The response ofPlantago lanceolata L. andP. major L. spp.major to nitrate depletion

To study aspects of the ecology of grassland species, in a comparative experiment, plants ofP. lanceolata andP. major were grown in pots in a greenhouse, and subjected to a gradual nitrate depletion for several weeks. Control plants were weekly supplied with nitrate. Growth, leaf appearance and disappearance, concentrations of cations and inorganic anions, soluble and insoluble reduced nitrogen concentrations,in vivo nitrate reductase activity (NRA) and the concentration of non-structural carbohydrates in several parts of the plants were followed. Depletion of nitrate caused a reduction of shoot growth, both in biomass and number of leaves. Withering of leaves increased. Accumulation of root dry matter was little (P. lanceolata), or not (P. major) affected. The concentration of reduced nitrogen in all tissues also decreased, both that of the soluble and that of the insoluble fraction. As a result, nitrogen use efficiency (NUE, g dry matter produced per mmol N incorporated) increased by nitrate depletion. NRA was higher in the roots than in the leaves, and decreased with increasing nitrate depletion. In control plants, nitrate became also limiting. This resulted in decreasing nitrate concentrations in leaves and roots. In the leaves, the decrease in nitrate concentration was preceded by a decrease in NRA. The decrease of the nitrate concentration was parallelled by an increase in the concentration of soluble sugar. No major differences in the response towards nitrate depletion were observed between the two species. Grassland Species Research Group, publication no. 129     

Nitrate production in the rhizosphere of Plantago species

Summary The production of nitrate in an old established dune grassland soil and its uptake by plants was studied by comparing amounts of mineral nitrogen and numbers of nitrifying bacteria in the rhizosphere on the one hand, and on the other accumulated nitrate and levels of nitrate reductase (NaR) of individual plants of three Plantago species,i. e., P. major, P. lanceolata andP. coronopus. For these three Plantago species andP. media basal levels of NaR in the absence of nitrate were determined in plants grown in culture solutions. The basal NaR levels ofP. major andP. media (species occurring on nutrient-rich soils) were significantly higher than those ofP. lanceolata andP. coronopus (species found on nutrient-poor soils). NaR activity increased in the presence of nitrate and was suppressed by ammonium.From the numbers of nitrifying bacteria in the rhizosphere and NaR activity in the leaves it was concluded that nitrate was produced in the root environments of the three Plantago species and that the compound was taken up by the plants. NaR activities and numbers of nitrifying bacteria were higher for individuals ofP. major than for those ofP. lanceolata andP. coronopus. No correlation was found between the ammonium levels and the numbers of nitrifying bacteria in the soil, and no indications of inhibition of nitrifying bacteria in the rhizosphere were obtained. For individuals ofP. lanceolata a correlation was found between the numbers of nitrifying bacteria in the soil and NaR activity in the leaves. The results are discussed in relation to the ecological habitats of the three species.Grassland Species Research Group Publication No.38.     

Heterotrophic nitrification in a health soil demonstrated by anin situ method

Summary Nitrate reductase activity (NRA) in the leaves of two subspecies ofHypochaeris radicata was taken as a parameter for nitrate production in the soilin situ. Ammonium addition to the soil ofH. radicata ssp.radicata (soil pH 6.2) resulted in an increase of NRA, thus indicating nitrate formation by chemolithotrophic nitrifiers after a certain time-lag. Addition of ammonium to the soil ofH. radicata ssp.ericetorum (soil pH 4.3) dit not affect NRA in the leaves. Tests based on the MPN method failed to demonstrate the occurrence of chemolithotrophic nitrifiers in this soil. However, the addition of peptone led to an increase of NRA within seven days, which indicates the presence of heterotrophic nitrifying organisms. The results obtainedin situ were confirmed in a laboratory experiment, where soil samples were incubated in the presence and absence of (NH₄)₂SO₄ or peptone. The addition of ammonium led to a decrease in the production of nitrate to zero as compared with the control in the acid soil of ssp.ericetorum, whereas the addition of peptone resulted in nitrate levels amounting to about twice the control value. In the soil of ssp.radicata nitrate formation showed a rapid increase, compared with the control, after the addition of ammonium as well as after the addition of peptone.Grassland species research group, publication no27     

Survival ofXanthomonas campestris pv.vesictoria in pepper seeds and roots in symptomless and dry leaves in non-host plants and in the soil

Summary A population ofXanthomonas campestris pv.vesicatoria developed as endophytes in the leaves and rhizosphere of apparently symptomless plants grown under mist but not under dry conditions. The pathogen survired for long periods on, and could be isolated from, the surface of infested dried seeds, inoculated sandy loam soil, dried leaves, and the rhizosphere of pepper and of other non-host plants. In addition, small numbers of the pathogen survived for 18 months in a field previously cropped with pepper diseased with bacterial scab. Healthy nursery or mature plants developed symptoms while growing in soil containing infested leaves, either buried or placed on the soil surface.     

Ammonium and nitrate nutrition inPlantago lanceolata L. andPlantago major L. ssp.major

With the aims (1) to test whether the different natural occurrence of twoPlantago species in grasslands is explained by a different preference of the species for nitrate or ammonium; (2) to test whether the different occurrence is explained by differences in the flexibility of the species towards changes in the nitrogen form; (3) to find suitable parameters as a tool to study ammonium and nitrate utilization of these species at the natural sites in grasslands, plants ofPlantago lanceolata andP. major ssp.major were grown with an abundant supply of nitrate, ammonium or nitrate+ammonium as the nitrogen source (0.5 mM). The combination of ammonium and nitrate gave a slightly higher final plant weight than nitrate or ammonium alone. Ammonium lowered the shoot to root ratio inP. major. Uptake of nitrate per g root was faster than that of ammonium, but from the mixed source ammonium and nitrate were taken up at the same rate. In vivo nitrate reductase activity (NRA) was present in both shoot and roots of plants receiving nitrate. When ammonium was applied in addition to nitrate, NRA of the shoot was not affected, but in the root the activity decreased. Thus, a larger proportion of total NRA was present in the shoot than with nitrate alone. In vitro glutamate dehydrogenase activity (GDHA) was enhanced by ammonium, both in the shoot and in the roots.In vitro glutamine synthetase activity (GSA) was highest in roots of plants receiving ammonium. Both GDHA and GSA were higher inP. lanceolata than inP. major. The concentration of ammonium in the roots increased with ammonium, but it did not accumulate in the shoot. The concentration of amino acids in the roots was also enhanced by ammonium. Protein concentration was not affected by the form of nitrogen. Nitrate accumulated in both the shoot and the roots of nitrate grown plants. When nitrate in the solution was replaced by ammonium, the nitrate concentration in the roots decreased rapidly. It also decreased in the shoot, but slowly. It is concluded that the nitrogen metabolism of the twoPlantago species shows a similar response to a change in the form of the nitrogen source, and that differences in natural occurrence of these species are not related to a differential adaptation of nitrogen metabolism towards the nitrogen form. Suitable parameters for establishing the nitrogen source in the field are thein vivo NRA, nitrate concentrations in tissues and xylem exudate, and the fraction of total reduced nitrogen in the roots that is in the soluble form, and to some extent thein vitro GDHA and GSA of the roots. Grassland Species Research Group. Publ. no 118.     

Nitrate reduction and nitrate content in ash trees (Fraxinus excelsior L.): distribution between compartments,site comparison and seasonal variation

Summary Nitrate reductase activity (NRA), nitrate content and biomass components of leaflets, leaf stalks, old stem, current-year stem and roots of ash trees (Fraxinus excelsior L.) growing in their natural habitats were investigated. In addition, NRA, total nitrogen and nitrate concentration were analyzed in the leaves and roots of ash trees from four different field sites. The highest NRA per gram biomass and also per total compartment biomass was found in the leaflets, even though root biomass was much higher than total leaflet biomass. The highest nitrate concentrations were found in the leaf stalks. Correlations between nitrate availability in the soil and NRA in leaves were not significant due to high variability of the actual soil nitrate concentrations. The seasonal variation in foliar NRA, nitrate concentration and total nitrogen concentration is much smaller in F. excelsior than reported for herbaceous species and is mainly caused by changes in the actual soil nitrate availability and by senescence of the leaves.     

Nitrate nutrition ofDeschampsia flexuosa (L.) Trin. in relation to nitrogen deposition in Sweden

Summary Current and maximally induced nitrate reductase activity (NRA), total-N, nitrate, K, P, Ca, Mg, Mo and sucrose in leaves ofDeschampsia flexuosa was measured three times during the vegetation period in forests along a deposition gradient (150 km) in south Sweden, in north Sweden where the nitrogen deposition is considerably lower, and at heavily N-fertilized plots. In addition, the interaction between nitrogen nutrition and light was studied along transects from clearings into forest in both south and north Sweden. Plants from sites with high nitrogen deposition had elevated current NRA compared to plants from less polluted sites, indicating high levels of available soil nitrate at the former. Current NRA and total N concentration in grass from sites with high deposition resembled those found at heavily N-fertilized plots. Under such circumstances, the ratio current NRA: maximally induced NRA as well as the concentration of nitrate was high, while the concentration of sucrose was low. This suggests that the grass at these sites was already utilizing a large portion of its capacity to assimilate nitrate. Light was found to play an important role in the assimilation of nitrate; leaf concentration of sucrose was found to be negatively correlated with both nitrate and total N. Consequently, grass growing under dense canopies in south Sweden is not able to dilute N by increasing growth. The diminished capacity of the grass to assimilate nitrate will increase leaching losses of N from forests approaching N saturation.     

Utilization of the ambient concentration as a criterion for steady state after exponential growth: Some culture experiments with optimum or suboptimum N nutrition

Culture experiments are described in which Plantago lanceolata L. was grown from seedling till flowering under steady state conditions of optimum or suboptimum nitrate nutrition. In the optimum treatment, plants had free access to nitrate. In two suboptimum treatments, nitrate was added with constant relative addition rates (RAR) of 0.18 or 0.10 d^–1 during the phase of constant relative uptake rates (RUR) of the plants and then with RAR's that were reduced stepwise from 0.18 to 0.07 d^–1 or 0.10 to 0.04 d^–1 when nutrient absorption gradually decreased. Reduction of the RAR's was aimed at maintenance of a balance between RAR and RUR. External nitrate concentrations were measured to monitor the reductions. In the vegetative phase, the relative growth rate (RGR) and the root weight ratio (RWR) of P. lanceolata were constant. In the reproductive phase, RGR's were constant, but lower, and RWR's decreased. Concentrations of organic-N in leaves were stable during the experimental period while those of the peduncles were lower and decreased with time. The ratio of reproductive to vegetative weight increased linearly with time. A number of plant parameters varied with N supply. ei]Section editor: T W Rufty     

毛竹各器官和根际土浸提液对杉木种子萌发的化感作用

随着毛竹杉木混交林面积的不断扩大,它们之间既会产生促进作用,又会产生抑制作用。本研究对比分析了不同浓度毛竹各器官(鲜叶、干叶、枝条、竹杆、竹鞭和鞭根)的浸提液及枯落物、根际土浸提液对杉木种子发芽率的影响,探讨了毛竹各器官浸提液及枯落物、根际土浸提液对杉木种子萌发的化感作用。结果表明:(1)处理前期,毛竹各器官浸提液对杉木种子萌发具有抑制作用,且当浸提液浓度为1∶25和1∶50的高浓度时,相对于低浓度浸提液对杉木种子萌发的抑制作用更为持久;(2)处理后期,低浓度的毛竹各器官浸提液对杉木种子萌发促进作用较快,而高浓度的浸提液仍呈现抑制作用,促进作用出现较为缓慢;(3)枯落叶浸提液总体上对杉木种子萌发具有抑制作用,在整个实验过程中低于对照组的发芽率,仅在1∶50浓度时略高对照组实验;而根际土浸提液对杉木种子萌发自始至终都具有促进作用,其中低浓度的促进作用十分显著。故在实际的毛竹杉木混交林生产营林过程中可选用毛竹根际土制备浸提液,促进杉木种子萌发,为毛竹杉木混交林的生产经营提供参考。     

Nitrate reductase activity in leaves and roots ofAlnus glutinosa (L) Gaertner

Summary Thein vivo nitrate reductase activity (NRA) was determined inAlnus glutinosa plants grown nonsymbiotically on ammonium, nitrate, a combination of both, or symbiotically with atmospheric nitrogen as the only nitrogen source. Root NRA was absent when ammonium or atmospheric nitrogen was the nitrogen source. With nitrate in the culture solution the roots showed a high NRA. However, the leaf NRA behaved quite differently: with negligible activities on all nitrogen sources except atmospheric nitrogen. The foliar NRA measured, however, is likely not due to the activity of the plant but of microbial origin. Methods commonly used to facilitate produced nitrite to leak out of the tissue, such as addition of propanol and cutting the plant material, did not increase the nitrite release from the leaves. A turbidity developed when testing the samples for nitrite which was positively correlated with the NRA. Populations of microorganisms in the phyllosphere did not differ between the nutritional treatments. Bacteria, able to grow on a low-nitrogen medium, were present on the leaves. Nitrifiers could not be detected. The bacteria on the leaves appear to produce nitrite when incubated with leaf material. Grassland Species Research Group, Publication no. 106     

Biomass production and nitrate metabolism of Atriplex hortensis L. (C3 plant) and Amaranthus retroflexus L. (C4 plant) in cultures at different levels of nitrogen supply

Summary Pure and mixed cultures of the dicotyledons Atriplex hortensis L. (C₃ plant) and Amaranthus retroflexus L. (C₄ plant) were maintained under open air conditions in standard soil at low and high nitrogen supply levels.A comparison of shoot dry weight and shoot length in the various series shows that the growth of the aboveground parts of both species was severely reduced under low N conditions. In both pure and mixed cultures the differences resulting from low N vs. high N conditions was less pronounced with Atriplex (C₃ plant) than with Amaranthus (C₄ plant). The root dry weight of the two species was not reduced so much under low N conditions as was the shoot dry weight. The low N plants were found to contain a larger proportion of their biomass in the roots than did the high N plants. In general the root proportion of Atriplex was greater than that of Amaranthus. The contents of organic nitrogen and nitrate and the nitrate reductase activity (NRA) per g dry weight of both species decreased continually throughout the experiments. With the exception of young plants, the low N plants always had tower contents of organic nitrogen and nitrate and nitrate reductase activities than did the high N plants. The highest values of NRA were measured in the leaf laminae. The eaves also exhibited the highest concentrations of organic nitrogen. The highest nitrate concentrations, however, were observed in the shoot axis, and in most cases the lowest nitrate values were found in the laminae. At the end of ne growing season this pattern was found to have been reversed with Atriplex, but not with Amaranthus. Thus Atriplex was able to maintain a higher NRA in the laminae than Amaranthus under low N conditions.The transpiration per leaf area of the C₄ plant Amaranthus during the course of a day was substantially lower than that of the C₃ plant Atriplex. There were no significant differences in transpiration between the low N and high N series of Amaranthus. The low N plants of Atriplex, however, clearly showed in most cases higher transpiration rates than the corresponding high N plants. These different transpiration rates of the high N and the low N Atriplex plants were also reflected in a distinct ¹³C discrimination.The sum of these results points to the conclusion that the C₃ plant Atriplex hortensis can maintain a better internal inorganic nitrogen supply than the C₄ plant Amaranthus retroflexus under low N conditions and an ample water supply, due to the larger root proportion and the more pronounced and flexible transpiration of the C₃ plant.Dedicated to Prof. Dr. Karl Mägdefrau, Deisenhofen, on the ocasion of his 80th birthday     

Is the isotopically exchangeable phosphate of a loamy soil the plant-available P?

Carduus nutans L. is an invasive pasture/grassland species which may undergo rapid population growth through positive feedback. Plants ofC. nutans produce a vegetative rosette, and after several months produce stems containing flower-heads, during which time the rosette leaves die and decompose. We investigated the influence ofC. nutans on the nitrogen-fixation ability ofTrifolium repens L. in three experiments. The first experiment was set up in a mixture design, and demonstrated that seedlings ofT. repens were more susceptible to competition with otherT. repens seedlings than toC. nutans seedlings. Nodule numbers and acetylene reduction per unit root, and acetylene reduction per unit nodules were adversely affected by increasingT. repens, but notC. nutans densities. The second experiment was of an additive design, with separate partitions to isolate above-ground and below-ground interference. FloweringC. nutans plants strongly inhibitedT. repens root growth, nodulation and acetylene reduction, but usually only when shoot interference was permitted. This appears to be due to decomposition of rosette leaves, which was maximal at this stage. The third experiment involved monitoring effects of taggedC. nutans individuals againstT. repens in the field. This experiment showed that acetylene reduction was severely influenced by floweringC. nutans (when rosette leaves were decomposing), even when only mild reduction ofT. repens growth was observed, and these effects persisted for some months after theC. nutans plants had died. The results of these experiments in combination suggest that decomposing rosette leaves have a strong potential to inhibitT. repens nitrogen fixation. It appears that allelopathy is involved, since alternative explanations (e.g. root competition byC. nutans; effects ofC. nutans on soil moisture, microbial nutrient immobilisation and light availability; facilitation of herbivores byC. nutans) can be effectively discounted. Although invasive species are often assumed to be associated with soil nitrogen build-up, we believe that some invasive species such asC. nutans have the potential to induce long-term decline of soil nitrogen input.     

Root biomass distribution in a moist tropical montane forest

Effects of N sources (ammonium, nitrate and ammonitrate) and VA mycorrhizae (Glomus intraradices) on rhizosphere soil characteristics (pH, exchangeable acidity, exchangeable cations, inorganic N concentrations) growth and nutrient acquisition of coffee seedlings (Coffea arabica L. cv guatemala) were investigated in a pot study with an acid soil (Red Bluff Loam) sterilized by autoclaving. Ammonium addition decreased rhizosphere pH while nitrate and ammonitrate additions both increased rhizosphere pH. Mycorrhizae induced a higher pH, a lower exchangeable acidity and higher values of exchangeable cations in the rhizosphere. Ammonium addition resulted in a lower mycorrhizal infection than the two other N sources. Mycorrhizal plants grew better and accumulated more N, Ca and Mg than non-mycorrhizal plants.     

nir1, a conditional-lethal mutation in barley causing a defect in nitrite reduction

Summary Eleven green individuals were isolated when 95000 M₂ plants of barley (Hordeum vulgare L.), mutagenised with azide in the M₁, were screened for nitrite accumulation in their leaves after nitrate treatment in the light. The selected plants were maintained in aerated liquid culture solution containing glutamine as sole nitrogen source. Not all plants survived to flowering and some others that did were not fertile. One of the selected plants, STA3999, from the cultivar Tweed could be crossed to the wild-type cultivar and analysis of the F₂ progeny showed that leaf nitrite accumulation was due to a recessive mutation in a single nuclear gene, which has been designated Nir1. The homozygous nir1 mutant could be maintained to flowering in liquid culture with either glutamine or ammonium as sole nitrogen source, but died within 14 days after transfer to compost. The nitrite reductase cross-reacting material seen in nitrate-treated wild-type plants could not be detected in either the leaf or the root of the homozygous nir1 mutant. Nitrite reductase activity, measured with dithionite-reduced methyl viologen as electron donor, of the nitrate-treated homozygous nir1 mutant was much reduced but NADH-nitrate reductase activity was elevated compared to wild-type plants. We conclude that the Nir1 locus determines the formation of nitrite reductase apoprotein in both the leaf and root of barley and speculate that it represents either the nitrite reductase apoprotein gene locus or, less likely, a regulatory locus whose product is required for the synthesis of nitrite reductase, but not nitrate reductase. Elevation of NADH-nitrate reductase activity in the nir1 mutant suggests a regulatory perturbation in the expression of the Narl gene.     

Changes in the activities of nitrogen assimilation enzymes ofLolium perenne L. during regrowth after cutting

The activities of key enzymes involved in N assimilation were investigated after defoliation of 6-week-old ryegrass plants grown in water culture conditions. In a first experiment, nitrate reductase, glutamine synthetase and glutamate dehydrogenase activities were measured in roots, stubble and leaves on the day of cutting and at 7-day intervals over the following 5-week period of regrowth. Ammonia assimilation enzymes showed little change whereas the nitrate reductase activity sharply decreased 2 weeks after clipping. In a second experiment, the nitrate reductase activity was measured at 2- or 3-day intervals 1 week before and 3 weeks after clipping.In vivo andin vitro assays both showed an increasing activity in leaves up to 8 days after cutting while root activity decreased. The opposite changes then occurred and both organs recovered their initial nitrate reductase activity levels after 12–14 days of regrowth. These fluctuations in nitrate reductase activity were considered to be related to the capacity for C assimilation and the nitrate availability.     

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.     

The effect of nitrate concentration in a flowing solution system on growth and nitrate uptake of twoPlantago species

Summary Juvenile plants ofPlantago lanceolata andP. major ssp.major were grown in a flowing solution system at 7.5 mM or 9.5 M NO₃. The parameters investigated were: RGR, shoot weight percentage, leaf length, length of main root axis, shoot concentrations of major ions and organic N, and the specific uptake rate for NO₃. At 9.5 M NO₃ growth ofP. major was not hampered, whereas shoot growth and leaf length ofP. lanceolata were reduced. The NO₃ concentration ofP. lanceolata decreased more than that ofP. major. The different performances of the species at 9.5 M NO₃ were associated with different specific uptake rates. In both treatments the root system ofP. major was shorter than that ofP. lanceolata. P. lanceolata accumulated more NO₃ in the leaves. The performance of thePlantago species is discussed in relation to the availability of nutrients in their habitats.Grassland Species Research Group. Publication no. 37.     

Nitrate reductase and nitrite reductase activity in free-living cells and bacteroids of Rhizobium loti

Cells of Rhizobium loti strains T1 and U226 cultured in defined medium with glutamate as the only nitrogen source and bacteroids isolated from root nodules of Lotus corniculatus, L. pedunculatus and L. tenuis did not show constitutive (non-nitrate induced) nitrate reductase activity (NRA). In contrast, nitrite reductase activity (NiRA) was present in both free-living cells and bacteroids of either strain T1 or U226. Constitutive NRA and NiRA were detected in the cytosol fraction from nodules of all three symbioses examined. An induced NRA was expressed in bacteroids after a 10 h incubation in the presence of nitrate.     

Soil nitrogen transformations and nitrate utilization by Deschampsia flexuosa (L.) Trin. at two contrasting heathland sites

Two Dutch heathland sites Hoorneboeg (HB) and Ede, dominated by Deschampsia flexuosa and differing in nitrate production, were sampled for an entire growing season. A large number of soil and plant parameters were monitored in an attempt to assess the contribution of nitrate in the N supply and its assimilation by Deschampsia.Average NO₃ ^– and NH₄ ⁺ concentrations (mg kg^–1) in the top 10-cm depth were 0.03 and 2.2, respectively, for HB, and 2.1 and 6.7, respectively, for Ede. Laboratory incubations of intact cores and experiments with FH-layer suspensions showed significantly higher mineralization and nitrification rates for the Ede site during most of the season. Nitrification was largely controlled by the rate of net N-mineralization, which in turn was highly affected by soil moisture. Nitrate production was virtually zero at HB and accounted for 25% of the net N-mineralization at Ede.Shoot chemical composition showed no essential differences for the two sites, but mean in vivo (current) foliar NRA was almost 2-fold higher at Ede than at HB, indicating some utilization of nitrate at the former location. At the HB site with essentially no nitrate production, however, enzyme activities were clearly higher than basal constitutive levels in NH₄ ⁺-fed plants. Apparently, shoot NRA at the HB site became positively affected by factors other than nitrate availability and/or showed disproportional increases in response to atmospheric nitrate inputs. Root NRA displayed the same low basal level at the two sites. Nitrate fertilization (100 kg N ha^–1) yielded maximally induced foliar NRAs similar to levels found in hydroponic nitrate plants. Although no accumulation of free NO₃ ^– was observed in shoots from fertilized plots, increases in foliar concentrations of both organic N and carboxylates clearly indicated nitrate assimilation. Root NRA showed no response to nitrate addition.It is concluded that current NRA measurements in Deschampsia at heathland sites are of limited value only, especially when interpreted in isolation. A combined approach, using concurrently conducted soil and plant analyses, will allow the extent of nitrate utilization in the field to be best characterized.Publication 2013 of the Netherlands Institute of Ecology.FAX no corresponding author: +31 8306 23227     

Effects of substrate nitrate concentration on symbiotic nodule formation in actinorhizal plants

The response ofAlnus glutinosa, Casuarina cunninghamiana, Elaeagnus angustifolia andMyrica cerifera to a range of substrate nitrogen levels in solution, in relation to plant growth, infection, nodulation and root fine structure was studied. Nine concentrations of potassium nitrate ranging from 0.05 to 3.0 mM, were tested on each of the species. Plants were inoculated withFrankia pure cultures after a two week exposure to one of the nine levels of added nitrate. After six more weeks with constant exposure to nitrate, plants were harvested and assayed. With the exception of Myrica, regression analyses of whole plant dry weights as a function of added nitrate were highly significant. There was a tendency for nodulated plants grown at intermediate levels of added nitrate to exhibit higher relative growth rates, probably due to the additive effect of substrate nitrogen and fixation of atmospheric nitrogen. The mean numbers of nodules per plant were, with the exception of Alnus, significantly higher at intermediate levels of added nitrate, as were mean nodule dry weights. A highly significant inverse relationship between nodule weight as a percentage of whole plant weight was found in Elaeagnus and Myrica. The observed response of Elaeagnus to added nitrate compared to other actinorhizal plants appears to demonstrate that root hair infected plants are much more sensitive to the inhibitory effects of added nitrate than plants infected by intercellular penetration. A sharp reduction in the presence of root hairs at high concentrations of nitrate was observed. This indicates that the inhibition of nodulation in some actinorhizal plant species results from nitrate induced root hair suppression.