rDNA magnification in D. melanogaster: state of rDNA copies following the first step.

Summary D. melanogaster males of bb/O genetic constitution undergoing rDNA magnification were mated singly to XXbb ⁺/O females, yielding bb/O male progeny, and to X_NO-w sn bb ⁺ fameles, yielding bb/X_NO- females. The male and female offspring were scored for the bb ⁺ phenotype.Results show that there is a higher percentage of bb ⁺ flies in the bb/O male progeny than in bb/X_NO- females progeny, in single crosses as well as in the combined data. rRNA/DNA hybridization experiments agree with this observation, by showing that the rDNA content in the progeny of premagnified flies was higher in the sons than in the daughters.These data indicate that the increase of ribosomal RNA genes is not due to a stable event such as an unequal mitotic sister exchange, whereas they do not contrast with the extracopy model.     

Check of Gene Number during the Process of rDNA Magnification

THE multiple sequences of rDNA (DNA complementary to ribosomal RNA) of the Drosophila genome are localized at the bobbed locus, located in the X chromosome, position 66 and in the short arm of the Y chromosome^1,2. Wild bobbed (bb⁺) is that locus which, without a partner, gives rise to a normal phenotype. That locus which in similar conditions is incapable of giving rise to a normal phenotype is called a bobbed mutation (bb) and contains fewer genes for rRNA. The number of genes for rRNA in different individuals can vary considerably. One mechanism for rDNA variation is unequal crossing over³. Another mechanism, described by Tartof⁴, becomes apparent when individual flies, carrying only one bobbed locus, are constructed and only if such a locus is on the X chromosome; that is, if one constructs Xbb⁺/O males (and also Xbb/O males) or Xbb⁺/X_NO- females. Such individuals show a higher rDNA content than expected from the analysis of the same locus in Xbb⁺/Xbb⁺ females or in Xbb⁺/Ybb⁺ males. The increase of rDNA in this case is not inheritable⁴.     

Regulation of rRNA gene number in Drosophila melanogaster: new aspects resulting from the use of free duplications

Summary We have isolated a bobbed (bb) mutant on the free duplication Dp(1; f)122bb ⁺ and we have measured the rDNA content of the bb ⁺ and the bb loci in genetic combinations in which none of the phenomena involved in the change of the rDNA redundancy occurs. We have also measured the rDNA content of the two bb loci carried by the free duplications in two different genetic combinations: (1) and females in which there are two attached X chromosomes completely deleted for the nucleolus organizer (NO) regions and there-fore the only rDNA is contributed by the free duplication; (2) X/Dp122bb ⁺ and X/Dp122bb males, in which there are two bb loci, one on the X chromosome and the other on the X free duplication.The bb ⁺ and the bb duplications produced an overall increase of the rDNA content in the two genetic conditions tested.These results are not in favour of both a cis and trans effect of the regulator locus (cr ⁺ locus) hypothesised as being involved in the disproportionate replication of rRNA genes.     

X-Y Exchange and the Coevolution of the X and Y Rdna Arrays in Drosophila melanogaster

The nucleolus organizers on the X and Y chromosomes of Drosophila melanogaster are the sites of 200-250 tandemly repeated genes for ribosomal RNA. As there is no meiotic crossing over in male Drosophila, the X and Y chromosomal rDNA arrays should be evolutionarily independent, and therefore divergent. The rRNAs produced by X and Y are, however, very similar, if not identical. Molecular, genetic and cytological analyses of a series of X chromosome rDNA deletions (bb alleles) showed that they arose by unequal exchange through the nucleolus organizers of the X and Y chromosomes. Three separate exchange events generated compound X·Y ^L chromosomes carrying mainly Y-specific rDNA. This led to the hypothesis that X-Y exchange is responsible for the coevolution of X and Y chromosomal rDNA. We have tested and confirmed several of the predictions of this hypothesis: First, X· Y^L chromosomes must be found in wild populations. We have found such a chromosome. Second, the X·Y^L chromosome must lose the Y^L arm, and/or be at a selective disadvantage to normal X⁺ chromosomes, to retain the normal morphology of the X chromosome. Six of seventeen sublines founded from homozygous X·Y^Lbb stocks have become fixed for chromosomes with spontaneous loss of part or all of the appended Y^L. Third, rDNA variants on the X chromosome are expected to be clustered within the X⁺ nucleolus organizer, recently donated (" Y") forms being proximal, and X-specific forms distal. We present evidence for clustering of rRNA genes containing Type 1 insertions. Consequently, X-Y exchange is probably responsible for the coevolution of X and Y rDNA arrays.     

Cytological localization of ribosomal cistrons in polytene chromosomes of Phaseolus coccineus

Tritiated ribosomal RNA (rRNA) was prepared from hypocotyls of Phaseolus coccineus grown in liquid culture in the dark and in presence of 5-³H-uridine. A mixture of the 18S and 25S ³H-rRNA fractions was used for hybridization with DNA in the polytene chromosome cells of the embryo suspensor of P. coccineus. It was shown that the ribosomal cistrons (rDNA) are located in the nucleolus organizing system (satellite, nucleolar constriction and organizer) of the satellited chromosome pairs I (S₁) and V (S₂), in the proximal heterochromatic segment of the long arm of chromosomes S₁ and in the terminal heterochromatic segment of chromosome pair II. The micronucleoli which are produced by the satellite and nucleolus organizer of the chromosome pair S₁ contain rDNA; on the contrary, no rRNA-DNA hybridization is found in the DNA containing granules which are produced by the satellite and nucleolus organizer of chromosome pair S₂. The DNA which is amplified during production of DNA puffs at some chromosomal regions apparently does not code for ribosomal RNA (no detectable rRNA-DNA hybridization).Publication no. 62 from the Laboratorio di Mutagenesi e Differenziamento, Consiglio Nazionale delle Ricerche, Pisa. Part of the investigation was supported by Contract SC 001/076-69-1 BIAN between the European Atomic Energy Community and the University of Pisa, Institute of Genetics.     

Nonselective amplification of ribosomal DNA repeat units in compensating genotypes of Drosophila melanogaster

Compensation is a mechanism by which the X-chromosome nucleolus organizer region of Drosophila melanogaster can increase its ribosomal DNA content up to twofold. It occurs in somatic cells under specific genetic conditions and is mediated by a defined genetic site, the compensatory response locus. The In and various type I ribosomal DNA repeat units were separated by restriction endonuclease digestion. Comparison of the percentages of these repeat unit types between compensating and noncompensating genotypes showed the same distribution. Therefore no selective amplification of these repeat unit types occurs during ribosomal DNA compensation. These results demonstrate that two processes of rDNA amplification in somatic cells, compensation and independent rDNA polytenization, are exclusive events.This research was supported by NIH Grant GM 28008.     

A dynamic whole-plant model of integrated metabolism of nitrogen and carbon. 1. Comparative ecological implications of ammonium-nitrate interactions

The dynamics of ammonium (NH₄ ⁺) and nitrate (NO₃ ^-) concentrations in the soil solution is an important determinant of the species composition of natural vegetation. A mathematical model of uptake, assimilation and translocation of NH₄ ⁺ and NO₃ ^- is presented to assess the performance of species with respect to NO₃ ^-/NH₄ ⁺ feeding characterised by physiologically defined parameters. Nitrate efflux is explicitly considered. The capacities of NO₃ ^-, [U _NM], and NH₄ ⁺ influx, [U _AM], and NO₃ ^- reduction, [A _NM], appear sufficient to characterise whole-plant N metabolism including NO₃ ^- translocation. The parameter space made up by these parameters is represented by 276 parameter combinations (`species'). Simulated total net N uptake rate and C costs for uptake and assimilation per mole total net N taken up are used to decide on how a species profits or suffers from NO<sub>3</sub> <sup>-</sup>+NH<sub>4</sub> <sup>+</sup> mixtures relative to pure N forms with similar total N concentration for external concentrations up to 1.6 mM. Five response categories were identified and contrasted with categories defined by Bogner (1968) on the basis of experimental results on forest plants. The largest category comprises species that respond positively to NO<sub>3</sub> <sup>-</sup> and positively or indifferently to NH<sub>4</sub> <sup>+</sup>. These species have intermediate to high [U <sub>NM</sub>] and [A <sub>NM</sub>] and variable [U <sub>AM</sub>] and correspond to woodland edge species and forest plants on rich soil including typical `nitrophilic' species. This category fades into a group of species that respond positively to NO₃ ^- and negatively to NH₄ ⁺. These species have high [U _NM] and low [U _AM] and [A _NM]; several species from oak-hornbeam woodland (Carpinion) belong to this group. Many parameter combinations were found that responded positively to NH₄ ⁺ and indifferently to NO₃ ^-: low to medium [U _NM], medium to high [U _AM] and variable [A _NM]. This category includes all heathland species. No species were found which responded negatively to NO₃ ^-. The physiological background of differences between the categories is explained with respect to the equilibrium NO₃ ^- concentration in roots, influx, efflux, translocation and assimilation of NO₃ ^- and uptake and assimilation of NH₄ ⁺. The relationship between NO₃ ^- accumulation capacity and morphology is discussed. Some slow-growing species with high [U _NM] and low [A _NM] use NO₃ ^- mainly as an osmotic solute. Respiratory costs in roots of inherently slow-growing species are discussed with respect to patterns in NH₄ ⁺ and NO₃ ^- availabilities of their habitat. This revised version was published online in June 2006 with corrections to the Cover Date.     

Male-Sterilizing Interactions between Duplications and Deficiencies for Proximal X-Chromosome Material in DROSOPHILA MELANOGASTER

The genetic limits of sixty-four deficiencies in the vicinity of the euchromatic-heterochromatic junction of the X chromosome were mapped with respect to a number of proximal recessive lethal mutations. They were also tested for male fertility in combination with three Y chromosomes carrying different amounts of proximal X-chromosome-derived material (B^SYy⁺, y⁺Ymal¹²⁶ and y⁺Ymal⁺). All deficiencies that did not include the locus of bb and a few that did were male-fertile in all male-viable Df(1)/Dp(1;Y) combinations. Nineteen bb deficiencies fell into six different classes by virtue of their male-fertility phenotypes when combined with the duplicated Y chromosomes. The six categories of deficiencies are consistent with a formalism that invokes three factors or regions at the base of the X, one distal and two proximal to bb, which bind a substance critical for precocious inactivation of the X chromosome in the primary spermatocyte. Free duplications carrying these regions or factors compete for the substance in such a way that, in the presence of such duplications, proximally deficient X chromosomes are unable to command sufficient substance for proper control of X-chromosome gene activity preparatory to spermatogenesis. We conclude that there is no single factor at the base of the X that is required for the fertility of males whose genotype is otherwise normal.     

The compensatory response locus deletion increases the number of nucleoli in Drosophila melanogaster polytene cells

The formation of ribosomal DNA (rDNA) not associated with the nucleolar organizer (NO) regions was studied in polytene cells of Drosophila melanogaster mutants, mal ¹² and bb ^2rl , heterozygous for deficiencies in the NO. In the mutant X chromosome mal ¹² a smaller part of the NO is deleted than in bb ^2rl but it comprises the compensatory response (cr) locus, controlling the compensatory synthesis of rDNA. We found that in the polytene cells of all tissues of mutant X/Xmal ¹² investigated (larval salivary gland, fat body and midgut; adult fly midgut) the number of nucleoli was increased compared with that in the corresponding cells of X/Xbb ^2rl and X/X (wild type). Using in situ hybridization we showed that in the salivary gland cells of the X/Xmal ¹² larvae chromosomal sites containing type I insertion sequences and scattered throughout the genome were more frequent than in the wild type and in X/Xbb ^2rl mutant cells.     

A Study of Rdna Magnification Phenomenon in a Repair-Recombination Deficient Mutant of DROSOPHILA MELANOGASTER

The rDNA magnification process consists of a rapid and inheritable rDNA increase occurring in bobbed males: in a few generations the bb loci acquire the wild-type rDNA value and reach a bb⁺ phenotype.—We have analyzed the rDNA magnification process in the repair-recombination-deficient mutant mei9^a, both at the phenotypical and rDNA content levels. In mei9^a bb double mutants the recovery of bb⁺ phenotype is strongly disturbed and the rDNA redundancy value fails to reach the wild-type level. The strong effect of this meiotic mutation on rDNA magnification suggests a close relationship between this phenomenon and the repair-recombination processes.     

Comparison between rDNA magnification and bb lethal mutation frequencies in Drosophila melanogaster

Summary Unequal mitotic sister strand crossing over has been evoked to explain the occurrence of phenotypically bb ^- males in the progeny of phenotypically bobbed males during magnification. If this is the case, complementary bb ¹ loci should be obtained together with the bb ¹. To test this hypothesis we compared the frequency of bb lethal mutations in the sperms of bb males with the percentage of phenotypically bb ⁺ males obtained during magnification of these bb males. We then compared these values with those occurring in phenotypically bb ⁺ control males. We found that, while the number of bb ⁺ males obtained during magnification, though variable, is high, the bb lethal mutation occurs at a very low frequency in all the genetic conditions, whatever the phenotype of the parental male.     

The effect of root temperature on the induction of nitrate reductase activities and nitrogen uptake rates in arctic plant species

We investigated whether six arctic plant species have the potential to induce nitrate reductase (NR) activity when exposed to NO₃ ^--nitrogen under controlled environment conditions, using an in vivo assay that uses the rate of NO₂ ^--accumulation to estimate potential NR activity. We also assessed the effect of low root temperatures on NR activity, growth and nitrogen uptake (using ¹⁵N applications) in two of the selected species. Five of the six species (Cerastium alpinum, Dryas intergrifolia, Oxyria digyna, Saxifraga cernua and Salix arctica) were capable of inducing NR activity when exposed to solutions containing 0.5 mM NO₃ ^- at 20°C for 10 days. Although in vivo NR activity was not induced in Saxifraga oppositifolia under controlled conditions, we conclude that it was capable of growing successfully on NO₃ ^-, due to the presence of moderate rates of NR activity observed in both NH₄ ⁺-grown and NO₃ ^--treated plants. Exposure of O. digyna and D. integrifolia to 3°C root temperatures for two weeks, with the shoots kept at 20°C, resulted in root and leaf NR activity rates of NO₃ ^--treated plants being reduced to rates exhibited by NH₄ ⁺-grown plants. Although these decreases in NR in both species appeared to be due to limitations in NO₃ ^--uptake and growth rate (rather than direct low-temperature inhibition of NR synthesis per se), direct low-temperature inhibition of root NR synthesis could not be ruled out. In contrast to the temperature insensitivity of NH₄ ⁺ uptake in D. integrifolia, NO₃ ^--uptake in D. integrifolia was inhibited by low root temperatures. We conclude that the selected arctic species have the genetic potential to utilize NO₃ ^--nitrogen, and that low root temperatures, in conjunction with other environmental limitations, may be responsible for the lack of induction of NR in D. integrifolia and Salix arctica under field conditions.     

UPTAKE OF INORGANIC NITROGEN BY CODIUM FRAGILE SUBSP. TOMENTOSOIDES (CHLOROPHYTA)1

The uptake of nitrate, nitrite and ammonium by Codium fragile subsp. tomentosoides (van Goor) Silva was measured at different combinations of temperature (6–30 C) and irradiance (0–140 μEin.m-^2. s^-1). Uptake of all three forms of N was greater at 12–24 C than at 6 and 30 C. Although uptake was stimulated by light, saturation occurred at relatively low irradiance (7–28 μEin m^-2 s^-1, depending on the N source and temperature). The Michaelis-Menten uptake constants (V_max K)varied with temperature. V_max was greatest at intermediate temperatures and K was lowest at lower temperatures. The V_maxfor NH₄⁺ was higher and the K, for NH₄⁺was lower than those for NO₃^-_- and NO₂^-_-. Codium was capable of simultaneously taking up all three forms of inorganic N although the presence of NH₄⁺ reduced the uptake of both NO₃^-_- and NO₂^-_-. The results of this study indicate that part of the ecological success of Codium in a N-limited environment may be due to its N uptake capabilities.     

Ribosomal RNA Cistrons of X Chromosomes Clonally Derived from D. MELANOGASTER Laboratory Populations: Redundancy, Organization and Stability

To determine whether heterogeneity exists in the organization or redundancy of the rRNA cistrons of inbred populations of Drosophila melanogaster, we have derived a number of sublines from the strains Oregon R and Canton S. These two strains were chosen because our previous studies have demonstrated a difference in the competence of these strains to exhibit a "compensatory response" of the rDNA. In each subline, the X chromosomes are descended from a single maternal X, that is, each line is homozygous for a particular nucleolus organizer (NO). These derivative lines have been characterized in terms of rDNA content and organization, using quantitative liquid hybridizations and Southern blot analyses, respectively. Our studies reveal that both of the highly inbred parent populations contained a heterogeneous array of X chromosomal rDNA contents. Once isogenized, the rDNA redundancy of a given X chromosomal NO can be shown to remain stable for at least 20 generations in culture. We detect no restriction pattern heterogeneity among X chromosomes isolated from a given strain, despite relatively large differences in their rDNA contents. This leads us to suggest that there is no significant clustering of intervening sequence-bearing (ivs ⁺) genes within the rDNA loci of chromosomes from the populations examined. Furthermore, we conclude that apparent alterations in rDNA redundancy known as the compensatory response are not related to the heterogeneity of rDNA content within a population.     

Differential magnification of rDNA gene types in bobbed mutants of Drosophila melanogaster

Summary In Drosophila melanogaster a partial loss of ribosomal genes leads to the bobbed phenotype. Magnification is a heritable increase in rDNA that may occur in males carrying a deleted X chromosome with a strong bobbed phenotype. The restriction patterns of X chromosome total rDNA, insertions and spacers from magnified bobbed strains were compared with those of the original bobbed mutations. It was found that magnification modifies restriction patterns and differentially affects gene types, increasing specific genes lacking insertions (INS^-). Increases in copy number of genes with type I insertions are generally lower than the total number of INS^- genes, while type II insertion genes are not perceptibly increased. The recovery of homogeneous progeny from a single premagnified male indicates that the magnification event might take place and become stable very early in the germ line, arguing against magnification being due to extrachromosomal amplification. Additionally, some gene types increase 3.5-fold while others are eliminated, indicating that they could not result from a single unequal cross-over. These results are in good agreement with the existence of partial clustering of rDNA genes according to type, and suggest that magnification could result from local amplification of genes.     

Interactive effect of salt (NaCl) and nitrogen form on growth, water relations and photosynthetic capacity of sunflower (Helianthus annum L.)

The effects of the ammonium (NH₄⁺) and nitrate (NO₃^-) forms of nitrogen and NaCl on the growth, water relations and photosynthesis performance of sunflower (Helianthus annuus L.) were examined under glasshouse conditions. Eight-day-old plants of cv. Hisun 33 were subjected for 21 days to Hoagland's nutrient solution containing 8 mol m^-3N as NH₄⁺or NO₃^-, and salinised with 0, 60, or 120 mol m^-3NaCl. Fresh weights of shoots and roots, and leaf area of NO₃^-supplied non-salinised plants were significantly greater than those of NH₄⁺-supplied non-salinised plants. But addition of NaCl to the rooting medium of these plants had more inhibitory effect on the growth of NO₃^--supplied plants than on NH₄⁺-supplied plants. Both leaf water and osmotic potentials of plants grown with NH₄⁺were lower than those of plants given NO₃^-under both non-saline and saline conditions. Chlorophylls a and b concentrations were higher in plants grown with NH₄⁺than N0₃^--supplied plants at the lower two levels of salinisation. The rate of photosynthesis in plants was considerably higher in non-salinised plants grown with NO₃^-than with NH₄⁺, but with increase in salinisation the photosynthesis rate decreased in NO₃^--supplied plants, but not in those given NH₄⁺. The rate of transpiration was increased significantly by salinisation in NO₃^--supplied plants, but not consistently so in NH₄⁺-supplied plants. The stomatal conductances were much higher in plants given NO₃^-than with NH₄⁺when grown under non-saline conditions, but not when salinised. As a consequence, water-use efficiency in NO₃^--supplied control plants was better than in NH₄⁺-supplied under non-saline conditions, but worse under saline conditions. The different forms of nitrogen and the addition of NaCl to the growing medium did not affect the relative intercellular concentrations of CO₂ (C_i/C_a). Overall, the NH₄⁺form of nitrogen inhibited the growth of sunflowers under non-saline conditions, but NO₃^-and NaCl interacted to inhibit growth more than did NH₄⁺under saline conditions.     

Nitrate and ammonium as nitrogen sources for lupins prior to nodulation

Two cultivars of Lupinus angustifolius L. were grown in a glasshouse in solutions containing NO₃ ^-, NH₄ ⁺ or NH₄NO₃ with a total nitrogen concentration of 2.8 M m^-3 in each treatment. One cultivar chosen (75A-258) was relatively tolerant to alkaline soils whereas the other (Yandee) was intolerant to alkalinity. Controlled experiments were used to assess the impact of cationic vs. anionic forms of nitrogen on the relative performance of these cultivars. Relative growth rates (dry weight basis) were not significantly different between the two cultivars when grown in the presence of NO₃ ^-, NH₄ ⁺ or NH₄NO₃. However, when NO₃ ^- was supplied, there was a modest decline in relative growth rates in both cultivars over time. When plants grown on the three sources of nitrogen for 9 days were subsequently supplied with ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ for 30 h, NH₄ ⁺ uptake was generally twice as fast as NO₃ ^- uptake, even for plants grown in the presence of NO₃ ^-. Low rates of NO₃ ^- uptake accounted for the decrease in growth rates over time when plants were grown in the presence of NO₃ ^-. It is concluded that the more rapid growth of 75A-258 than Yandee in alkaline conditions was not due to preferential uptake of NH₄ ⁺ and acidification of the external medium. In support of this view, acidification of the root medium was not significantly different between cultivars when NH₄ ⁺ was the sole nitrogen source.     

温度和水分变化对冻土区泥炭地土壤氮循环功能基因丰度的影响

以大兴安岭多年冻土区泥炭地为研究对象,通过室内模拟增温实验,研究温度升高对不同深度（0-150 cm）土壤氮循环功能基因丰度的影响。同时针对0-20 cm和20-40 cm土壤设置两个水分处理,分别为土壤原始含水量和淹水状态,研究水分变化对表层土壤氮循环功能基因丰度的影响。结果表明温度升高显著提高了活动层（0-60 cm）、过渡层（60-80 cm）、永冻层（80-100 cm）中nifH、nirK基因丰度,温度升高显著提高了活动层（0-40 cm）和过渡层（60-80 cm）中nirS基因丰度。温度升高显著提高了过渡层（60-80 cm）NH₄⁺-N和较深永冻层（140-150 cm）NO₃^--N的含量,但降低了过渡层（60-80 cm）NO₃^--N和较深永冻层（120-150 cm）NH₄⁺-N的含量,相关性分析表明,NH₄⁺-N含量与nifH和nirS基因丰度呈显著正相关,NO₃^--N含量与nirK基因丰度呈显著正相关,说明温度升高能够通过改变微生物丰度促进过渡层固氮作用和反硝化作用。在增温条件下,淹水处理使表层土壤nirS和nirK基因丰度及NH₄⁺-N含量降低,但提高了NO₃^--N含量,说明淹水造成了过度还原的条件使反硝化底物浓度降低,降低反硝化微生物活性进而抑制了土壤反硝化作用。该结果对于明确未来气候变化影响下冻土区泥炭地土壤氮循环过程具有重要意义。     

Growth,chemical composition,and nitrate reductase activity of Rumex species in relation to form and level of N supply

Growth, chemical composition, and nitrate reductase activity (NRA) of hydroponically cultured Rumex crispus, R. palustris, R. acetosa, and R. maritimus were studied in relation to form (NH₄ ⁺, NO₃ ^-, or both) and level of N supply (4 mM N, and zero-N following a period of 4mM N). A distinct preference for either NH₄ ⁺ or NO₃ ^- could not be established. All species were characterized by a very efficient uptake and utilization of N, irrespective of N source, as evident from high concentrations of organic N in the tissues and concurrent excessive accumulations of free NO₃ ^- and free NH₄ ⁺. Especially the accumulation of free NH₄ ⁺ was unusually large. Generally, relative growth rate (RGR) was highest with a combination of NH₄ ⁺ and NO₃ ^-. Compared to mixed N supply, RGR of NO₃ ^-- and NH₄ ⁺-grown plants declined on average 3% and 9%, respectively. Lowest RGR with NH₄ ⁺ supply probably resulted from direct or indirect toxicity effects associated with high NH₄ ⁺ and/or low Ca²⁺ contents of tissues. NRA in NO₃ ^- and NH₄NO₃ plants was very similar with maxima in the leaves of ca 40 μmol NO₂ ^- g^-1 DW h^-1. ‘Basal’ NRA levels in shoot tissues of NH₄ ⁺ plants appeared relatively high with maxima in the leaves of ca 20 μmol NO₂ ^- g^-1 DW h^-1. Carboxylate to organic N ratios, (C-A)/N_org, on a whole plant basis varied from 0.2 in NH₄ ⁺ plants to 0.9 in NO₃ ^- plants. After withdrawal of N, all accumulated NO₃ ^- and NH₄ ⁺ was assimilated into organic N and the organic N redistributed on a large scale. NRA rapidly declined to similar low levels, irrespective of previous N source. Shoot/root ratios of -N plants were 50–80% lower than those from +N plants. In comparison with +N, RGR of -N plants did not decline to a large extent, decreasing by only 15% in -NH₄ ⁺ plants due to very high initial organic-N contents. N-deprived plants all exhibited an excess cation over anion uptake (net proton efflux), and whole-plant (C-A)/N_org ratios increased to values around unity. Possible difficulties in interpreting the (C-A)/N_org ratio and NRA of plants in their natural habitats are briefly discussed.     

氮肥配施生化抑制剂组合对黄泥田土壤氮素淋溶特征的影响

揭示尿素类肥料添加生化抑制剂组合后,在黄泥田土壤中硝态氮(NO~-_3-N)和铵态氮(NH~+_4-N)的淋溶损失规律。采用室内土柱淋溶培养试验,研究脲酶抑制剂N-丁基硫代磷酰三胺(NBPT)和硝化抑制剂2-氯-6-(三氯甲基)吡啶(CP)单独添加及配合施用对尿素和尿素硝铵(300 kg N/hm~2)中氮(N)素在土体中淋溶损失的影响。结果表明:尿素和尿素硝铵处理淋溶液中NH~+_4-N和NO~-_3-N浓度均呈先升后降的变化趋势,而出峰时间不一。NH~+_4-N和NO~-_3-N淋失量随着时间的延长,处理间差异逐渐变大。NBPT处理可以减缓尿素水解,有效抑制NH~+_4-N生成,延缓其出峰时间,减少NH~+_4-N流失;CP处理可以有效抑制NH~+_4-N向NO~-_3-N转化,减少NO~-_3-N流失。与单独添加NBPT和CP处理相比,两者配施对N素淋溶损失有明显的协同抑制效果在黄泥田土壤中,既能减缓尿素水解,保持土壤中较高NH~+_4-N含量,又能降低淋溶液中NO~-_3-N浓度。培养结束时(第72天),UAN处理中NO~-_3-N、NH~+_4-N、矿质态N淋失总量及硝化率较U处理高34.39%、5.32%、31.72%和15.71%。U+NBPT、U+CP和U+NBPT+CP处理较U处理分别显著降低NO~-_3-N淋失总量达15.58%、114.77%和73.45%;UAN+NBPT、UAN+CP和UAN+NBPT+CP处理较UAN处理分别显著降低达15.88%、54.87%和37.46%。不同处理NO~-_3-N淋失总量大小表现为:UAN UAN+NBPT U UAN+NBPT+CP U+NBPT UAN+CP U+NBPT+CP U+CP CK。在一定施肥量条件下,NBPT和CP单独施用或配施均可降低黄泥田土壤中NO~-_3-N累积淋失量。对各处理淋溶液中NO~-_3-N淋失量(y)随时间(x)的变化进行拟合,其中以线性方程(y=ax+b)的拟合度较高,且各抑制剂处理a、b值均存在明显差异。总体认为,在黄泥田土壤中施用CP及其与NBPT配施可以显著降低土壤NO~-_3-N淋溶损失,减少N素淋失风险,提高肥料利用率。