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.     

Interaction of nitrate uptake and nitrogen fixation in faba beans

An experiment was carried out to determine the relationship between nitrate uptake and nitrogen fixation of faba beans. Therefore inoculated and uninoculated faba beans were grown in nutrient solution with different nitrate concentrations. Nitrate uptake was measured every two days during the growing period. At the end of the experiment the nitrate uptake kinetics were determined with a short time depletion technique and nitrogen fixation was measured with the acetylene reduction method. A limitation of nitrate uptake due to nitrogen fixation was relatively small. Nitrate concentrations of approximately 1 mol m^–3 and 5 mol m^–3 decreased nitrogen fixation to values of 16% and 1% of the control plants which received no nitrate nitrogen. A reduction of nitrogen fixation was mainly due to a decrease of specific nitrogen fixation per unit nodule weight and to a lesser extent due to a reduction of nodule growth. Only the maximum nitrate influx (I_max) seemed to be influenced by nitrogen fixation. Michaelis-Menten constants (K_m) and minimum NO _inf3 ^– -concentrations (C_min) were not significantly influenced by nitrogen fixation.     

Regulation of nitrate uptake by amino acids in maize cell suspension culture and intact roots

The effect of amino acids on nitrate transport was studied in Zea mays cell suspension cultures and in Zea mays excised roots. The inclusion of aspartic acid, arginine, glutamine and glycine (15mM total amino acids) in a complete cell-culture media containing 1.0 mM NO₃ ^- strongly inhibited nitrate uptake and the induction of accelerated uptake rates. The nitrate uptake rate increased sharply once solution amino acid levels fell below detection limits. Glutamine alone inhibited induction in the cell suspension culture. Maize seedlings germinated and grown for 7 days in a 15 mM mixture of amino acids also had lower nitrate uptake rates than seedlings grown in 0.5 mM Ca(NO₃)₂ or 1 mM CaCl₂. As amino acids are the end product of nitrate assimilation, the results suggest an end-product feed-back mechanism for the regulation of nitrate uptake.     

Role of sugars in nitrate utilization by roots of dwarf bean

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.     

Kinetic parameters of nitrate uptake by different catch crop species: effects of low temperatures or previous nitrate starvation

The pollution of aquifers by NO^?₃ in temperate environments is aggravated by farming practices that leave the ground bare during winter. The use of catch crops during this time may decrease nitrate loss from the soil. Nitrate uptake by several catch crop species (Brassica napus L., Sinapis alba L., Brassica rapa L., Raphanus sativus L., Trifolium alexandrinum L., Trifolium incarnatum L., Phacelia tanacetifolia Benth., Lolium perenne L., Lolium multiflorum Lam. and Secale cereale L.) was here studied in relation to transpiration rate and low temperatures applied to the whole plant or to roots only. The Michaelis constant (K_m), maximum uptake rate (V_max), time of induction and contributions of inducible and constitutive mechanisms were estimated from measurements of NO^?₃ depletion in the uptake medium. There were large differences between species, with K_m (μM) values ranging between 5.12 ± 0.64 (Trifolium incarnatum) and 36.4 ± 1.97 (Lolium perenne). Maximum NO^?₃ uptake rates expressed per unit root weight were influenced by ageing, temperature and previous NO^?₃ nutrition. They were also closely correlated with water flow through the roots and with shoot/root ratio of these species. The combined results from all species and treatments showed that V_max increased with shoot/root ratio, suggesting a regulatory role for the shoots in NO^?₃ uptake. Overall, the results showed a great diversity in NO^?₃ uptake characteristics between species in terms of kinetic parameters, contribution of the constitutive system (100% of total uptake in ryegrass, nil in Fabaceae) and time of induction.     

The effects of temperature and nutrient concentrations on nitrate and phosphate uptake in different species of Porphyra from Long Island Sound (USA)

Uptake rates of nitrate and phosphate were measured for four species and one variety of Porphyra from Long Island Sound (USA) at two temperatures and two nutrient medium concentrations at increasing intervals over a 24- or 48-h period. Maximum uptake rates found were: V_{30 μM}^{0-1 h}=73.8 μmol NO₃ g⁻¹ DW h⁻¹ and V_{3 μM}^{0-1 h}=16.7 μmol PO₄ g⁻¹ DW h⁻¹, in the two thinnest Porphyra. We found that the nitrate uptake rates were significantly greater at 30 μM than 3 μM NO₃ concentration, and that the uptake rates decreased with time of exposure. Temperature (5, 15, and 25 °C) did not have as strong an effect on nitrate uptake rates as did nutrient concentration. Q₁₀ values and uptake rates at four different nitrate concentrations indicated that nutrient uptake at 5 °C was initially an active process. After 24 h, the processes involved appeared passive as Q₁₀ values were between 1.0 and 1.3 and nitrate uptake curves were linear. Nitrate uptake rates correlated positively with the surface area/volume (SA/V) ratio. No coherent trends were found for uptake of phosphate, except that the uptake rates were significantly higher in 30 μM NO₃ medium as opposed to 3 μM NO₃. We did not find any significant difference in uptake rate and pattern between the summer species Porphyra purpurea (Roth.) C. Agardh, the eurythermic Porphyra suborbiculata Kjellm., the winter species Porphyra rosengurttii J. Coll and J. Cox, and the two varieties of Porphyra leucosticta Thur. Le Jol. (both winter species).     

Sink-stimulated photosynthesis and sink-dependent increase in nitrate uptake: nitrogen and carbon relations of the parasitic association Cuscuta reflexa–Ricinus communis

Ricinus communis L. was grown under limiting N supply in quartz sand culture, fed with 0.2, 1 or 5 mol m^?3 NO₃^?, or in liquid culture with 0.022, 0.05 or 0.5 mol m^?3 NO₃^?. Some of the plants were infected with Cuscuta reflexa Roxb. As occurred for the host, dry matter production and growth of C. reflexa were severely depressed with decreasing N supply to the host. When parasitized by C. reflexa, the shoot and root dry weight of Ricinus was diminished at all levels of N nutrition, but the total dry weight of host plus parasite was almost the same as that of uninfected Ricinus. In contrast to the situation in Lupinus albus (Jeschke et al. 1994b), infection by Cuscuta resulted in increased tissue N levels in the host and the N content of the system Ricinus plus C. reflexa was the same or even somewhat larger than that of uninfected plants. This indicated a sink-dependent stimulation of nitrate uptake. As a result of decreased root weights, nitrate uptake g^?1 FW was stimulated by 80, 60 or only 40% at 0.2, 1 or 5 mol m^?3 nitrate supply. Increased nitrate uptake was reflected, particularly at low N supply, in xylem transport; xylem sap nitrate concentrations were substantially elevated, while those of amino acids were decreased in parasitized plants. This indicated an inhibition of nitrate assimilation in roots of parasitized plants under limiting N supply. Besides these effects on N relations, C. reflexa induced a substantial sink-dependent stimulation of net photosynthesis in host leaves and a concomitant increase in stomatal opening and transpiration. This stimulation depended on the relative sink size induced by Cuscuta, on nitrogen nutrition and on leaf age, indicating that delayed senescence of leaves contributes to the overall effects of Cuscuta on its host. The Cuscuta-induced inhibition of nitrate assimilation in the roots and the increase in nitrate uptake suggest that nitrate reduction was shifted towards the leaves in the presence of C. reflexa. The stimulating effects of C. reflexa in the Ricinus-Cuscuta association are compared with the strongly inhibitory effects occurring in the tripartite association L. albus–Rhizobium–Cuscuta reflexa.     

Effects of aluminium on nitrate uptake and assimilation

A study was conducted to examine the hypothesis that the effects of external Al on NO₃^? uptake and assimilation depend upon the concentration of Al present. Young soybean seedlings [Glycine max (L.) Merrill, cv. Essex], growing under moderate acidity stress at pH 4-2, were exposed to a range of {A1³⁺} in solution for 3d, and to labelled 99 atom %¹⁵NO₃^? during the final hour of Al exposure. Uptake of ¹⁵NO₃^?g^?1 root dry weight was increased by about 28% in the presence of Al at {A1³⁺} below 10 mmolm^?3, and NO₃^? uptake was decreased by about 12% when the {A1³⁺} increased to 44mmoln^?3. The stimulation phase closely paralleled stimulation of root elongation. At higher {A1³⁺}, the inhibition of root elongation was much more severe than that of NO₃^? uptake. There was no indication of a separate effect of Al on root ¹⁵NO₃^? reduction in situ, as the accumulation of reduced ¹⁵N in the root remained a similar percentage of ¹⁵NO₃^? uptake at all {A1³⁺}. At higher {A1³⁺}, the atom %¹⁵N enrichment of the insoluble reduced-N (protein) fraction of root tips increased. This suggests that the Al inhibition of root elongation did not result from disruption of the N supply to the root apex.     

The nitrogen balance of Raphanus sativus X raphanistrum plants. I. Daily nitrogen use under high nitrate supply

Abstract Growth-chamber cultivated Raphanus plants accumulate nitrate during their vegetative growth. After 25 days of growth at a constant supply to the roots of 1 mol m^?3 (NO^?₃) in a balanced nutrient solution, the oldest leaves (eight-leaf stage) accumulated 2.5% NO^?₃-nitrogen (NO₃-N) in their lamina, and almost 5% NO₃-N in their petioles on a dry weight basis. This is equivalent to approximately 190 and 400 mol^?3 m^?3 concentration of NO^?₃ in the lamina and the petiole, respectively, as calculated on a total tissue water content basis. Measurements were made of root NO^?₃ uptake, NO^?₃ fluxes in the xylem, nitrate uptake by the mesophyll cells, and nitrate reduction as measured by an in vivo test. NO^?₃ uptake by roots and mesophyll cells was greater in the light than in the dark. The NO^?₃ concentration in the xylem fluid was constant with leaf age, but showed a distinct daily variation as a result of the independent fluxes of root uptake, transpiration and mesophyll uptake. NO^?₃ was reduced in the leaf at a higher rate in the light than in the dark. The reduction was inhibited at the high concentrations calculated to exist in the mesophyll vacuoles, but reduction continued at a low rate, even when there was no supply from the incubation medium. Sixty-four per cent of the NO^?₃ influx was turned into organic nitrogen, with the remaining NO^?₃ accumulating in both the light and the dark.     

Time-course of Nitrate Uptake and Nitrate Reductase Activity in Nitrogen-depleted Dwarf Bean

The rate of nitrate uptake by N-depleted French dwarf bean (Phaseolus vulgaris L. cv. Witte Krombek) increased steadily during the first 6 h after addition of NO₃ -After this initial phase the rale remained constant for many hours. Detached root systems showed the same time-course of uptake as roots of intact plants. In vivo nitrate reductase activity (NRA) was assayed with or without exogenous NO₃^- in the incubation medium and the result ing activities were denoted potential and actual level, respectively. In roots the difference between actual and potential NRA disappeared within 15 min after addition of nitrate, and NRA increased for about 15 h. Both potential and actual NRA were initially very low. In leaves, however, potential NRA was initially very high and was not affected by ambient nitrate (0.1–5 mol m^-3) for about 10 h. Actual and potential leaf NRA became equal after the same period of time. In the course of nitrate nutrition, the two nitrate reductase activities in leaves were differentially inhibited by cycloheximide (3.6 mmol m^-3) and tungstate (1 mol m^-3). We suggest that initial potential NRA reflects the activity of pre-existing enzyme, whereas actual NRA depends on enzyme assembly during NO₃^- supply. Apparent induction of nitrate uptake and most (85%) of the actual in vivo NRA occurred in the root system during the first 6 h of nitrate utilization by dwarf bean.     

Induction of nitrate uptake and nitrate reductase activity in trembling aspen and lodgepole pine

¹³NO₃^– influx into the roots and in vivo nitrate reductase activity (NRA) in the roots and leaves have been measured in trembling aspen (Populus tremuloides Michx.) and lodgepole pine (Pinus contorta Dougl.) seedlings after exposure to either 0·1 or 1·5 mol m^–3 NO₃^– for varying periods up to 20 d. Both NO₃^– influx and NRA were inducible in these species and, in trembling aspen, peak induction of nitrate influx and NRA were achieved within 12 h, compared to 2–4 d for influx and 4–12 d for NRA in lodgepole pine. In trembling aspen, ≈ 30% of the total ¹³N absorbed during a 10 min influx period followed by 2 min of desorption was translocated to the shoot. In lodgepole pine, by contrast, translocation of ¹³N to the shoot was undetectable during the same time period. Root NRA as well as NO₃^– influx from 0·1 mol m^–3 NO₃^– were substantially higher in trembling aspen than in lodgepole pine at all stages of NO₃^– exposure, i.e. during the uninduced, the peak induction, and steady-state stages. In order to examine whether the lower rates of NO₃^– influx and NRA were related to proportionately fewer young (unsuberized) roots in lodgepole pine, we determined these parameters in young and old (suberized) roots of this species separately. Induction of influx and NRA were initially greater in young roots but at steady-state there were only minor differences between the young and the old roots. However, even the elevated initial rates in the young roots of lodgepole pine were substantially lower than those of aspen. In pine, influx at 1·5 mol m^–3 NO₃^– was ~ 6-fold higher than at 0·1 mol m^–3 NO₃^– and appeared to be mostly via a constitutive system. By contrast, in aspen, steady-state influxes at 0·1 and 1·5 mol m^–3 were not significantly different, being similar to the rate attained by pine at only the higher [NO₃^–]. In aspen, leaf NRA was ~ 2-fold higher than that of roots. In lodgepole pine NRA of the needles was below the detection limit. These results show that trembling aspen seedlings are better adapted for NO₃^– acquisition and utilization than lodgepole pine seedlings.     

Effect of root volume and nitrate solution concentration on growth,fruit yield,and temporal N and water uptake rates by apple trees

Meager information is available on the specific effects of root volume (V) and N concentration in the water (C_N) on uptake rates of water and N by apple trees, as related to fruit yield and tree growth. To investigate this relationship, Golden Delicious/Hashabi trees were grown for 5 years in containers of 200, 50 and 101. Trees in the 200–1 containers were irrigated with a nutrient solution containing 10.7±1.3, 7.1±1.5 or 2.5±1.0 mM NO₃. Trees in the remaining two container-volume treatments were uniformly supplied with a solution of 7.1±1.5 mM NO₃. Elevated C_N had no effect on the rate of water uptake, but increased the rate of N absorption by the trees from 2.4 to 4.8 g N tree⁻¹ day⁻¹ during July. The stimulated N uptake rate stemmed from enhanced fluxes of N uptake by the roots. C_N had a negligible effect on root weight and root permeability to NO₃ and water. The elevated N uptake rate did not result in greater fruit yield and growth, or greater N content in tree organs, indicating considerable release of N from living and decaying roots to the growth medium. Reducing the container volume decreased yield, total dry matter production and N and water uptake rates, but increased root permeability to NO₃ and water, and total soluble solids in fruits. The all-season average C_N in the irrigation solution above which N concentration in the transpiration stream was lower than the inflowing C_N was 4.2 mM NO₃.     

The effect of endogenous and externally supplied nitrate on nitrate uptake and reduction in sugarbeet seedlings

The pericarp of the dormant sugarbeet fruit acts as a storage reservoir for nitrate, ammonium and -amino-N. These N-reserves enable an autonomous development of the seedling for 8–10 d after imbibition. The nitrate content of the seed (1% of the whole fruit) probably induces nitrate-reductase activity in the embryo enclosed in the pericarp. Nitrate that leaks out of the pericarp is reabsorbed by the emerging radicle. Seedlings germinated from seeds (pericarp was removed) without external N-supply are able to take up nitrate immediately upon exposure via a low-capacity uptake system (v_max = 0.8 mol NO ₃ ^- ·(g root FW)^–1·h^–1; K_s = 0.12 mM). We assume that this uptake system is induced by the seed nitrate (10 nmol/seed) during germination. Induction of a high-capacity nitrate-uptake system (v_max = 3.4 mol NO ₃ ^- ·(g root FW)^–1·h^–1; K_s = 0.08 mM) by externally supplied nitrate occurs after a 20-min lag and requires protein synthesis. Seedlings germinated from whole fruits absorb nitrate via a highcapacity uptake mechanism induced by the pericarp nitrate (748 nmol/pericarp) during germination. The uptake rates of the high-capacity system depend only on the actual nitrate concentration of the uptake medium and not on prior nitrate pretreatments. Nitrate deprivation results in a decline of the nitrate-uptake capacity (t_1/2 of v_max = 5 d) probably caused by the decay of carrier molecules. Small differences in K_s but significant differences in v_max indicate that the low- and high-capacity nitrate-uptake systems differ only in the number of identical carrier molecules.Abbreviations NR nitrate reductase - pFPA para-fluorophenylalanine This work was supported by a grant from Bundesministerium für Forschung und Technologie and by Kleinwanzlebener Saatzucht AG, Einbeck.     

Effects of nitrite, chlorate, and chlorite on nitrate uptake and nitrate reductase activity

Effects of NO₂⁻, ClO₃⁻, and ClO₂⁻ on the induction of nitrate transport and nitrate reductase activity (NRA) as well as their effects on NO₃⁻ influx into roots of intact barley (Hordeum vulgare cv Klondike) seedlings were investigated. A 24-h pretreatment with 0.1 mol m⁻³ NO₂⁻ fully induced NO₃⁻ transport but failed to induce NRA. Similar pretreatments with ClO₃⁻ and ClO₂⁻ induced neither NO₃⁻ transport nor NRA. Net ClO₃⁻ uptake was induced by NO₃⁻ but not by ClO₃⁻ itself, indicating that NO₃⁻ and ClO₃⁻ transport occur via the NO₃⁻ carrier. At the uptake step, NO₂⁻ and ClO₂⁻ strongly inhibited NO₃⁻ influx; the former exhibited classical competitive kinetics, whereas the latter exhibited complex mixed-type kinetics. ClO₃⁻ proved to be a weak inhibitor of NO₃⁻ influx (K_i = 16 mol m⁻³) in a noncompetitive manner. The implications of these findings are discussed in the context of the suitability of these NO₃⁻ analogs as screening agents for the isolation of mutants defective in NO₃⁻ transport.     

Changes in NO3− and K+ uptake by four species in flowing solution culture in response to increased irradiance

Net rates of NO₃^? and K⁺ uptake were compared for oilseed rape (Brassica napus L. cv. Jet neuf), perennial ryegrass (Lolium perenne L. cv. S23), Italian ryegrass (Lolium multiflorum Lam. cv. Augusta) and wheat (Triticum aestivum L. cv. Fen-man) in flowing solution culture during a 4-day sequence of low-low-high-high natural irradiance. Concentrations of NO₃^? (10 μM) and K⁺ (2.5 μM) in solutions were maintained automatically and hourly variation in net uptake of these ions was measured. During the 2 days of low irradiance (<1 MJ m^?2 day^?1) the uptake rates of both ions by all species were low at <1 mmol NO₃^?, m^?2 h^?1 and <0.4 mmol K⁺ m^?2 h^?1. Uptake increased in each species during the first day of high irradiance (7.90 MJ m^?2 day^?1) to >4 mmol NO₃^? m^?2 h^?1 and >1.4 mmol K⁺ m^?1 h^?1. These higher rates were maintained throughout the following night. The lag-time between maximum irradiance and the onset of the highest acceleration in uptake was greater for NO₃^? (5–8 h) than for K⁺ (≤1 h) in rape, wheat and Italian ryegrass. Uptake of NO₃^?, by perennial ryegrass showed an almost constant acceleration for 18 h following maximum irradiance. In all species the measured maximum inflows (uptake rate per unit root length) of both ions were greater than theoretical maximum potential inflows to a non-competing infinite-sink root in soil, by factors of 7 and 36, respectively, for NO₃^? and K⁺, averaged over all species.     

Productivity and nutrient uptake of water hyacinth,Eichhornia crassipes I. Effect of nitrogen source

Water hyacinth,Eichhornia crassipes, growth and nutrient uptake rates, as influenced by different N sources and N transformations, were measured using microcosm aquaculture systems. Net productivity was highest in the system receiving equal amounts of NH₄ ⁺ and NO₃ ^- (at 10 mg N 1^-1 each) and decreased in the order of NO₃ ^-, NH₄ ⁺, urea (added at 20 mg N 1^-1 each), and methane digestor effluent (at 6 mg N 1^-1). During the first 7-wk study (average ambient air temperature was 26–28°C), biomass yields were in the range of 19–53 g dry wt m^-2 day^-1, while between the 8th and 12th wk (average ambient air temperature was 16–22°C), biomass yields were in the range of 10–33 g dry wt m^-2 day^-1. In the systems with either NH₄ ⁺ or NO₃ ^-, or both added in equal proportions, about 14–20% of the total yield was contributed by roots, whereas in the system with urea and digestor effluent, roots contributed about 23 and 44% of the total yield, respectively. Nitrogen and P uptake per unit area followed trends similar to biomass yields. Nitrogen uptake rates were in the range of 533–2, 161 mg N m^-2 day^-1 for the systems receiving NH₄ ⁺, NO₃ ^-, and urea, while uptake rates were in the range of 124–602 mg N m^-2 day^-1 for the system receiving methane digestor effluent. Phosphorus uptake rates were found to be in the range of 59–542 mg P m^-2 day^-1. Under the most favorable conditions, maximum recorded biomass yield was 53 g dry wt m^-2 day^-1, with N and P removal rate of 2,161 mg N m^-2 day^-1 and 542 mg P m^-2 day^-1, indicating the potential of water hyacinth to produce large amounts of biomass which can be potentially used as a feedstock to produce methane.     

Growth and development of seminal and crown root systems in N-limited barley,and their contributions to nitrate acquisition during vegetative and generative growth

Barley (Hordeum vulgare L., cvs Golf and Laevigatum) was grown under nitrogen limitation, controlled by the relative rate of nitrate-N addition (RA), in solution culture. The seminal and crown root systems were kept apart, but in contact with the same nutrient solution throughout culturing. Growth, nitrate uptake, and in vitro nitrate reductase (NR) activity in the different root parts were studied at plant ages from 40 (late vegetative stage) to 110 (mid grain-filling) days. The RA was during this time interval stepwise decreased from 0.08 day^–1 to 0.005 day^–1. The ratio between seminal root dry weight and total plant dry weight decreased drastically during post-anthesis growth, whereas the contribution by crown roots remained unchanged. Tissue nitrogen concentrations in seminal roots did not change with time, but decreased in crown roots after day 80. The NR activity decreased with age in both seminal and crown roots. The V_max for net nitrate uptake decreased throughout the experiment in the seminal root system, but not in the crown root system. The kinetic properties (V_max and K_M) were used to calculate the nitrate concentration required to maintain a relative rate of nitrate-N uptake that equals the relative addition rate. These concentrations (2 to 5 mmol m^–3) were found to closely match actually measured nitrate concentrations in the nutrient solution (1 to 6 mmol m^–3). From uptake kinetics, it was deduced that the contribution by seminal roots to total nitrate uptake at these concentrations decreased from more than 50% in vegetative plants, to about 20% just after main shoot anthesis, and to less than 5% during grain-filling. ei]Section editor: H Lambers     

A comparative study between two citrus rootstocks: Effect of nitrate on the root morpho-topology and net nitrate uptake

Root morpho-topology and net nitrate uptake of two citrus seedlings, Volkamer Lemon and Carrizo Citrange, grown at two nitrogen supplies (NO₃-N 5 M and 1000 M, respectively) were studied. Root morphological and topological parameters were gauged by an image-specific analysis system (WinRHIZO). Net nitrate uptake was estimated using the nitrate depletion method. The main findings showed that Carrizo seedlings had a dichotomous branching root system characterized by high root tip numbers and long 2^nd order lateral roots. Conversely, Volkamer root systems had a herringbone structure with a long tap root and 1^st order lateral root. Nitrate treatment did not seem to affect the pattern of the two genotypes, except for the 2^nd order lateral roots (Carrizo more than Volkamer) and root/shoot ratio and root mass ratio (Volkamer more than Carrizo) that were significantly different at low nitrate supply. Nitrate treatments induced a diverse net nitrate uptake regulation between citrus rootstocks. Indeed, at low nitrate supply, Carrizo showed a more efficient nitrate acquisition process in terms of: 1) higher net nitrate uptake maximum of the inducible high affinity transport system or full induction (A), (2) higher cumulative nitrate uptake (A_t) and (3) lower t₁ parameter defined as the half time of the net nitrate uptake rate of the inducible transport system during the induction phase, compared to Volkamer. Conversely, at the high nitrate level, only the genotypical difference of the t₁ parameter was maintained. The results suggested that, at the low nitrate level, the morphological root traits such as higher 2^nd order lateral roots and greater root tip numbers of the Carrizo compared with Volkamer seedlings, enhance the capacity to absorb nitrate from nutrient solution.     

Effect of temperature and irradiance on the uptake of ammonium and nitrate by <Emphasis Type="Italic">Laurencia brongniartii</Emphasis> (Rhodophyta,Ceramiales)

The effects of temperature (20, 24 and 28 °C) and irradiance (15 and 40 μmol photon m⁻² s⁻¹) on the nitrate and ammonium uptake rates of the subtropical red alga, Laurencia brongniartii, were investigated to prepare for tank cultivation. Nitrate uptake followed saturation kinetics and was faster at higher irradiances and temperatures. In contrast, ammonium uptake was linear over the experimental range and was not affected by an increase in temperature. A parameter, β, was calculated to compare substrate uptake rates of nitrate along the linear portion of the uptake curve with that of ammonium. For nitrate, β was lower at low irradiance and higher at high irradiance (β = 0.007 ± 0.003 and 0.030 ± 0.002 [μmol N L⁻¹ (μmol N g_ww⁻¹ d⁻)⁻¹], respectively). However, β was 0.023 ± 0.002 and 0.034 ± 0.002 [μmol N L⁻¹ (μmol N g_ww⁻¹ d⁻¹)⁻¹] for ammonium, suggesting a preference for ammonium over nitrate.