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.     

Simultaneous influx of ammonium and potassium into maize roots: kinetics and interactions

The interaction between ammonium and potassium during influx was examined in roots of dark-grown decapitated corn seedlings (Zea mays L., cv. Pioneer 3369A). Influx was measured during a 10-min exposure to either (¹⁵NH₄)₂SO₄ ranging from 10 to 200 M NH ₄ ⁺ with and without 200 M K(⁸⁶Rb)Cl or to K(⁸⁶Rb)Cl ranging from 10 to 200 M K⁺ with and without 200 M NH ₄ ⁺ as (¹⁵NH₄)₂SO₄. The simple Michaelis-Menten model described the data well only for potassium influx in the presence of ambient ammonium. For the other three instances, the data were improved by assuming that a second influx mechanism became operative as the low-concentration phase approached saturation. Two distinct mechanisms are thus indicated for both ammonium and potassium influx within the range of 10 to 200 M.The influx mechanism operating at low concentrations showed greater affinity for potassium than for ammonium, even though the capacity for ammonium transport was twice as large as that for potassium. It is suggested that this phase involved a common transport system for the two ions and that localized low acidity next to the internal surface, following H⁺ extrusion, favored ammonium deprotonation and dissociation from the transport system-ammonium complex. Parallel decreases in V _max and increases in K_m of the low-concentration saturable phase occurred for ammonium influx when ambient potassium was present and for potassium influx when ambient ammonium was present. The data support a mixed-type inhibition in each case. Simultaneous measurement of potassium and ammonium influx showed that they were highly negatively correlated at the lower concentrations, indicating that the extent to which influx of the inhibited ion was restricted was associated with influx of the inhibitor ion. Presence of ambient ammonium eliminated the second phase of potassium influx. In contrast, the presence of ambient potassium decreased the concentration at which the second phase of ammonium influx was initiated but did not restrict the rate.Paper no. 11131 of the Journal Series of the North Carolina Agricultural Research ServiceThe use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named, nor criticism of similar ones not mentioned     

The mechanism of nitrate transport across the tonoplast of barley root cells

Nitrate-selective microelectrodes were used to measure not only nitrate activity in the cytoplasm and vacuole of barley (Hordeum vulgare L.) root cells, but also the tonoplast electrical membrane potential. For epidermal cells, the mean cytoplasmic and vacuolar pNO₃ (-log₁₀ [NO₃]) values were 2.3±0.04 (n=19) and 1.41±0.03 (n=35), respectively, while for cortical cells, the mean cytoplasmic and vacuolar nitrate values were 2.58±0.18 (n=4) and 1.17±0.06 (n=13), respectively. These results indicate that the accumulation of nitrate in the vacuole must be an active process. Proton-selective microelectrodes were used to measure the proton gradient across the tonoplast to assess the possibility that nitrate transport into the vacuole is mediated by an H⁺/NO ₃ ^– antiport mechanism. For epidermal cells, the mean cytoplasmic and vacuolar pH values were 7.12±0.06 (n=10) and 4.93±0.11 (n=22), respectively, while for cortical cells, the mean cytoplasmic and vacuolar pH values were 7.24±0.07 (n=3) and 5.09±0.17 (n=7), respectively. Calculations of the energetics for this mechanism indicate that the observed gradient of nitrate across the tonoplast of both epidermal and cortical cells could be achieved by an H⁺/NO ₃ ^– antiport with a 11 stoichiometry.Abbreviations and Symbols G/F free-energy change for H⁺/NO ₃ ^– antiport - F Faraday constant - pH_c cytoplasmic pH - pH_v vacuolar pH - p[NO₃]_c log₁₀ (cytoplasmic [NO ₃ ^– ]) - P[NO₃]_v -log₁₀ (vacuolar [NO₃]) We wish to thank Dr. K. Moore for assistance with statistical analysis.     

The role of ion antiporters in the maintenance of intracellular pH in rat vascular smooth muscle cells

Vascular smooth muscle intracellular pH is maintained by the Na⁺/H⁺ and Cl^–/HCO ₃ ^– antiporters. The Na⁺/H⁺ exchanger is a major route of H⁺ extrusion in most eukaryotic cells and is present in vascular smooth muscle cells in a similar capacity. It extrudes H^– into the extracellular space in exchange for Na⁺. The Cl^–/HCO ₃ ^– exchanger plays an analogous role to lower the pH of vascular smooth muscle cells when increases in intracellular pH occur. Its activity has also been demonstrated in A7r5 and A10 vascular smooth muscle cells. The Na⁺/H⁺ exchanger is regulated by a number of agents which act through inositol trisphosphate/diacylglycerol, to stimulate the antiporter. Calcium-calmodulin dependent protein kinase may also activate the antiporter in vivo. Phosphorylation of the Cl^–/HCO ₃ ^– exchanger has also been observed but its physiological role is not known. Both these antiporters exist in the plasma membrane as integral proteins with free acidic cytoplasmic termini. These regions may be important in sensing changes in intracellular pH, to which these antiporters respond.Abbreviations CaM Calmodulin - DCCD Dicylohexyl-Carbodiimide - DG Diacylglycerol - DIDS-4 4-Diisthiocyanostilbene-2,2-Disulfonic Acid - IP₃ Inositol Trisphosphate - PKC protein Kinase C - SITS-4 4-Acetamido-4-Isothiocyanstilbene-2,2-Disulfonate - VSMC Vascular Smooth Muscle Cell     

Potassium channel in rabbit corneal endothelium activated by external anions

Summary The apical membrane of the rabbit corneal endothelium contains a potassium-selective ionic channel. In patch-clamp recordings, the probability of finding the channel in the open state (P _o) depends on the presence of either HCO ₃ ^– or Cl^– in the bathing medium. In a methane sulfonate-containing bath,P _o is <0.05 at all physiologically relevant transmembrane voltages. With 0mm [HCO ₃ ^– ] _o at +60 mV,P _o was 0.085 and increased to 0.40 when [HCO ₃ ^– ] _o was 15mm. With 4mm [Cl^–] _o at +60 mV,P _o was 0.083 and with 150mm Cl^–,P _o increased to 0.36. LowP _o's are also found when propionate, sulphate, bromide, and nitrate are the primary bath anions. The mechanism of action of the anion-stimulated K⁺ channel gating is not yet known, but a direct action of pH seems unlikely. The alkalinization of cytoplasm associated with the addition of 10mm (NH₄)₂SO₄ to the bath and the acidification accompanying its removal do not result in channel activation nor does the use of Nigericin to equilibrate intracellular pH with that of the bath over the pH range of 6.8 to 7.8. Channel gating also is not affected by bathing the internal surface of the patch with cAMP, cGMP, GTP--s, Mg²⁺ or ATP. Blockers of Na/H⁺ exchange, Na⁺–HCO ₃ ^– cotransport, Na⁺–K⁺ ATPase and carbonic anhydrase do not block the HCO ₃ ^– stimulation ofP _o. Several of the properties of the channel could explain some of the previously reported voltage changes that occur in corneal endothelial cells stimulated by extracellular anions.     

Quantification of N2-fixation and survival of inoculated diazotrophs associated with roots of Kallar grass

White clover plants were grown for 97 days under two temperature regimes (20/15°C and 8/5°C day/night temperatures) and were supplied with either small amounts (a total of 80 mg N pot^–1) of ammonium (NH ₄ ⁺ ) or nitrate (NO ₃ ^– ) nitrogen, or received no mineral N and relied on N₂ fixation. Greatest growth and total leaf area of clover plants occurred in N₂ fixing and NO ₃ ^– -fed plants grown at 20/15°C and poorest growth occurred in NH ₄ ⁺ -fed plants grown at 8/5°C. Nodule mass per plant was greater at 8/5°C due to increased nodule numbers rather than increased dry weight per nodule. This compensated to some extent for the reduced N₂-fixing activity per unit dry weight of nodule tissue found at the low growth temperature up to 116 d after sowing, but thereafter both activity per nodule dry weight and activity per plant were greater at the low temperature. Highest nitrate reductase activity (NRA) per g fresh weight and total activity per leaf, petiole or root occurred in NO ₃ ^– -fed plants at 8/5°C. Low growth temperature resulted in a greater partitioning of total plant NRA to the roots of NO ₃ ^– -fed plants. The results are considered in relation to the use of N fertiliser in the spring under field conditions.     

Intracellular pH regulation in the mouse lacrimal gland acinar cells

Summary Intracellular pH (pH _i ) of the acinar cells of the isolated, superfused mouse lacrimal gland has been measured using pH-sensitive microelectrodes. Under nonstimulated condition pH _i was 7.25, which was about 0.5 unit higher than the equilibrium pH. Alterations of the external pH by ±0.4 unit shifted pH _i only by ±0.08 unit. The intracellular buffering value determined by applications of 25mm NH ₄ ⁺ and bicarbonate buffer solution gassed with 5% CO₂/95% O₂ was 26 and 46mm/pH, respectively Stimulation with 1 m acetylcholine (ACh) caused a transient, small decrease and then a sustained increase in pH _i . In the presence of amiloride (0.1mm) or the absence of Na⁺, application of ACh caused a significant decrease in pH _i and removal of amiloride or replacement with Na⁺-containing saline, respectively, rapidly increased the pH _i . Pretreatment with DIDS (0.2mm) did not change the pH _i of the nonstimulated conditions; however, it significantly enhanced the increase in pH _i induced by ACh. The present results showed that (i) there is an active acid extrusion mechanism that is stimulated by ACh; (ii) stimulation with ACh enhances the rate of acid production in the acinar cells; and (iii) the acid extrusion mechanism is inhibited by amiloride addition to and Na⁺ removal from the bath solution. We suggest that both Na⁺/H⁺ and HCO ₃ ^– /Cl^– exchange transport mechanisms are taking roles in the intracellular pH regulation in the lacrimal gland acinar cells.     

在不同pH值土壤中铵硝比对南方高丛蓝浆果生长的影响

以南方高丛蓝浆果(Vaccinium corymbosum hybrids)品种‘南月’(‘Southmoon’)优选系A47的1年生扦插苗为实验材料,采用盆栽实验方法,研究在pH 4.5和pH 6.0的土壤中按照摩尔比100∶0、75∶25、50∶50、25∶75和0∶100施用铵态氮与硝态氮,对扦插苗根、茎和叶干质量、茎基径和株高的影响。结果表明:在pH 4.5的土壤中,5个施肥处理组幼苗的根、茎和叶干质量均无显著差异(P>0.05);而多数处理组间幼苗的茎基径和株高也无显著差异。在pH 6.0的土壤中,铵硝摩尔比100∶0处理组幼苗的根、茎和叶干质量显著高于其他处理组(P<0.05),其余处理间均无显著差异;茎基径随铵态氮比例的提高而增大,其中铵硝摩尔比100∶0处理组幼苗的茎基径显著高于其他处理组;多数处理组间幼苗的株高无显著差异。与pH 4.5的土壤相比,在pH 6.0的土壤中各施肥处理组的根、茎和叶干质量均显著降低;除铵硝摩尔比100∶0处理组外,其他处理组的茎基径均显著降低;除铵硝摩尔比50∶50和0∶100处理组外,其他处理组的株高显著降低。综合分析结果表明:在pH 4.5的土壤中施用硝态氮和铵态氮对优选系A47生长的影响总体上无明显差异,但在pH 6.0的土壤中施用铵态氮肥较有利于植株生长;优选系A47适宜栽植于pH 4.5的酸性土壤中。     

Effect of ammonium and nitrate on growth and appearance of nitrate reductase and nitrite reductase in dark- and light-grown mustard seedlings

Nitrate-induced and phytochrome-modulated appearance of nitrate reductase (NR; EC 1.6.6.1) and nitrite reductase (NIR; EC 1.7.7.1) in the cotyledons of the mustard (Sinapis alba L.) seedling is strongly affected by externally supplied ammonium (NH ₄ ⁺ ). In short-term experiments between 60 and 78 h after sowing it was found that in darkness NH ₄ ⁺ —simultaneously given with NO ₃ ^- —strongly inhibits appearance of nitrate-inducible NR and NIR whereas in continuous far-red light—which operates exclusively via phytochrome without significant chlorophyll formation —NH ₄ ⁺ (simultaneously given with NO ₃ ^- ) strongly stimulates appearance of NR. The NIR levels are not affected. This indicates that NR and NIR levels are regulated differently. In the absence of external NO ₃ ^- appearance of NR is induced by NH₄ in darkness as well as in continuous far-red light whereas NIR levels are not affected. On the other hand, in the absence of external NO ₃ ^- , exogenous NH ₄ ⁺ strongly inhibits growth of the mustard seedling in darkness as well as in continuous far-red light. This effect can be abolished by simultaneously supplying NO ₃ ^- . The adverse effect of NH ₄ ⁺ on growth (NH ₄ ⁺ -toxicity) cannot be attributed to pH-changes in the medium since it was shown that neither the growth responses nor the changes of the enzyme levels are related to pH changes in the medium. Non-specific osmotic effects are not involved either.Abbreviations c continuous - D darkness - FR far-red light - NIR nitrite reductase (EC 1.7.7.1) - NR nitrate reductase (EC 1.6.6.1)     

A field method of determining NH 4 + and NO 3 - uptake kinetics in intact roots: Effects of CO2 enrichment on trees and crop species

Models describing plant and ecosystem N cycles require an accurate assessment of root physiological uptake capacity for NH ₄ ⁺ and NO ₃ ^- under field conditions. Traditionally, rates of ion uptake in field-grown plants are determined by using excised root segments incubated for a short period in an assay solution containing N either as a radioactive or stable isotope tracer (e.g., ³⁶ClO₃ as a NH ₄ ⁺ analogue, ¹⁴CH₃NH₃ as an NO ₃ ^- analogue or ¹⁵NH ₄ ⁺ and ¹⁵NO ₃ ^- ). Although reliable, this method has several drawbacks. For example, in addition to radioactive safety issues, purchase and analysis of radioactive and stable isotopes is relatively expensive and can be a major limitation. More importantly, because excision effectively interrupts exchange of compounds between root and shoot (e.g., carbohydrate supply to root and N transport to shoot), the assay must be conducted quickly to avoid such complications. Here we present a novel field method for simultaneous measurements of NH ₄ ⁺ and NO ₃ ^- uptake kinetics in intact root systems. The application of this method is demonstrated using two tree species; red maple (Acer rubrum) and sugar maple (Acer saccharum) and two crop species soybean (Glycine max) and sorghum (Sorghum bicolor). Plants were grown in open-top chambers at either ambient or elevated levels of atmospheric CO₂ at two separate US national sites involved in CO₂ research. Absolute values of net uptake rates and the kinetic parameters determined by our method were found to be in agreement with the literature reports. Roots of the crop species exhibited a greater uptake capacity for both N forms relative to tree species. Elevated CO₂ did not significantly affect kinetics of N uptake in species tested except in red maple where it increased root uptake capacity, V, for NH ₄ ⁺ . The application, reliability, advantages and disadvantages of the method are discussed in detail. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of S20787, a novel Cl--HCO3- exchange inhibitor, on intracellular pH regulation in guinea pig ventricular myocytes

S20787 has recently been proposed to be a selective Cl--HCO3- anion exchange (AE) inhibitor in rat cardiomyocytes. The AE transporter mediates sarcolemmal acid influx but is only one part of the cardiac cell's dual acid loading mechanism, the other part being a sarcolemmal Cl--OH- exchanger (CHE). We have therefore (1) investigated the differential effects of S20787 on the AE and CHE transporters in isolated guinea pig ventricular myocytes and (2) re-examined the influence of the drug on other sarcolemmal acid transporters by monitoring its effect on intracellular pH (pH(i)) recovery from alkali or acid loads. The pH(i) was measured using microspectrofluorimetry (carboxy-SNARF-1). The results indicate that CHE activity was unaffected by the drug (1-20 microM), whereas up to 78% of AE activity was blocked (K(i) = 3.9 microM). Thus, S20787 targets only the AE component of the dual acid influx system. Activities of other acid-transporting carriers, such as Na+-H+ exchange, Na+-HCO3- co-transport and the monocarboxylic acid transporter, were unaffected by the drug. The inhibitory efficacy of S20787 for AE in guinea pig cardiomyocytes appears to be considerably higher (approximately 78%) than proposed previously for rat cardiomyocytes (50%). This is most likely because, in both cells, a significant fraction (20-30%) of acid influx is mediated through the S20787-insensitive CHE transporter. Previous studies made no allowance for the CHE component, which would result in an underestimation. S20787 is thus a highly selective AE inhibitor which may be useful as an experimental tool and a potential cardiac protective agent in the heart.     

pH Regulation in tissue-cultured bovine lens epithelial cells

Summary The intracellular pH (pH _i ) of tissue-cultured bovine lens epithelial cells was measured in small groups of 6 to 10 cells using the trapped fluorescent dye 2,7-bis-(2-,carboxyethyl)-5 (and 6)carboxyfluorescein (BCECF). When perifused at 35°C with artificial aqueous humour solution (AAH) containing 16 mM HCO ₃ ^- and 5% CO₂, pH 7.25, pH _i was 7.19±0.02 (sem, n = 95). On removing HCO ₃ ^- and CO₂ there was an initial transient alkalinization followed by a fall in pH to a steady value of 6.97±0.03 (sem, n = 54). Addition of 0.25 mM 4,4-diisothiocyanatostilbene2, 2-disulfonic acid (DIDS) to AAH containing HCO ₃ ^- and CO₂ led to a rapid and pronounced fall in pH. Exposure to Na⁺-free AAH again led to a marked fall in pH _i , but in this case the addition of DIDS did not produce a further fall. Substitution of the impermeant anion gluconate for Cl^– in the presence of HCO ₃ ^- led to a rise in pH _i , while substitution in the absence of HCO ₃ ^- led to a fall in pH _i . The above data indicate a significant role for a sodium-dependent Cl^–-HCO ₃ ^- exchange mechanism in the regulation of pH _i . Addition of 1 mM amiloride to control AAH in both the presence and absence of HCO ₃ ^- led to a marked fall in pH _i , indicating that a Na⁺/H⁺ exchange mechanism also has a significant role in the regulation of pH _i . There is evidence for a lactic acid transport mechanism in bovine lens cells, as addition of lactate to the external medium produced a rapid fall in pH _i . Larger changes in pH _i were observed in control compared to HCO ₃ ^- -free AAH and in the latter case a pronounced alkalinizing overshoot was obtained on removing external lactate. Tissue-cultured bovine lens cells thus possess at least three membrane transport mechanisms that are involved in pH regulation. The buffering capacity of the lens cells was measured by perturbing pH _i with either NH ₄ ⁺ or procaine. The values obtained were similar in both cases and the intrinsic buffering capacity measured in the absence of external HCO ₃ ^- was 5 mm/pH unit (procaine). However, in the presence of HCO ₃ ^- and CO₂ the buffer capacity increases approximately fourfold, indicating that HCO ₃ ^- is the principal intracellular buffer.We acknowledge financial support from the Wellcome Trust and the Humane Research Trust for this project. M.R. Williams was in receipt of a Science & Engineering Research Council studentship.     

Characterization of the nitrate transporter in root plasma membranes of Cucumis sativus L

Nitrate uptake in right-side out plasma membrane vesicles isolated from cucumber roots was characterized. Nitrate uptake into vesicles was driven by an artificially imposed pH gradient. The uptake was strongly inhibited by phenylglyoxal, an arginyl residue modificator. Only a slight repression of NO ₃ ⁻ transport in vesicles was observed in the presence of NEM, a thiol group reagent. pCMBS, an other thiol reagent and DEPC, an effector of histidine residue, had no effect on the nitrate transport in plasma membranes. ATP-driven proton transport in vesicles was not significantly affected in the presence of both, phenylglyoxal and DEPC, whereas pCMBS and NEM abolished it almost completely. The possible role of the particular amino acids residues in the active nitrate transport is discussed. NO ₃ ⁻ uptake into vesicles isolated from both, nitrate-induced and nitrate-depleted plant material was higher than that observed in the vesicles obtained from uninduced plants. Thus, isolated vesicles reflect the well-known in vivo response of intact plants on the exogenous nitrogen regime.     

Intracellular pH regulatory mechanism in human atrial myocardium: functional evidence for Na(+)/H(+) exchanger and Na(+)/HCO(3)(-) symporter

Intracellular pH (pH(i)) exerts considerable influence on cardiac contractility and rhythm. Over the last few years, extensive progress has been made in understanding the system that controls pH(i) in animal cardiomyocytes. In addition to the housekeeping Na(+)-H(+) exchanger (NHE), the Na(+)-HCO(3)(-) symporter (NHS) has been demonstrated in animal cardiomyocytes as another acid extruder. However, whether the NHE and NHS functions exist in human atrial cardiomyocytes remains unclear. We therefore investigated the mechanism of pH(i) recovery from intracellular acidosis (induced by NH(4)Cl prepulse) using intracellular 2',7'-bis(2-carboxethyl)-5(6)-carboxy-fluorescein fluorescence in human atrial myocardium. In HEPES (nominally HCO(3)(-)-free) Tyrode solution, pH(i) recovery from induced intracellular acidosis could be blocked completely by 30 microM 3-methylsulfonyl-4-piperidinobenzoyl, guanidine hydrochloride (HOE 694), a specific NHE inhibitor, or by removing extracellular Na(+). In 3% CO(2)-HCO(3)(-) Tyrode solution, HOE 694 only slowed the pH(i) recovery, while addition of HOE 694 together with 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (an NHS inhibitor) or removal of extracellular Na(+) inhibited the acid extrusion entirely. Therefore, in the present study, we provided evidence that two acid extruders involved in acid extrusion in human atrial myocytes, one which is HCO(3)(-) independent and one which is HCO(3)(-) dependent, are mostly likely NHE and NHS, respectively. When we checked the percentage of contribution of these two carriers to pH(i) recovery following induced acidosis, we found that the activity of NHE increased steeply in the acid direction, while that of NHS did not change. Our present data indicate for the first time that two acid extruders, NHE and NHS, exist functionally and pH(i) dependently in human atrial cardiomyocytes.     

Adaptation of plasma membrane H+-ATPase of rice roots to low pH as related to ammonium nutrition

The preference of paddy rice for NH₄⁺ rather than NO₃^- is associated with its tolerance to low pH since a rhizosphere acidification occurs during NH₄⁺ absorption. However, the adaptation of rice root to low pH has not been fully elucidated. This study investigated the acclimation of plasma membrane H⁺-ATPase of rice root to low pH. Rice seedlings were grown either with NH₄⁺ or NO₃^-. For both nitrogen forms, the pH value of nutrient solutions was gradually adjusted to pH 6.5 or 3.0. After 4 d cultivation, hydrolytic H⁺-ATPase activity, V _max, K _m, H⁺-pumping activity, H⁺ permeability and pH gradient across the plasma membrane were significantly higher in rice roots grown at pH 3.0 than at 6.5, irrespective of the nitrogen forms supplied. The higher activity of plasma membrane H⁺-ATPase of adapted rice roots was attributed to the increase in expression of OSA1, OSA3, OSA7, OSA8 and OSA9 genes, which resulted in an increase of H⁺-ATPase protein concentration. In conclusion, a high regulation of various plasma membrane H⁺-ATPase genes is responsible for the adaptation of rice roots to low pH. This mechanism may be partly responsible for the preference of rice plants to NH₄⁺ nutrition.     

Optimizing environmental factors for large-scale multiplication of chrysanthemum (<Emphasis Type="Italic">Chrysanthemum grandiflorum</Emphasis>) in balloon-type bioreactor culture

Summary We have developed optimum culture conditions for the large-scale propagation of chrysanthemum in balloon-type bioreactors to achieve vigorous growth and quality. The effects of NH ₄ ⁺ /NO ₃ ⁻ ratio, air volume, air temperature, photosynthetic photo flux, and an inoculation density on the growth and quality of plantlets were investigated. The best production conditions were an NH ₄ ⁺ :NO ₃ ⁻ ratio of 20∶40 mM, air exchange of 0.1 vvm min⁻¹, air temperature 25°C, photosynthetic photo flux (PPF) at 100 μmol m⁻² s⁻¹, and an inoculation density of 40 nodes Chrysanthemum grandiflorum. Under each of these conditions, the maximum growth rate reached 279.0, 260,0, 20.0, 23.3, and 94.5 (g-fresh weight per plantlet d⁻¹), respectively, at 12 wk of culture. These results specify the key environmental factors that can be regulated to improve the quality and quantity of flowers and increase yield in large-scale bioreactor cultures of chrysanthemum.     

Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Immobilization of ammonium (NH ₄ ⁺ ) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of¹⁵NH ₄ ⁺ solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH ₄ ⁺ , we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old,¹⁵N enriched nitrate (NO ₃ ⁻ ) was applied in the 0–150 mm soil layer. After 24 h, > 30 times more¹⁵NH ₄ ⁺ was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH ₄ ⁺ -N present at this time in the soil with roots had been converted from NO ₃ ⁻ . We estimated rates of conversion of NO ₃ ⁻ to NH ₄ ⁺ , and rates ofNH ₄ ⁺ immobilization by plants and microbes, by simulating N-flow of¹⁴⁺¹⁵N and¹⁵N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO ₃ ⁻ reduction to NH ₄ ⁺ (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO ₃ ⁻ uptake, plant NH ₄ ⁺ uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO ₃ ⁻ to NH ₄ ⁺ via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH ₄ ⁺ than generally assumed.     

Relationship between intracellular pH and N metabolism in maize (Zea mays L.) roots

In vivo ³¹P nuclear magnetic resonance (NMR) was used to characterize the effect of the N form (NO₃ vs. NH₄) and the external pH (4, 6, and 8), on the intracellular pH of root tips (0–5 mm) and root segments (5–30 mm). Ammonium-grown root tips were the most sensitive to changes in the external pH. In vivo ¹⁵N NMR was used to characterize the pathway of primary ammonium assimilation in the ammonium-grown roots and to compare the activity of the apical and more-basal root parts. The kinetics of ¹⁵NH₄ ⁺ incorporation showed that primary assimilation in both root tips and root segments followed the glutamine synthetase (GS) pathway. In agreement with the reported gradient of GS along the seminal root of maize, incorporation of label into glutamine amide was more rapid in tips than in segments. It is suggested that this higher GS activity increases the endogenous proton production and thus contributes to the greater dependence of the cytoplasmic pH on the external pH in the ammonium-treated root tips.