模拟氮沉降增加条件下土壤团聚体对酶活性的影响

氮沉降增加改变了森林土壤生态系统物质输入,影响土壤生物及酶活性,而土壤团聚体内相对稳定的微域生境可能减弱或延缓土壤生物和酶对氮沉降增加的响应强度。以广东省东莞大岭山森林公园荷木人工林为研究对象,用模拟N沉降方法,分析了2011年12月到2012年11月一年内氮沉降增加条件下表层混合土壤和土壤团聚体内脲酶、蔗糖酶和酸性磷酸酶活性的变化及影响因素,旨在理解氮沉降增加条件下土壤团聚体对酶活性的影响。结果表明:氮沉降增加对表层混合土壤中脲酶和蔗糖酶的抑制作用不显著,而酸性磷酸酶受氮沉降显著影响,表现为低氮(50 kg N hm-2a-1)促进,高氮(300 kg N hm-2a-1)抑制的规律。表层土壤团聚体内脲酶活性随氮沉降增加而降低,N300处理显著低于对照;蔗糖酶和酸性磷酸酶活性随氮沉降增加先降低后增加,N100处理最低,分别比其他处理降低了6.46%—25.53%和42.33%—68.25%。试验区内各粒径土壤团聚体内酶活性高于混合土壤,且随团聚体粒径增加酶活性均为先增加后降低。不同粒径土壤团聚体的3种酶活性均以2—5 mm最高,但脲酶、酸性磷酸酶在各团聚体粒径间差异不显著,蔗糖酶活性2—5 mm显著高于5—8 mm。土壤酶相对活性指数和相对活性综合指数结果显示,超过85%的团聚体粒径内的相对酶活性指数大于1,而土壤酶相对活性综合指数均大于1。以上结果表明,氮沉降增加条件下土壤团聚体对其团聚体内的土壤酶活性有隔离保护作用,但其隔离保护效果与酶的种类和土壤团聚体粒径有关。     

Spatial variability of soil nutrients and microbiological properties after the establishment of leguminous shrub Caragana microphylla Lam. plantation on sand dune in the Horqin Sandy Land of Northeast China

The objective of this study is to determine the spatial variability of nutrients, microbial biomass, and enzyme activities of soil due to the establishment of shrub plantation on moving sandy dunes, as part of an effort to understand the microenvironmental factors that control the soil microbiological properties. Caragana microphylla Lam., an indigenous leguminous shrub, is the dominant plant species used to control desertification in the semi-arid Horqin Sandy Land of Northeast China. In a 26-year-old C. microphylla plantation, soil samples were collected from three soil depths (0-5 cm, 5-10 cm, and 10-20 cm), three slope positions (windward slope, top slope, and leeward slope), and two microsites (under shrubs and between shrubs). The results showed significant differences in soil EC, nutrient content (except for total K), microbial biomass C and N, and the activities of dehydrogenase, urease, and protease at different slopes, soil depths, and microsites. Significant differences in pH at different microsites and slopes, soil moisture and polyphenol oxidase activity at different soil depths and slopes, and activities of phosphomonoesterase and nitrate reductase at different soil depths were also observed. The soil nutrient contents and microbiological activities were greater in the surface soil layer and decreased with the increase of soil depth. Soil organic C, total N, total P, available P and K, microbial biomass C and N, and the activities of enzymes tested (except for protease) under shrubs were higher than those in between shrubs. Furthermore, significant correlations among soil organic C, microbial biomass C and N, the activities of phosphomonoesterase, dehydrogenase, urease, protease, and nitrate reductase were observed, and correlations were also found among EC, total N, total P, available P and K, enzyme (except for polyphenol oxidase) activities, and microbial biomass C and N contents. These results suggest that microenvironmental factors (slope, soil depth and microsite) have significant influences on the spatial distribution of soil nutrients and microbiological properties when the C. microphylla sand-fixing plantation is established in the moving sand dunes in the Horqin Sandy Land.     

Protein volume changes on cosolvent denaturation

A thermodynamic relationship is obtained which links the effect of a cosolvent on the denaturation equilibrium of a protein to the effect of the cosolvent on the change in partial molar volume (pmv) of a protein on denaturation. The relationship uses the concept of preferential interactions and is exact for an infinitely dilute protein. Analysis of the literature data on protein volume changes suggests that many of the observed volume changes are thermodynamically inconsistent with the corresponding free energy changes, especially at low cosolvent concentrations. It is argued that the most reasonable explanation for this involves cosolvent induced changes in the degree of protein-protein association.     

Effects of Cosolvent and Temperature on the Desorption of Polynuclear Aromatic Hydrocarbons from Contaminated Sediments of Chien-Jen River,Taiwan

This study evaluated the effects of the water-miscible cosolvent and temperature on the sorption-desorption of polynuclear aromatic hydrocarbons (PAHs) from contaminated sediments in Chien-Jen River, Taiwan. Sediment samples from five sampling stations of downstream section were utilized in this study. Phenanthrene and anthracene were selected as target compounds. The cosolvent effect on sorption of phenanthrene and anthracene was examined by the addition of various volume fractions of methanol (i.e., 0.3, 0.5, 0.7, and 0.9, respectively) in the sediment/water systems. The utility of the log-linear cosolvency model for predicting PAH sorption from solvent mixtures was evaluated. An inverse relationship was observed for sorption coefficients of phenanthrene and anthracene as a function of increasing cosolvent. The effect of temperature on sorption of phenanthrene and anthracene was conducted at temperature from 10°C to 40°C. The use of elevated temperatures in desorption experiments increased the PAH release from sediments. It was observed that sorption of phenanthrene and anthracene onto sediments decreased when temperature increased. The decrease of sorption coefficient of phenanthrene was more sensitive than that of anthracene. The magnitude of decreased sorption was attributed by the increased desorption rate constant, solubility, and heterogeneities of sediments.     

Water Concentration and Activity Effects on Aminoacylase in Aqueous/Organic One-Liquid-Phase Systems

Aminoacylase has been employed as a model system to study its catalytic properties at low water concentrations/water activities with different water-miscible organic cosolvents. Cosolvents assayed were alcohols and polyols with pure logarithm of the partition coefficient (log P) values, on the standard water/octanol system, ranging between -5.2 and 0.24.

Experimental hydrolysis equilibrium constants (K_app), at a constant water concentration, decreased with the fall in log P of the cosolvent, as well as with reduction of the water concentration/water activity, as would be expected. The enzyme hydrolytic and synthetic activities, measured at a constant water concentration/water activity value, followed a sigmoidal dependence on log P of the cosolvent employed when the water concentration or water activity values were lower than 50% (w/w) or 0.66, respectively. This became a hyperbolic relationship at higher water concentration/water activity values. A linear relationship between the logarithm of the limiting water activity necessary to maintain enzyme activity and log P was obtained. Both hydrolytic and synthetic activities were suppressed for water activities higher than 0.66 and cosolvents with log P lower than -1.6.     

Cosolvent control of substrate inhibition in cosolvent stimulation of beta-glucuronidase activity.

Bacterial β-glucuronidase exhibits a considerable increase in specific activity, upon addition of suitable organic cosolvents to aqueous systems. The effect is especially marked with three to five carbon alcohols; a nine fold increase was observed in this study using t-butanol in the range of 10% by volume of t-butanol in water. Most such increases were previously ascribed to cosolvent effects on V_max, i.e. to the catalytic steps. However, it is shown here that this enzyme is susceptible to marked substrate inhibition, and that half of the cosolvent induced activity increase is due to decreasing this inhibition. The other 50% of the activity increase probably does reside in cosolvent stimulation of V_max.     

Temporal dynamics of exchangeable K, Ca and Mg in acidic bulk soil and rhizosphere under Norway spruce (Picea abies Karst.) and beech (Fagus sylvatica L.) stands

Anthropogenic nitrogen (N) deposition significantly affects forest soil microbial biomass and extracellular enzymatic activities (EEA). However, the influence of mixed N fertilizations on soil microbial biomass and EEA remains unclear. In this work, NH₄NO₃ was chosen as inorganic N, while urea and glycine were chosen as organic N. They were used to fertilize subtropical forest soil monthly for 1 year with different ratios (inorganic N : organic N?=?10 : 0, 7 : 3, 3 : 7 and 1 : 9 respectively.) and N inputs were equivalent to 7.2 g?N?m^?2?y^?1. Soil samples were harvested every 2 months. Subsequently, soil microbial biomass and enzymatic activities were assayed. Multiple regression analysis (MRA) and principle components analysis (PCA) were utilized to illustrate the relationship between soil microbial biomass and EEA. Results showed that soil EEA displayed different changes in response to various mixed N fertilizations. Invertase, cellulase, cellobiohydrolase, alkaline phosphatase, and catalase activities under mixed N fertilization were higher than those of single inorganic N (NH₄NO₃) fertilization. Polyphenol oxidase activities were depressed after inorganic N fertilization and accelerated after mixed N fertilization. Acid phosphatase activities were accelerated in all N fertilization plots, while the influence of various mixed N fertilizations were not significant. Soil microbial biomass was enhanced by mixed N fertilization, while no significant changes were observed after inorganic N fertilization. The result revealed that although N fertilization may alleviate soil N-limitation, single inorganic N fertilization may disturb the balance of inorganic N and organic N, and depress the increases of soil enzymatic activities and microbial biomass in the end. Soil enzymes activities and microbial biomass showed the highest activities after medium organic N fertilization (inorganic : organic N?=?3 : 7), which might be the most suitable N fertilizer for soil microbes. Meanwhile, PCA showed that the alleviation of N-limited reached a maximum after medium organic N fertilization. All results indicated that soil EEA, microbial biomass, and their relationship are all affected by N type and inorganic to organic N ratio.     

Cosolvent effects on gel-entrapped oxidoreductase: the glucose oxidase model

An intrinsic problem often involved in biotransformations carried out by immobilized cells is the poor solubility of substrate and product in water. Increase in hydrophobic substrate availability to such gel-entrapped cells may be attained by the replacement of a fraction of the aqueous medium by water-miscible solvents (cosolvents). The introduction of cosolvents results in increased solubility, but may simultaneously affect enzymic activity and stability. Recently, criteria and guidelines for cosolvent selection on the basis of its effect on intracellular enzyme stability were reported (Freeman, A., and Lilly, M.D. (1987) Appl. Microbiol. Biotechnol. 25, 495-501). In order to understand the impact of the preferable or unsuitable cosolvents on enzyme kinetics and stability, the effects of 1-5 M concentrations of a series of cosolvents (e.g., ethylene glycol, dimethylsulfoxide, N,N-dimethylformamide, ethanol) on a well-characterized, highly specific enzyme model (glucose oxidase) were investigated. The presence of 1-5 M of the cosolvents studied imposed 10-50% reduction in Vmax of the enzyme, but Km was only mildly affected (+/- 25%). This inhibition was attributed to cosolvent effect on small, reversible, conformational changes in the enzyme native structure. Determination of the rate constant of thermal inactivation (at 55 degrees C) of glucose oxidase, in the presence of cosolvents, was employed for the quantitative evaluation of cosolvent effect on enzyme stability. A clear pattern of cosolvent preference in respect to its denaturing effect was obtained, which was identical to the pattern previously observed in a study of oxidoreductases operating from within a whole cell. In both cases diols (e.g., ethylene glycol) were found to be the preferable group of cosolvents. Our results indicate that a soluble enzyme and an intracellular enzyme operating from a whole cell are affected by cosolvents via the same mechanism.     

Enthalpies of DNA melting in the presence of osmolytes

The melting of DNA in the presence of osmolytes has been studied with the intention of obtaining information about how base pair stability is affected by changes in solution conditions. In previous investigations, the melting enthalpies were assumed to be constant as osmolalities change, but no systematic evaluation of whether this condition is true has been offered. This paper presents calorimetric data on the melting of two synthetic DNA samples in the presence of a number of common osmolytes. Poly(dAdT)*poly(dTdA) and poly(dGdC)*poly(dCdG) melting have been examined by differential scanning calorimetry in solutions containing ethylene glycol, glycerol, sucrose, urea, betaine, PEG 200 and PEG 1450 at increasing osmolalities. The results show small, but significant changes in the enthalpy of melting of the two polynucleotides that are different, depending on the structure of the cosolvent. The polyols, ethylene glycol, glycerol, PEG 200 and also urea all show decreases in melting enthalpy, while betaine and sucrose display increases with increasing concentration of cosolvent. The large stabilizing PEG 1450 shows no change within the experimental errors. Using concepts relating to preferential interactions of the cosolvents with the DNA base pairs, it is possible to interpret some of the observed changes in the thermodynamic properties of melting. The results indicate that there is strong entropy-enthalpy compensation upon melting base pairs, but entropy increases dominate to cause the decreases in stability with increased cosolvent concentration. Excess hydration parameters are evaluated and their magnitudes discussed in terms of changes in cosolvent interactions with the DNA base pairs.     

Continuous non-aerated Δ1-dehydrogenation of hydrocortisone by PAAH-bead entrapped Arthrobacter simplex

Summary A continuous non-aerated process for the ¹ of hydrocortisone by gel-entrapped Arthrobacter simplex was developed. The process employs PAAH-bead entrapped cells for the continuous conversion of up to 1.6 g/l hydrocortisone solutions in cosolvent containing buffer. Employing ethyleneglycol (10–20% (v/v)) as the cosolvent of choice and menadione sodium bisulfite as effective, non-toxic, sole electron acceptor, efficient non-aerated continuous production of prednisolone in a packed bed conlumn could be maintained, at least for the 30–40 day period of continuous operation tested. The high operational stability observed was made possible by the combined effect of immobilization technique (PAAH bead entrapment), wisely selected cosolvent and artificial electron acceptor, and elimination of aeration and vigorous mixing.     

玉米生长期间土壤微生物量与土壤酶变化及其相关性研究

研究了玉米生长期间土壤微生物量碳、氮与土壤过氧化氢酶、蔗糖酶、脲酶、蛋白酶活性变化及其相关性．结果表明，玉米生长前期和中期，土壤微生物量碳、氮及酶活性迅速上升，并逐渐达到最大值；玉米生长后期，土壤微生物量碳、氮、酶活性下降至某一值后并逐渐趋于平稳．几种处理相比较，以秸秆＋尿素处理的土壤微生物量碳、氮及酶活性为最大．除玉米生长后期，土壤微生物量碳、氮与碱解氮、活性腐殖质、土壤ｐＨ不相关外，土壤微生物量碳、氮与土壤过氧化氢酶、蔗糖酶、脲酶、蛋白酶活性及速效养分在玉米生长期间均相关或极相关     

Low-temperature reactions of trypsin with p-nitroanilide substrates: tetrahedral intermediate formation or enzyme isomerization

The reactions of trypsin with the p-nitroanilides of N alpha-carbobenzoxy-L-lysine, N alpha-carbobenzoxy-L-arginine, and N alpha-benzoyl-L-arginine have been studied in the 0 to -30 degrees C temperature region, over a range of pH values, using 65% (v/v) aqueous dimethyl sulfoxide cryosolvent. At alkaline pH, -30 degrees C, the catalytic reaction appears as a slow "burst" of product (or intermediate) followed by turnover. For all three substrates, the rate of the burst phase is identical. Preincubation of the enzyme at -30 degrees C abolishes the burst. On addition of trypsin to the cryosolvent at -30 degrees C, a time-dependent decrease in fluorescence emission is observed with the same rate as that of the burst with the anilides. The burst phase is thus interpreted as reflecting a temperature/solvent-induced isomerization of trypsin to a less catalytically efficient form, rather than the previously suggested formation of a tetrahedral intermediate [Compton, P. D., & Fink, A. L. (1980) Biochem. Biophys. Res. Commun. 93, 427-431]. The isomerization is not observed at high temperature (greater than or equal to 0 degree C) or at neutral pH. The burst phase was not observed with aqueous methanol cryosolvent, indicating that it is sensitive to both cosolvent and temperature.     

Influence of ionic liquid cosolvent on transgalactosylation reactions catalyzed by thermostable beta-glycosylhydrolase CelB from Pyrococcus Furiosus

The synthesis of glycosides by enzymatic transglycosylation is a kinetically controlled reaction performed in the context of a non-favorable thermodynamic equilibrium. An unreactive organic cosolvent which increases the selectivity of the enzyme for glycosyl transfer to the acceptor nucleophile compared with water (Ksel) could improve maximum product yield. Here we report on the effect of the ionic liquid 1,3-dimethylimidazoliummethylsulfate on hydrolase and transferase activities of the hyperthermostable beta-glycosidase CelB from the archaeon Pyrococcus furiosus. CelB retained full catalytic efficiency for lactose hydrolysis at 80 degrees C in a 50% (by vol.) solution of ionic liquid in sodium citrate buffer, pH 5.5. It was inactive but not irreversibly denatured at 70% ionic liquid. Using lactose (0.15 M) as galactosyl donor, values of Ksel for a representative series of eight acceptor alcohols were determined in kinetic assays at 80 degrees C and found to increase between 1.3-fold (D-xylose) and 3.1-fold (glycerol) in 45% ionic liquid. Enhancement of Ksel was dependent on ionic liquid concentration and higher than expected from the decrease in water activity caused by the cosolvent. Experimental molar ratios of D-glucose and D-galactose produced during enzymatic conversion of lactose (75-150 mM) in the presence of D-xylose (0.5 M) or glycerol (0.5 M) showed excellent agreement with predictions based on Ksel values and confirm a significant, yet moderate effect of 45% ionic liquid on increasing the yield of D-galactoside product, by < or = 10%.     

A Screening Analysis of the High‐Pressure Extraction of Anthocyanins from Red Grape Pomace with Carbon Dioxide and Cosolvent

In the work described here, a set of screening factorial design experiments was carried out in order to study the extraction process of anthocyanins from red grape pomace with carbon dioxide, along with either methanol or water as cosolvent, at high pressure. The variables studied were pressure, temperature, solvent flow‐rate, cosolvent percentage, cosolvent type (methanol or water) and extraction time. The quantification of the total amount of anthocyanins was performed by a colorimetric method. The results of the ANOVA study show a strong influence of the type and percentage of cosolvent. The best results were obtained with 20 mol‐% of methanol, 100 bar, 60 °C and a flow‐rate of 22 mmol/min after two hours of extraction.     

Chemoenzymatic synthesis of enantiomerically enriched α-hydroxyamides

A study on a chemoenzymatic synthesis of model α-hydroxyamide was performed. Special attention was paid to the optimization of the enzymatic process, both on the selection of enzyme and cosolvent. An intriguing influence of cosolvent on the enantioselectivity of Wheat Germ Lipase and Amano PS Lipase catalyzed hydrolysis was observed, as the results obtained proved that enzyme's enantioselectivity is directly correlated with cosolvent's hydrophobicity. In the best example (Wheat Germ lipase, Et₂O used as a cosolvent), the reaction proceeded with E = 55, and the target compound was obtained in 33% yield with 92.7%ee.     

Enzyme-polyelectrolyte noncovalent complexes as catalysts for reactions in binary mixtures of polar organic solvents with water

Summary The formation of non-covalent complexes with polyelectrolytes has been suggested to enhance the resistance of enzymes towards inactivation by organic solvents in their homogeneous mixtures with water. Existence of such complexes in water-cosolvent media was proved by experiments with a fluorescence dye, eosin. In the case of catalysis by -chymotrypsin, formation of the complex with polyelectrolytes produced two major eflects: i) considerable increase in enzyme activity at concentrations of ethanol and N,N-dimethylformamide of 10–30 % v/v; ii) conservation of the enzymatic activity at cosolvent concentrations of more than 40% v/v, where the native enzyme is completely inactive. General character of the observed activation and stabilization phenomena was shown by example of several experimental systems.     

Helix-enhancing propensity of fluoro and alkyl alcohols: influence of pH, temperature and cosolvent concentration on the helical conformation of peptides.

We have analyzed the effects of trifluoroethanol (TFE) and three other alcohols(1-propanol, 2-propanol and hexafluoro-2-propanol) on S-peptide (residues 1-20) of ribonuclease A, an analog of S-peptide (QHM-->AAA, Sa-peptide) and TC-peptide (residues 295-316) of thermolysin to assess the helix-enhancing propensity of fluoro and alkyl alcohols under different environmental conditions of cosolvent concentration, pH and temperature by circular dichroism (CD). The dependence of cosolvent concentration on helix-induction showed a plateauing effect in all cases. 1-Propanol and 2-propanol were as effective as TFE in all the three peptides. Hexafluoro-2-propanol (HFIP) was a better helix enhancer in all cases however, the relative effectiveness varied with the peptide sequence. The alcohol transitions were analyzed assuming a two-state transition. The free energy decreased linearly in the cosolvent concentration range of 0-5 m for all the three peptides. The m-value (constant of proportionality) varied between peptides but was similar for any given peptide for TFE, 1-propanol or 2-propanol. The m-values of HFIP for all three peptides was much higher compared to other cosolvents. The isothermal cosolvent helix-induction curves for the three peptides exhibited similar features of shape and character for 1-propanol, 2-propanol and TFE. The additivity of cosolvent-induced helix formation was observed for different blends of alkyl and/or fluoro cosolvents. The pH-dependence of helix formation was observed in both TFE and 1-propanol solutions for S-peptide and TC-peptide, respectively, while in Sa-peptide, which was designed to perturb the pH-effect, helix formation was unaffected. The overall results provide some insight into the mechanism of cosolvent-mediated helix-enhancement in protein segments and are likely to facilitate optimization of conditions for cosolvent usage in chemistry and biology.     

Synthesis of lovastatin with immobilized Candida rugosa lipase in organic solvents: Effects of reaction conditions on initial rates

Lipase from Candida rugosa immobilized on a nylon support has been used to synthesize lovastatin, a drug which lowers serum cholesterol levels, by the regioselective acylation of a diol lactone precursor with 2-methylbutyric acid in mixtures of organic solvents. Analogs of lovastatin having a different side chain were also obtained through this method by reacting the diol substrate with different carboxylic acids. The selection of reaction conditions that maximize the initial reaction rate is investigated. Since the diol substrate has very low solubility in non-polar solvents, reaction solvents consisting of mixtures of hexane with a different, more polar cosolvent are considered. For each of the cosolvent mixtures studied, the reaction rate is maximum for an intermediate percentage of cosolvent in hexane. With total concentrations of the diol lactone in the range 6.25-12.5 mM, maximum initial rates correspond approximately to those cosolvent concentrations that permit a complete solubilization of the substrate. At higher cosolvent concentrations, lower rates are obtained. When considering the same dissolved substrate concentration, the reaction rate was found to increase with increasing values of logP(mix) and decreasing values of the dielectric constant, when varying the composition of a binary solvent mixture. However, when comparing different cosolvents, no general trend with respect to these properties was observed. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56:671-680, 1997.     

Response of Dendrobium officinale Kimura et Migo,a Prized Medicinal Plant,to Continuous UV-B Irradiation at Different C/N Ratios

The response of Dendrobium officinale Kimura et Migo (D. officinale) to continuous UV-B irradiation at different carbon to nitrogen ratios (C/N ratios) was investigated. Seedlings grown for 60 days were incubated under aseptic conditions with UV-B irradiation (15.6 µW cm⁻²) at different C/N ratios: control group (CK; C/N 30 without UV-B), UV-B + CK (C/N 30 with UV-B irradiation, similarly hereafter), UV-B + C/N 120, UV-B + C/N 60, UV-B + C/N 15, UV-B + C/N 10, UV-B + C/N 7.5. Growth parameters (the defoliation rate and the sprout number), photosynthetic pigments (carotenoids, chlorophyll a and chlorophyll b), total polysaccharides, total alkaloids, and activities of antioxidant enzymes were determined following 4, 8, 12, and 16 days of continuous UV-B exposure. Results indicated that UV-B irradiation increased the defoliation rate and the content of carotenoids, total polysaccharides and total alkaloids, as well as the activities of antioxidant enzymes. Conversely, UV-B irradiation reduced the sprout number and chlorophyll content in D. officinale. Compared with UV-B + CK, lower C/N ratio treatments (UV-B + C/N 15, UV-B + C/N 10 and UV-B + C/N 7.5) enhanced the defoliation rate and sprout number, but decreased antioxidant enzyme activities and total polysaccharide content during the whole period, and reduced total alkaloid content after 4 days of UV-B exposure. Following initial UV-B irradiation, lower C/N ratios increased the contents of carotenoid and chlorophyll b, while after 8 days, a reversal in carotenoid content was observed, and after 12 days, a reversal in chlorophyll b content was observed. Optimizing the C/N ratio (C/N 60) resulted in lower defoliation rate, higher photosynthetic pigments and total polysaccharides, and increased activities of antioxidant enzymes, whereas no significant change in sprout number and total alkaloid content was recorded under long-term UV-B irradiation. Furthermore, the UV-B + C/N 120 treatment negatively affected D. officinale in terms of an increased defoliation rate and reduced sprout number, photosynthetic pigments, and total alkaloids. Therefore, results suggested that an appropriate C/N ratio (C/N 60) could ameliorate the adverse effects of continuous UV-B irradiation on D. officinale.

     

Response of enzymes and storage proteins of maize endosperm to nitrogen supply

To examine the effects of N nutrition upon endosperm development, maize (Zea mays) kernels were grown in vitro with either 0, 3.6, 7.1, 14.3, or 35.7 millimolar N. Kernels were harvested at 20 days after pollination for determination of enzyme activities and again at maturity for quantification of storage products and electrophoretic separation of zeins. Endosperm dry weight, starch, zein-N, and nonzein-N all increased in mature kernels as N supply increased from zero to 14.3 millimolar. The activities of sucrose synthase, aldolase, phosphoglucomutase, glutamate-pyruvate transaminase, glutamate-oxaloacetate transaminase, and acetolactate synthase increased from 1- to 2.5-fold with increasing N supply. Adenosine diphosphate-glucose pyrophosphorylase and both ATP- and PPi-dependent phosphofructokinases increased to lesser extents, while no significant response was detected for hexose kinases and glutamine synthetase. Nitrogen-induced changes in enzyme activities were often highly correlated with changes in final starch and/or zein-N contents. Separation of zeins indicated that these peptides were proportionately enhanced by N supply, with the exception of C-zein, which was relatively insensitive to N. These data indicate that at least a portion of the yield increase in maize produced by N fertilization is induced by a modification of kernel metabolism in response to N supply.