Ethanolamine Metabolism in Plant Tissues

Ethanolamine is readily metabolized by oat, pea, wheat, apple and carrot tissue preparations. Ethanolamine-1,2 (14)C was incorporated into the lipid fraction, and (14)C activity was distributed in the organic acid, sugar, acid volatile, carbon dioxide and insoluble residue fractions. The distribution varied with the particular tissue. Incorporation into the lipid fraction occurred in tissue homogenates in the absence of ATP by a Ca(++) activated system similar to that reported for animal preparations. The initial step in ethanolamine oxidation involves an amine oxidase. Glycolaldehyde and glyoxylic acid are metabolic intermediates, the former in the conversion of ethanolamine to carbon dioxide. No evidence was obtained for the operation of an ethanolamine transaminase or for the involvement of phosphorylated intermediates in the conversion of ethanolamine to carbon dioxide.     

川西高海拔增温和加氮对红杉凋落物有机组分释放的影响

对川西高山树线红杉新鲜凋落物中有机组分于11月进行自然条件(对照)、加氮(2 g N·m^-2)、增温(顶开式培养室)、加氮+增温4个处理的原位培养,并监测凋落物中有机组分的分解动态.结果表明: 在试验开始后4个月内,增温、加氮以及加氮+增温处理比对照显著促进了红杉凋落物中水溶性糖、水溶性酚和多酚的分解,但随着培养时间的延长,累积分解量的差异逐渐缩小.与对照相比,增温、加氮和增温+加氮处理均抑制红杉凋落物中CH₂Cl₂提取组分、酸溶碳水化合物、酸溶木质素和非酸溶木质素分解,其中增温处理抑制作用最强,加氮处理抑制效果最弱,增温+加氮处理介于二者之间;增温处理对非酸溶木质素和CH₂Cl₂提取组分的半分解周期延长1倍以上,热水溶组分的半分解周期延长50%以上.在原位培养条件下,红杉新鲜凋落物中水溶性糖、水溶性酚、多酚、酸溶碳水化合物、酸溶木质素是较容易分解的有机组分,半分解周期分别为182、159、127、154和190 d;热水溶组分、CH₂Cl₂提取组分和非酸溶木质素是较难分解的有机组分,半分解周期分别是209、302和318 d;尽管低温季节(11月至次年3月)极其寒冷,气温均低于0 ℃,常被认为是微生物活性最弱、有机物分解最慢的时期,但结果显示低温季节期间红杉凋落物各有机组分却分解最快.因此,氮沉降和升温将迟滞该区域高寒红杉林凋落物的分解.这将有利于高寒森林生态系统的土壤碳固持.     

不同温度条件下杉木、桤木和火力楠细根分解对土壤活性有机碳的影响

通过室内培养试验,研究了不同温度(9 ℃、14 ℃、24 ℃和28 ℃)条件下桤木、杉木和火力楠细根分解对土壤活性有机碳的影响.结果表明,不同树种细根的分解率不同,树种间差异显著,大小依次为火力楠>桤木>杉木.细根分解率随着培养温度的增加而增大,随着培养时间的延长而降低.添加细根的种类、培养温度和培养时间均对实验系统中土壤微生物碳和水溶性有机碳的含量产生影响.3个树种细根分解使土壤微生物碳和水溶性有机碳含量显著高于对照,大小依次为火力楠>桤木>杉木>对照; 培养中期以及中等培养温度条件下细根分解对应着较高的土壤微生物碳和水溶性有机碳含量.细根分解对土壤易氧化碳含量无显著影响.     

Biochemical origin and refractory properties of humic acid extracted from maize plants: the contribution of lignin

The soil organic carbon (SOC) pool is the largest terrestrial reservoir of carbon and plant residues play an important role in its maintenance. Up to 70–80% of SOC in arable soil is composed of humic substances (HS). In these soils post-harvested residues, left in arable soil after harvesting the crops, are the basic source of humus. Previous research indicated that maize plants residue contain a humic acid (HA) fraction possessing recalcitrant compounds that contributed to soil-HA fraction. This study presents updated results obtained using Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS) to provide an indication of the contribution of the lignin to the soil HA. Results obtained indicated that the HAs from maize plants were mainly composed of lignin-derived moieties that were likely derived from the partial hydrolysis of p-coumaric and ferulic acid that are linked to lignin, polysaccharides or other biopolymers of the cell wall. Lignin composing the HAs derived from plants and incubated in soil was substantially preserved. Nevertheless the modification of the syringyl/guaiacyl ratio and the oxidation of the side-chains of lignin, suggested a turnover of lignin-derived molecules in soil-HA fraction. This fact indicated an involvement of the alkali insoluble fraction of maize plant residue (humin) in the soil-HA formation, up-dating our previous knowledge.     

Proteomics Analysis of the Nacre Soluble and Insoluble Proteins from the Oyster Pinctada margaritifera

Shell nacre is laid upon an organic cell-free matrix, part of which, paradoxically, is water soluble and displays biological activities. Proteins in the native shell also constitute an insoluble network and offer a model for studying supramolecular organization as a means of self-ordering. Consequently, difficulties are encountered in extraction and purification strategies for protein characterization. In this work, water-soluble proteins and the insoluble conhiolin residue of the nacre of Pinctada margaritifera matrix were analyzed via a proteomics approach. Two sequences homologous to nacre matrix proteins of other Pinctada species were identified in the water-soluble extract. One of them is known as a fundamental component of the insoluble organic matrix of nacre. In the conchiolin, the insoluble residue, four homologs of Pinctada nacre matrix proteins were found. Two of them were the same as the molecules characterized in the water-soluble extract. Results established that soluble and insoluble proteins of the nacre organic matrix share constitutive material. Surprisingly, a peptide in the conchiolin residue was found homologous to a prismatic matrix protein of Pinctada fucata, suggesting that prismatic and nacre matrices may share common proteins. The insoluble properties of shell matrix proteins appear to arise from structural organization via multimerization. The oxidative activity, found in the water-soluble fraction of the nacre matrix, is proposed as a leading process in the transformation of transient soluble proteins into the insoluble network of conchiolin during nacre growth.     

Differential phytotoxicity among species and cultivars of the genus Brassica to wheat

Classhouse and laboratory studies were conducted to investigate the effect of microorganisms on the activity of water-soluble phytotoxins from Brassica residues, and on the persistence of the phytotoxins. Warm temperatures (20–24°C) and long incubation periods (20 to 40 days) were conductive to reduced phytotoxicity while low temperatures (0–4°C, regardless of incubation time), or short incubation time (regardless of temperature) resulted in levels of phytotoxicity similar to that found without incubation. The removal of microbial populations by micro-filtration resulted in the maintenance of phytotoxicity, regardless of the incubation conditions. The quantity of residues or extracts from residues used in a pot experiment and the soil type (sand or clay soil) determined the degree of phytotoxicity. Generally, the greater the quantity of residues or extracts, the greater the toxicity, with residues being more toxic than extracts from the same rate of residue. These observations agree with the general literature that the level of toxicity is determined by the quantity of residue present, and the rate of decline in the toxicity of water-soluble toxins is dependent on the microbial populations present, and their level of activity.     

Ecologically significant effects of Pseudomonas putida PPO301(pRO103), genetically engineered to degrade 2,4-dichlorophenoxyacetate, on microbial populations and processes in soil

Pseudomonas putida PPO301 (pRO103), genetically engineered to degrade 2,4-dichlorophenoxyacetate, affected microbial populations and processes in a nonsterile xeric soil. In soil amended with 2,4-dichlorophenoxyacetate (500 micrograms/g soil) and inoculated with PPO301 (pRO103), the rate of evolution of carbon dioxide was retarded for approximately 35 days; there was a transient increase in dehydrogenase activity; and the number of fungal propagules decreased below detection after 18 days. In unamended soil inoculated with PPO301(pRO103), the rate of evolution of carbon dioxide and the dehydrogenase activity were unaffected, and the numbers of fungal propagules were reduced by about two orders of magnitude. The numbers of total, spore-forming, and chitin-utilizing bacteria were reduced transiently in soil either amended or unamended with 2,4-dichlorophenoxyacetate and inoculated with PPO301(pRO103). The activities of arylsulfatases and phosphatases in soil were not affected by the presence of PPO301(pRO103), either in the presence or absence of 2,4-dichlorophenoxyacetate. In soil amended with 2,4-dichlorophenoxyacetate and inoculated with the parental strain (PPO301) or not inoculated, the evolution of carbon dioxide, the numbers of fungal propagules and of total, spore-forming, and chitin-utilizing bacteria, and the dehydrogenase activity were not affected as in soil inoculated with PPO301(pRO103). These results demonstrated that a genetically engineered microorganism, in the presence of the substrate on which its novel genes can function, is capable of inducing measurable ecological effects in soil.     

Energy metabolism of beating rat heart cell cultures: II. - Glucose metabolism

Glycogen metabolism, lactate excretion and carbon dioxide production from uniformly labelled glucose were studied on beating rat heart cell cultures in relation to medium replenishment. Only a small fraction of labelled glucose was incorporated into glycogen and then released at a high rate, while exogenous glucose was still continuously taken up by the cells. When glucose in the medium was nearly depleted, glycogen degradation slowed down. The rate of carbon flow through phosphopentose shunt showed a 600 p. cent increase several hours before DNA synthesis. Up to 24 hours after medium renewal, the cells excreted high amounts of lactate; then lactate was reutilized, probably through transaminating processes. Up to 10 hours after medium renewal, labelled carbon dioxide production from exogenous labelled glucose decreased strongly and then increases. Only one of these variations (pentose phosphate shunt) could be attributed to the release of topoinhibition by serum. The other three also occured in the absence of serum and probably resulted from the variation in substrate avaibility induced by medium replenishment.     

Bound auxin formation in growing stems

The term “bound auxin” is herein used to describe auxin conjugates insoluble in organic solvents which dissolve indoleacetic acid (IAA) and its derivatives, but hydrolyzable by NaOH to release IAA. Bound auxin from pea stems was fractionated into water-soluble, water-insoluble/NaOH-hydrolyzable, and insoluble components. Formation of bound auxin commenced with 15 minutes of applying exogenous labeled IAA, and progressively increased in amount, relative to IAA uptake, over 6 hours. Formation was not restricted to any particular zone of the stem and occurred in both light- and dark-grown stems. A greater quantity of bound auxin was formed in light-grown stems, reaching 4.2 and 7.7%, of the IAA taken up, in the water-soluble and water-insoluble/NaOH-hydrolyzable fractions after 6 hours. The presence of sucrose, during either the IAA treatment or an aging pretreatment had no effect, though 6 hours aging did cause a subsequent increase in the water-insoluble fraction of the bound auxin. Bound auxin formation in light-grown stems was dependent on respiratory metabolism, being reduced by KCN. It was also reduced, compared to total uptake, by inhibitors of RNA, and protein synthesis (6-methylpurine and cycloheximide) but only when the inhibitors preceded auxin addition and were present for a 4-hour period. Addition of inhibitors following auxin had no effect, suggesting an early inductive effect of auxin on bound auxin formation. Inhibitors of cell elongation had no effect. Deoxyglucose, an inhibitor of glucan synthesis, had only a small effect on the water-soluble fraction. Bound auxin is an important auxin product in growing plants. Its function is unknown, but some possibilities are discussed.     

Nitrogen released from different plant residues of the Loess Plateau and their additions on contents of microbial biomass carbon, nitrogen in soil

An incubation method was used to investigate the nitrogen release characteristics from the residue of ten plant species which commonly grow in the northern part of the Loess Plateau. The effect of the residue on soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN) was also determined. There were significant differences in the total N content and the C/N ratios among the different types of plant residue. The total N content of the residues ranged from 6.61 to 32.78 g kg^?1. The C/N ratio of the residue ranged from 14 to 65. There was an immediate increase in soil N after alfalfa, erect milkvetch, and korshinsk peashrub residue was added to the soil. In contrast, soil N decreased after elm, sea buckthorn, and wild peach residue was added to the soil. The soil N content remained relatively low for 14–34 days and then increased. This indicated that N immobilization occurred during the early portion of the incubation period when elm, sea buckthorn and wild peach residue was added to the soil. Soil N levels were low during the entire incubation period when simon poplar, locust, Stipa bungeana, and old world bluestem residue were added to the soil. The addition of plant residue significantly increased SMBC and SMBN in all treatments. The SMBC and SMBN values were greatest in treatments containing plant residue with high total N content and low C/N ratios. The C/N ratios of korshinsk peashrub, sea buckthorn, and wild peach residues were similar, but the amount of N released from these residues and the effects of the residue on SMBC and SMBN in soil were significantly different. This indicates that not only the C/N ratio but also the chemical composition of the plant residue affected decomposition. It is important to consider C and N release characteristics from plant residue in order to adjust the C and N balance of soil when revegetating degraded ecosystems.     

玉米和大豆秸秆还田初期对黑土CO2排放的影响

通过恒温培养试验,研究了不同类型秸秆还田后的土壤CO₂排放特征及其与秸秆C、N含量的关系,以明晰黑土区不同类型秸秆还田后的分解特征,探明还田秸秆的C、N含量对固碳效果的影响.结果表明： 在61 d的培养试验中,土壤CO₂排放速率随时间呈现出“下降 稳定 增大(出现‘较高值’) 下降”的过程.不同类型秸秆还田后土壤CO2排放速率随时间变化的特征存在明显差异,主要体现在“较高值”出现和持续的时间不同.秸秆类型对土壤CO₂累积排放量具有显著影响,前21 d和前61 d的土壤CO2累积排放量对秸秆添加的响应不同.在前21 d,玉米根、玉米茎下部、玉米叶、大豆叶的CO₂累积排放量(约160 μmol·g^-1)显著大于其他秸秆;而除大豆叶外,大豆秸秆61 d的CO₂累积排放量均比玉米秸秆大.前21 d CO₂累积排放量与秸秆含碳量的比值(CR)和秸秆的C/N、含氮量之间均呈显著的线性相关;而61 d的CO₂累积排放量与秸秆的C、N含量之间不存在线性关系.综上,在还田条件下,秸秆类型对土壤CO₂的排放有明显影响;大豆秸秆比玉米秸秆容易分解,但与长时间分解不同,大豆秸秆还田最初阶段的分解速率小于玉米秸秆;秸秆的C/N、含氮量只对还田最初阶段的土壤CO₂排放有较大影响.     

玉米和大豆秸秆还田初期对黑土CO2排放的影响

通过恒温培养试验,研究了不同类型秸秆还田后的土壤CO₂排放特征及其与秸秆C、N含量的关系,以明晰黑土区不同类型秸秆还田后的分解特征,探明还田秸秆的C、N含量对固碳效果的影响.结果表明： 在61 d的培养试验中,土壤CO₂排放速率随时间呈现出“下降 稳定 增大(出现‘较高值’) 下降”的过程.不同类型秸秆还田后土壤CO2排放速率随时间变化的特征存在明显差异,主要体现在“较高值”出现和持续的时间不同.秸秆类型对土壤CO₂累积排放量具有显著影响,前21 d和前61 d的土壤CO2累积排放量对秸秆添加的响应不同.在前21 d,玉米根、玉米茎下部、玉米叶、大豆叶的CO₂累积排放量(约160 μmol·g^-1)显著大于其他秸秆;而除大豆叶外,大豆秸秆61 d的CO₂累积排放量均比玉米秸秆大.前21 d CO₂累积排放量与秸秆含碳量的比值(CR)和秸秆的C/N、含氮量之间均呈显著的线性相关;而61 d的CO₂累积排放量与秸秆的C、N含量之间不存在线性关系.综上,在还田条件下,秸秆类型对土壤CO₂的排放有明显影响;大豆秸秆比玉米秸秆容易分解,但与长时间分解不同,大豆秸秆还田最初阶段的分解速率小于玉米秸秆;秸秆的C/N、含氮量只对还田最初阶段的土壤CO₂排放有较大影响.     

Translocation of C in the sugarcane plant during the day and night

The time-course of translocation of ¹⁴C from the blades of the sugarcane plant was investigated by analysis and radioactive counting of successive samples punched from a single blade. In 1 experiment, the time-course was studied by determining the specific activity of the carbon dioxide respired by the roots.

The rate of translocation, expressed as percentage, was highest immediately after the application of the radioactive carbon dioxide. Morning-made photosynthate translocated a higher percentage during the morning than during the afternoon in 90-minute periods in the light. Afternoon-made photosynthate translocated as well or better than morning-made photosynthate for the first hour in the light.

The leaf-disk data and the specific activity of the carbon dioxide respired by the roots corresponded by showing lower rates of translocation by night than by day for several successive days. Also, the translocation of ¹²C sucrose was slower at night.

The ¹⁴C sucrose translocated by day was made primarily by photosynthesis; the sucrose translocated by night was made primarily by the conversion of other labeled compounds, e.g. organic acids, organic phosphates, and insoluble residue.

The radioactive constituent of the residue, which was converted to sucrose, was tentatively identified as a glucose-xylose-glucuronic acid hemicellulose, with most or all of the ¹⁴C in the glucose moiety.

Translocation of sucrose may be triggered by different mechanisms during the night than the day. The conversion of insoluble residue to sucrose by increasing the osmotic potential at the source would favor a pressure-flow mechanism for nocturnal translocation; whereas translocation by day is thought to be a process of phototranslocation, a photoactivation of the translocation mechanism.

     

Use of a sequencing batch reactor to study the biodegradation of 4-chlorophenol in soil

Summary A sequencing batch reactor (SBR) was used to study the biodegradation of 4-chlorophenol (4CP) in a soil slurry. The SBR system was controlled by measuring the carbon dioxide evolution rate (CER) in the gas phase. The biodegradation rate was increased from 3.2 to 67 mg 4CP/l·h after 13 cycles.     

Turnover of cell wall polysaccharides in elongating pea stem segments

Turnover of cell wall polysaccharides and effects of auxin thereon were examined after prelabeling polysaccharides by feeding pea (Pisum sativum var. Alaska) stem segments ¹⁴C-glucose, then keeping the tissue 7 hours in unlabeled glucose with or without indoleacetic acid. There followed an extraction, hydrolysis, and chromatography procedure by which labeled monosaccharides and uronic acids were released and separated with consistently high recovery. Most wall polymers, including galacturonan and cellulose, did not undergo appreciable turnover. About 20% turnover of starch, which normally contaminates cell wall preparations but which was removed by a preliminary step in this procedure, occurred in 7 hours. Quantitatively, the principal wall polymer turnover process observed was a 50% decrease in galactose in the pectinase-extractable fraction, including galactose attached to a pectinase-resistant rhamnogalacturonan. Other pectinase-resistant galactan(s) did not undergo turnover. No turnover was observed in arabinans, but a doubling of radioactivity in arabinose of the pectinase-resistant, hot-acid-degradable fraction occurred in 7 hours, possibly indicating conversion of galactan into arabinan. None of the above changes was affected by indoleacetic acid, but a quantitatively minor turnover of a pectinase-degradable xyloglucan was found to be consistently promoted by indole-acetic acid. This was accompanied by a reciprocal increase in water-soluble xyloglucan, suggesting that indoleacetic acid induces conversion of wall xyloglucan from insoluble to water-soluble form. The results indicate a highly selective pattern of wall turnover processes with an even more specific influence of auxin.     

Sulfide Alleviation of the Acetylene Inhibition of Nitrous Oxide Reduction in Soil

The alleviation of the acetylene blockage of nitrous oxide reduction by sulfide was studied in anaerobically incubated Brookston soil to better characterize the process. Removal of nitrate-derived nitrous oxide from soil amended with acetylene and sulfide occurred earlier in the presence of glucose than it did in its absence. This was attributed to the influence of glucose on nitrous oxide production rather than reduction during the early stages of the soil incubation. Glucose was found to have no effect on reduction of injected nitrous oxide in the presence of acetylene- and sulfide-amended soil, whereas carbon dioxide significantly stimulated reduction. It is suggested that the microorganism(s) involved may use carbon dioxide as a cellular carbon source. The sulfide added to the soil probably did not act solely as an electron donor, as the number of electrons required to reduce the added nitrous oxide in our systems was greater than the amount supplied by the sulfide. The soil pH at which the alleviation occurred was 6.7 and was not affected by the sulfide treatment.     

Residue fractionation and decomposition: The significance of the active fraction

This paper describes an incubation experiment with homogeneously ¹⁴C labeled maize-straw and its insoluble fraction. The role of the soluble fraction in the decomposition process was assessed, using three independently measured characteristics: (1) fractionation of the maize-straw, resulting in kinetically different fractions; (2) microbial biomass C and its ¹⁴C activity determined by a fumigation extraction method, and (3) the ¹⁴C activity of the released CO₂-C. The fumigation extraction method was proved to be useful from 9 days after the application of the maize-straw onwards. The fractionation method yielded a soluble (48%), a (hemi) cellulosic (47%), and a lignin fraction (1%). Nine days after addition of either the complete residue or its insoluble fraction, the microbial biomass C increased from 53 to 337 and 217 mg C kg^-1 dry soil, respectively. Similar values were maintained up to day 40. The large increase in microbial activity was accompanied by a N-immobilization of 65 and 29 mg N Kg^-1 dry soil for the maize-straw treatment and its insoluble fraction, respectively, resulting in biomass C/N values of 5.5 and 5.6 A genuine priming effect (10 and 7% of the total CO₂-C production) on the mineralization of native soil organic C was caused by an increase in decomposition of the native C rather than by an increase in turnover of the microbial biomass in the soil amended with maize straw. The soluble fraction caused a priming effect on the decomposition of the less decomposable cell-wall fraction. Calculations by nonlinear regression confirmed this observation.     

Determination of carbon dioxide evolution rate using on-line gas analysis during dynamic biodegradation experiments

Respirometry is a precious tool for determining the activity of microbial populations. The measurement of oxygen uptake rate is commonly used but cannot be applied in anoxic or anaerobic conditions or for insoluble substrate. Carbon dioxide production can be measured accurately by gas balance techniques, especially with an on-line infrared analyzer. Unfortunately, in dynamic systems, and hence in the case of short-term batch experiments, chemical and physical transfer limitations for carbon dioxide can be sufficient to make the observed carbon dioxide evolution rate (OCER) deduced from direct gas analysis very different from the biological carbon dioxide evolution rate (CER).To take these transfer phenomena into account and calculate the real CER, a mathematical model based on mass balance equations is proposed. In this work, the chemical equilibrium involving carbon dioxide and the measured pH evolution of the liquid medium are considered. The mass transfer from the liquid to the gas phase is described, and the response time of the analysis system is evaluated.Global mass transfer coefficients (K(L)a) for carbon dioxide and oxygen are determined and compared to one another, improving the choice of hydrodynamic hypotheses. The equations presented are found to give good predictions of the disturbance of gaseous responses during pH changes.Finally, the mathematical model developed associated with a laboratory-scale reactor, is used successfully to determine the CER in nonstationary conditions, during batch experiments performed with microorganisms coming from an activated sludge system. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 53: 243-252, 1997.     

Effect of Oxygen Free Radicals on Corneal Collagen

Corneal collagen was labeled in vivo by injection of ¹⁴C-proline into the anterior chamber of rabbit eyes. The isolated corneal collagen was incubated in iron-free phosphate buffered saline (pH 7.4) containing I mM axorbate and 0.1 mM CuSO₄ for either 1 hour or 3 hours at 37°. Addition of 2 volumes of 8 M urea-I mM dithiothreitol and heating for 1 min at 100° solubilized virtually all of the collagen in the control incubations but left a significant amount of insoluble collagen in specimens exposed to the hydroxyl radical generating system. This residue amounted to 19% and 38% of the initial radioactivity in samples incubated for 1 h and 3 h, respectively. The chromatographic profiles (gel filtration on CL-4B) of the soluble fraction showed an increase in both aggregation and degradation products of collagen in the 1 h incubation mixture, whereas after 3 h there was an increase only in degradation products. These observations suggest that additional crosslinking of the soluble collagen aggregates observed at 1 h may be responsible for their subsequent disappearance at 3 h, with concomitant increase of the insoluble fraction. Collagen degradation by OH may play a role in corneal ulceration, whereas hydroxyl radical-mediated crosslinking is consistent with age-dependent increases in insoluble collagen.     

Control of photosynthesis during nitrogen depletion and recovery in a non-nitrogen-fixing cyanobacterium

When cells of Synechocystis strain PCC 6308 are starved for nitrogen, the amount of stored carbohydrate increases, the phycocyanin to chlorophyll a ratio decreases, and the rates of oxygen evolution and of carbon dioxide fixation decrease. When nitrate-nitrogen is replenished, the amount of carbohydrate decreases, the rate of oxygen evolution increases immediately, preceeding the increase in phycocyanin or carbon dioxide fixation. The rate of respiration first increases and then decreases upon nitrogen addition. Nitrogen-starved cells show no variable fluorescence; variable fluorescence recovered in parallel with oxygen evolution. This suggests that photosystem II is inactive in nitrogen depleted cells and not blocked by a build up of metabolic endproducts. Since carbon dioxide fixation does not increase until two to four hours after nitrate is replenished to nitrogen starved cells, it is suggested that reducing power may first be needed within the cell for some other process than photosynthesis, such as nitrate reduction.