Interactions between species change the uptake of ammonium and nitrate in Abies faxoniana and Picea asperata.

The functional traits of roots play an important role in nutrient acquisition in plants, which affects the outcome of plant–plant interactions. However, few studies have comprehensively investigated the plastic responses of plant root traits to plant–plant interactions. A pot experiment was conducted to quantify the effects of intraspecific and interspecific interactions on seedling growth and on multiple root traits of two coniferous species, Picea asperata Mast. and Abies faxoniana Rehd. et Wils. The results showed that plant–plant interactions changed the root physiology of these two species but had no effect on the morphological, architectural, and biotic traits of their root system. Intraspecific interaction resulted in lower root nitrogen content and stronger resource competition than interspecific interaction. Under intraspecific interaction, P. asperata had lower root vigor and nitrate reductase activity, which impeded the acquisition and utilization of the limited resources, and thus resulted in marginally decreased total biomass, where the total biomass for A. faxoniana was not significantly affected. Under interspecific interaction, the high total biomass of A. faxoniana could be explained by rhizosphere interactive effects and reduced metabolic (carbon and nitrogen) costs due to lower root exudative outputs. Our results demonstrate that root physiological responses can explain the effects of short-term plant–plant interactions on plant growth.

We examined the effect of pretreatments (18 h at 5 //mol dm-3) with abscisic acid, the ethylene-releasing substance 'Ethephon', gibberellic acid, indoleacetic acid, kinetin and zeatin on nitrate uptake and in vivo nitrate reductase activity (NRA) in roots of nitrogen-depleted Phaseolus vulgaris L. Nitrate uptake showed an apparent induction pattern with a steady state after about 6 h, in all treatments. The nitrate uptake rate after 6 h was unaffected or at most 30% lower after treatments with the plant growth regulators. Gibberellic acid, kinetin and zeatin induced substantial NRA in roots in the absence of nitrate, whereas Ethephon enhanced NRA only during nitrate nutrition. Kinetin-induced NRA (Ki-NRA) was maximal after a pretreatment at 1 //mol dm~3, and showed a lag phase of 6-8 h. Ki-NRA was additive to nitrate-induced NRA (NOj-NRA) for at least 24 h, independent of the induction sequence. After full induction, Ki-NRA approximated 20% of NO7-NRA. Abscisic acid counteracted the development of Ki-NRA, but not of NOj-NRA. Cycloheximide and tungstate were equally effective to suppress the development of nitrate reductase activity after supply of kinetin or NO3. Our data are consistent with the operation of two independent enzyme fractions (Ki-NRA and NO7-NRA) with apparently identical properties but with separate control mechanisms. The absence of major effects of plant growth regulators on the time-course and rate of nitrate uptake suggests that exogenous regulators, and possibly endogenous phytohormones are of minor importance for initial nitrate uptake. The differential effect of some regulators on nitrate uptake and root NRA furthermore indicates that the processes of uptake and reduction of NOj are not obligatory or exclusively coupled to each other.

The influence of elevated [CO2] on the uptake and assimilation of nitrate and ammonium was investigated by growing tobacco plants in hydroponic culture with 2 mm nitrate or 1 mm ammonium nitrate and ambient or 800 p.p.m. [CO2]. Leaves and roots were harvested at several times during the diurnal cycle to investigate the levels of the transcripts for a high‐affinity nitrate transporter (NRT2), nitrate reductase (NIA), cytosolic and plastidic glutamine synthetase (GLN1, GLN2), the activity of NIA and glutamine synthetase, the rate of 15N‐nitrate and 15N‐ammonium uptake, and the levels of nitrate, ammonium, amino acids, 2‐oxoglutarate and carbohydrates. (i) In source leaves of plants growing on 2 mm nitrate in ambient [CO2], NIA transcript is high at the end of the night and NIA activity increases three‐fold after illumination. The rate of nitrate reduction during the first part of the light period is two‐fold higher than the rate of nitrate uptake and exceeds the rate of ammonium metabolism in the glutamate: oxoglutarate aminotransferase (GOGAT) pathway, resulting in a rapid decrease of nitrate and the accumulation of ammonium, glutamine and the photorespiratory intermediates glycine and serine. This imbalance is reversed later in the diurnal cycle. The level of the NIA transcript falls dramatically after illumination, and NIA activity and the rate of nitrate reduction decline during the second part of the light period and are low at night. NRT2 transcript increases during the day and remains high for the first part of the night and nitrate uptake remains high in the second part of the light period and decreases by only 30% at night. The nitrate absorbed at night is used to replenish the leaf nitrate pool. GLN2 transcript and glutamine synthetase activity rise to a maximum at the end of the day and decline only gradually after darkening, and ammonium and amino acids decrease during the night. (ii) In plants growing on ammonium nitrate, about 30% of the nitrogen is derived from ammonium. More ammonium accumulates in leaves during the day, and glutamine synthetase activity and glutamine levels remain high through the night. There is a corresponding 30% inhibition of nitrate uptake, a decrease of the absolute nitrate level, and a 15–30% decrease of NIA activity in the leaves and roots. The diurnal changes of leaf nitrate and the absolute level and diurnal changes of the NIA transcript are, however, similar to those in nitrate‐grown plants. (iii) Plants growing on nitrate adjust to elevated [CO2] by a coordinate change in the diurnal regulation of NRT2 and NIA, which allows maximum rates of nitrate uptake and maximum NIA activity to be maintained for a larger part of the 24 h diurnal cycle. In contrast, tobacco growing on ammonium nitrate adjusts by selectively increasing the rate of ammonium uptake, and decreasing the expression of NRT2 and NIA and the rate of nitrate assimilation. In both conditions, the overall rate of inorganic nitrogen utilization is increased in elevated [CO2] due to higher rates of uptake and assimilation at the end of the day and during the night, and amino acids are maintained at levels that are comparable to or even higher than in ambient [CO2]. (iv) Comparison of the diurnal changes of transcripts, enzyme activities and metabolite pools across the four growth conditions reveals that these complex diurnal changes are due to transcriptional and post‐transcriptional mechanisms, which act several steps and are triggered by various signals depending on the condition and organ. The results indicate that nitrate and ammonium uptake and root NIA activity may be regulated by the sugar supply, that ammonium uptake and assimilation inhibit nitrate uptake and root NIA activity, that the balance between the influx and utilization of nitrate plays a key role in the diurnal changes of the NIA transcript in leaves, that changes of glutamine do not play a key role in transcriptional regulation of NIA in leaves but instead inhibit NIA activity via uncharacterized post‐transcriptional or post‐translational mechanisms, and that high ammonium acts via uncharacterized post‐transcriptional or post‐translational mechanisms to stabilize glutamine synthetase activity during the night.

Although it is known that plant–plant interaction is an important factor influencing plant nitrogen (N) uptake and biomass productivity, its effects on seasonal inorganic N uptake, preference, and allocation remain unclear. In this study, two conifer species (Picea asperataandAbies faxoniana) were planted in three different planting modes (i.e., single, intraspecific, and interspecific interaction). Using15N stable isotope tracer, we quantified plant biomass, ammonium (NH4+), and nitrate (NO3−) uptake rate (mass) and allocation in the middle (July) and the end (September) of the growing season, respectively, followed by analyses of root traits and soil properties so as to explore the underlying mechanism. Across the two seasons, intraspecific interaction decreased plant biomass and inorganic N‐uptake rate, which triggered intense competition for both species. Intraspecific competition ofP. asperatawas stronger than that ofA. faxoniana. In contrast, interspecific interaction revealed significant facilitative effects onA. faxoniana, particularly in September. From the middle to the late growing season, the inorganic N‐uptake rate ofP. asperatareduced, whereas that ofA. faxonianaincreased under interspecific interaction. The seasonal variation in plant N uptake was regulated by changes in root traits (such as root nitrogen concentration, specific root length, and branching intensity) and soil N availabilities. Both species indicated a preference for NO3−across seasons. Furthermore, we observed that15N allocation to shoots ofA. faxonianaunder interspecific interaction was higher than that ofP. asperataand declined from July to September. These findings on how plant–plant interactions affect plant N uptake seasonally can facilitate our understanding of species co‐existence and community assembly in forest ecosystems.

Soil fungi are more sensitive than bacteria to short-term plant interactions of Picea asperata and Abies faxoniana

The efficiency of nitrate use by turfgrasses is likely related to its efficiency of absorption by roots and its rate of metabolism in roots and shoots. This study was conducted to quantify the relationship between nitrate uptake rate and nitrate reductase activity with N use efficiency in a cool‐season turfgrass. Six cultivars of Kentucky bluegrass (Poa pratensis L.), which differ markedly in field performance, were used to measure intraspecific variation in nitrate uptake, in vivo nitrate reductase activity of roots and leaves, and N use efficiency expressed as clipping mass per unit N in clippings. Companion field studies compared N use efficiency and metabolism among 14 Kentucky bluegrass cultivars established on an Enfield silt loam (Coarse loamy over sandy skeletal, mixed, mesic, Typic Dystrochrepts). Nitrate uptake rate was determined by an in situ nitrate depletion method. Nitrate reductase activity was assayed by an optimized in vivo method. Significant differences among cultivars were observed for nitrate absorption, nitrate reductase activity in roots and leaves, and N use efficiency. Ambient nitrate concentrations influenced these parameters and their intraspecific differences. Nitrate uptake and reduction were saturable at external nitrate concentrations in excess of 1 mM. Regression analyses demonstrated that nitrate reductase activity in roots and leaves was strongly influenced by nitrate uptake rate. Nitrogen use efficiency was negatively related to ambient nitrate levels, nitrate uptake rate and nitrate reductase activity, with nitrate reductase activity in leaves having the strongest negative effect on use efficiency. These results suggest that the efficiency of N use by Kentucky bluegrass may be increased by genetically altering nitrate reductase activity and its partitioning between roots and shoots.

The effect of nitrogen form (NH(4)-N, NH(4)-N + NO(3) (-), NO(3) (-)) on nitrate reductase activity in roots and shoots of maize (Zea mays L. cv INRA 508) seedlings was studied. Nitrate reductase activity in leaves was consistent with the well known fact that NO(3) (-) increases, and NH(4) (+) and amide-N decrease, nitrate reductase activity. Nitrate reductase activity in the roots, however, could not be explained by the root content of NO(3) (-), NH(4)-N, and amide-N. In roots, nitrate reductase activity in vitro was correlated with the rate of nitrate reduction in vivo. Inasmuch as nitrate reduction results in the production of OH(-) and stimulates the synthesis of organic anions, it was postulated that nitrate reductase activity of roots is stimulated by the released OH(-) or by the synthesized organic anions rather than by nitrate itself. Addition of HCO(3) (-) to nutrient solution of maize seedlings resulted in a significant increase of the nitrate reductase activity in the roots. As HCO(3) (-), like OH(-), increases pH and promotes the synthesis of organic anions, this provides circumstantial evidence that alkaline conditions and/or organic anions have a more direct impact on nitrate reductase activity than do NO(3) (-), NH(4)-N, and amide-N.

Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen‐depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium.The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m−3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m−3).When plants were decapitated after an 18 h NO3− pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG−3 uptake. Presumably due to a difference in NO3− due to a difference in NO3− uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar‐stimulation of, oxygen consumption as well as the release of 14CO2 from freshly absorbed (U‐14C) sugar was the same for glucose and fructose. Therefore, we propose a glucose‐specific effect on NO3− uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose‐fructose effect on nitrate reductase activity independent of the effect on NO3− uptake was not indicated.A constant level of NRA occurred in roots of NO3− induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half‐life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots.

The author studied the effect of different nickel concentrations (0, 0.4, 40 and 80 μM Ni) on the nitrate reductase (NR) activity of New Zealand spinach (Tetragonia expansa Murr.) and lettuce (Lactuca sativa L. cv. Justyna) plants supplied with different nitrogen forms (NO3 −–N, NH4 +–N, NH4NO3). A low concentration of Ni (0.4 μM) did not cause statistically significant changes of the nitrate reductase activity in lettuce plants supplied with nitrate nitrogen (NO3 −–N) or mixed (NH4NO3) nitrogen form, but in New Zealand spinach leaves the enzyme activity decreased and increased, respectively. The introduction of 0.4 μM Ni in the medium containing ammonium ions as a sole source of nitrogen resulted in significantly increased NR activity in lettuce roots, and did not cause statistically significant changes of the enzyme activity in New Zealand spinach plants. At a high nickel level (Ni 40 or 80 μM), a significant decrease in the NR activity was observed in New Zealand spinach plants treated with nitrate or mixed nitrogen form, but it was much more marked in leaves than in roots. An exception was lack of significant changes of the enzyme activity in spinach leaves when plants were treated with 40 μM Ni and supplied with mixed nitrogen form, which resulted in the stronger reduction of the enzyme activity in roots than in leaves. The statistically significant drop in the NR activity was recorded in the aboveground parts of nickel-stressed lettuce plants supplied with NO3 −–N or NH4NO3. At the same time, there were no statistically significant changes recorded in lettuce roots, except for the drop of the enzyme activity in the roots of NO3 −-fed plants grown in the nutrient solution containing 80 μM Ni. An addition of high nickel doses to the nutrient solution contained ammonium nitrogen (NH4 +–N) did not affect the NR activity in New Zealand spinach plants and caused a high increase of this enzyme in lettuce organs, especially in roots. It should be stressed that, independently of nickel dose in New Zealand spinach plants supplied with ammonium form, NR activity in roots was dramatically higher than that in leaves. Moreover, in New Zealand spinach plants treated with NH4 +–N the enzyme activity in roots was even higher than in those supplied with NO3 −–N.

Strawberries (Fragaria xananassa Duch. 'Osogrande') were grown hydroponically with three NO3-N concentrations (3.75, 7.5, or 15.0 mM) to determine effects of varying concentration on NO3-N uptake and reduction rates, and to relate these processes to growth and fruit yield. Plants were grown for 32 weeks, and NO3-N uptake and nitrate reductase (NR) activities in roots and shoots were measured during vegetative and reproductive growth. In general, NO3-N uptake rates increased as NO3-N concentration in the hydroponics system increased. Tissue NO3- concentration also increased as external NO3-N concentration increased, reflecting the differences in uptake rates. There was no effect of external NO3-N concentration on NR activities in leaves or roots during either stage of development. Leaf NR activity averaged approximately 360 nmol NO2 formed/g fresh weight (FW)/h over both developmental stages, while NR activity in roots was much lower, averaging approximately 115 nmol NO2 formed/g FW/h. Vegetative organ FW, dry weight (DW), and total fruit yield were unaffected by NO3-N concentration. These data suggest that the inability of strawberry to increase growth and fruit yield in response to increasing NO3-N concentrations is not due to limitations in NO3-N uptake rates, but rather to limitations in NO3- reduction and/or assimilation in both roots and leaves.

Differential regulatory role of nitric oxide in mediating nitrate reductase activity in roots of tomato (Solanum lycocarpum).

Nitrate reductase (NR) activity was followed in root and nodule from Glycine max (L.) Merr. (Cv. Tracy) inoculated with Rhizobium japonicum. Initially, a plus NO3‐ in vivo assay was used. When chlorate‐resistant mutants were used as inoculum, nodule NR activity was reduced by about 90%. indicating that the bacteroid accounts for much of the normal nodule's NR. With plants 3 to 15 weeks of age nodule NR activity (g fresh weight)‐1 was highest in young plants and root activity highest in old plants. Root and nodule total NR activity increased with plant age and were often not greatly different. Root NR activity correlated with plant NO3‐ supply and increased from 0.8 to 11.4 μmol plant‐1 h‐1 as NO3‐ was increased from 0 to 3 mM. In contrast, nodule NR activity was high in plants grown without NO3‐ and did not appear to increase as nitrate supply to the plant was increased. Nodule activity was 6 to 14 μmol NO2‐ plant‐1 h‐1. Use of a minus NO3‐ in vivo assay had little affect on root NR activity, but greatly reduced nodule activity. Root tissue was found to have 5 to 38 times more NO3‐ than nodule tissue. It is concluded that low nitrate levels within the nodule limit NR activity and that it is improbable that the nodule is a major site of plant nitrate reduction.

In vivo nitrate reductase (NR) activity in roots of both cucumber (Cucumis sativus L. cv. Suyo) and figleaf gourd (Cucurbita ficifolia Bouche) grown at a root temperature of 20°C was greatly reduced by low temperature of the enzyme assay medium. In contrast, NR activity in roots of plants grown at either 13° or 20°C root temperature did not differ, based on the temperature of incubation. Activity of root NR was very low compared to that of leaf NR, particularly in figleaf gourd.The fate of nitrate nitrogen (N) in plants exposed for 2h to K15NO3 solutions at either 13° or 20°C root temperature was determined by chase experiments for 8h in the light after transfer of plants to non-labeled media at the same temperatures as those used for exposure to 15N. Absorption of nitrate-15N at 13°C root temperature was significantly lower than that at 20°C, especially in cucumber. However, in both species the assimilation and translocation of absorbed nitrate-15N were little affected by root temperature. Roots of both species accumulated very slight amounts of reduced-15N. Most of 15N translocated to leaves was assumed to be in the form of nitrate.These results suggest that nitrate assimilation takes place predominantly in leaves in both cucumber and figleaf gourd, and that the nitrate assimilating capacity is not affected by low root temperature and therefore not responsible for the differential root-chilling tolerance of these plant species.

Recent studies suggest that changes in leaf traits due to interactions between plants affect resource utilisation by herbivores, as well as herbivore distribution. However, this has not yet been confirmed experimentally. Here, we investigated the effects of phenotypic plasticity in leaf traits of Rumex obtusifolius (host plant) in response to intraspecific and interspecific interactions on the distribution of two leaf beetles, Gastrophysa atrocyanea (specialist herbivore) and Galerucella grisescens (generalist herbivore). We investigated the local population density of R. obtusifolius plants and the presence of leaf beetles on the plants at five study sites. Leaf chemicals (condensed tannins and total phenolics) were compared between aggregated and solitary R. obtusifolius plants. To clarify the effects of the interaction environment of R. obtusifolius plants on their leaf traits and on resource utilisation by the leaf beetles, we compared leaf chemicals and preferences of adult leaf beetles among treatments where R. obtusifolius experienced intraspecific interaction, interspecific interaction or no interaction in cultivation experiments. Finally, we evaluated the independent and combined effects of patch size and intraspecific interaction on leaf beetle distribution in mesocosm experiments. In the field, the presence of the specialist leaf beetle G. atrocyanea was positively correlated with the local population density (rosette overlap ratio) of R. obtusifolius plants; however, there was no correlation in the case of the generalist leaf beetle G. grisescens. In the cultivation experiments, plants in the intraspecific interaction treatment increased their leaf concentrations of condensed tannins and total phenolics, and G. atrocyanea consumed more of these leaves than leaves in other treatments. Similar results were observed in the field. In the mesocosm experiments, larger numbers of G. atrocyanea were distributed on R. obtusifolius plants exposed to below‐ground intraspecific interaction than on plants not exposed to intraspecific interaction. Our results provide experimental evidence that leaf‐trait changes in response to intraspecific interaction between host plants influence specialist herbivore distribution. This highlights the need to integrate plant–plant interactions into our understanding of plant–animal interactions. A free Plain Language Summary can be found within the Supporting Information of this article.

According to the ability to assimilate nitrate all plants are usually divided into three groups, including those with high nitrate reductase activity in plant root, low nitrate reductase activity in plant root and approximately equal nitrate reductase activity in root and leaves.The authors studied the assimilation of nitrates by different organs in plants of Brassicaceae ( Raphanus sativus convar. oleifera, Raphanus sativus loba, Raphanus sativus, garden radish of the Zhara, Saksa, Ledyanaya sosul’ka varieties), Poaceae (spring wheat, winter wheat, barley, maize) and Fabaceae ( Pisum sativum, Soja hispida, Lupinus perennis, garbanzo) in controlled conditions of development. The nitrate reductase activity, the content of nitrates in organs, and also the mass of dump matter were determined in 15-days plants. The most activity of nitrate reductase in all organs was revealed in Raphanus sativus convar. oleifera. This activity was highest in leaves almost in all studied species, which carry out the basic role in process of reduction of nitrates. So grouping the plants on their ability to reduce nitrates did not have clear boundaries. The species differ in this index even within the range of one family.