THE ESSENTIAL ROLE OF CALCIUM ION IN POLLEN GERMINATION AND POLLEN TUBE GROWTH

Brewbaker, James L., and Beyoung H. Kwack. (U. Hawaii, Honolulu.) The essential role of calcium ion in pollen germination and pollen tube growth. Amer. Jour. Bot. 50(9): 859–865. Illus. 1963.—A pollen population effect occurs whenever pollen grains are grown in vitro. Small pollen populations germinate and grow poorly if at all, under conditions which support excellent growth of large pollen populations. The pollen population effect is overcome completely by a growth factor obtained in water extracts of many plant tissues. This factor is shown to be the calcium ion, and its action confirmed in 86 species representing 39 plant families. Other ions (K⁺, Mg⁺⁺, Na⁺) serve in supporting roles to the uptake or binding of calcium. The high requirement of calcium (300–5000 ppm, as Ca (NO₃)₂·4H₂O, for optimum growth) and low calcium content of most pollen may conspire to give calcium a governing role in the growth of pollen tubes both in vitro and in situ. It is suspected that ramifications of this role extend to the self-incompatibilities of plants and to the curious types of arrested tube growth distinguishing, for example, the orchids. A culture medium which proved its merit in a wide variety of pollen growth studies included, in distilled water, 10% sucrose, 100 ppm H₃BO₃, 300 ppm Ca (NO₃)₂·4H₂O, 200 ppm MgSO₄·7H₂O and 100 ppm KNO₃.     

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.     

Role of cytokinin in enhanced productivity of maize supplied with NH4 + and NO3 −

Supplying both N forms (NH₄ ⁺+NO₃ ⁻) to the maize (Zea mays L.) plant can optimize productivity by enhancing reproductive development. However, the physiological factors responsible for this enhancement have not been elucidated, and may include the supply of cytokinin, a growth-regulating substance. Therefore, field and gravel hydroponic studies were conducted to examine the effect of N form (NH₄ ⁺+NO₃ ⁻ versus predominantly NO₃ ⁻) and exogenous cytokinin treatment (six foliar applications of 22 μM 6-benzylaminopurine (BAP) during vegetative growth versus untreated) on productivity and yield of maize. For untreated plants, NH₄ ⁺+NO₃ ⁻ nutrition increased grain yield by 11% and whole shoot N content by 6% compared with predominantly NO₃ ⁻. Cytokinin application to NO₃ ⁻-grown field plants increased grain yield to that of NH₄ ⁺+NO₃ ⁻-grown plants, which was the result of enhanced dry matter partitioning to the grain and decreased kernel abortion. Likewise, hydroponically grown maize supplied with NH₄ ⁺+NO₃ ⁻ doubled anthesis earshoot weight, and enhanced the partitioning of dry matter to the shoot. NH₄ ⁺+NO₃ ⁻ nutrition also increased earshoot N content by 200%, and whole shoot N accumulation by 25%. During vegetative growth, NH₄ ⁺+NO₃ ⁻ plants had higher concentrations of endogenous cytokinins zeatin and zeatin riboside in root tips than NO₃ ⁻-grown plants. Based on these data, we suggest that the enhanced earshoot and grain production of plants supplied with NH₄ ⁺+NO₃ ⁻ may be partly associated with an increased endogenous cytokinin supply.     

γ‐Aminobutyric acid addition alleviates ammonium toxicity by limiting ammonium accumulation in rice (Oryza sativa) seedlings

Excessive use of nitrogen (N) fertilizer has increased ammonium (NH₄⁺) accumulation in many paddy soils to levels that reduce rice vegetative biomass and yield. Based on studies of NH₄⁺ toxicity in rice (Oryza sativa, Nanjing 44) seedlings cultured in agar medium, we found that NH₄⁺ concentrations above 0.75 mM inhibited the growth of rice and caused NH₄⁺ accumulation in both shoots and roots. Use of excessive NH₄⁺ also induced rhizosphere acidification and inhibited the absorption of K, Ca, Mg, Fe and Zn in rice seedlings. Under excessive NH₄⁺ conditions, exogenous γ‐aminobutyric acid (GABA) treatment limited NH₄⁺ accumulation in rice seedlings, reduced NH₄⁺ toxicity symptoms and promoted plant growth. GABA addition also reduced rhizosphere acidification and alleviated the inhibition of Ca, Mg, Fe and Zn absorption caused by excessive NH₄⁺. Furthermore, we found that the activity of glutamine synthetase/NADH‐glutamate synthase (GS; EC 6.3.1.2/NADH‐GOGAT; EC1.4.1.14) in root increased gradually as the NH₄⁺ concentration increased. However, when the concentration of NH₄⁺ is more than 3 mM, GABA treatment inhibited NH₄⁺‐induced increases in GS/NADH‐GOGAT activity. The inhibition of ammonium assimilation may restore the elongation of seminal rice roots repressed by high NH₄⁺. These results suggest that mitigation of ammonium accumulation and assimilation is essential for GABA‐dependent alleviation of ammonium toxicity in rice seedlings.     

Elevated CO2 plus chronic warming reduce nitrogen uptake and levels or activities of nitrogen‐uptake and ‐assimilatory proteins in tomato roots

Atmospheric CO₂ enrichment is expected to often benefit plant growth, despite causing global warming and nitrogen (N) dilution in plants. Most plants primarily procure N as inorganic nitrate (NO₃^?) or ammonium (NH₄⁺), using membrane‐localized transport proteins in roots, which are key targets for improving N use. Although interactive effects of elevated CO₂, chronic warming and N form on N relations are expected, these have not been studied. In this study, tomato (Solanum lycopersicum) plants were grown at two levels of CO₂ (400 or 700 ppm) and two temperature regimes (30 or 37°C), with NO₃^? or NH₄⁺ as the N source. Elevated CO₂ plus chronic warming severely inhibited plant growth, regardless of N form, while individually they had smaller effects on growth. Although %N in roots was similar among all treatments, elevated CO₂ plus warming decreased (1) N‐uptake rate by roots, (2) total protein concentration in roots, indicating an inhibition of N assimilation and (3) shoot %N, indicating a potential inhibition of N translocation from roots to shoots. Under elevated CO₂ plus warming, reduced NO₃^?‐uptake rate per g root was correlated with a decrease in the concentration of NO₃^?‐uptake proteins per g root, reduced NH₄⁺ uptake was correlated with decreased activity of NH₄⁺‐uptake proteins and reduced N assimilation was correlated with decreased concentration of N‐assimilatory proteins. These results indicate that elevated CO₂ and chronic warming can act synergistically to decrease plant N uptake and assimilation; hence, future global warming may decrease both plant growth and food quality (%N).     

Response of grain sorghum to enhanced-ammonium supply

Two greenhouse experiments were conducted to examine the effects of increased levels of soil NH ₄ ⁺ on the growth and yield of grain sorghum (Sorghum bicolor (L.) Moench). Nitrogen was supplied as urea plus the nitrification inhibitor nitrapyrin (enhanced-NH ₄ ⁺ supply) or as a 41 molar ratio of CA(NO₃)₂ and Mg(NO₃)₂ at rates of 0 to 450 mg N kg^–1 soil in 37.5 mg N increments. Enhanced-NH ₄ ⁺ supply, in comparison to the NO₃ treatment, increased grain yield 15 and 18% in the two experiments. In one experiment this yield increase occurred through increased number of kernels and in a second experiment, through increased weight of kernels. During the first 28 days after plant emergence, the number of leaves, stalk width, plant weight, and plant N content were greater with enhanced-NH ₄ ⁺ supply than with NO ₃ ^– . However, at harvest total plant weight and plant N content were minimally affected by enhanced-NH ₄ ⁺ supply.     

The stimulating effect of ammonium on nodulation in Pisum sativum L. is not long lived once ammonium supply is discontinued

Although mineral N generally has a negative effect on legume-rhizobia symbioses, experiments in hydroponic culture in our laboratory (Waterer et al., 1992) have shown that low concentrations of NH⁺ ₄ can stimulate nodulation in pea (Pisum sativum L.). The objectives of the current study were to determine the immediate and residual effects of NH⁺ ₄ on nodulation and N₂ fixation in pea in sand culture. Peas (cv. Express) were exposed to 0.0, 0.5, 1.0, and 2.0 mM of ¹⁵N-labelled (NH₄)₂SO₄ for 28 days after inoculation (DAI). From 28 to 56 DAI the plants were grown on a NH⁺ ₄-free nutrient solution. Plants were harvested at 7, 14, 21, 28 and 56 DAI and nitrogenase activity was measured by gas exchange at 28 and 56 DAI. Root, shoot, and nodule dry weight (DW) and total N content were obtained, in addition to nodule counts and ¹⁵N enrichment of plant composites. The 1.0 and 2.0 mM NH⁺ ₄ treatments consistently resulted in higher total plant DW accumulation than the control (0.0 mM NH⁺ ₄). At 28 DAI, plants exposed to 1.0 and 2.0 mM NH⁺ ₄ had 1.8 to 2.8 times more nodules plant^-1, respectively, and plants exposed to 2.0 mM NH⁺ ₄ had 1.7 fold higher specific nodulation (nodule number g^-1 root DW). However, individual nodule DW was greater in control plants, such that there were no differences in nodule DW per plant among treatments. Ammonium treatment resulted in more nitrogen derived from the atmosphere (NDFA) in peas early in the experiment, but by 28 DAI there were no treatment effects on NDFA. Whole plant and nodule specific nitrogenase activity (µmol H₂ g^-1 nodule DW h^-1) was higher in control plants at 28 DAI. However, by 56 DAI, after an additional 4 weeks of NH⁺ ₄-free nutrition, no differences in nitrogenase activity nor whole plant or specific nodulation were detectable. This study indicates that nodulation in pea is stimulated in sand culture while exposed to NH⁺ ₄. However, once NH⁺ ₄ is removed, relative growth rate, nodulation and nitrogenase activity becomes similar to plants that were never exposed to NH⁺ ₄.     

Prediction of nitrogen availability and rice yield in lowland soils: Nitrogen mineralization parameters

Field experiments were conducted under flooded soil conditions using Maahas clay amended with urea and rice straw-sesbania mixtures during the wet and dry seasons. Parallel laboratory incubation tests were done. The objectives were 1) to determine N mineralization patterns and establish the relationship between mineralization parameters and either N availability or grain yield, and 2) to correlate the results of organic N mineralization studies in the laboratory with data from field experiments. The N mineralization patterns of flooded soils in the laboratory followed a logistic function. In laboratory studies, mineralization potential was positively correlated with extractable soil NH₄ ⁺-N at the end of the incubation period (cumulative available N). Likewise, mineralization potential calculated from laboratory studies was positively correlated with N uptake and grain yield from field studies. Extractable (NH₄ ⁺+NO₃ ^–)-N in the field correlated positively with extractable NH₄ ⁺-N in the laboratory. The extractable NH₄ ⁺-N from laboratory incubations at 14 days after transplanting, panicle initiation, and maturity was also highly and positively correlated with grain yield from field experiments.     

Fertilizer nitrogen budget of Na15NO3 and (15NH4)2SO4 split-applied to winter wheat in microplots on a loam soil

Labelled fertilizer N applied to winter wheat as Na¹⁵NO₃ and (¹⁵NH₄)₂SO₄ at a total N dressing of 100kg ha⁻¹ was used in a microplot balance study to investigate the fate of each split fraction at three growth stages: end of tillering, heading and beginning of flowering. Results indicated that while the percentage utilization of the applied N by the grain and total crop increased considerably from the first to the third split application, these values diminished steadily in the straw. Grain recovery values for the first, second and third split applications were 34.2%, 51.5% and 55.7% for the NO₃ and 32.3%, 48.4% and 52.5% for the NH₄ carrier, respectively. The corresponding recovery values for the whole plant were 54.6%, 67.8% and 69.9% for the NO₃ and 51.7%, 63.5% and 66.1% for the NH₄ carrier. A greater proportion of the fertilizer N applied at the end of tillering stage was found in the vegetative plant components as compared with the grain. The reverse occurred for the N applied at the heading and at the beginning of the flowering stages. The residual fertilizer N found in the soil amounted to 18.0%, 10.4% and 11.6% of the applied NO₃−N and to 22.5%, 12.7% and 15.2% of the applied NH₄−N for the respective split applications. No differences were found for each split application between the two carriers as far as the unaccounted fertilizer N was concerned. The losses were 26.6%, 22.3% and 18.6% of the applied N for the three split applications, respectively. The application of fertilizer N did not lead to any increase in soil N uptake by the crop.     

Changing nitrogen source on the yield and nitrogen content of soybeans

Summary Soybean plants were grown in nutrient culture solutions containing 150 ppm of N either as an equal concentration of NH₄ ⁺ or NO₃ ^–, or all NO₃ ^–. At the R2 stage of growth for some plants, the N form was changed to either all NO₃ ^– or all NH₄ ⁺, but at the same total N concentration as before. Highest seed yield was obtained with all NO₃ ^– over the entire growth period, the poorest when the N form was switched from an equal ratio of NH₄ ⁺ and NO₃ ^– to all NH₄ ⁺ at the R2 stage. Kjeldahl N concentrations in the plant leaves and seed were highest when NH₄ ⁺ was part or all of the N source in the nutrient solution. These results may partially explain why the literature is inconsistent on the effect of added fertilizer N on soybean seed yield, and may pose a problem in using leaf Kjeldahl N concentration to determine plant N sufficiency.     

Yield of submerged paddy and uptake of Zn,P and N as affected by liming and Zn fertilizers

The effects of CaCO₃, Zn sources and levels on the yield of submerged paddy and uptake of Zn, P and N to paddy were studied in green-house at Haryana Agricultural University, Hissar. Powdered CaCO₃ was mixed at 0,4 and 8 per cent and Zn was added at 0,5 and 10 ppm through ZnSO₄.7H₂O, ZnO and Zn EDTA separately. Dry weight at tillering and heading and grain and straw at maturity decreased significantly with 4 and 8 per cent CaCO₃ in comparison to the control. Increasing Zn application increased the dry weight and grain yield. Zn EDTA gave highest yield of paddy followed by ZnSO₄.7H₂O and ZnO.Increasing the application of CaCO₃ from 0–8 per cent decreased the concentration and uptake of Zn and increasing Zn application from 0–10 ppm increased concentration and uptake of Zn in paddy at tillering, heading and maturity. Zn EDTA gave the highest concentration and uptake of Zn followed by ZnSO₄.7H₂O and ZnO. There was interaction between Zn sources and CaCO₃.The concentration and uptake of N and P in paddy dry matter at tillering and heading and straw and grain at maturity decreased as compared to control with increasing CaCO₃ addition. The concentration and uptake of N increased and that of P decreased in paddy dry matter straw and grain with increasing Zn application. The highest concentration of N was observed with ZnO, followed by ZnSO₄.7H₂O and Zn EDTA. But highest uptake of N was observed with Zn EDTA followed by ZnSO₄.7H₂O and ZnO. As regards concentration and uptake of P, it was highest with ZnO followed by ZnSO₄.7H₂O and Zn EDTA.     

Effects of NH4+ concentration on growth,morphology and NH4+ uptake kinetics of Salvinia natans

Many plants develop toxicity symptoms and have reduced growth rates when supplied with ammonium (NH₄⁺) as the only source of inorganic nitrogen. In the present study, the growth, morphology, NH₄⁺ uptake kinetics and mineral concentrations in the tissues of the free-floating aquatic plant Salvinia natans (water fern) supplied exclusively with NH₄⁺–N at concentrations of 0.25–15 mM were investigated. S. natans grew well, with relative growth rates of c. 0.25 g g^?1 d^?1 at external NH₄⁺ concentrations up to 5 mM, but at higher levels growth was suppressed and the plants had small leaves and short roots with stunted growth. The high-affinity transport system (HATS) that mediate NH₄⁺ uptake at dilute NH₄⁺ levels was downregulated at high NH₄⁺ concentrations with lower velocities of maximum uptake (V_max) and higher half-saturation constants (K_1/2). High NH₄⁺ levels also barely affected the concentrations of mineral cations and anions in the plant tissue. It is concluded that S. natans can be characterized as NH₄⁺-tolerant in line with a number of other species of wetland plants as growth was unaffected at NH₄⁺ concentrations as high as 5 mM and as symptoms of toxicity at higher concentrations were relatively mild. Depolarization of the plasma membrane to the equilibrium potential for NH₄⁺ at high external concentrations may be a mechanism used by the plant to avoid excessive futile transmembrane cycling. S. natans is tolerant to the high NH₄⁺ levels that prevail in domestic and agricultural wastewaters, and the inherent high growth rate and the ease of biomass harvesting make S. natans a primary candidate for use in constructed wetland systems for the treatment of various types of nitrogen-rich wastewaters.     

丛枝菌根真菌和施加不同形态氮肥对杉木幼苗养分吸收的影响

为了解丛枝菌根真菌(AMF)和不同形态氮对杉木(Cunninghamia lanceolata)生长和养分吸收的影响,以1 a生杉木幼苗接种摩西球囊霉(Glomus mosseae)和添加不同形态氮(NH₄⁺-N和NO₃^–-N),对其养分元素和生长状况的变化进行研究。结果表明,AMF显著提高了杉木的苗高和生物量,促进了杉木对N、P、K、Ca、Mg、Fe和Na的吸收,AMF对微量元素Fe、Na的促进作用总体上要强于大量元素K、Ca。与NO₃^–-N相比,AMF显著提高了NH₄⁺-N处理杉木的生物量、总C和N、Ca、Mg、Mn含量,而且这种显著性在叶中普遍高于根和茎。接种AMF可以促进杉木幼苗的生长和对养分元素的吸收,且添加NH₄⁺-N处理的促进作用要强于NO₃^–-N。     

Nitrogen limitation in mycorrhizal Norway spruce (Picea abies) seedlings induced mycelial foraging for ammonium: implications for Ca and Mg uptake

Although many studies support the importance of the external mycelium for nutrient acquisition of ectomycorrhizal plants, direct evidence for a significant contribution to host nitrogen nutrition is still scarce. We grew nonmycorrhizal seedlings and seedlings mycorrhizal with Paxillus involutus (Batsch) Fr. in a sand culture system with two compartments separated by a 45-m Nylon mesh. Hyphae, but not roots, can penetrate this net. Nutrient solutions were designed to limit seedling growth by nitrogen. Hyphal density in the hyphal compartment, host N status and shoot growth of mycorrhizal seedlings significantly increased in response to NH₄ ⁺ addition to the hyphal compartment. Labeling the compartment only accessible to hyphae with ¹⁵NH₄ ⁺ showed that the increase in N uptake in the mycorrhizal seedlings was a result of hyphal N acquisition from the hyphal compartment. These results indicate that hyphae of P. involutus may actively forage into N-rich patches and improve host N status and growth. In the mycorrhizal seedlings, which received additional NH₄ ⁺ via their external mycelium, the increase in NH₄ ⁺ supply less negatively affected Ca and Mg uptake than in nonmycorrhizal seedlings, where the additional NH₄ ⁺ was directly supplied to the roots. This was most likely due to the close link of NH₄ ⁺ uptake and H⁺ extrusion, which, in the nonmycorrhizal seedlings, lead to a strong acidification in the root compartment, and subsequently reduced Ca and Mg uptake, whereas in the mycorrhizal seedlings the site of intensive NH₄ ⁺ uptake and acidification was in the hyphal and not in the root compartment. Our data support the idea that the ectomycorrhizal mycelium connected to an N-deficient host may actively forage for N. The mycelium may also be important as a biological buffer system ameliorating negative influence of high NH₄ ⁺ supply on cation uptake.     

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.     

Soil acidification as caused by the nitrogen uptake pattern of Scots pine (Pinus sylvestris)

Three-year-old Scots pine (Pinus sylvestris) trees were grown on a sandy forest soil in pots, with the objective to determine their NH₄/NO₃ uptake ratio and proton efflux. N was supplied in three NH₄-N/NO₃-N ratios, 3:1, 1:1 and 1:3, either as ¹⁵NH₄+¹⁴NO₃ or as ¹⁴NH₄+¹⁵NO₃. Total N and ¹⁵N acquisition of different plant parts were measured. Averaged over the whole tree, the NH₄/NO₃ uptake ratios throughout the growing season were found to be 4.2, 2.5, and 1.5 for the three application ratios, respectively. The excess cation-over-anion uptake value (C_a-A_a) appeared to be linearly related to the natural logarithm of the NH₄/NO₃ uptake ratio. Further, this uptake ratio was related to the NH₄/NO₃ ratio of the soil solution. From these relationship it was estimated that Scots pine exhibits an acidifying uptake pattern as long as the contribution of nitrate to the N nutrition is lower than 70%. Under field circumstances root uptake may cause soil acidification in the topsoil, containing the largest part of the root system, and soil alkalization in deeper soil layers.     

The kinetics of NH4 + and NO3 − uptake by Douglas fir from single N-solutions and from solutions containing both NH4 + and NO3 −

The kinetics of NH₄ ⁺ and NO₃ ⁻ uptake in young Douglas fir trees (Pseudotsuga menziesii [Mirb.] Franco) were studied in solutions, containing either one or both N species. Using solutions containing a single N species, the V_max of NH₄ ⁺ uptake was higher than that of NO₃ ⁻ uptake. The K_m of NH₄ ⁺ uptake and K_m of NO₃ ⁻ uptake differed not significantly. When both NH₄ ⁺ and NO₃ ⁻ were present, the V_max for NH₄ ⁺ uptake became slightly higher, and the K_m for NH₄ ⁺ uptake remained in the same order. Under these conditions the NO₃ ⁻ uptake was almost totally inhibited over the whole range of concentrations used (10–1000 μM total N). This inhibition by NH₄ ⁺ occurred during the first two hours after addition. ei]{gnA C}{fnBorstlap}     

Yield and crop parameters of wetland rice as influenced by soil and fertilizer nitrogen

Summary The behavior of soil N, fertilizer N and plant N was studied in a greenhouse experiment with 2 plant densities of rice (IR 36) under flooded conditions. Increasing plant density from 25 hills m² to 50 hills m² increased tiller number and panicle number but had no influence on grain yield. The yield of grain was linearly related to N content of the above ground dry matter at harvest (r²=.96) and thus the effect of manipulating the N supply on yield was directly related to N uptake.Mixing of (NH₄)₂SO₄ with the soil volume before transplanting resulted in increases in N in the aboveground dry matter equal to 87% of the applied N. When (NH₄)₂SO₄ was broadcast into the flood water at 4 stages of growth beginning 25 DAT, the corresponding increase was 77% of the applied N. When (NH₄)₂SO₄ was split between shallow mixing before transplanting and a broadcast application of 32 DAT, the corresponding increase was 42%. Thus several applications of fertilizer N increased grain production per unit of applied N.Inorganic N extractable by KCl was a useful but not an infailible guide to the behavior of the soil and fertilizer inorganic N.     

Interactions between K+ and NH4+: effects on ion uptake by rice roots

Interactive effects of K⁺ and N (principally NH₄⁺) on plant growth and ion uptake were investigated using hydroponically grown rice (Oryza sativa L. cv. M202) seedlings by varying the availability of NH₄⁺ or NO₃^? and K⁺ during an 18d growth period, a 3d pretreatment period and during flux measurements. Plants grew best in media containing 100 mmol m^?3 NH₄⁺ and 200mmolm^?3 K⁺ (N100/K200), followed by N2/K200 < N100/K2 < N2/K2. ⁸⁶Rb⁺(K⁺) fluxes were increased by exposure to N during the 18 d growth period and the 3 d of pretreatment, but decreased by the presence of NH₄⁺ during flux measurements. This inhibition was a function of prior N/K provision and the [NH₄⁺]₀ present during flux determinations. NH₄⁺ was least inhibitory to 86Rb⁺(K⁺) influx in high-N/low-K plants. Pretreatments with K⁺ failed to stimulate NH₄⁺ uptake, and the presence of K⁺ in the uptake solutions reduced NH₄⁺ fluxes only in high-N/low-K plants.