Urease Activity in Aspergillus tamarii

Previous (unpublished) studies of the vegetative and conidialdevelopment of several fungi of the genus Aspergillus grownon different nitrogenous substrates showed that urea servedas a good source of nitrogen in all cases investigated. Moredetailed work carried out with Aspergillus tamarii IMI 61268has failed to establish the presence of extracellular or cellbound urease activity but shows high levels of activity in aqueousextracts of this fungus. In surface-grown, sporulating cultures,urease activity increased prior to conidiation and disappearedrapidly during the reproductive phase. Similar patterns of enzymicactivity were obtained with cultures grown on urea and ammoniaas sole sources of nitrogen. In submerged, shaken flask cultures using ammonia as the solenitrogenous substrate, conidiation was inhibited and ureaseactivity continued to increase until such time as the residualammonia in the medium became depleted. Thereafter the ureaseactivity as well as total soluble protein showed a sharp decrease.The results suggest that the enzyme showing urease activityin this organism is not substrate-inducible and one of its functionsmay be that of a storage protein whose breakdown products assistin the continued growth and especially in conidial development.     

Growth of Phaseolus vulgaris on Various Nitrogen Sources: The Importance of Urease

Rhizobium-inoculatcd plants of Phaseolus vulgaris L. were grownwith different N-sources (nitrate, ammonium, urea) and differentconcentrations of urea. The distribution of growth between plantparts varied with N-sources. Nitrate and ammonium were moreinhibitory to nodulation than urea, which at 40 mol m^–3N had no effect. Urease activity varied in amount and locationover a range of urea concentrations. At higher concentrations,more urea was transported to and increased urease activity wasfound in the shoot Lower levels of activity in plants relianton N₂-fixation were consistent with a ureide-degradation pathwaynot involving urea. Moderate doses of urea could be assimilatedconcomitantly with N₂-fixation. At higher levels of appliedurea, nodulation and ureide transport to the shoots were reduced,although increased growth could not be maintained at concentrationsof applied urea greater than 6.0 mol m^–3 urea N. Key words: Phaseolus vulgaris, growth, nitrogen source, urease     

Nitrogen Utilization in N-limited Barley During Vegetative and Generative Growth: III. POST-ANTHESIS KINETICS OF NET NITRATE UPTAKE AND THE ROLE OF THE RELATIVE ROOT SIZE IN DETERMINING THE CAPACITY FOR NITRATE ACQUISITION

Barley (Hordeum vulgare L., cvs Golf, Mette, and Laevigatum)was grown under nitrogen limitation in solution culture untilnear maturity. Three different nitrogen addition regimes wereused: in the ‘HN’ culture the relative rate of nitrate-Naddition (RA) was 0·08 d^–1 until day 48 and thendecreased stepwise to, finally, 0·005 d^–1 duringgrain-filling; the ‘LN’ culture received 45% ofthe nitrogen added in HN; the ‘CN’ culture was maintainedat RA 0·0375 d^–1 throughout. Kinetics of net nitrateuptake were measured during ontogeny at 30 to 150 mmol m^–3external nitrate. V_max (which is argued to reflect the maximuminflux rate in these plants) declined with age in both HN andLN cultures. A pronounced transient drop was observed just beforeanthesis, which correlated in time with a peak in root nitrateconcentration. Similar, but less pronounced, trends were observedin CN. The relative V_max (unit nitrogen taken up per unit nitrogenin plants and day) in all three cultures declined from 1·3–2·3d^–1 during vegetative growth to 0·1–0·7d^–1 during generative growth. These values are in HN andLN cultures 15- to more than 100-fold in excess of the demandset by growth rates throughout ontogeny. Predicted balancingnitrate concentrations (defined as the nitrate concentrationrequired to support the observed rate of growth) were below6·0 mmol m^–3 in HN and LN cultures before anthesisand then decreased during ontogeny. In CN cultures the balancingnitrate concentration increased during grain-filling. Apartfrom the transient decline during anthesis, most of the effectof ageing on relative V_max can be explained in terms of reducedcontribution of roots to total biomass (R:T). The loss in uptakeper unit root weight is largely compensated for by the declinewith time in average tissue nitrogen concentrations. The quantitativerelationships between relative V_max and R:T in ageing plantsare similar to those observed for vegetative plants culturedat different RAs. The data support the contention that the capacity for nitrateacquisition in N-limited plants is under general growth control,rather than controlled by specific regulation of the biochemicalpathway of nitrate assimilation. Key words: Barley, nitrogen concentration, root: total plant biomass ratio, V_max     

Studies in the Physiology of Parasitism: XXI. The Production and Properties of Pectic Enzymes secreted by Fusarium moniliforme Sheldon

Fusarium moniliforme secreted macerating enzymes in liquid mediaonly when these contained certain natural extracts, pectic substances,or galacturonic acid. Apple extract was unsuitable for enzymesecretion and also inhibited enzyme secretion in synthetic mediaotherwise suitable. Protopectinase activity of solutions was highest in the pH range8·0–9·0, was rapidly lost at temperaturesabove 50–60° C., and was reduced by concentrationsof phosphate higher than 0·02 M. The enzyme was partiallypurified by precipitation in 60 per cent. acetone at pH 6·0. Protopectinase solutions also contained an enzyme which reducedthe viscosity of solutions of various pectic substances. Theproperties of this enzyme were, in general, similar to thoseof protopectinase. When activity of enzyme solutions was measured by the liberationof reducing groups, pectate solutions were more rapidly degradedthan were solutions of a high methoxyl pectin, particularlyin the early stages of the reaction. Paper chromatography ofthe products formed showed that pectate and pectin were degradedin different ways. Although the pathogen readily secreted protopectinase in potatoextract, potato tubers were not readily parasitized. In contrast,Fusarium avenaceum which readily attacked tubers, secreted littleprotopectinase in potato extract.     

Microbiological Synthesis of Fat: THE FORMATION OF FAT FROM SUCROSE

1. Forty-three strains of moulds, selected from ten species,were examined to ascertain their capabilities as producers offat, when grown in four different solutions of nutrient inorganicsalts together with sucrose. The three highest yields of feltwere given by Penicillium javanicum van Beyma, P. Soppi Zaleski,and Aspergillus nidulans Eidam, in that order. The fat contenton felt weight was maximal at 34·8 per cent, with P.Soppi, at 28·4 per cent. with Fusarium lini(1), and at25·8 per cent. with A. nidulans; on sugar utilized itwas maximal at 11·4 per cent. with P. Soppi, at 7·9per cent, with A. nidulans, and at 5·6 per cent. withF. lini(1). 2. Other moulds considered worthy of further trial were P. spinulosum,P. aurantiobrunneum, F. graminearum, A. flavus, F. oxysporum,A. clavatus, and P. oxalicum.     

Studies on the control and properties of ornithine transcarbamylase in germinated spores of Geotrichum candidum

The effects of arginine and urea on the levels of ornithinetranscarbamylase (OTC) were investigated in relation to thephysiological functions of this enzyme in Geotrichum candidum.OTC was repressed in germinated spores to half of its initiallevel when exogenous arginine exceeded 12 mM. The repressionof OTC could not be correlated with intracellular arginine concentration.The addition of urea at the final concentration of 0.035 M increasedthe specific activity of OTC by 5.5 and 2.5 fold as comparedto enzyme levels in arginine-repressed spores and control sporesrespectively. Simultaneous addition of urea and arginine duringgermination prevented either arginine repression or urea inductionof OTC. The enzyme was partially purified from germinated sporesand isolated as a single protein band after disc electrophoresis.Two distinct pH optima for the forward reaction (pH 8.8–9)and backward reaction (pH 7.8) were found. Km values for ornithineand carbamyl phosphate were 5 x 10^–3 M and 6.8 x 10^–4so respectively. The Km for citrulline in the catabolic directionwas 1 x 10^–2 M. Enzymes obtained from cell-free extractsof germinated spores could synthetize ATP from citrulline andADP under physiological conditions by coupling the phosphorolysisof citrulline with carbamate kinase activity. The initial rateof germination was stimulated in the presence of citrullineas the sole nitrogen source, as compared to arginine, glutamineor yeast extract. These observations suggest that citrullinemay be catabolized during germination by means of OTC ratherthan via the energy-consuming urea cycle. (Received June 26, 1971; )     

Determination of Nitrate Reductase Activity in Barley Leaves and Roots

The inactivation of nitrate reductase in the leaves and rootsof barley (Hordeum vulgare L. cv. Mazurka) during and afterextracting was investigated. At 0 °C in the absence of casein,25 per cent of ‘total’. i.e. maximal in vitro, nitratereductase activity was lost during the 2 min extraction process,followed by a slower loss of activity while the extract wasstored in ice. Activity was maintained by adding a minimum of1 per cent casein to the extraction medium containing 0·1M phosphate (pH 7·5), 1 mM EDTA and 1 mM dithiothreitol.Nitrate reductase was stable for several hours in these extracts,but declined in a first order manner in the absence of dithiothreitol.Casein also prevented the initial loss while making root extracts,but had less effect during storage. Using casein and thiols, nitrate reductase activity in light,(as product of maximal in vitro rates and wt g^–1) in leaveswas 98 per cent of the total activity in 31-day-old plants grownwith full nutrient in water culture and 60-day-old field-grownplants receiving no fertilizer. Field-grown plants, however,exhibited only 17 per cent of the activity of culture-grownplants. Nitrate reductase in leaves of barley plants grown in waterculture had a diurnal rhythm. During the first 3 h of the lightperiod, activity increased to 1·3 x the ‘dark’value. This was followed by a temporary decrease and then byanother increase to a maximum of 1·7 x the ‘dark’value, occurring about 8 h after illumination. Activity thendecreased during the rest of the light period and in darkness. Hordeum vulgare L., barley, nitrate reductase     

The Structure and Function of Pine Forest in Central Himalaya. I. Dry Matter Dynamics

The present study deals with structure and function of fourareas of Himalayan chir pine forest. Tree layer was monospecificon all sites with varied density and basal cover in the rangeof 540–1630 individuals per ha and 25·0–47·2m²ha^–1, respectively. Shrubs having low density were sparselydistributed. All allometric equations relating to biomass ofdifferent components, to circumference at breast height (cbh)were significant, with the exception of that for cone biomass.Total vegetation biomass (115–236 t ha^–1) was distributedas 113–283 t ha^–1 in trees. 0·56–0·82t ha^–1 in shrubs and 1·63–2·57 t ha^–1in herbs. Total forest floor biomass including herbaceous litterranged between 9·6 and 13·6 t ha^–1. Of thetotal annual litter fall (4·26–7·38 t ha^–1),60·3–75·1 per cent was distributed in leaflitter and 24·9–39·7 per cent in wood litter.Turnover rate of tree litter varied from 0·45 to 0·53,whereas rates for shrubs and herbs were assumed to 1. Net primaryproduction of total vegetation ranged between 9·91 and21·2 t ha^–1 year^–1, of which the contributionof trees, shrubs and herbs was 76·5– 88·1per cent 0·6–1·8 per cent and 11·3–21·5per cent, respectively. A compartment model of dry matter onthe basis of mean data across sites was developed to show drymatter storage and flow of dry matter within the ecosystem. Pinus roxburghii forest, biomass, litter fall, net primary production, compartmental transfer     

The Carbon Economy of Rubus chamaemorus L. II. Respiration

Respiratory activity and seasonal changes in carbohydrate contentof the storage organs of Rubus chamaemorus L. have been investigated.Leaf dark respiration rate increases in a non-linear mannerfrom 0·7 mg CO₂ evolved dm^–2 h^–1 at 0 °Cto 4·6 rng CO₂ evolved dm^–2 hh^–1 at 30 °C.Root and rhizome respiration rates increase from 1 µ1O₂ uptake g^–1 fresh weight h^–1 at 0.7 ° C to10 µ10, uptake g^–1 f. wt h^–1 at 20 °C.Rhizome carbohydrate reserves decline from a September peakof 33 per cent alcohol insoluble d. wt to 16 per cent in May. The circumpolar distribution of R. chamaemorus is discussedin relation to the evidence presented here and in the precedingpaper of the series.     

Distribution of Nitrogen during Growth of Sunflower (Helianthus annuus L.)

The accumulation, distribution and redistribution of dry matterand nitrogen is described for Helianthus annuus L. cv. Hysun21 grown on 6 mM urea in glasshouse culture. Seed dry matterand nitrogen were transferred to seedlings with net efficienciesof 40 and 86 per cent respectively. At flowering, the stem hadmost of the plant's dry matter and the leaves most of its nitrogen.About 35 per cent of the plant's nitrogen accumulated afterthree-row anthesis. The amount of protein in vegetative parts,especially leaves, declined after flowering. Concentrationsof free amino compounds also decreased during growth. Matureseeds had 38 per cent of the total plant dry weight and 68 percent of the total nitrogen. Seeds acquired 33 per cent of theirdry matter and nitrogen from redistribution from above-groundplant parts. The stem was most important for storage of carbohydrate,leaves the most important for nitrogen. Over 50 per cent ofthe nitrogen in the stem and leaves was redistributed. Plantsthat received 6 mM nitrate accumulated more dry matter thanurea-grown plants. Seeds from nitrate-grown plants were heavier(58 mg) than those of urea-grown plants (46 mg), and their percentageoil was greater (50 and 41 respectively). The amount of nitrogenper seed was the same. Little or no urea was detected in xylem sap of plants suppliedwith 5 mM urea, but it was detected in sap of plants which received25 mM. Concentrations of urea and amino compounds in the sapdecreased up the stem. Plants supplied with nitrate had mostof the nitrogen in xylem sap as NO₂^–, suggesting littlenitrate reduction in roots. Plants grown on 6 mM nitrate andchanged to high levels of urea-nitrogen for 14 days still hadhigh levels of nitrate; little nitrate remained in plants receivinglow levels of urea. When urea is applied in irrigation waterto field-grown sunflower, the nitrogen is subsequently takenup as nitrate due to rapid nitrogen transformations in the soil. Helianthus annuus L., sunflower, urea, nitrate, nitrogen transport, xylem sap, nitrogen accumulation nitrogen distribution     

Characterization of the 'In Vivo' Nitrate Reductase Activity in the Roots and Leaves of Eichhornia crassipes

Studies of in vivo nitrate reductase activity (NRA) in Eichhorniacrassipes showed that adding 5% isopropanol gave highest NRAin all parts of the plant, but it was also necessary to flushthe assay media containing leaves with N₁ to obtain the maximalactivity. There were differences in the pH optima for NRA, theroot optimum being 6·5 leaf 7·5–8·0,and the petiole showing a major peak at 6·5 and a minorpeak at 7·.5–8·0. Possible significancesof these differences are discussed. Nitrate reductase activity, Eichhornia crassipes     

Isocitrate Lyase from Germinating Soybean Cotyledons: Purification and Characterization

Ruchti, M. and Widmer, F. 1986. Isocitrate lyase from germinatingsoybean cotyledons: purification and characterization.—J.exp. Bot. 37: 1685–1690. Isocitrate lyase (E.C. 4.1.3.1 [EC] ) was purified from the cotyledonsof 7-d-old soybean seedlings. Three molecular forms were detectedwith pi values of 6·46, 6·25 and 6·0. Themain form (pl = 6·46) had an approximate Mr of 130000,a pH optimum of 8·0, a K_m (isocitrate) close to 2·0mol m^–3 and a molecular activity of 615 min ^–1 at25 °C. The purified enzyme is not a glycoprotein and isheat labile. Key words: Isocitrate lyase, soybean     

Variations in Lipid and Fatty Acid Content in Relation to Acetyl CoA Carboxylase in the Marine Prymnesiophyte Isochrysis galbana

Lipid and fatty acid content was determined in Isochrysis galbanacultures grown under various environmental conditions in steadystate continuous cultures. Lipid and fatty acid accumulationwas observed under severe nitrogen limiting conditions. Therate of in situ lipid synthesis, determined from ¹⁴C bicarbonateincorporation into lipid fraction, decreased under nitrogenlimited growth (µ<0.72day^–1), concomitant witha reduction in the in vitro activity of the enzyme acetyl CoAcarboxylase. Cellular lipid and fatty acid content remainedfairly constant over a wide range of irradiance levels. Therate of lipid synthesis, however, increased as irra-diance levelwas elevated to a maximal value at a light intensity of 150µmolquanta m^–2s^–1 and slightly decreased at a higherphoton flux. The in vitro activity of ACCase roughly followedthe same pattern of lipid synthesis in response to light intensity.The relative abundance of acetyl CoA carboxylase significantlydecreased in nitrogen limited cultures grown at low dilutionrates (µ<0.72day^–1). A fairly good linear correlationwas measured between the cellular content of ACCase and theenzyme activity in cultures grown under nitrogen limiting conditions.Furthermore, in nitrogen limited cultures, the cellular fattyacid content was linearly related to the cell capacity to producemalonyl CoA, the end product of ACCase and the building blockin fatty acid synthesis. (Received November 20, 1990; Accepted January 17, 1991)     

Effect of Applied Nitrogen on N2 Fixation, Assimilation of Nitrate and Ammonia in Nodules of Field-grown Moong (Vigna radiata)

A field experiment was conducted to study the effect of nitrogenapplication at 15, 30 and 45 kg ha^–1 of urea at pre-flowering(PF) and pod initiation (PI) stages on the activity of nitrogenase(N₂ase), nitrate reductase (NR) and other related parametersin the nodules of moong (Vigna radiata). Nitrogen applied atPF or PI stage was found to be inhibitory to N₂ase and glutaminesynthetase (GS) activities except at 15 kg N ha^–1 whenapplied at PF in the case of N₂ase. At both the stages therewas increase in NR and glutamate dehydrogenase (GDH) activitieswith the application of nitrogen. Seed yield increased by 18per cent with the application of 15 kg N ha^–1 at PI stagewhereas nitrogen application at PF stage only increased strawyield significantly. Nitrate reductase, nitrogenase, nitrogen application, ammonia assimilation, Vigna radiata     

Cryopreservation and Plant Regeneration from Embryogenic Cultures of Larch (Larixxeurolepis) and Black Spruce (Picea mariana)

A method has been developed for the routine cryopreservationof embryogenic cultures of hybrid larch (Larixxeurolepis) andblack spruce (Picea mariana Mill.). The method involves growingthe cultures in the presence of sorbitol and then briefly exposingthem to DMSO followed by controlled cooling to –40°C.The cultures were then submerged and stored in liquid nitrogen.Growth of the embryogenic cultures was monitored for 14 d afterrapid thawing and plating on to media. The highest relativeincrease in the tissue fresh weight, after storage in liquidnitrogen, was observed when embryogenic cultures of both specieswere pregrown for 24 h in a medium with 0·4 M sorbitoland then treated with 10% DMSO. This pretreatment also ensuredthe shortest lag phase in resuming the growth. The post-thawcultures gave rise to mature somatic embryos which developedinto plants Key words: Larixxeurolepis, Picea mariana, cryopreservation, embryogenic tissue, plant regeneration     

Microbiological Synthesis of Fat: THE EFFECTS OF NITROGEN LEVEL ON THE METABOLISM OF PENICILLIUM LILACINUM THOM IN A SUCROSE MEDIUM

1. A study has been made of the relationships between the synthesesof carbohydrate, protein, and fat by Penicillium lilacinum Thomin presence of different amounts of sodium nitrate us a definedsucrose salts medium.
2. Under the defined experimental conditionsincreases in the concentrationof NO^–₂ in the medium werefollowed by increases in therates at which nitrogen and sugarwere taken up by the fungus,in the quantities assimilated,and in total and protein nitrogenin the felt. These conditionsprevailed so long as unassimilatedsugar was available.
3. Mediaof lower NO^–₃ concentration (for example, 0·32or 0·64 per cent. (w/v) NaNO^–₂;) yielded feltsricher in carbohydrate than were those grown in media of higherNO^–₂; content (0·96 or 1·28 per cent. (w/v)NaNO₃ The carbohydrate content of the felts increased graduallyuntil the sugar in the medium was exhausted; carbohydrate contentthen decreased.
4. Media of lower NO^–₃; concentration weremore conduciveto fat synthesis than those of higher NO^–₃;content.

     

Acid Phosphatase Activity in Karri (Eucalyptus diversicolor F. Muell.) in Relation to Soil Phosphate and Nitrogen Supply

O'Connell, A. M. and Grove, T. S. 1985. Acid phosphatase activityin karri (Eucalyptus diversicolor F. Muell.) in relation tosoil phosphate and nitrogen supply.—J. exp. Bot. 36: 1359–1372 Soluble acid phosphatase activity was measured in tissues ofkarri (Eucalyptus diversicolor F. Muell.) seedlings and fiveyear old karri trees to which P and N fertilizer had been applied.Addition of P from 0 to 1250 mg P kg^–1 soil with a basaltreatment of other nutrients produced significant increasesin growth, P content and P concentration of karri seedlings.In each of five plant components (shoot tips, partly expandedleaves, mature leaves, young stems and old stems) soluble acidphosphatase activity was greatest at low levels of added P anddecreased with increasing soil P supply. The range of acid phosphataseactivity (0·5-6·5 µmol NPP g^–1 f.wt.min^–1) was similar to that reported for a number of agriculturaland horticultural plants. Enzyme activity was highest for shoottips and lowest for old stems. However, the relative changein activity with decreasing soil P supply was greatest for stems(4·3 fold) and least for shoot tips (2·7 fold) Mature leaves of seedlings grown in ‘high P’ and‘low P’ soil at four levels of added N showed, inaddition to the effect of P, a significant N-P interaction onsoluble acid phosphatase activity. In leaf samples from fiveyear old karri trees there was a significant decrease in solubleacid phosphatase with increasing P fertilization. Addition ofN fertilizer had no significant effect on enzyme activity, probablybecause added N had little effect on foliar N concentrations Exponential models relating (1) plant growth to enzyme activityand (2) plant growth to P concentration in stems and matureleaves of plants grown in soil with a range of added P accountedfor 78–92% and 63–87%, respectively, of the variationin top dry weight. The results suggest that for the diagnosisof plant P status, (1) stem components may be the most appropriatetissue to sample, and (2) nutrient and enzyme assays may complementeach other, P concentration being most useful where P supplyis adequate and phosphatase activity where P supply limits growth Key words: Phosphatase activity, Eucalyptus diversicolor, nutrients, phosphorus, nitrogen, forests     

Microclimate, Photosynthesis and Growth of Lucerne (Medicago sativa L.). II. Canopy Structure and Growth

The growth of lucerne var. Europe was examined in the fieldduring 1976. The annual dry matter production of unirrigatedlucerne during 1976, with no nitrogen fertilizer application,was 82.5 per cent greater than unirrigated S.24 perennial ryegrass,with a nitrogen application of 384 kg ha^–1. The mean aboveground growth rate of lucerne was 7.3 g DM m^–2 day^–1between March and early June, of which stem material contributeda maximum of 76.5 per cent. Significant losses of leaves andstems occurred from the end of April, indicating a loss of potentialforage material. Nitrogen analyses of the above ground crop suggested that in56 days lucerne yielded 10.7 per cent more nitrogen than didS.24 annually with a nitrogen fertilizer addition of 280 kgha^–1. Between 13 and 57 per cent of the daily photosynthate is translocatedbelow ground. Medicago sativaL, lucerne, dry matter production, canopy structure, nitrogen analyses     

Some Observations on Invertase Activity in Roots of Vicia faba L.

Invertase activity has been determined at intervals along primaryroots of Vicia faba as they elongated from 0·5 to 8 cm.Little activity was evident in 0·5–1·0 cmlong primaries but in those 2–8 cm in length the mainpeak of enzyme activity was associated with the region of cellelongation. Changes took place in the pattern of invertase activityalong the primary roots as they lengthened and these changeshave been correlated with fluctuations in both the rate of rootelongation and the supply of sucrose to the root from the cotyledons.The presence of a root cap did not increase the activity ofthis enzyme in the apical 1 mm of these roots. Invertase activity was higher in lateral root primordia thanin most parts of the primary root basal to the meristem, presumablybecause of the presence of sucrose in the adjacent cavity inthe cortex of the primary root. The peaks of invertase activityfound basal to the region of cell elongation in 3–8 cmlong primary roots probably resulted from the development ofroot pnmordia in these parts of the root.     

Influence of Ni Supply on Growth and Nitrogen Metabolism ofBrassica napusL. Grown with NH4NO3or Urea as N Source

The significance of nickel (Ni), which is essential for ureaseactivity, for growth and nitrogen (N) metabolism ofBrassicanapusgrown in nutrient solution with either NH₄NO₃or urea assole N source was investigated. Although Ni contents were below25 µg kg^-1d. wt, growth of plants relying on NH₄NO₃wasnot affected by the Ni status. However, supplementing the growthmedium with 0.04 µMNi enhanced dry matter production ofurea-grown plants significantly. Urease activity was significantlyreduced in leaves and roots of plants grown without supplementaryNi irrespective of N source. Plants grown with urea withoutadditional Ni accumulated large amounts of urea and had loweramino acid contents indicating impaired usage of the N supplied,while those grown with NH₄NO₃under Ni-deprived conditions accumulatedendogenous urea in their older leaves. It is suggested thatNi may not be strictly essential for plants receiving mineralN, or that the critical level is well below 25 µg kg^-1d.wt. These results confirm that Ni is required for urease activityand thus for growth of plants on urea-based media, as well asfor recycling endogenous urea.Copyright 1999 Annals of BotanyCompany. Brassica napusvar.annua, amino acids, N nutrition, nickel, spring rape, urea, urease activity.