Relation between electron exchange and nuclear vibrations in the H 2 + +H 2 + → H 3 + +p exothermic ion-molecular reaction

The effective cross section for the H ₂ ⁺ +H ₂ ⁺ → H ₃ ⁺ +p reaction in the energy range 5.7–11.5 eV is measured by the split beam method. The maximum of the cross section at an energy of ～8 eV is related to the production of the H ₄ ⁺⁺ compound system. The reaction threshold W _thr≈5 eV provides evidence in favor of the classical model of the H ₂ ⁺ ion with the charge fixed on one of the nuclei throughout the collision event.     

Production of H 3 + ions in low-energy collisions between hydrogen molecular ions

The effective cross section for the H ₂ ⁺ + H ₂ ⁺ → H ₃ ⁺ + p reaction in the energy range 5.7–11.5 eV is measured by the split beam method. The cross-section maximum at an energy of ～8 eV is related to the production of the H ₄ ⁺⁺ compound system. The reaction threshold W _thr ≈5 eV provides evidence in favor of the classical model with the H ₂ ⁺ ion charge fixed on one of the two nuclei during the entire collision event.     

Effect of cadmium on the distribution of hydroxyl radical, superoxide and hydrogen peroxide in barley root tip

In the present study, we investigated the alteration of reactive oxygen species production along the longitudinal axis of barley root tips during Cd treatment. In unstressed barley root tips, H₂O₂ production decreased from the root apex towards the differentiation zone where again, a slight increase was observed towards the more mature region of root. An opposite pattern was observed for O ₂ ^?? and OH^? generation. The amount of both O ₂ ^?? and OH^? was highest in the elongation zone, decreased in the root apex and at the differentiation zone of root, then increased again towards the more mature region of root. An elevated Cd-induced O ₂ ^?? production started in the elongation zone and increased further along the differentiation zone of barley root tip. In contrast, Cd-induced H₂O₂ production was localised to the root elongation zone and to the beginning of the differentiation zone. In contrast to Cd-induced H₂O₂ and O ₂ ^?? production, Cd reduced OH^? production along the whole barley root tip. Our results suggest that not only an increase but also the spatial distribution of reactive oxygen species production is involved in the Cd-induced stress response of barley root tip.     

Superoxide production pathways in aortas of diabetic rats: beneficial effects of insulin therapy and endurance training

Superoxide (O ₂ ^·? ) overproduction, by decreasing the nitric oxide (^·NO) bioavailability, contributes to vascular complications in type 1 diabetes. In this disease, the vascular O ₂ ^·? can be produced by the NADPH oxidase (NOX), nitric oxide synthase (NOS), and xanthine oxidase (XO). This study aimed to determine the contribution of each enzymatic pathway in hyperglycemia-induced O ₂ ^·? overproduction, and the effects of an endurance training program and insulin therapy, associated or not, on the O ₂ ^·? production (amount and related enzymes) in diabetic rats. Forty male Wistar rats were divided into diabetic (D), diabetic treated with insulin (D-Ins), diabetic trained (D-Tr), or diabetic insulin-treated and trained (D-Ins + Tr) groups. An additional healthy group was used as control. Insulin therapy (Glargine Lantus, Sanofi) and endurance training (treadmill run: 60 min/day, 25 m/min, 5 days/week) started 1 week after diabetes induction by streptozotocin (45 mg/kg), and lasted for 8 weeks. At the end of the protocol, the O ₂ ^·? production in aorta rings was evaluated by histochemical analyses (DHE staining). Each production pathway was studied by inhibiting NOX (apocynin), NOS (L-Name), or XO (allopurinol) before DHE staining. Diabetic rats exhibited hyperglycemia-induced O ₂ ^·? overproduction, resulting from NOX, NOS, and XO activation. Insulin therapy and endurance training, associated or not, decreased efficiently and similarly the O ₂ ^·? overproduction. Insulin therapy reduced the hyperglycemia and decreased the three enzymatic pathways implicated in the O ₂ ^·? production. Endurance training decreased directly the NOS and XO activity. While both therapeutic strategies activated different pathways, their association did not reduce the O ₂ ^·? overproduction more significantly.     

Oxidative reactions in axenically seedling roots of olive (Olea europaea, L.)

The production of reactive oxygen species (ROS) plays important roles in the life cycle and in the stress response and defence mechanisms of plants. Various enzyme systems are involved in the formation of ROS in the apoplast, including plasmalemma NADPH oxidase and apoplastic peroxidases. The production of O ₂ ^·? and apoplastic peroxidase and exogenous NADH oxidation activities are all strongly dependent on the age of roots??the younger the root, the greater the activity. Apoplastic production of ROS is shown in the root by using specific histochemical probes, this ROS production is growing zone dependent. In the present study, using olive seedlings, differences were also observed between cultivars, especially in O ₂ ^·? production by the Verdial cultivar which was well above that of other cultivars studied. In all the cultivars, treatment of roots with methyl jasmonate (MeJA) or methyl salicylate (MeSA) increased O ₂ ^·? production. Similar results were observed for peroxidase activity, but not for the oxidation of exogenous NADH which was either unaffected (MeJA) or even partially inhibited (MeSA). A conclusion was that MeJA or MeSA induced apoplastic production of ROS does not use exogenous NADH. Treatment with diphenylene iodonium (DPI) reduced the formation of O ₂ ^·? , but affected neither peroxidase nor NADH oxidation activities. Cyanide inhibited O ₂ ^·? production and peroxidase and NADH oxidation activities. Treatment with MnCl₂ had a strong stimulatory effect on peroxidase and NADH oxidation activities, but much less on O ₂ ^·? production. Finally, azide greatly reduced all activities, but especially O ₂ ^·? production. Together, these results indicate a relationship between oxidative activities and the processes of root growth, and that those activities are also dependent on the cultivar, as well as an involvement of peroxidases and plasmalemma NADPH oxidase in apoplast ROS production which is sensitive to DPI, azide, and cyanide but relatively insensitive to MnCl₂, while exogenous NADH oxidation is linked to peroxidase activity.     

Mechanism of primary and secondary ion-radical pair formation in photosystem I complexes

The mechanisms of the ultrafast charge separation in reaction centers of photosystem I (PS I) complexes are discussed. A kinetic model of the primary reactions in PS I complexes is presented. The model takes into account previously calculated values of redox potentials of cofactors, reorganization energies of the primary P700⁺A ₀ ^- and secondary P700⁺A ₁ ^- ion-radical pairs formation, and the possibility of electron transfer via both symmetric branches A and B of redox-cofactors. The model assumes that the primary electron acceptor A₀ in PS I is represented by a dimer of chlorophyll molecules Chl2A/Chl3A and Chl2B/Chl3B in branches A and B of the cofactors. The characteristic times of formation of P700⁺A ₀ ^- and P700⁺A ₁ ^- calculated on the basis of the model are close to the experimental values obtained by pump-probe femtosecond absorption spectroscopy. It is demonstrated that a small difference in the values of redox potentials between the primary electron acceptors A_0A and A_0B in branches A and B leads to asymmetry of the electron transfer in a ratio of 70: 30 in favor of branch A. The secondary charge separation is thermodynamically irreversible in the submicrosecond range and is accompanied by additional increase in asymmetry between the branches of cofactors of PS I.     

The effect of transpiration on selenium uptake and mobility in durum wheat and spring canola

Aims

The objective of this study was to determine the relative importance of transpirational pull, Se speciation, sulfate and species on Se accumulation by plants, in order to determine which of these factors must be considered in the future development of models to predict Se accumulation by plants.

Methods

Seedlings of durum wheat (Triticum turgidum L. var durum cv ‘Kyle’) and spring canola (Brassica napus L. var Hyola 401) were grown hydroponically and exposed to SeO ₄ ^2- (selenate) with or without SO ₄ ^2- (sulfate), or to HSeO ₃ ^- (biselenite) under different transpiration regimes altered through ‘low’ (~50%) or ‘high’ (~78%) relative humidity (RH). Plants were harvested after 0, 8, 16, or 24?h exposures, digested, and analyzed for Se by GFAAS.

Results

Accumulation and distribution of Se by plants is dependent on plant species, Se speciation in the nutrient solution, SO ₄ ^2- competition, and transpiration regimes. Canola accumulated and translocated more Se than wheat. In wheat and canola, the greatest accumulation and translocation of Se occurred when plants were exposed to SeO ₄ ^2- without SO ₄ ^2- compared to solutions of SeO ₄ ^2- with SO ₄ ^2- or HSeO ₃ ^2- . Wheat plants exposed to SeO ₄ ^2- and SO ₄ ^2- had an increased Se accumulation and translocation under increased transpiration rates than when exposed to SeO ₄ ^2- without SO ₄ ^2- or HSeO ₃ ^2- . On the other hand, increases in transpiration increased the translocation of Se to canola shoots when exposed to HSeO ₃ ^- more than any other treatments.

Conclusions

Overall, our results suggest that plant species is the most important factor influencing Se accumulation and translocation, but that these endpoints can be modified by climate and specific soil Se or S content. Models to predict accumulation of Se by plants must consider all of these factors to accurately calculate the mechanisms of uptake and translocation.     

NO 3 − transport across the plasma membrane of Arabidopsis thaliana root hairs: Kinetic control by pH and membrane voltage

High-affinity nitrate transport was examined in intact root hair cells of Arabidopsis thaliana using electrophysiological recordings to characterise the response of the plasma membrane to NO ₃ ^? challenge and to quantify transport activity. The NO ₃ ^? -associated membrane current was determined using a three-electrode voltage clamp to bring membrane voltage under experimental control and to compensate for current dissipation along the longitudinal cell axis. Nitrate transport was evident in the roots of seedlings grown in the absence of a nitrogen source, but only 4–6 days postgermination. In 6-day-old seedlings, additions of 5–100 μm NO ₃ ^? to the bathing medium resulted in membrane depolarizations of 8–43 mV, and membrane voltage (V _m) recovered on washing NO ₃ ^? from the bath. Voltage clamp measurements carried out immediately before and following NO ₃ ^? additions showed that the NO ₃ ^? -evoked depolarizations were the consequence of an inward-directed current that appeared across the entire range of accessible voltages (?300 to +50 mV). Both membrane depolarizations and NO ₃ ^? -evoked currents recorded at the free-running voltage displayed quasi-Michaelian kinetics, with apparent values for K_m of 23 ± 6 and 44 ± 11 μm, respectively and, for the current, a maximum of 5.1 ± 0.9 μA cm^?2. The NO ₃ ^? current showed a pronounced voltage sensitivity within the normal physiological range between ?250 and ?100 mV, as could be demonstrated under voltage clamp, and increasing the bathing pH from 6.1 to 7.4–8.0 reduced the current and the associated membrane depolarizations 3- to 8-fold. Analyses showed a well-defined interaction between the kinetic variables of membrane voltage, pH_o and [NO ₃ ^? ]_o. At a constant pH_o of 6.1, depolarization from ?250 to ?150 mV resulted in an approximate 3-fold reduction in the maximum current but a 10% rise in the apparent affinity for NO ₃ ^? . By contrast, the same depolarization effected an approximate 20% fall in the K_m for transport as a function in [H⁺]_o. These, and additional characteristics of the transport current implicate a carrier cycle in which NO ₃ ^? binding is kinetically isolated from the rate-limiting step of membrane charge transit, and they indicate a charge-coupling stoichiometry of 2(H⁺) per NO ₃ ^? anion transported across the membrane. The results concur with previous studies showing a high-affinity NO ₃ ^? transport system in Arabidopsis that is inducible following a period of nitrogen-limiting growth, but they underline the importance of voltage as a kinetic factor controlling NO ₃ ^? transport at the plant plasma membrane.     

Ionic balance,proton efflux,nitrate reductase activity and growth ofHippophaë rhamnoides L. ssp.rhamnoides as influenced by combined-N nutrition or N2 fixation

Growth of 2-month-old nonnodulatedHippophaë rhamnoides seedlings supplied with combined N was compared with that of nodulated seedlings grown on zero N. Plant growth was significantly better with combined N than with N₂ fixation and, although not statistically significant for individual harvests, tended to be highest in the presence of NH ₄ ⁺ , a mixture of NH ₄ ⁺ and NO ₃ ^? producing the highest yields. Growth was severely reduced when solely dependent on N₂ fixation and, unlike the combined-N plants, shoot to root ratios had only slightly increased after an initial decrease. An apparently insufficient nodule mass (nodule weight ratio <5 per cent) during the greater part of the experimental period is suggested as the main cause of the growth reduction in N₂-fixing plants. Thein vivo nitrate reductase activity (NRA) of NO ₃ ^? dependent plants was almost entirely located in the roots. However, when grown with a combination of NO ₃ ^? and NH ₄ ⁺ , root NRA was decreased by approximately 85 per cent.H. rhamnoides demonstrated in the mixed supply a strong preference for uptake of N as NH ₄ ⁺ , NO ₃ ^? contributing only for approximately 20 per cent to the total N assimilation. Specific rates of N acquisition and ion uptake were generally highest in NO ₃ ^? +NH ₄ ⁺ plants. The generation of organic anions per unit total plant dry weight was approximately 40 per cent less in the NH ₄ ⁺ plants than in the NO ₃ ^? plants. Measured extrusions of H⁺ or OH^? (HCO ₃ ^? ) were generally in good agreement with calculated values on the basis of plant composition, and the acidity generated with N₂ fixation amounted to 0.45–0.55 meq H⁺. (mmol N_org)^?1. Without acidity control and in the presence of NH ₄ ⁺ , specific rates of ion uptake and carboxylate generation were strongly depressed and growth was reduced by 30–35 per cent. Growth of nonnodulatedH. rhamnoides plants ceased at the lower pH limit of 3.1–3.2 and deterioration set in; in the case of N₂-fixing plants the nutrient solution pH stabilized at a value of 3.8–3.9 without any apparent adverse effects upon plant performance. The chemical composition of experimental and field-growing plants is being compared and some comments are made on the nitrogen supply characteristics of their natural sites.     

Biofortification of rice grain with zinc through zinc fertilization in different countries

Aims and background

The ability to suppress soil nitrification through the release of nitrification inhibitors from plant roots is termed ‘biological nitrification inhibition’ (BNI). Earlier, we reported that sorghum roots release higher BNI-activity when grown with NH ₄ ⁺ , but not with NO ₃ ^- as N source. Also for BNI release, rhizosphere pH of <5.0 is needed; beyond this, a negative effect on BNI release was observed with nearly 80% loss of BNI activity at pH >7.0. This study is aimed at understanding the inter-functional relationships associated with NH ₄ ⁺ uptake, rhizosphere-pH and plasma membrane H⁺-ATPase (PM H⁺-ATPase) activity in regulating the release of BNIs (biological nitrification inhibitors) from sorghum roots.

Methods

Sorghum was grown hydroponically and root exudates were collected from intact plants using a pH-stat system to separate the secondary acidification effects by NH ₄ ⁺ uptake on BNIs release. A recombinant luminescent Nitrosomonas europaea bioassay was used to determine BNI-activity. Root plasma membrane was isolated using a two-phase partitioning system. Hydrolytic H⁺-ATPase activity was determined. Split-root system setup was deployed to understand the localized responses to NH ₄ ⁺ , H⁺-ATPase-stimulator (fusicoccin) or H⁺-ATPase-inhibitor (vanadates) on BNI release by sorghum.

Results

Presence of NH ₄ ⁺ in the rhizosphere stimulated the expression of H⁺-ATPase activity and enhanced the release of BNIs from sorghum roots. Fusicoccin, which stimulates H⁺-ATPase activity, also stimulated BNIs release in the absence of NH ₄ ⁺ ; vanadate, which suppresses H⁺-ATPase activity, also suppressed the release of BNIs. NH ₄ ⁺ levels (in rhizosphere) positively influenced BNIs release and root H⁺-ATPase activity in the concentration range of 0-1.0 mM, indicating a close relationship between BNI release and root H⁺-ATPase activity with a possible involvement of carrier-mediated transport for the release of BNIs in sorghum.

Conclusion

Our results suggest that NH ₄ ⁺ uptake, PM H⁺-ATPase activity, and rhizosphere acidification are functionally inter-connected with BNI release in sorghum. Such knowledge is critical to gain insights into why BNI function is more effective in light-textured, mildly acidic soils compared to other soil types.     

Influence of different mineral nitrogen sources (NO 3 − -N vs. NH 4 + -N) on arbuscular mycorrhiza development and N transfer in a Glomus intraradices–cowpea symbiosis

Labeled nitrogen (¹⁵?N) was applied to a soil-based substrate in order to study the uptake of N by Glomus intraradices extraradical mycelium (ERM) from different mineral N (NO ₃ ^? vs. NH ₄ ⁺ ) sources and the subsequent transfer to cowpea plants. Fungal compartments (FCs) were placed within the plant growth substrate to simulate soil patches containing root-inaccessible, but mycorrhiza-accessible, N. The fungus was able to take up both N-forms, NO ₃ ^? and NH ₄ ⁺ . However, the amount of N transferred from the FC to the plant was higher when NO ₃ ^? was applied to the FC. In contrast, analysis of ERM harvested from the FC showed a higher ¹⁵?N enrichment when the FC was supplied with ¹⁵NH ₄ ⁺ compared with ¹⁵NO ₃ ^? . The ¹⁵?N shoot/root ratio of plants supplied with ¹⁵NO ₃ ^? was much higher than that of plants supplied with ¹⁵NH ₄ ⁺ , indicative of a faster transfer of ¹⁵NO ₃ ^? from the root to the shoot and a higher accumulation of ¹⁵NH ₄ ⁺ in the root and/or intraradical mycelium. It is concluded that hyphae of the arbuscular mycorrhizal fungus may absorb NH ₄ ⁺ preferentially over NO ₃ ^? but that export of N from the hyphae to the root and shoot may be greater following NO ₃ ^? uptake. The need for NH ₄ ⁺ to be assimilated into organically bound N prior to transport into the plant is discussed.     

Characterization of ammonium and nitrate uptake and assimilation in roots of tea plants

It has been pointed out that tea (Camellia sinensis (L.) O. Kuntze) prefers ammonium (NH ₄ ⁺ ) over nitrate (NO ₃ ^? ) as an inorganic nitrogen (N) source. ¹⁵N studies were conducted using hydroponically grown tea plants to clarify the characteristics of uptake and assimilation of NH ₄ ⁺ and NO ₃ ^? by tea roots. The total ¹⁵N was detected, and kinetic parameters were calculated after feeding ¹⁵NH ₄ ⁺ or ¹⁵NO ₃ ^? to tea plants. The process of N assimilation was studied by monitoring the dynamic ¹⁵N abundance in the free amino acids of tea plant roots by GC-MS. Tea plants supplied with ¹⁵NH ₄ ⁺ absorbed significantly more ¹⁵N than those supplied with ¹⁵NO ₃ ^? . The kinetics of ¹⁵NH ₄ ⁺ and ¹⁵NO ₃ ^? influx into tea plants followed a classic biphasic pattern, demonstrating the action of a high affinity transport system (HATS) and a low affinity transport system (LATS). The V _max value for NH ₄ ⁺ uptake was 54.5 nmol/(g dry wt min), which was higher than that observed for NO ₃ ^? (39.3 nmol/(g dry wt min)). K_M estimates were approximately 0.06 mM for NH ₄ ⁺ and 0.16 mM for NO ₃ ^? , indicating a higher rate of NH ₄ ⁺ absorption by tea plant roots. Tea plants fed with ¹⁵NH ₄ ⁺ accumulated larger amounts of assimilated N, especially glutamine (Gln), compared with those fed with ¹⁵NO ₃ ^? . Gln, Glu, theanine (Thea), Ser, and Asp were the main free amino acids that were labeled with ¹⁵N under both conditions. The rate of N assimilation into Thea in the roots of NO ₃ ^? -supplied tea plants was quicker than in NH ₄ ⁺ -supplied tea plants. NO ₃ ^? uptake by roots, rather than reduction or transport within the plant, seems to be the main factor limiting the growth of tea plants supplied with NO ₃ ^? as the sole N source. The NH ₄ ⁺ absorbed by tea plants directly, as well as that produced by NO ₃ ^? reduction, was assimilated through the glutamine synthetase-glutamine oxoglutarate aminotransferase pathway in tea plant roots. The ¹⁵N labeling experiments showed that there was no direct relationship between the Thea synthesis and the preference of tea plants for NH ₄ ⁺ .     

Production of reactive oxygen species in decoupled, Ca2+-depleted PSII and their use in assigning a function to chloride on both sides of PSII

Extraction of Ca²⁺ from the oxygen-evolving complex of photosystem II (PSII) in the absence of a chelator inhibits O₂ evolution without significant inhibition of the light-dependent reduction of the exogenous electron acceptor, 2,6-dichlorophenolindophenol (DCPIP) on the reducing side of PSII. The phenomenon is known as “the decoupling effect” (Semin et al. Photosynth Res 98:235–249, 2008). Extraction of Cl^? from Ca²⁺-depleted membranes (PSII[–Ca]) suppresses the reduction of DCPIP. In the current study we investigated the nature of the oxidized substrate and the nature of the product(s) of the substrate oxidation. After elimination of all other possible donors, water was identified as the substrate. Generation of reactive oxygen species HO, H₂O₂, and O ₂ ^·? , as possible products of water oxidation in PSII(–Ca) membranes was examined. During the investigation of O ₂ ^·? production in PSII(–Ca) samples, we found that (i) O ₂ ^·? is formed on the acceptor side of PSII due to the reduction of O₂; (ii) depletion of Cl^? does not inhibit water oxidation, but (iii) Cl^? depletion does decrease the efficiency of the reduction of exogenous electron acceptors. In the absence of Cl^? under aerobic conditions, electron transport is diverted from reducing exogenous acceptors to reducing O₂, thereby increasing the rate of O ₂ ^·? generation. From these observations we conclude that the product of water oxidation is H₂O₂ and that Cl^? anions are not involved in the oxidation of water to H₂O₂ in decoupled PSII(–Ca) membranes. These results also indicate that Cl^? anions are not directly involved in water oxidation by the Mn cluster in the native PSII membranes, but possibly provide access for H₂O molecules to the Mn₄CaO₅ cluster and/or facilitate the release of H⁺ ions into the lumenal space.     

The interrelationship between HCO 3 − , NO 3 − , and Cl− as effective anions in the photosynthetic electron transport

Thylakoids were isolated from the leaves of three different plants (Pisum sativium L., Lactuca sativa L., and Raphanus sativus L.). The addition of HCO ₃ ^? to a suspension of salt-and HCO ₃ ^? -epleted thylakoids (suspended in salt-free medium) raised the rate of O₂ evolution up to fourfold. This stimulation could be partially replaced by the addition of chloride or nitrate ions. However, the addition of HCO ₃ ^? in the presence of Cl^? or NO ₃ ^? resulted in a higher stimulation of O₂ evolution (sixfold in the presence of nitrate and sevenfold in the presence of chloride). On the other hand, the addition of HCO ₃ ^? to the thylakoids depleted from salt only raised the rate of O₂ evolution by 10–15%, whereas 40–70% was obtained by the addition of nitrate or chloride ions. The fluorescence induction studies indicated a significant decrease in the yield of the variable fluorescence of the salt- and HCO ₃ ^? -depleted thylakoids. A partial increase in the fluorescence yield was obtained by the addition of HCO ₃ ^? alone. A typical fluorescence induction curves were obtained by the addition of HCO ₃ ^? in the presence of Cl^? or NO ₃ ^? ions. The data obtained suggest a similar role for chloride and nitrate ions in O₂ evolution in the Hill-reaction, which is restricted at the donor side of photosystem II, whereas bicarbonate plays its role at both sides (acceptor and donor sides). The presented data are those obtained for the thylakoids of P. sativium, which were more or less similar to those obtained for L. sativa and R. sativus.     

Possible pathways of heat resistance induction in plant cells by exogenous nitrogen oxide

The mechanisms of exogenous nitrogen oxide (NO) influence on heat resistance of wheat coleoptiles have been studied. The treatment of plant cells with an NO donor (sodium nitroprusside) resulted in an increase in the generation of superoxide anion radicals (O ₂ ^·? ) after 10 min. The inhibitor of protein biosyn-thesis??cycloheximide??did not inhibit the O ₂ ^·? generation by coleoptiles induced by the NO donor, whereas the inhibitor of phosphatidic acid formation (butanol-1) inhibited it partially. Treatment of coleoptiles with calcium ionophore (A23187) or with an activator of the inositol cycle (inositol) compensated for the suppression effect of butanol-1 on NO-dependent O ₂ ^·? formation. Butanol-1 also leveled out the increase in the heat resistance of coleoptiles caused by the NO donor, whereas calcium ionophore and inositol almost completely removed the butanol-1 effect. The possible participation mechanisms of reactive oxygen species, phosphatidic acid, and calcium ions in the realization of NO physiological effects are discussed.     

Dark induction of nitrate reductase in the halophilic alga Dunaliella salina

The effect of nitrogen starvation on the NO₃-dependent induction of nitrate reductase (NR) and nitrite reductases (NIR) has been investigated in the halophilic alga Dunaliella salina. When D. salina cells previously grown in a medium with NH ₄ ⁺ as the only nitrogen source (NH ₄ ⁺ -cells) were transferred into NO ₃ ^? medium, NR was induced in the light. In contrast, when cells previously grown in N-free medium were transferred into a medium containing NO ₃ ^? , NR was induced in light or in darkness. Nitrate-dependent NR induction, in darkness, in D. salina cells previously grown at a photon flux density of 500 umol · m^?2 s^?1 was observed after 4 h preculture in N-free medium, whilst in cells grown at 100 umol · m^?2 s^?1 NR induction was observed after 7–8 h. An inhibitor of mRNA synthesis (6-methylpurine) did not inhibit NO ₃ ^? -induced NR synthesis when the cells, previously grown in NH ₄ ⁺ medium, were transferred into NO ₃ ^? medium (at time 0 h) after 4-h-N starvation. However, when 6-methylpurine was added simultaneously with the transfer of the cells from NH ₄ ⁺ to NO ₃ ^? medium (at time 0 h), NO ₃ ^? induced NR synthesis was completely inhibited. The activity of NIR decreased in N-starved cells and the addition of NO ₃ ^? to those cells greatly stimulated NIR activity in the light. The ability to induce NR in darkness was observed when glutamine synthetase activity reached its maximal level during N starvation. Although cells grown in NO ₃ ^? medium exhibited high NR activity, only 0.33% of the total NR was found in intact chloroplasts. We suggest that the ability, to induce NR in darkness is dependent on the level of N starvation, and that NR in D. salina is located in the cytosol. Light seems to play an indirect regulatory role on NO ₃ ^? uptake and NR induction due to the expression of NR and NO ₃ ^? -transporter mRNAs.     

Spectroscopic Studies on the Binding of Cobalt(II) 1,10-Phenanthroline Complex to Bovine Serum Albumin

The binding interaction of the cobalt(II) 1,10-phenanthroline complex (Co(phen) ₃ ²⁺ , phen = 1,10-phenanthroline) with bovine serum albumin (BSA) was investigated by fluorescence spectroscopy combined with UV–Vis absorption and circular dichroism measurements under simulative physiological conditions. The experiment results showed that the fluorescence intensity of BSA was dramatically decreased owing to the formation of Co(phen) ₃ ²⁺ –BSA complex. The corresponding association constants (K _a) between Co(phen) ₃ ²⁺ and BSA at four different temperatures were calculated according to the modified Stern–Volmer equation. The enthalpy change (ΔH°) and entropy change (ΔS°) were calculated to be ?2.73 kJ mol^?1 and 82.27 J mol^?1?K^?1, respectively, which suggested that electrostatic interaction and hydrophobic force played major roles in stabilizing the Co(phen) ₃ ²⁺ –BSA complex. Site marker competitive experiments indicated that the binding of Co(phen) ₃ ²⁺ to BSA primarily took place in site I of BSA. A value of 4.11 nm for the average distance r between Co(phen) ₃ ²⁺ (acceptor) and tryptophan residues of BSA (donor) was derived from Förster’s energy transfer theory. The conformational investigation showed that the presence of Co(phen) ₃ ²⁺ resulted in the change of BSA secondary structure and induced the slight unfolding of the polypeptides of protein, which confirmed the microenvironment and conformational changes of BSA molecules.     

Positive feedback between acidification and organic phosphate mineralization in the rhizosphere of maize (Zea mays L.)

To test the hypothesis that rhizosphere acidification would enhance the hydrolyzation of organic phosphates by increasing phosphatase activity. A Petri dish experiment with sterile agar and a pot experiment with a low P soil were used. In the Petri dish experiment, roots of each plant were cultured in two compartments, each of which contained agar with one of three nitrogen combinations: NH ₄ ⁺ /N0 (N0 = nitrogen free), NH ₄ ⁺ /NO ₃ ^- , and NO ₃ ^- /N0. Phytin was supplied as the sole phosphorus (P) source to all compartments. In the pot experiment, the soil in each pot was treated with N0, KNO₃, or (NH₄)₂SO₄) together with 0 or 75 mg kg^?1 phytin-P. Dry weight, P concentration, and P content of roots were highest in the NH ₄ ⁺ compartments in the Petri dish experiment. In the pot experiment, dry weight, P concentration, and P content of both shoots and roots were higher with NH ₄ ⁺ than with NO ₃ ^- . NH ₄ ⁺ treatments reduced rhizosphere pH, promoted the hydrolization of phytin, enhanced acid phosphatase activity in the rhizosphere, and increased phytin-P utilization relative to N0 and NO ₃ ^- treatments. Phosphatase activity was negatively correlated with rhizosphere pH but was positively correlated with plant P content in both experiments. Rhizosphere acidification optimized the activity of acid phosphatase excreted by maize roots and promoted phytin mineralization. NH ₄ ⁺ -induced acidification in the maize rhizosphere improved the growth of maize roots by improving P uptake from phytin; the improved growth, in turn, increased NH ₄ ⁺ uptake and acidification.     

Interaction between NO 3 − and abscisic acid in the regulation of lateral root elongation in Zea mays L.

Lateral root (LR) elongation rate of 7–8-day maize seedlings depends on the availability of NO ₃ ^? , NO ₂ ^? , and abscisic acid (ABA) in an environment. Four-hour exposure to 0.01–1.5 mM NO ₂ ^? increases the relative LR elongation rate; in the case of NO ₂ ^? , the stimulation occurs only at an NO ₂ ^? concentration equal to 0.01 mM. Exogenous ABA (10^?6 M) inhibits the LR elongation process. In the case of a combined influence of NO ₃ ^? and ABA or NO ₂ ^? and ABA, the character of the response elongation reaction is different. The NO role in the regulation of LR elongation is discussed.