sulfur relations in wild-type tobacco and the transformant. Both transformation and nitrogen nutrition strongly affected sulfur pools and sulfur fluxes. Transformation decreased the rate of sulfate uptake in nitrate-grown plants and root sulfate and total sulfur contents in root biomass, irrespective of N nutrition. Nevertheless, glutathione levels were enhanced in the roots of transformed plants. This may be a consequence of enhanced APR activity in the leaves that also resulted in enhanced organic sulfur content in the leaves of the tranformants. The lack of nitrate reductase in the roots in the transformants caused regulatory changes in sulfur metabolism that resembled those observed under nitrogen deficiency. Nitrate nutrition reduced total sulfur content and all the major fractions analysed in the leaves, but not in the roots, compared to ammonium nitrate supply. The enhanced organic sulfur and glutathione levels in ammonium nitrate-fed plants corresponded well to elevated APR activity. But foliar sulfate contents also increased due to decreased re-allocation of sulfate into the phloem of ammonium nitrate-fed plants. Further studies will elucidate whether this decrease is achieved by downregulation of a specific sulfate transporter in vascular tissues.     

Ammonia Inhibition of Plasmid pRmeGR4a Conjugal Transfer between Rhizobium meliloti Strains

We have examined nutritional factors influencing conjugal transfer of the two nonsymbiotic large plasmids, pRmeGR4a and pRmeGR4b, of Rhizobium meliloti GR4. To monitor transfer, each plasmid was tagged with a different antibiotic resistance marker. Transfer of plasmid pRmeGR4b was dependent upon the presence of plasmid pRmeGR4a on the same donor cell. Transconjugants for pRmeGR4b were obtained at frequencies 5-to 10-fold higher than transconjugants carrying both plasmids, indicating that mobilization of pRmeGR4b by pRmeGR4a probably occurred in trans. Conjugal transfer of the tagged plasmids between R. meliloti strains was tested on minimal medium supplemented with single amino acids, nitrate, or ammonium as the single nitrogen source. A higher number of transconjugants was obtained when glutamate was the only nitrogen source, whereas conjugation was virtually undetectable on ammonium. No relationship was found between donor or recipient growth rate and plasmid transfer rate on a given nitrogen source. Furthermore, in media containing both glutamate and ammonium as nitrogen sources, transfer was reduced almost 100-fold compared with that in media containing glutamate alone. Inhibition was readily detected at 2.5 mM or higher concentrations of either ammonium chloride or ammonium sulfate and appeared to be specific for exogenously supplied ammonium. Inhibition of conjugal transfer between R. meliloti strains by ammonium was only observed for rhizobial plasmids, not for a heterologous plasmid such as RP4. Apparently, ammonium did not affect the plasmid-encoded transfer machinery, as it had no influence on rhizobial plasmid transfer from R. meliloti to Agrobacterium tumefaciens. The effect of ammonium seemed to take place on R. meliloti recipient cells, thereby reducing the efficiency of plasmid conjugation, probably by affecting mating pair formation or stabilization.