Ribulose Diphosphate Carboxylase Research Articles

The complex stress of waterlogging and high temperature accelerated maize leaf senescence and decreased photosynthetic performance at different growth stages

AbstractThe greenhouse effect caused by global warming was becoming more and more obvious, resulting in increased frequency of high temperature and high humidity, which significantly affected maize productivity. However, it was poorly understood how the interactions of high temperature and high humidity affected leaf senescence, photosynthetic performance and yield of summer maize. Three stress treatments including (a) high temperature stress (T), (b) waterlogging stress (W) and (c) complex stress (T‐W) were set at the third leaf stage (V3), the sixth leaf stage (V6) and the tasselling stage (VT) in 2019–2020 to explore the influence mechanism of complex stress. Each stress treatment period lasted 6 days. Non‐stressed plants served as control. Yield, antioxidant enzyme activity, photosynthetic characteristics, and dry matter accumulation were determined. Our study found that the activity of antioxidant enzymes was significantly decreased, while malonyldialdehyde (MDA) accumulation was increased under each stress treatment. As a result, the photosynthetic characteristics were impaired, manifested in a significant decrease in net photosynthetic rate (Pn), enzyme activities of phosphoenolpyruvate carboxylase (PEPCase) and ribulose diphosphate carboxylase (RUBPCase). The decrease in photosynthetic intensity affected by each stress treatment led to a significant decrease in total dry matter accumulation and grain yield. The most significant effects of waterlogging and combined stresses on yield occurred at the V3 stage, followed by the V6 and VT stages. However, the most significant effects of high temperature occurred at the VT stage, followed by the V6 and V3 stages. Moreover, the compound stress exacerbated damage to leaf senescence and photosynthetic properties of summer maize compared to the single stress of high temperature or waterlogging.

Summer dormancy of Myricaria laxiflora to escape flooding stress: Changes in phytohormones and enzymes induced by environmental factors

Dormancy is an adaptation mechanism of plants to environmental stress. Myricaria laxiflora undergoes a long period of flooding stress every year. In order to determine whether this species escapes flooding stress through dormancy, young branches and leaves were collected at different time points before the onset of flooding, and changes in the content/activity of hormones/enzymes that are closely involved in plant growth were monitored. The inducing environmental factors of summer dormancy were identified. The branches and leaves of M. laxiflora showed the following trends as summer flooding approached: (1) gradual increase in the abscisic acid content; (2) gradual decrease in the gibberellin and cytokinin contents; and (3) a continuous decrease in the activities of malate dehydrogenase (MDH), ribulose diphosphate carboxylase (RuBisCo), and glycolate oxidase (GLO). Pearson correlation analysis revealed (1) daylight duration was highly correlated with the hormone content and enzyme activity; (2) the daily mean air temperature (DMAT) was significantly correlated with the cytokinin content. These findings suggest that daylight duration was the main environmental factor leading to changes in the phytohormone content and enzyme activity as well as leading to summer dormancy. M. laxiflora undergoes dormancy before the onset of summer flooding to escape summer flooding stress. Our data indicate that summer flooding does not impede the survival and growth of M. laxiflora.

Heat Stress After Pollination Reduces Kernel Number in Maize by Insufficient Assimilates.

Global warming has increased the occurrence of high temperature stress in plants, including maize, resulting in decreased the grain number and yield. Previous studies indicate that heat stress mainly damages the pollen grains and thus lowered maize grain number. Other field studies have shown that heat stress after pollination results in kernel abortion. However, the mechanism by which high temperature affect grain abortion following pollination remains unclear. Hence, this study investigated the field grown heat-resistant maize variety “Zhengdan 958” (ZD958) and heat-sensitive variety “Xianyu 335” (XY335) under a seven-day heat stress treatment (HT) after pollination. Under HT, the grain numbers of XY335 and ZD958 were reduced by 10.9% (p = 0.006) and 5.3% (p = 0.129), respectively. The RNA sequencing analysis showed a higher number of differentially expressed genes (DEGs) between HT and the control in XY335 compared to ZD958. Ribulose diphosphate carboxylase (RuBPCase) genes were downregulated by heat stress, and RuBPCase activity was significantly lowered by 14.1% (p = 0.020) in XY335 and 5.3% (p = 0.436) in ZD958 in comparison to CK. The soluble sugar and starch contents in the grains of XY335 were obviously reduced by 26.1 and 58.5%, respectively, with no distinct change observed in ZD958. Heat stress also inhibited the synthesis of grain starch, as shown by the low activities of metabolism-related enzymes. Under HT, the expression of trehalose metabolism genes in XY335 were upregulated, and these genes may be involved in kernel abortion at high temperature. In conclusion, this study revealed that post-pollination heat stress in maize mainly resulted in reduced carbohydrate availability for grain development, though the heat-resistant ZD958 was nevertheless able to maintain growth.

Decision Tree Algorithm–Generated Single-Nucleotide Polymorphism Barcodes of rbcL Genes for 38 Brassicaceae Species Tagging

DNA barcode sequences are accumulating in large data sets. A barcode is generally a sequence larger than 1000 base pairs and generates a computational burden. Although the DNA barcode was originally envisioned as straightforward species tags, the identification usage of barcode sequences is rarely emphasized currently. Single-nucleotide polymorphism (SNP) association studies provide us an idea that the SNPs may be the ideal target of feature selection to discriminate between different species. We hypothesize that SNP-based barcodes may be more effective than the full length of DNA barcode sequences for species discrimination. To address this issue, we tested a ribulose diphosphate carboxylase (rbcL) SNP barcoding (RSB) strategy using a decision tree algorithm. After alignment and trimming, 31 SNPs were discovered in the rbcL sequences from 38 Brassicaceae plant species. In the decision tree construction, these SNPs were computed to set up the decision rule to assign the sequences into 2 groups level by level. After algorithm processing, 37 nodes and 31 loci were required for discriminating 38 species. Finally, the sequence tags consisting of 31 rbcL SNP barcodes were identified for discriminating 38 Brassicaceae species based on the decision tree–selected SNP pattern using RSB method. Taken together, this study provides the rational that the SNP aspect of DNA barcode for rbcL gene is a useful and effective sequence for tagging 38 Brassicaceae species.

Sink-source relationship of potato plants and its role involved in the reduction of tuber yield in continuous cropping system

The Yellow River irrigation area in middle Gansu Province is one of the main production bases for processing potato and potato tuber seeds in China. However, continuous potato cropping (CPC), resulting from intensive cultivation, has been affecting the sound development of the potato industry. A long-term field experiment was carried out in order to reveal the mechanisms of CPC obstacles. Five treatments, with different years of continuous potato cropping, were designed marking as 0-5 a, 0 a was maize-potato rotation, used as the control (CK). The present study focused on how to change in sink size and sink activity as well as source activity of potato plants under CPC conditions, especially their roles involved in the reduction of tuber yield. There were no significant differences in tuber yield under short-term CPC (1-2 a) compared with CK, however, significant decline by 28.6%-32.8% occurred under long-term CPC (3-5 a), which was mainly derived from the decline in fresh mass of each tuber. Compared with CK, long-term CPC significantly decreased sink size by 38.4%-53.0%. In addition, long-term CPC not only postponed the potato development progress, by postponing the formation and development of potato tubers, but reduced dry matter accumulation in tubers as well. Long-term CPC significantly decreased source activity, showing that plant height, branch numbers per main stem, chlorophyll content, and dry-matter content of leaf were significantly lower than those of CK, besides, morphological development of root system was also restrained. Compared with CK, root vigor, ribulose diphosphate carboxylase (RuBP Case) and sucrose phosphate synthase (SPS) activities of leaves under long-term CPC significantly decreased by 28.6%-63.1%, 52.6%-64.6% and 26.3%-53.4%, respectively. Long-term CPC caused signi-ficant decline in production capability of source, consequently, reduced the production of assimilation product by a large margin, and contributed to the deficiency in translocation amount of assimilates into tuber during post-anthesis, which finally led to the reduction in tuber yield. In conclusion, the unbalance of sink-source relationship of potato plants was the main cause for CPC obstacles in the Yellow River irrigation area in middle Gansu Province.

DNA barcode for the identification of the sand fly Lutzomyia longipalpis plant feeding preferences in a tropical urban environment.

Little is known about the feeding behavior of hematophagous insects that require plant sugar to complete their life cycles. We studied plant feeding of Lutzomyia longipalpis sand flies, known vectors of Leishmania infantum/chagasi parasites, in a Brazilian city endemic with visceral leishmaniasis. The DNA barcode technique was applied to identify plant food source of wild-caught L. longipalpis using specific primers for a locus from the chloroplast genome, ribulose diphosphate carboxylase. DNA from all trees or shrubs within a 100-meter radius from the trap were collected to build a barcode reference library. While plants from the Anacardiaceae and Meliaceae families were the most abundant at the sampling site (25.4% and 12.7% of the local plant population, respectively), DNA from these plant families was found in few flies; in contrast, despite its low abundance (2.9%), DNA from the Fabaceae family was detected in 94.7% of the sand flies. The proportion of sand flies testing positive for DNA from a given plant family was not significantly associated with abundance, distance from the trap, or average crown expansion of plants from that family. The data suggest that there may indeed be a feeding preference of L. longipalpis for plants in the Fabaceae family.

Potassium Internal Use Efficiency Relative to Growth Vigor, Potassium Distribution, and Carbohydrate Allocation in Rice Genotypes

Potassium (K) deficiency in soils is widespread and has become one of the most nutritional limiting factors for increasing rice yield in China and Asia. Improving K use efficiency by exploiting genetic potential largely depends on understanding physiological and genetic mechanisms underlying K efficiency in lowland rice. In this study, the K internal use efficiency (KIUE) at low K supply relative to K accumulation and distribution, tillering and grain filling rates, dry matter accumulation and allocation were examined under field conditions. The relative K concentrations in upper and lower leaves at booting and grain filling stages decreased with decreasing K internal use efficiency among the rice genotypes, indicating that K-efficient genotypes had greater K translocation ability than the K-inefficient genotypes at low K. The KIUE in biomass production at seedling and tillering stages was closely and positively correlated with the KIUE in grain production at the harvest stage. Significant positive correlations were noted between the KIUE at low K and the relative dry matter yield at the tillering stage and the harvest index among the rice genotypes. The K-efficient genotypes had a greater relative tillering rate during the tillering stage and a greater relative grain filling rate at the late grain filling stage, as compared with the inefficient genotypes. The relative net photosynthetic rate in the upper, and especially in the lower leaves was much lower in the K-inefficient genotypes than in the efficient ones at the grain filling stage, which may partly be due to the fact that the efficient genotype could maintain greater stomatal conductance and ribulose diphosphate carboxylase (RuBPcase) activity in functioning leaves under low K. The results indicate that (i) the K internal use efficiency could be an important index for measuring rice tolerance to low K stress, and (ii) K internal use efficiency is closely associated with efficient translocation and distribution of both K and carbohydrate within rice plants.

A multi-laboratory evaluation of a common in vitro pepsin digestion assay protocol used in assessing the safety of novel proteins

Rationale. Evaluation of the potential allergenicity of proteins derived from genetically modified foods has involved a weight of evidence approach that incorporates an evaluation of protein digestibility in pepsin. Currently, there is no standardized protocol to assess the digestibility of proteins using simulated gastric fluid. Potential variations in assay parameters include: pH, pepsin purity, pepsin to target protein ratio, target protein purity, and method of detection. The objective was to assess the digestibility of a common set of proteins in nine independent laboratories to determine the reproducibility of the assay when performed using a common protocol. Methods. A single lot of each test protein and pepsin was obtained and distributed to each laboratory. The test proteins consisted of Ara h 2 (a peanut conglutin-like protein), β-lactoglobulin, bovine serum albumin, concanavalin A, horseradish peroxidase, ovalbumin, ovomucoid, phosphinothricin acetyltransferase, ribulose diphosphate carboxylase, and soybean trypsin inhibitor. A ratio of 10 U of pepsin activity/μg test protein was selected for all tests (3:1 pepsin to protein, w:w). Digestions were performed at pH 1.2 and 2.0, with sampling at 0.5, 2, 5, 10, 20, 30, and 60 min. Protein digestibility was assessed from stained gels following SDS–PAGE of digestion samples and controls. Results. Results were relatively consistent across laboratories for the full-length proteins. The identification of proteolytic fragments was less consistent, being affected by different fixation and staining methods. Overall, assay pH did not influence the time to disappearance of the full-length protein or protein fragments, however, results across laboratories were more consistent at pH 1.2 (91% agreement) than pH 2.0 (77%). Conclusions. These data demonstrate that this common protocol for evaluating the in vitro digestibility of proteins is reproducible and yields consistent results when performed using the same proteins at different laboratories.

The methionine salvage pathway in Bacillus subtilis

BackgroundPolyamine synthesis produces methylthioadenosine, which has to be disposed of. The cell recycles it into methionine through methylthioribose (MTR). Very little was known about MTR recycling for methionine salvage in Bacillus subtilis.ResultsUsing in silico genome analysis and transposon mutagenesis in B. subtilis we have experimentally uncovered the major steps of the dioxygen-dependent methionine salvage pathway, which, although similar to that found in Klebsiella pneumoniae, recruited for its implementation some entirely different proteins. The promoters of the genes have been identified by primer extension, and gene expression was analyzed by Northern blotting and lacZ reporter gene expression. Among the most remarkable discoveries in this pathway is the role of an analog of ribulose diphosphate carboxylase (Rubisco, the plant enzyme used in the Calvin cycle which recovers carbon dioxide from the atmosphere) as a major step in MTR recycling.ConclusionsA complete methionine salvage pathway exists in B. subtilis. This pathway is chemically similar to that in K. pneumoniae, but recruited different proteins to this purpose. In particular, a paralogue or Rubisco, MtnW, is used at one of the steps in the pathway. A major observation is that in the absence of MtnW, MTR becomes extremely toxic to the cell, opening an unexpected target for new antimicrobial drugs. In addition to methionine salvage, this pathway protects B. subtilis against dioxygen produced by its natural biotope, the surface of leaves (phylloplane).

Immunization with Purified Natural and Recombinant Allergens Induces Mouse IgG1 Antibodies That Recognize Similar Epitopes as Human IgE and Inhibit the Human IgE-Allergen Interaction and Allergen-Induced Basophil Degranulation

Molecular characterization of allergens by recombinant DNA technology has made rapid progress in the recent few years. In the present study we immunized mice with aluminum hydroxide-adsorbed purified recombinant major timothy grass pollen allergens (rPhl p 1, rPhl p 2, rPhl p 5), dog albumin, a major animal dander allergen, and proteins with low (beta-lactoglobulin) or no (ribulose diphosphate carboxylase) allergenic potential in humans. Allergens that bind high levels of IgE in humans (Phl p 1, Phl p 5, dog albumin) induced high IgE and IgG1 levels in mice, whereas proteins with little or no allergenic activity in humans failed to induce significant IgE and IgG1 levels in mice. Continuous immunization for a period of 27 wk resulted in the production of mouse IgG1 Abs that recognized recombinant allergen fragments/epitopes defined by IgE Abs of allergic patients. As a consequence, allergen-specific mouse Abs strongly inhibited human IgE binding to the allergens and suppressed the allergen-induced histamine release from human basophils. In summary, our data indicate that 1) the allergenic potency of a protein may be related to its overall immunogenicity and 2) prolonged immunization with single purified recombinant allergens induces protective IgG Abs. The presented experimental in vivo/in vitro system allows the evaluation of Ag preparations (e.g., recombinant allergens) to be used for immunotherapy in humans.

Clone bank of Nicotiana tabacum chloroplast DNA: Mapping of the alpha, beta and epsilon subunits of the ATPase coupling factor, the large subunit of ribulosebisphosphate carboxylase, and the 32-kDal membrane protein

All of the Pst I restriction fragments of the chloroplast DNA of Nicotiana tabacum have been cloned in the plasmid vector pBR322. The cloned fragment sizes range from 0.8 to 26 kb, are stable, and can be amplified by chloramphenicol with varying efficiencies. Using these clones we have detailed a PstI physical map of the tobacco chloroplast genome. Selected clones of SalI, BamHl and PstI fragments were used to localize the map positions of the α, β, and ε subunits of the chloroplast ATPase coupling factor, the large subunit of ribulosediphosphate carboxylase and the 32-kDal membrane protein. The gene products of these clones were characterized by RNA transcript sizing, immunoprecipitation of maxicell-directed protein synthesis, and hybrid-arrested translation.

The metabolism of photosynthetically assimilated 14CO 2 under different concentrations of carbon dioxide

Bean and maize leaves were treated with 14CO 2 under different concentrations (300 or 3000 ppm) and the distribution of 14C in products of steady state photosynthesis was studied. In bean leaves an increase the distribution of 14C in products of steady state photosynthesis was studied. In bean leaves an increase in the CO 2 concentration significantly enhanced the incorporation of 14C into starch and into amino and organic acids synthesized via phosphoglyceric acid. Due to the depression of the oxygenase function of ribulose diphosphate carboxylase, the incorporation of 14C into glycolic acid, glycine and serine was essentially diminished at higher CO 2 tension. However, the synthesis of glycolic acid was not completely inhibited. Under the saturating CO 2 concentration (3000 ppm) and a low O 2 concentration (∼1%) when the oxygenase reaction was blocked, the incorporation of 14C into glycolic acid was 15–40% of that in the normal atmosphere (300 ppm CO 2, 21% O 2). It has been suggested that in photosynthesizing cells glycolic acid may be formed as a product of different reactions. Distinct from the bean, the synthesis of glycolic acid in maize leaves was not markedly affected by the CO 2 concentration. This was probably a consequence of the CO 2 pump operating in leaves of C 4 plants, which created a high CO 2 concentration in bundle sheath cells and, in this way, depressed the oxygenase reaction under the normal CO 2 concentration of the environment.

90] Formate dehydrogenase from Pseudomonas oxalaticus

This chapter describes the assay method, purification, and properties of formate dehydrogenase isolated from Pseudomonas oxalaticus . When Pseudomonas oxalaticus is grown on formate as the main carbon and energy source, formate dehydrogenase is the key enzyme that generates nicotinamide adenine dinucleotide dehydrogenase (NADH) and CO 2 . The CO 2 enters the ribulose diphosphate carboxylase reaction. The continuous spectrophotometric assay is based on formate oxidation by nicotinamide adenine dinucleotide kinase (NAD + ) or ferricyanide. The appearance of NADH is followed at 334 or 365 nm; ferricyanide decrease is followed at 405 nm. The NADH oxidase activity of the enzyme does not interfere in the presence of 5 m M sulfide. The steps involved in the purification of the enzyme are also discussed in the chapter. All steps are performed at 0°C. Stored enzyme solutions and all solutions for chromatographic steps are kept in an atmosphere of argon. Essential conditions of the purification procedure of the enzyme are (1) the exclusion of light and oxygen, (2) the absence of formate, (3) low temperature, and (3) rapid performance, with possible interruption only after precipitation by ammonium sulfate of the eluate of the hydroxyapatite chromatography.

Compartmental modelling of photorespiration and carbon metabolism of water stressed leaves

Abstract Carbon fluxes in photosynthesis and photorespiration of water stressed leaves have been analysed in a steady state model based on the ribulose diphosphate carboxylase (RuDP carboxylase) and RuDP oxygenase enzyme activities and the CO2 and O2 concentrations in the leaf. Agreement between predicted and observed photorespiration (Lawlor & Fock, 1975) and C flux in the glycollate pathway is good over much of the range of water stress, but not at severe stress. An alternative source of respiratory CO2 is suggested to explain the discrepancy. The model suggests that resistance to CO2 fixation is mainly in the carboxylation reactions, not in CO2 transport. Using the steady state model, the kinetics of 14C incorporation into photosynthetic and photorespiratory intermediates are simulated. The predicted rate of 14C incorporation is faster than observed and delay terms in the model are used to simulate the slow rates of mixing and metabolic reactions. Inactive pools of glycine and serine are suggested to explain the observed specific activities of glycine and serine. Three models of carbon flux between the glycollate pathway, the photosynthetic carbon reduction cycle and sucrose synthesis are considered. The most satisfactory simulation is for glycollate pathway carbon feeding into the PCR cycle pool of 3-phosphoglyceric acid which provides the carbon for sucrose synthesis. Simulation of the specific activity of CO2 released in photorespiration suggests that a source of unlabelled carbon may contribute to photorespiration.

The Role of Abscisic Acid (ABA) in Regulation of Some Photosynthetic Enzyme Activities in Apple Seedlings in Relation to Embryonal Dormancy

Changes in RuDPC and PEPC activities were investigated in non-dormant apple embryos grown in water as well as in solutions of abscisic acid (ABA) at various concentrations. At all concentrations applied ABA caused a decrease in ribulose-1.5-diphosphate carboxylase (RuDPC) activity and an increase in phosphoenolpyruvate carboxylase (PEPC) activity. These effects were correlated with a decrease in germinability as well as with a decrease in growth rate and a decline in chlorophyll content. The results reported suggest that endogenous ABA in dormant embryos regulates the carboxylating enzyme activities and chlorophyll content.

Photosynthesis, chlorophyll content and ribulose diphosphate carboxylase activity in relation to yield in wheat genotypes

SummaryPhotosynthetic rates, chlorophyll content and activity of ribulose 1,5-diphosphate carboxylase (RuDPC) were determined for 18 genotypes of wheat (Triticum aestivum L.) in pot culture. Measurements were made at various growth stages. Correlation studies between physiological characteristics and seed yield were made at each stage separately.Genotypic variations were observed at different growth and developmental stages in photosynthetic rates (1·8–2·7-fold), chlorophyll contents (1·2-fold) and enzyme activities (2-fold). Chlorophyll content and enzymic activity increased in plant leaves with advancing age while apparent rates of photosynthesis decreased.RuDPC activity in flag leaf at anthesis showed significant positive correlation with grain yield (r = 0·55). Other attributes did not show significant association with grain yield.

Ribulosediphosphate and Phosphoenolpyruvate Carboxylase Activities in Winter Rape as Related to Cold Acclimation

Summary A marked decrease of ribulosediphosphate carboxylase (RuDPC) activity was observed in leaves of winter rape plants ( Brassica napus L., var. oleifera , cv. Gorczainski) during two weeks of leaf growth at cold (2 °C) or ambient temperature. On the other hand, the activity of phosphoenolpyruvate carboxylase (PEPC) increased considerably in cold-treated leaves while it remained almost unchanged in leaves of plants grown at ambient temperature. The ratio of activities of PEPC to RuDPC was found to increase during plant acclimation to low temperature.

Isolation and Enzymic Characterization of Euglena Proplastids

Organelles were isolated from dark-grown Euglena gracilis Klebs by sucrose density gradient centrifugation. Plastids, identified by triosephosphate isomerase and NADP glyoxylate reductase were present at an equilibrium density of 1.24 grams per cubic centimeter clearly separated from mitochondria at an equilibrium density of 1.22 grams per cubic centimeter. Assay for choline phosphotransferase and glucose-6-phosphatase showed that endoplasmic reticulum membranes were present at a density of 1.12 grams per cubic centimeter. The plastid fraction contained phosphofructokinase, pyruvate kinase, triosephosphate isomerase and aldolase indicating the operation of a glycolytic pathway. During regreening pyruvate kinase and phosphofructokinase in the developing proplastid decreased, neither enzyme being present in the mature chloroplast. However, plastids were present in the photosynthetic cell as shown by a peak of glycolysis enzymes at an equilibrium density of 1.24 grams per cubic centimeter.The integrity of isolated plastids was demonstrated by their capacity for protein synthesis. Plastids isolated from dark-grown cells rapidly incorporated [(35)S]methionine into protein with an absolute dependence on added ATP. The large subunit of ribulose diphosphate carboxylase was the major polypeptide synthesized by these isolated plastids.

Some aspects of carbon relations in the barley – Helminthosporium teres complex. I. The effects of infection upon carboxylation in vivo and in vitro

First leaves of seedlings of the barley varieties Parkland (susceptible) and C.I. 5791 (resistant) were inoculated with the net blotch organism Helminthosporium teres. In Parkland, over the 7-day infection cycle (inoculation to sporulation), (1) ribulosediphosphate (RuDP) carboxylase activity, chlorophyll levels, and carbon fixation in the light all fell to ca. 50% of control levels and (2) the activities of phosphoenolpyruvate (PEP) carboxylase and the malic enzyme, and carbon fixation in the dark (which was 0.5% that in the light in healthy tissue), rose 200–400%, peaking just before sporulation of the pathogen. Quantitative analytical studies of the early products of 14CO2 fixation in the light indicated that carbon entered metabolism predominantly via the Calvin cycle in healthy and infected plants. However, β-carboxylation, which in all samples was higher in the light than in the dark, showed a significant increase in susceptible tissues upon H. teres infection. In C.I. 5791 (hypersensitive reaction), the metabolic changes were similar to those in Parkland, with the magnitude of the metabolic response being related to the extent of fungal development; the effect persisted until the growth of the pathogen was inhibited. The changes induced in carbon fixation in situ in response to infection could be reproduced to a limited extent by physical damage, but the response to pathogenesis, especially in susceptible tissues, was probably more specific. The possible significance of this response is discussed.

CHEMICAL CHANGES IN THE GROWTH MEDIUM OFMICROCYSTIS, BATCH CULTURES GROWN AT STRESS AND NON-STRESS LIGHT INTENSITIES

SUMMARY The modified BG 11 growth medium used in this study has a poor buffer ability and large pH changes occurred during growth of Microcystis. Inhibition of growth in cultures grown at stress light intensity (26 μEinst m−2 sec−1) could be ascribed to depletion of the total inorganic carbon content of the medium rather than to light intensity per se, pH or high O2 concentration. In cell suspensions grown under stress light intensity an unfavourably low CO2: O2 concentration ratio developed due to rapid photosynthesis and insufficient replenishment of CO2 by diffusion from air which may have induced oxygenation instead of carboxylation by ribulose diphosphate carboxylase.