The influence of NO3 - and NH4 + nutrition on the carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) plants

The carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) grown hydroponically at a constant pH on either 4 mM or 12 mM NO₃ ^- or NH₄ ⁺ nutrition were investigated using either ¹⁴C or ¹⁵N techniques. Greater allocation of ¹⁴C to amino-N fractions occurred at the expense of allocation of ¹⁴C to carbohydrate fractions in NH₄ ⁺-compared to NO₃ ^--fed plants. The [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were 1.5-fold and 2.0-fold greater in shoots and roots respectively of 12 mM NO₃ ^--compared to 12 mM NH₄ ⁺-fed wheat. In both 4 mM and 12 mM N-fed maize the [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were approximately 1.7-fold and 2.0-fold greater in shoots and roots respectively of NO₃ ^--compared to NH₄ ⁺-fed plants. Similar results were observed in roots of wheat and maize grown in split-root culture with one root-half in NO₃ ^--and the other in NH₄ ⁺-containing nutrient media. Thus the allocation of carbon to the amino-N fractions occurred at the expense of carbohydrate fractions, particularly within the root. Allocation of ¹⁴N and ¹⁵N within separate sets of plants confirmed that NH₄ ^--fed plants accumulated more amino-N compounds than NO₃ ^--fed plants. Wheat roots supplied with ¹⁵NH₄ ⁺ for 8 h were found to accumulate ¹⁵NH₄ ⁺ (8.5 g ¹⁵N g^-1 h^-1) whereas in maize roots very little ¹⁵NH₄ ⁺ accumulated (1.5 g ¹⁵N g^-1 h^-1)It is proposed that the observed accumulation of ¹⁵NH₄ ⁺ in wheat roots in these experiments is the result of limited availability of carbon within the roots of the wheat plants for the detoxification of NH₄ ⁺, in contrast to the situation in maize. Higher photosynthetic capacity and lower shoot: root ratios of the C₄ maize plants ensure greater carbon availability to the root than in the C₃ wheat plants. These differences in carbon and nitrogen partitioning between NO₃ ^--and NH₄ ⁺-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO₃ ^- and NH₄ ⁺ nutrition.     

Al avoidance and Al tolerance ofMucuna pruriens var.utilis: Effects of a heterogeneous root environment and the nitrogen form in the root environment

InMucuna pruriens var.utilis, grown with nitrate-N in a hydroponic split-root system, an Al avoidance reaction of root growth was observed, which was ascribed to local P stress in the Al containing compartment. The Al avoidance reaction was similar to the avoidance ofMucuna roots of acid subsoil in the field where roots grew preferentially in the topsoil. In the present paper the effect of different N forms (NO₃ ^– and NH₄ ⁺) on the reactions ofMucuna to Al were studied, since in acid soils N is present as a mixture of NO₃ ^– and NH₄ ⁺. No interaction between the N form and Al toxicity was found. A hydroponic split-root experiment with NH₄NO₃ nutrition, which is comparable to the situation in the field, showed that under these conditions Al avoidance did not occur. It is concluded that a relation between the Al avoidance reaction ofMucuna and P stress is still likely.Abbreviations D_r root diameter - L_pr total root length per plant - L_rw specific root length - NRA nitrate reductase activity - S/R shoot: root ratio     

Effects of different nitrogen forms and combination with foliar spraying with 6-benzylaminopurine on growth,transpiration, and water and potassium uptake and flow in tobacco

NH₄ ⁺-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO₃ ^–, NH₄NO₃, NH₄ ⁺ or NH₄ ⁺+6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO₃ ^–-N by NH₄ ⁺-N (NH₄NO₃) did not change growth, transpiration, uptake and flow of water and K⁺ compared with the NO₃ ^–-N treatment. However, NH₄ ⁺-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO₃ ^–-N and NH₄NO₃ treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (¹³C) compared with the NO₃ ^– treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K⁺ uptake to 10%. Under NH₄ ⁺ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K⁺ currently taken up, indicating massive mobilisation of K⁺ from lower leaves and even the roots. Phloem retranslocation of K⁺ from the shoot and cycling through the root contributed 67% to the xylem transport of K⁺, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K⁺) caused by NH₄ ⁺-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10^–8 M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH₄ ⁺-fed plants are discussed.     

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake,mineral composition and yield of citrus

In short-term water culture experiments with different ¹⁵N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH₄ ⁺ at a higher rate than NO₃ ^–. Maximum NO₃ ^– uptake by the whole plant occurred at 120 mg L^–1 NO₃ ^–-N, whereas NH₄ ⁺ absorption was saturated at 240 mg L^–1 NH₄ ⁺-N. ¹⁵NH₄ ⁺ accumulated in roots and to a lesser degree in both leaves and stems. However, ¹⁵NO₃ ^– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH₄ ⁺ (15–60 mg L^–1 N) to nutrient solutions containing 120 mg L^–1 N as ¹⁵N labeled nitrate reduced ¹⁵NO₃ ^– uptake. Maximum inhibition of ¹⁵NO₃ ^– uptake was about 55% at 2.14 mM NH₄ ⁺ (30 mg L^–1 NH₄ ⁺-N) and it did not increase any further at higher NH₄ ⁺ proportions.In a long-term experiment, the effects of concentration and source of added N (NO₃ ^– or NH₄ ⁺) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH₄ ⁺ versus NO₃ ^– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO₃ ^–-N:NH₄ ⁺-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L^–1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO₃ ^– or NH₄ ⁺ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO₃ ^–-N/NH₄ ⁺-N ratio. With increasing proportions of NH₄ ⁺ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH₄ ⁺, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO₃ ^–-N:NH₄ ⁺-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH₄ ⁺ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO₃ ^–:NH₄ ⁺ ratio.     

Impact of NH4NO3 on microbial biomass C and N and extractable DOM in raised bog peat beneath Sphagnum capillifolium and S. recurvum

Regular bi-weekly additions of NH₄NO₃, equivalent to a rate of 3 g N m^–2 yr^–1, were applied to cores of Sphagnum capillifolium, inhabiting hummocks and S. recurvum a pool and hollow colonizer, in a raisedbog in north east Scotland. Microbial biomass C and N,both measured by chloroform extraction, showed similarseasonal patterns and, for most depths, the effects ofadded N on microbial biomass C and N changed withtime. The addition of inorganic N had greatest effectduring October when the water table had risen to thesurface and microbial C and N in the untreated coreshad decreased. Microbial C and N were maintained at75 g C m^–2 and 8.3 g N m^–2 above the values in the untreated cores and far exceeded the amounts of N that had been added up to that date (1 g N m^–2) as NH₄NO₃. This increased microbial biomass was interpreted as leaching of carbonaceous material from the NH₄NO₃ treated moss resulting in greater resistance of the microbialbiomass to changes induced by the rising water table.Treatment with N also caused significant reductions inextractable dissolved organic N (DON) at 10–15 cmdepth, beneath the surface of the moss, but at lowerdepths to 25 cm no changes were observed. Extracteddissolved organic carbon (DOC) was not affected by Ntreatment and showed less seasonal variation than DON,such that the C:N ratio of dissolved organic matter(DOM) in all depths increased from approximately 4 inJuly to around 30 in December.     

Relationship between morphological and physiological responses to waterlogging and salinity in Sporobolus virginicus (L.) Kunth

The effects of waterlogging and salinity on morphological and physiological responses in the marsh grass Sporobolus virginicus (L.) Kunth were investigated in a 4×2 factorial experiment. Plants were subjected to four salinity levels (0, 100, 200 and 400 mol m^–3 NaCl) and two soil inundation conditions (drained and flooded) for 42 days. Flooding at 0 mol m^–3 NaCl caused initiation of adventitious surface roots, increased internal acration and plant height, induced alcohol dehydrogenase activity (ADH), and decreased belowground biomass and the number of culms per plant. Salinity increase from 0 to 400 mol m^–3 NaCl under drained conditions increased leaf and root proline concentrations and decreased photosynthesis, aboveground biomass, number of culms per plant and number of internodes per culm. Concurrent waterlogging and salinity induced ADH activity and adventitious surface roots but decreased plant height and aboveground biomass. Internal air space increased with waterlogging from 0 to 100 mol m^–3 NaCl but further increases in salinity to 400 mol m^–3 reduced air space. Combined waterlogging and salinity stresses, however, had no effect on photosynthesis or on the concentrations of proline in leaves or roots. These results are discussed in relation to the widespread colonization by S. virginicus of a wide range of coastal environments varying in soil salinity and in the frequency and intensity of waterlogging.     

The significance of Cognettia sphagnetorum(Enchytraeidae) on nitrogen availability and plant growth in wood ash-treated humus soil

A laboratory microcosm experiment was established to study whether the role of Cognettia sphagnetorum (Enchytraeidae) in affecting Scots pine (Pinus sylvestris) seedling growth is influenced by wood ash-amendment, i.e., neutralisation of the raw humus soil. Coniferous forest soil, inoculated with soil microbes and nematodes, was either treated with wood ash or left as ash-free control. Wood ash (corresponding to an amount of 5000 kg ha^–1) was either spread on the soil surface or mixed into the soil. Enchytraeid and pine seedling biomass, abundance of nematodes, and water leachable NH₄ ⁺-N and NO₃ ^–-N were measured 26 and 51 weeks after initiation of the experiment and root length and N concentration of needles were measured 51 weeks after initiation of the experiment. Wood ash when mixed into the soil, reduced the biomass of C. sphagnetorum. Nematodes were unaffected by the treatments. In the ash-free soils C. sphagnetorum had little influence on pine growth, but it did decrease root length and root to shoot ratio. In the absence of enchytraeids wood ash decreased pine biomass production and root length. However, the presence of enchytraeids in the ash-treated soil compensated the ash-induced negative effects on the pine performance. Enchytraeids increased and wood ash decreased water leachable NH₄ ⁺-N in the presence but not in the absence of enchytraeids, while water leachable NO₃ ^–-N was not affected by the treatments. It was concluded that C. sphagnetorum can be important in ensuring nutrient cycling and plant growth in situations when an ecosystem encounters disturbances.     

Variation inRhizobium leguminosarum response to short term application of NH4NO3 to nodulatedPisum sativum L.

Summary This study was conducted to determine the effect of short term application of NH₄NO₃ on nodule function and to determine whether the rhizobial isolate used was a significant factor in this effect. Pea plants were inoculated with 10 differentRhizobium leguminosarum isolates and grown for 3 weeks in N-free medium before addition of 0, 1, 2 or 5 mM NH₄NO₃ for 2 to 7 days. Acetylene reduction and leghemoglobin content decreased with increasing exposure time to NH₄NO₃ and with increasing concentration of NH₄NO₃. NH ₄ ⁺ and NO ₃ ⁻ depletion from the nutrient medium were assayed in plants exposed to 5 mM NH₄NO₃ and mean uptake rates were similar for each ion. There were significant differences among isolates in the rate of decrease of C₂H₂ reduction with increasing NH₄NO₃ concentration (C₂H₂ reduction responsiveness to NH₄NO₃) 4 and 7 days after addition of NH₄NO₃ but no differences after 2 days of exposure to NH₄NO₃. There were significant differences among isolates in NH ₄ ⁺ depletion from the nutrient medium but these differences were not correlated with the differences observed in C₂H₂ reduction. Ranking of the isolates for C₂H₂ reduction responsiveness to NH₄NO₃ applied to plants with nodules was different from that obtained when NH₄NO₃ was applied at seeding. Isolates with varying sensitivity to NH₄NO₃ may be useful tools for determining the mechanisms responsible for inhibition of symbiotic N₂ fixation by combined nitrogen. NRCC paper no. 25863.     

Plant-mediated processess to acquire nutrients: nitrogen uptake by rice plants

The ways in which root–soil interactions can control nutrient acquisition by plants is illustrated by reference to the N nutrition of rice. Model calculations and experiments are used to assess how uptake is affected by root properties and N transport through the soil. Measurements of the kinetics of N absorption and assimilation and their regulation, and of interactions between NH₄ ⁺ and NO₃ ^– nutrition, are described. It is shown that uptake of N from the soil–-as opposed to N in ricefield floodwater which can be absorbed very rapidly but is otherwise lost by gaseous emission–-will often be limited by root uptake properties. Rice roots are particularly efficient in absorbing and assimilating NO₃ ^–, and NH₄ ⁺ absorption and assimilation are stimulated by NO₃ ^–. The uptake of NO₃ ^– formed in the rice rhizosphere by root-released O₂ may be more important than previously thought, with beneficial consequences for rice growth. Other root-induced changes in the rice rhizosphere and their consequences are discussed.     

Relationships between the kinetics of NH4+ and NO3− absorption and growth in the cultivated tomato (Lycopersicon esculentum Mill, cv T-5)

Tomato growth was examined in solution culture under constant pH and low levels of NH₄⁺ or NO₃^?. There were five nitrogen treatments: 20 mmoles m^?3 NH₄⁺, 50 mmoles m^?3 NO₃^?, 100 mmoles m^?3 NH₄⁺ 200 mmoles m^?3 NO₃^?, and 20 mmoles m^?3 NH₄₊+ 50 mmoles m^?3 NO₃^?. The lower concentrations (20 mmoles m^?3 NH₄⁺ and 50 mmoles m^?3 NO₃^?) were near the apparent K_m for net NH₄⁺ and NO₃^? uptake; the higher concentrations (100 mmoles m^?3 NH₄⁺ and 200 mmoles m^?3 NO₃^?) were near levels at which the net uptake of NH₄⁺ or NO₃^? saturate. Although organic nitrogen contents for the higher NO₃^? and the NH₄⁺+ NO₃^? treatments were 22.2–30.3% greater than those for the lower NO₃^? treatment, relative growth rates were initially only 10–15% faster. After 24 d, relative growth rates were similar among those treatments. These results indicate that growth may be only slightly nitrogen limited when NH₄⁺ or NO₃^? concentrations are held constant over the root surface at near the apparent K_m concentration. Relative growth rates for the two NH₄⁺ treatments were much higher than have been previously reported for tomatoes growing with NH₄⁺ as the sole nitrogen source. Initial growth rates under NH₄⁺ nutrition did not differ significantly (P≥ 0.05) from those under NO₃^? or under combined NH₄⁺+ NO₃^?. Growth rates slowed after 10–15 d for the NH₄⁺ treatments, whereas they remained more constant for the NO₃^? and mixed NH₄⁺+ NO₃^? treatments over the entire observation period of 24–33 d. The decline in growth rate under NH₄⁺ nutrition may have resulted from a reduction in Ca²⁺, K⁺, and/or Mg²⁺ absorption.     

Growth responses of the perennial legume Sesbania sesban to NH4 and NO3 nutrition and effects on root nodulation

Preference for NH₄⁺ or NO₃⁻ nutrition by the perennial legume Sesbania sesban (L.) Merr. was assessed by supplying plants with NH₄⁺ and NO₃⁻ alone or mixed at equal concentrations (0.5 mM) in hydroponic culture. In addition, growth responses of S. sesban to NH₄⁺ and NO₃⁻ nutrition and the effects on root nodulation and nutrient and mineral composition of the plant tissues were evaluated in a hydroponic setup at a range of external concentration of NH₄⁺ and NO₃⁻ (0, 0.1, 0.2, 0.5, 2 and 5 mM). Seedlings of S. sesban grew equally well when supplied with either NH₄⁺ or NO₃⁻ alone or mixed and had high relative growth rates (RGRs) ranging between 0.19 and 0.21 d⁻¹. When larger plants of S. sesban were supplied with NH₄⁺ or NO₃⁻ alone, the RGRs and shoot elongation rates were not affected by the external concentration of inorganic N. At external N concentrations up to 0.5 mM nodulation occurred and contributed to the N nutrition through fixation of gaseous N₂ from the atmosphere. For both NH₄⁺ and NO₃⁻-fed plants the N concentration in the plant tissues, particularly water-extractable NO₃⁻, increased at high supply concentrations, and concentrations of mineral cations generally decreased. It is concluded that S. sesban can grow without an external inorganic N supply by fixing atmospheric N₂ gas via root nodules. Also, S. sesban grows well on both NH₄⁺ and NO₃⁻ as the external N source and the plant can tolerate relatively high concentrations of NH₄⁺. This wide ecological amplitude concerning N nutrition makes S. sesban very useful as a N₂-fixing fallow crop in N deficient areas and also a candidate species for use in constructed wetland systems for the treatment of NH₄⁺ rich waters.     

Simultaneous measurement of ammonium,nitrate and proton fluxes along the length of eucalypt roots

Knowledge of the preferred source of N for Eucalyptus nitens will lead to improved fertiliser management practices in plantations. Ion selective microelectrodes were used non-invasively to measure simultaneously net fluxes of NH₄ ⁺, NO₃ ^– and H⁺ along the tap root of solution-cultured E. nitens. Measurements were conducted in solutions containing 100 m NH₄NO₃. The pattern of fluxes was such that there was a large influx of NH₄ ⁺, a smaller influx of NO₃ ^– and large H⁺ efflux. The ratio of these fluxes was constant, according to the ratio 3:1:–6 (NH₄ ⁺:NO₃ ^–:H⁺). Within the region 20–60 mm from the root apex of E. nitens seedlings there was spatial and temporal variation in fluxes but flux patterns remained constant. Root hair density did not affect fluxes nor did proximity to lateral roots. Variation was less than that found in previous studies of localised root fluxes using similar high-resolution measurement techniques. It was concluded that small-scale spatial variation in fluxes may have confounded previous studies. There were associations between fluxes of all three ions, the strongest associations being between NH₄ ⁺ and H⁺, and NH₄ ⁺ and NO₃ ^–. Overall, these results are consistent with NH₄ ⁺ being the preferred source N for E. nitens.     

Effect of the nitrogen form used in the growth medium (NO3 −, NH4 +) on alkaloid production in Datura stramonium L.

Plants of Datura stramonium var. tatula L. Torr. were cultivated on vermiculite and received two different mineral solutions. In one treatment only NO₃ ^–-nitrogen was added, while in the other NO₃ ^–-nitrogen was partly (20%) replaced by NH₄ ⁺-nitrogen. Total dose of nitrogen as well as interionic ratios were kept constant in both treatments. With the combined treatment (NO₃ ^–-NH₄ ⁺) a significant higher hyoscyamine content was found at the time when highest biomass was reached. This was apparently the result of an increased alkaloid biosynthesis. Also scopolamine content was positively influenced, but only at a point past maximal biomass yield.No significant differences in amounts of nitrogen bound per plant were found between both treatments.The higher alkaloid content observed with the combined treatment was associated with a higher relative proportion of bound nitrogen present in the alkaloids. It seems that more nitrogen is available for secondary metabolism when NH₄ ⁺-nitrogen is present in the culture medium.     

Combination Effect of NaCl Salinity and Nitrogen Form on Mineral Composition of Sunflower Plants

The effect of two N-forms (NH₄ ⁺ and NO₃ ^–) and NaCl on pattern of accumulation of some essential inorganic nutrients was examined in sunflower (Helianthus annuus L.) cv. Hisun 33. Eight-day-old plants of were subjected for 21 d to Hoagland's nutrient solution containing 8 mM N as NH₄ ⁺ or NO₃ ^·, and salinized with and addition of NaCl to the growth medium had no significant effect on total leaf N. However, root N of NH₄-supplied plants decreased significantly with increase in NaCl concentration, whereas that of NO₃-supplied plants remained unaffected. There was no significant effect of NaCl on leaf or root P, but the NO₃-supplied plants had significa concentration of leaf P than that of NH₄-supplied plants at varying salt treatments. Salinity of the rooting med did not show any significant effect on Na⁺ concentrations of leaves or roots of plants subjected to two differen N. NH₄-treated plants generally had greater concentrations of Cl^– in leaves and roots and lower K⁺ content in leaves than NO₃-supplied plants. Ca²⁺ concentrations of leaves and roots and Mg²⁺ concentrations of leaves decreased in NH₄-supplied plants due to NaCl, but they remained unaffected in NO₃-treated plants.     

Root respiration associated with ammonium and nitrate absorption and assimilation by barley

We examined nitrate assimilation and root gas fluxes in a wild-type barley (Hordeum vulgare L. cv Steptoe), a mutant (nar1a) deficient in NADH nitrate reductase, and a mutant (nar1a;nar7w) deficient in both NADH and NAD(P)H nitrate reductases. Estimates of in vivo nitrate assimilation from excised roots and whole plants indicated that the nar1a mutation influences assimilation only in the shoot and that exposure to NO₃⁻ induced shoot nitrate reduction more slowly than root nitrate reduction in all three genotypes. When plants that had been deprived of nitrogen for several days were exposed to ammonium, root carbon dioxide evolution and oxygen consumption increased markedly, but respiratory quotient—the ratio of carbon dioxide evolved to oxygen consumed—did not change. A shift from ammonium to nitrate nutrition stimulated root carbon dioxide evolution slightly and inhibited oxygen consumption in the wild type and nar1a mutant, but had negligible effects on root gas fluxes in the nar1a;nar7w mutant. These results indicate that, under NH₄⁺ nutrition, 14% of root carbon catabolism is coupled to NH₄⁺ absorption and assimilation and that, under NO₃⁻ nutrition, 5% of root carbon catabolism is coupled to NO₃⁻ absorption, 15% to NO₃⁻ assimilation, and 3% to NH₄⁺ assimilation. The additional energy requirements of NO₃⁻ assimilation appear to diminish root mitochondrial electron transport. Thus, the energy requirements of NH₄⁺ and NO₃⁻ absorption and assimilation constitute a significant portion of root respiration.     

Variability of leaf photosynthetic characteristics in rice and its relationship with resistance to water stress under different nitrogen nutrition regimes

The negative effects of water stress on rice can be alleviated by NH₄⁺ nutrition. However, the effects of mixed nitrogen (N) nutrition (NO₃^? + NH₄⁺) on resistance to water stress are still not well known. To investigate the response of rice growth to water stress and its relationship with photosynthetic characteristics, a hydroponic experiment supplying different N forms was conducted. Compared with NO₃^? nutrition, mixed‐N and NH₄⁺ nutrition greatly alleviated the reduction of leaf area, chlorophyll content, and photosynthesis under water stress, whilst subsequently maintaining higher biomass. In contrast, water stress inhibited the root‐shoot ratios in NH₄⁺‐ and mixed‐N‐supplied plants, indicating reduced root growth and higher photosynthate availability to shoots. The following key observations were made: (1) a similar stomatal limitation and low proportion of activated Rubisco were observed among the three different N nutrition regimes; (2) increased mesophyll conductance in NH₄⁺‐ and mixed‐N‐supplied plants simultaneously stimulated leaf photosynthesis and improved the water use efficiency and (3), the maximum carboxylation rate and actual photochemical efficiency of photosystem II in NH₄⁺‐ and mixed‐N‐supplied plants were significantly higher than that in NO₃^?‐supplied plants, thus resulting in higher photochemical efficiency under water stress. In conclusion, mixed‐N and NH₄⁺ nutrition may be used to develop strategies for improved water stress resistance and stimulated biomass production under conditions of osmotic stress and possibly drought.     

Ammonia-oxidizing bacteria on root biofilms and their possible contribution to N use efficiency of different rice cultivars

Ammonia-oxidizing bacteria (AOB) populations were studied on the root surface of different rice cultivars by PCR coupled with denaturing gradient gel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH). PCR-DGGE of the ammonium monooxygenase gene (amoA) showed a generally greater diversity on root samples compared to rhizosphere and unplanted soil. Sequences affiliated with Nitrosomonas spp. tended to be associated with modern rice hybrid lines. Root-associated AOB observed by FISH were found within a discrete biofilm coating the root surface. Although the total abundance of AOB on root biofilms of different rice cultivars did not differ significantly, there were marked contrasts in their population structure, indicating selection of Nitrosomonas spp. on roots of a hybrid cultivar. Observations by FISH on the total bacterial community also suggested that different rice cultivars support different bacterial populations even under identical environmental conditions. The presence of active AOB in the root environment predicts that a significant proportion of the N taken up by certain rice cultivars is in the form of NO₃ ^–-N produced by the AOB. Measurement of plant growth of hydroponically grown plants showed a stronger response of hybrid cultivars to the co-provision of NH₄ ⁺ and NO₃ ^–. In soil-grown plants, N use efficiency in the hybrid was improved during ammonium fertilization compared to nitrate fertilization. Since ammonium-fertilized plants actually receive a mixture of NH₄ ⁺ and NO₃ ^– with ratios depending on root-associated nitrification activity, these results support the advantage of co-provision of ammonium and nitrate for the hybrid cultivar.     

Differential effect of ammonium on the induction of nitrate and nitrite reductase activities in roots of barley (Hordeum vulgare) seedlings

The effect of exogenous NH₄⁺ on the induction of nitrate reductase activity (NRA; EC 1.6.6.1) and nitrite reductase activity (NiRA; EC 1.7.7.1) in roots of 8-day-old intact barley (Hordeum vulgare L.) seedlings was studied. Enzyme activities were induced with 0.1, 1 or 10 mM NO₃⁺ in the presence of 0, 1 or 10 mM NH₄⁺, Exogenous NH₄⁺ partially inhibited the induction of NRA when roots were exposed to 0.1 mM, but not to 1 or 10 mM NO₃⁺, In contrast, the induction of NiRA was inhibited by NH₄⁺ at all NO₃⁺ levels. Maximum inhibition of the enzyme activities occurred at 1.0 mM NH₄⁺ Pre-treatment with NH₄⁺ had no effect on the subsequent induction of NRA in the absence of additional NH₄⁺ whereas the induction of NiRA in NH₄⁺-pretreated roots was inhibited in the absence of NH₄⁺ At 10 mM NO₃⁺ L-methionine sulfoximine stimulated the induction of NRA whether or not exogenous NH₄⁺ was present. In contrast, the induction of NiRA was inhibited by L-methionine sulfoximine irrespective of NH₄⁺ supply. During the postinduction phase, exogenous NH₄⁺ decreased NRA in roots supplied with 0.1 mM but not with 1mM NH₃⁺ whereas, NiRA was unaffected by NH₄⁺ at either substrate concentration. The results indicate that exogenous NH₄⁺ regulates the induction of NRA in roots by limiting the availability of NO₃⁺. Conversely, it has a direct effect, independent of the availability of NO₃⁺, on the induction of NiRA. The lack of an NH₄⁺ effect on NiRA during the postinduction phase is apparently due to a slower turnover rate of that enzyme.     

Effects of light quality,CO2 tensions and NO 3 +− concentrations on the inorganic nitrogen metabolism of Chlamydomonas reinhardii

The blue light dependent utilization of nitrate by green algae under common air and high irradiances, besides its assimilatory nature, is associated with the release of NO₂ ^– and NH₄ ⁺ to the culture medium. If the CO₂ content of the sparging air was increased up to 2%, previously excreted NO₂ ^– and NH₄ ⁺ were rapidly assimilated. When under air and high irradiances the cell density in the culture reached values corresponding to 25 g Ch 1.ml^-1, no further growth was observed and the highest values of NO₃ ^– consumption and NO₂ ^– and NH₄ ⁺ release were attained. Besides low CO₂ tensions, increasing NO₃ ^– concentrations in the medium stimulated the release of NO₃ ^– and NH₄ ⁺. Under CO₂-free air the consumption of NO₃ ^– and the release of NO₂ ^– and NH₄ ⁺ on a total N bases were almost stoichiometric and their rates saturated at much lower irradiances than under air. Under CO₂-free air high rates of NO₂ ^– release were only observed under the blue radiations that were effectively absorbed by photosynthetically active pigments, i.e. 460 nm, but not under 404 and 630 nm radiations. However, the simultaneous illumination of the cells with 404 and 630 nm monochromatic light showed a remarkable synergistic effect on NO₂ ^– release.The results are discussed in terms of the close relationship between C and N metabolism, the photosynthetic reducing power required to convert NO _inf3 ^sup± -N into R – NH₂-N and the blue light activation of nitrate reductase.     

Improvement of degraded physical properties of a saline-sodic soil by reclamation with kallar grass (Leptochloa fusca)

A field experiment was conducted to evaluate the effectiveness of growing salt tolerant plants to improve the physical characteristics of a saline-sodic soil. Kallar grass [Leptochloa fusca (L.) Kunth], a species tolerant to salinity, sodicity and alkalinity, was irrigated for five years with poor quality ground water (EC = 0.14 S m^–1, SAR_adj=19.3, RSC = 9.7 meq L^–1). The soil physical properties of plant available water, saturated hydraulic conductivity, structural stability, bulk density and porosity were determined at the end of each year. The growth of kallar grass for three years significantly improved the physical properties of the soil and these were maintained with further growth of grass up to five years. Kallar grass significantly increased plant available water with time (r=0.97^**). The available water was highly correlated (r=0.92^**) with increases in soil organic matter content, porosity (r=0.99^**) and other physical properties. Soil hydraulic conductivity increased substantially with time from 0.035 to 55.6 mm d^–1 in the topsoil (0–20 cm) in five years and was significantly correlated with porosity, water retention, structural stability and organic matter content of soil. The soil structural stability index improved significantly from 32 to 151 with kallar grass and showed greater increases in the surface soil than at depth. The cropping of kallar grass resulted in a linear increase of soil organic matter content (r=0.92^**) which improved porosity and other soil physical properties (r0.82^*). This study confirmed that kallar grass is effective for rehabilitation and restoration of soil fertility in saline-sodic areas on a sustainable basis.