Effects of nitrogen dioxide and nitrate nutrition on growth and nitrate assimilation in bean leaves

The influence of nutrient nitrate level (0-20 millimolar) on the effects of NO₂ (0-0.5 parts per million) on growth, K, photosynthetic pigment, N contents, and the activities of enzymes of N assimilation was studied in bean (Phaseolus vulgaris L. cv Kinghorn Wax) leaves. Exposing 7-day old bean seedlings for 5 days continuously to 0.02 to 0.5 parts per million NO₂ increased plant height, fresh weight, chlorophyll, carotenoid, organic N and nitrate contents, and nitrate reductase and glutamate synthase activities in the leaves of seedlings supplied with no external N. At 20 millimolar nitrate, most of the parameters examined were inhibited except for organic N and nitrate contents and glutamate synthase activity which increased in most cases. Generally, with an increase in NO₂ concentration, the stimulatory effect declined and/or the inhibitory effect increased. A 3-hour exposure of 12-day-old bean seedlings to 0.1 to 2.0 parts per million NO₂ increased nitrate content and nitrate reductase activity at each nutrient nitrate level except for a slight inhibition of enzyme activity during exposure to 2.0 parts per million NO₂ at 20 millimolar nitrate. The experiments demonstrated that the effect of NO₂ is strongly influenced by nutrient N level and that NO₂ is assimilated into organic nitrogenous compounds to serve as a source of N, only to a limited extent.     

Carbon and nitrogen limitations on soybean seedling development

Carbon and nitrogen limitations on symbiotically grown soybean seedlings (Glycine max [L.] Merr.) were assessed by providing 0.0, 1.0, or 8.0 millimolar NH₄NO₃ and 320 or 1,000 microliters CO₂/liter for 22 days after planting. Maximum development of the Rhizobium-soybean symbiosis, as determined by acetylene reduction, was measured in the presence of 1.0 millimolar NH₄NO₃ under both levels of CO₂. Raising NH₄NO₃ from 0.0 to 8.0 millimolar under 320 microliters CO₂/liter increased plant dry weight by 251% and Kjeldahl N content by 287% at 22 days after planting. Increasing NH₄NO₃ from 1.0 to 8.0 millimolar under 320 microliters CO₂/liter increased total dry weight and Kjeldahl N by 100 and 168%, respectively, on day 22. Raising CO₂ from 320 to 1,000 microliters CO₂/liter during the same period had no significant effect on Kjeldahl N content of plants grown with 0.0 or 1.0 millimolar NH₄NO₃. The maximum CO₂ treatment effects were observed in plants supplied with 8.0 millimolar NH₄NO₃, where dry weight and Kjeldahl N content were increased 64% and 20%, respectively. An increase in shoot CO₂-exchange rate associated with the CO₂-enrichment treatment was reflected in a significant increase in leaf dry weight and starch content for plants grown with 1,000 microliters CO₂/liter under all combined N treatments. These data show directly that seedling growth in symbiotically grown soybeans was limited primarily by N availability. The failure of the CO₂-enrichment treatment to increase total plant N significantly in Rhizobium-dependent plants indicates that root nodule development and functioning in such plants was not limited by photosynthate production.     

Nitrogen metabolism of soybeans: I. Effect of tungstate on nitrate utilization, nodulation, and growth

The effects of N source (6 mm nitrogen as NO₃⁻ or urea) and tungstate (0, 100, 200, 300, and 400 μm Na₂ WO₄) on nitrate metabolism, nodulation, and growth of soybean (Glycine max [L.] Merr.) plants were evaluated. Nitrate reductase activity and, to a lesser extent, NO₃⁻ content of leaf tissue decreased with the addition of tungstate to the nutrient growth medium. Concomitantly, nodule mass and acetylene reduction activity of NO₃⁻-grown plants increased with addition of tungstate to the nutrient solution. In contrast, nodule mass and acetylene reduction activity of urea-grown plants decreased with increased nutrient tungstate levels. The acetylene reduction activity of nodulated roots of NO₃⁻-grown plants was less than 10% of the activity of nodulated roots of urea-grown plants when no tungstate was added. At 300 and 400 μm tungstate levels, acetylene reduction activity of nodulated roots of NO₃⁻-grown plants exceeded the activity of comparable urea-grown plants.     

Nitrate inhibition of nodulation can be overcome by the ethylene inhibitor aminoethoxyvinylglycine

Previously, we reported (a) a positive correlation between the nitrate concentrations in growth medium and ethylene evolved from uninoculated and inoculated alfalfa (Medicago sativa) roots and (b) a negative correlation between ethylene evolution and nodulation. Here, we report that the inhibitory effect of NO₃⁻ on nodulation of alfalfa can be eliminated by the ethylene inhibitor aminoethoxyvinylglycine (AVG). This effect was probably related to the strong inhibition (90%) of ethylene biosynthesis caused by AVG in these inoculated and NO₃⁻-treated roots. These results support our hypothesis that the inhibitory effect of NO₃⁻ is mediated through the phytohormone ethylene. A possible role of endogenous ethylene in the autoregulation of nodulation also is discussed. AVG at 10 micromolar significantly (P < 0.05) increased total nitrogenase activity (acetylene reduction) in 2.5 and 5 millimolar NO₃⁻-fed plants probably as a result of the very high stimulation of nodulation.     

Growth and metabolic activity of lemon juice vesicle explants in vitro

The relationship between the relative abundance of ureides ([ureide-N/ureide-N plus nitrate-N] × 100) in the shoot axis (stems plus petioles), nodulated roots and leaflets of “Bragg” soybean (Glycine max [L.] Merrill) and the symbiotic dependence of these plants was examined under glass-house conditions. Plants, inoculated with effective Rhizobium japonicum CB1809, were grown with their roots exposed continuously to a nutrient solution containing either 0, 1.5, 3.0, 6.0 or 12.0 millimolar NO₃-N per liter. Nodulation and N₂-acetylene fixation were correlated inversely with the level of nitrate. Seasonal acetylene reduction profiles for each of the nitrate treatments were integrated and the symbiotic dependence ([N₂ fixed per total plant N] × 100) determined using a conversion ratio of 1.5:1 (acetylene reduced:N₂ fixed), calculated from the zero NO₃ treatment. Examination of the nitrogenous solutes of the shoot axis and nodulated roots showed linear relationships between the relative abundance of ureides and the symbiotic dependence of the plants. Two standard curves, depicting these relationships during vegetative and reproductive growth, were drawn for each plant part. The overriding effect of plant age invalidated any attempt to develop a standard relationship for leaflets. Data from two diurnal studies suggested that relative ureides were insensitive to diurnal fluctuations, thus simplifying sampling procedures. Plant material could be stored at ambient temperatures (20-30°C) for up to 24 h without affecting the relative concentration of ureides and nitrate. It is suggested that the shoot axis provides the most suitable target organ when using this technique as a quantitative assay for N₂ fixation because of ease of sampling of these tissues, especially with field-grown plants.     

Effect of inoculation and nitrogen on isoflavonoid concentration in wild-type and nodulation-mutant soybean roots

The isoflavones, daidzein and genistein, have been isolated and identified as the major inducers of nod genes of Bradyrhizobium japonicum. The common nod genes of rhizobia are in turn responsible for stimulating root hair curling and cortical root cell division, the earliest steps in the host response. This study evaluated whether there was a relationship between root isoflavonoid production and the hypernodulation phenotype of selected soybean (Glycine max [L.] Merr.) mutants. Three independently selected hypernodulating soybean mutants (NOD1-3, NOD2-4, and NOD3-7) and a nonnodulating mutant (NN5) were compared with the Williams parent for isoflavonoid concentrations. High performance liquid chromatographic analyses of soybean root extracts showed that all lines increased in daidzein, genistein, and coumestrol concentrations throughout the 12-day growth period after transplanting of both inoculated and noninoculated plants; transplanting and inoculation were done 6 days after planting. No significant differences were detected in the concentration of these compounds among the three noninoculated hypernodulating mutants and the Williams parent. In response to inoculation, the three hypernodulating mutants had higher isoflavonoid concentrations than did the Williams control at 9 to 12 days after inoculation when grown at 0 millimolar N level. However, the inoculated nonnodulating mutant also had higher isoflavonoid concentrations than did Williams. N application [urea, (NH₄)₂SO₄ and NO₃⁻] decreased the concentration of all three isoflavonoid compounds in all soybean lines. Application of NO₃⁻ was most inhibitory to isoflavonoid concentrations, and inhibition by NO₃⁻ was concentration dependent. These results are consistent with a conclusion that differential NO₃⁻ inhibition of nodulation may be partially due to changes in isoflavonoid levels, although the similar response of the nonnodulating mutant brings this conclusion into question. Alternatively, the nodulation control in the NN5 mutant may be due to factors totally unrelated to isoflavonoids, leaving open the possibility that isoflavonoids play a role in differential nodulation of lines genetically competent to nodulate.     

Economy of Carbon and Nitrogen in Nodulated and Nonnodulated (NO(3)-grown) Cowpea [Vigna unguiculata (L.) Walp.

The response of non-nodulated cowpea (Vigna unguiculata (L.) Walp. cv Caloona) to a wide range of NO₃ levels in the rooting medium was studied 40 days after sowing by in vitro assays of plant organs for NO₃ reductase (EC 1.6.6.1) and analyses of root bleeding (xylem) sap for nitrogenous solutes. Plants fed 1, 5, 10, 20, and 40 millimolar NO₃ showed, respectively, 64, 92, 94, and 91% of their total reductase activity in shoots and 34, 30, 66, 62, and 58% of the total N of their xylem sap as NO₃. These data, and the absence in the plants of significant pools of stored NO₃, indicated that shoots were major organs of NO₃ assimilation, especially at levels of NO₃ (10 to 40 millimolar) that maintained plant growth at near maximum rates. Partitioning and utilization of C and N were studied in nodulated, minus NO₃ plants and non-nodulated plants fed 10 or 20 millimolar NO₃, the levels of NO₃ which gave rates of growth and N assimilation closest to those of the symbiotic plants. The conversion of the C of net photosynthate to dry matter was similar in nodulated plants (67%) and NO₃-grown plants (64%), but greater proportions of photosynthate were translocated to below ground parts of nodulated plants (37%) than of NO₃-fed plants (23 to 26%). Greater photosynthate consumption by nodulated roots was associated with proportionately greater root growth and respiration and 2-fold greater export of C in xylem than in the NO₃-fed plants. Theoretical considerations suggest that the elevated CO₂ output of nodulated roots was due not only to CO₂ loss associated with nodule function, but also to a much greater nonassimilatory component of respiration in the supporting root of the nodulated plant compared to roots of the NO₃-fed plants. Data are compared with previously published information from other legumes.     

Root respiration associated with nitrate assimilation by cowpea

Nitrate uptake by roots of cowpea (Vigna unguiculata) was measured using ¹⁵NO₃⁻, and the energy cost to the root was estimated by respirometry. Roots of 8-day-old cowpea seedlings respired 0.6 to 0.8 milligram CO₂ per plant per hour for growth and maintenance. Adding 10 millimolar NO₃⁻ to the root medium increased respiration by 20 to 30% during the following 6 hours. This increase was not observed if the shoots were in the dark. Removal of NO₃⁻ from the root medium slowed the increase of root respiration. The ratios of additional respiration to the total nitrogen uptake and reduced nitrogen content in roots were 0.4 gram C per gram N and 2.3 grams C per gram N, respectively. The latter value is close to theoretical estimates of nitrate assimilation, and is similar to estimates of 1 to 4 grams C per gram N for the respiratory cost of symbiotic N₂ fixation.     

Restricted nitrate influx and reduction in corn seedlings exposed to ammonium

The effect of ambient ammonium (0.5 millimolar [¹⁴NH₄]₂SO₄) added to a nutrient solution containing 1.0 millimolar K¹⁵NO₃, 99 atom per cent ¹⁵N, upon [¹⁵N]nitrate assimilation and utilization of previously accumulated [¹⁴N]nitrate was investigated. Corn seedlings, 5-day-old dark-grown decapitated (experiment I) and 10-day-old light-grown intact (experiment II), which had previously been grown on K¹⁴NO₃ nutrient solution, were used. In both experiments, the presence of ambient ammonium decreased [¹⁵N]nitrate influx (20% after 6 hours) without significantly affecting the efflux of previously accumulated [¹⁴N]nitrate. In experiment I, relative reduction of [¹⁵N]nitrate (reduction as a percentage of influx) was inhibited more than was [¹⁵N]nitrate influx. Nevertheless, in experiment I, where all reduction could be assigned to the root system, the absolute inhibition of reduction during the 12 hours (13 micromoles/root) was less than the absolute inhibition in influx (24 micromoles/root). The data suggest that the influence of ammonium on [¹⁵N]nitrate influx could not be totally accounted for by the decrease in the potential driving force which resulted from restricted reduction; an additional impact on the influx process is indicated. Reduction of [¹⁵N]nitrate in experiment II after 6 hours accounted for 30 and 18% of the tissue excess ¹⁵N in the control and ammonium treatments, respectively. Relative distribution of ¹⁵N between roots and exudate (experiment I), or between roots and shoots (experiment II) was not affected by ammonium. On the other hand, the accumulation of [¹⁵N]nitrate in roots, shoots, and xylem exudate was enhanced by ammonium treatment compared to the control, whereas the accumulation of reduced ¹⁵N was inhibited.     

Nitrogen uptake,assimilation and transport in barley in the presence of atmospheric nitrogen dioxide

Summary Exposure of the leaves of young barley plants to nitrogen dioxide (NO₂) was shown to affect the rate of translocation of N, the form in which it is transported in the xylem stream and the partitioning of N between roots and shoots. Following its entry through the leaves, NO₂ is assimilated by the plant into reduced nitrogenous compounds which accounted for the major increases in plant N content and growth. The various effects of atmospheric NO₂ upon barley seedlings were strongly influenced by nitrate supply to the roots.     

Water Stress Effects on Nitrogen Assimilation and Growth of Trifolium subterraneum L. Using Dinitrogen or Ammonium Nitrate

The relative effects of water stress on growth parameters of subterranean clover (Trifolium subterraneum L. cv. Woogenellup) dependent on either N₂ or 8 millimolar NH₄NO₃ for N were examined. Whole-plant carbon exchange rate (CER), acetylene reduction (AR), dry matter production, and Kjeldahl N accumulation were measured on uniform, intact swards of clover that were maintained under adequately watered conditions or were subjected to three cycles of water stress (leaf water potential ≤−30 bar) over an 18-day period. In the absence or presence of water stress, growth rate, net N accumulation rate, and total N concentration of plants dependent on N₂ were 25 to 26, 45 to 50, and 20 to 21% less, respectively, than plants supplied with 8 millimolar NH₄NO₃. The water stress treatment produced less than a 50% decrease in CER regardless of plant N source, a 90% inhibition of AR in plants dependent on N₂, and a 41% decline in dry matter production on both N sources. Water stress decreased reduced N accumulation 55% in N₂-dependent plants and 50% in NH₄NO₃-dependent plants. Changes in growth and N accumulation caused a 10 to 11% decrease in total plant N concentration of water-stressed plants compared to adequately irrigated controls, but water stress decreased the N concentration of tissue synthesized during the 18-day treatment period in N₂-grown plants more than in plants supplied 8 millimolar NH₄NO₃. Thus, the relative effect of water stress on growth under the two N regimes was similar, but N accumulation by N₂-dependent clover was inhibited to a slightly greater extent (P ≤ 0.001) than in NH₄NO₃-dependent plants.     

Transport of nitrogen in the xylem of soybean plants

Experiments were conducted to characterize the distribution of N compounds in the xylem sap of nodulated and nonnodulated soybean plants through development and to determine the effects of exogenous N on the distribution of N compounds in the xylem. Xylem sap was collected from nodulated and nonnodulated greenhouse-grown soybean plants (Glycine max [L.] Merr. “Ransom”) from the vegetative phase to the pod-filling phase. The sum of the nitrogen in the amino acid, nitrate, ureide (allantoic acid and allantoin), and ammonium fractions of the sap from both types of plants agreed closely with total N as assayed by a Kjeldahl technique. Sap from nodulated plants supplied with N-free nutrient solution contained seasonal averages of 78 and 20% of the total N as ureide-N and amino acid-N, respectively. Sap from nonnodulated plants supplied with a 20 millimolar KNO₃ nutrient solution contained seasonal averages of 6, 36, and 58% of total N as ureide-N, amino acid-N, and nitrate-N, respectively. Allantoic acid was the predominant ureide in the xylem sap and asparagine was the predominant amino acid. When well nodulated plants were supplied with 20 millimolar KNO₃, beginning at 65 days, C₂H₂ reduction (N₂ fixation) decreased relative to nontreated plants and there was a concomitant decrease in the ureide content of the sap. A positive correlation (r = 0.89) was found between the ureide levels in xylem sap and nodule dry weights when either exogenous nitrate-N or urea-N was supplied at 10 and 20 millimolar concentrations to inoculated plants. The results demonstrate that ureides play a dominant role in N transport in nodulated soybeans and that the synthesis of ureides is largely dependent upon nodulation and N₂ fixation.     

Insights into the growth response and nitrogen accumulation and use efficiency of the Poaceae grass <Emphasis Type="Italic">Brachypodium distachyon</Emphasis> to high nitrogen availability

Optimization of nitrogen (N) use by grasses is a central issue of the current work. The effects of different N concentrations (0, 0.25, 0.5, 1.0, 2.5, and 5.0 mM NH₄NO₃) on growth of Brachypodium distachyon were assessed on controlled hydroponic culture. Maximal growth (132% of control) was obtained at 0.5 mM NH₄NO₃, critical N level, and was maintained at higher N concentrations. The highest N level (5.0 mM) has a similar effect on growth as 0.5 mM NH₄NO₃. It has no significant effects on water status, and total and reduced N contents in shoots while, increased those in roots, compared to plants receiving 0.5 mM NH₄NO₃. The high N availability, however, increased nitrate contents in shoots and roots by 3- and 20-folds, respectively, compared to those of plant receiving 0.5 mM NH₄NO₃. In addition, high N availability reduced the nitrogen use efficiency (NUE) by 18% compared to that of plant receiving only 0.5 NH₄NO₃. In view of B. distachyon productivity and environmental concerns, it is concluded that the critical level of N application should be 0.5 mM NH₄NO₃ and the excess fertilization led to a high nitrate accumulation.     

Changing nitrogen source on the yield and nitrogen content of soybeans

Summary Soybean plants were grown in nutrient culture solutions containing 150 ppm of N either as an equal concentration of NH₄ ⁺ or NO₃ ^–, or all NO₃ ^–. At the R2 stage of growth for some plants, the N form was changed to either all NO₃ ^– or all NH₄ ⁺, but at the same total N concentration as before. Highest seed yield was obtained with all NO₃ ^– over the entire growth period, the poorest when the N form was switched from an equal ratio of NH₄ ⁺ and NO₃ ^– to all NH₄ ⁺ at the R2 stage. Kjeldahl N concentrations in the plant leaves and seed were highest when NH₄ ⁺ was part or all of the N source in the nutrient solution. These results may partially explain why the literature is inconsistent on the effect of added fertilizer N on soybean seed yield, and may pose a problem in using leaf Kjeldahl N concentration to determine plant N sufficiency.     

Regulation by fixed nitrogen of host-symbiont recognition in the Rhizobium-clover symbiosis

Either NO₃⁻ (16 millimolar) or NH₄⁺ (1 millimolar) completely inhibited infection and nodulation of white clover seedlings (Trifoliin repens) inoculated with Rhizobium trifolii. The binding of R. trifolii to root hairs and the immunologically detectable levels of the plant lectin, trifoliin, on the root hair surface had parallel declining slopes as the concentration of either NO₃⁻ or NH₄⁺ was increased in the rooting medium. This supports the role of trifoliin in binding R. trifolii to clover root hairs. Agglutination of R. trifolii by trifoliin from seeds was not inhibited by these levels of NO₃⁻ or NH₄⁺. The results suggest that these fixed N ions may play important roles in regulating an early recognition process in the Rhizobium-clover symbiosis, namely the accumulation of high numbers of infective R. trifolii cells on clover root hairs.     

Nitrate regulation of nitrate uptake and nitrate reductase expression in barley grown at different nitrate:ammonium ratios at constant relative nitrogen addition rate

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day^?1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate: ammonium ratios. In 3- to 4-week-old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g^?1 root dry weight h^?1, whereas the total N flux was 21.8 ± 0.25 μmol g^?1 root dry weight h^?1 for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate:ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot. The apparent V_max for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/or plant. The method used in this study may facilitate separation of nitrate-specific responses from the nutritional effect of nitrate.     

Elevated CO2 plus chronic warming reduce nitrogen uptake and levels or activities of nitrogen‐uptake and ‐assimilatory proteins in tomato roots

Atmospheric CO₂ enrichment is expected to often benefit plant growth, despite causing global warming and nitrogen (N) dilution in plants. Most plants primarily procure N as inorganic nitrate (NO₃^?) or ammonium (NH₄⁺), using membrane‐localized transport proteins in roots, which are key targets for improving N use. Although interactive effects of elevated CO₂, chronic warming and N form on N relations are expected, these have not been studied. In this study, tomato (Solanum lycopersicum) plants were grown at two levels of CO₂ (400 or 700 ppm) and two temperature regimes (30 or 37°C), with NO₃^? or NH₄⁺ as the N source. Elevated CO₂ plus chronic warming severely inhibited plant growth, regardless of N form, while individually they had smaller effects on growth. Although %N in roots was similar among all treatments, elevated CO₂ plus warming decreased (1) N‐uptake rate by roots, (2) total protein concentration in roots, indicating an inhibition of N assimilation and (3) shoot %N, indicating a potential inhibition of N translocation from roots to shoots. Under elevated CO₂ plus warming, reduced NO₃^?‐uptake rate per g root was correlated with a decrease in the concentration of NO₃^?‐uptake proteins per g root, reduced NH₄⁺ uptake was correlated with decreased activity of NH₄⁺‐uptake proteins and reduced N assimilation was correlated with decreased concentration of N‐assimilatory proteins. These results indicate that elevated CO₂ and chronic warming can act synergistically to decrease plant N uptake and assimilation; hence, future global warming may decrease both plant growth and food quality (%N).     

Selection and initial characterization of partially nitrate tolerant nodulation mutants of soybean

Since NO₃⁻ availability in the rooting medium seriously limits symbiotic N₂ fixation by soybean (Glycine max [L.] Merr.), studies were initiated to select nodulation mutants which were more tolerant to NO₃⁻ and were adapted to the Midwest area of the United States. Three independent mutants were selected in the M₂ generation from ethyl methanesulfonate or N-nitroso-N-methylurea mutagenized Williams seed. All three mutants (designated NOD1-3, NOD2-4, and NOD3-7) were more extensively nodulated (427 to 770 nodules plant⁻¹) than the Williams parent (187 nodules plant⁻¹) under zero-N growth conditions. This provided evidence that the mutational event(s) affected autoregulatory control of nodulation. Moreover, all three mutants were partially tolerant to NO₃⁻; each retained greater acetylene reduction activity when grown hydroponically with 15 millimolar NO₃⁻ than did Williams at 1.5 millimolar NO₃⁻. The NO₃⁻ tolerance did not appear to be related to an altered ability to take up or metabolize NO₃⁻, based on solution NO₃⁻ depletion and on in vivo nitrate reductase assays. Enhanced nodulation appeared to be controlled by the host plant, being consistent across four Bradyrhizobium japonicum strains tested. In general, the mutant lines produced less dry weight than the control, with root dry weights being more affected than shoot dry weights. The nodulation trait has been stable through the M₅ generation in all three mutants.     

Effects of varied soil nitrogen supply on Norway spruce (Picea abies [L.] Karst.)

During a seven-month period the effect of different nitrogen (N) availability in soil on growth and nutrient uptake was studied in three-year-old Norway spruce (Picea abies [L.] Karst.) trees. The plants were grown in pots on N-poor forest soil supplied with various amounts and forms (inorganic and organic) of N. Increasing supply of inorganic N (as NH₄NO₃) increased the formation of new shoots and shoot dry weight. The root/shoot dry weight ratio of new growth was drastically decreased from 1.6 in plants without N supply to 0.5 in plants supplied with high levels of NH₄NO₃. This decrease in root/shoot dry weight ratio was associated with distinct changes in root morphology in favour of shorter and thicker roots. The addition of keratin as organic N source did neither affect growth nor root morphology of the trees. The amount of N taken up by plants was closely related to the supply of inorganic N, and trees supplied with highest levels of NH₄NO₃ also had the highest N contents in the dry matter of needles and roots. In contrast, N contents in needles of trees grown without additional N, or with keratin supply, were in the deficiency range. Supply of NH₄NO₃ decreased the contents of phosphate (P) and potassium (K) and therefore markedly increased N/P and N/K ratios in the needles. On the other hand, the contents of calcium (Ca), magnesium (Mg), and manganese (Mn) in the needles were increased in the plants supplied with inorganic N, suggesting high soil availability and promotion of uptake of these divalent cations by high nitrate uptake. The observed effects on root/shoot dry weight ratio, root morphology, and mineral nutrient composition of the needles indicated that high inorganic N supply may increase above-ground productivity but at the same time decrease the tolerance of trees against soil-borne (e.g. deficiency of other mineral nutrients) stress factors. Deceased 21 September 1996 Deceased 21 September 1996