Infradian cycles of oxygen consumption in diapausing pupae of the flesh fly, Sarcophaga crassipalpis, monitored by a scanning microrespirographic method.

Fine details of the infradian O2 consumption cycles that characterize pupal diapause in flesh flies have been monitored by a newly designed microrespirographic method coupled with an electronically regulated O2 generator. During the 4-5 days between the peaks of elevated O2 consumption, the diapausing pupae maintained a very low and fairly constant respiratory rate (13 microl O2 x g-1.h-1). During the intercalated peaks of increased respiratory metabolism, which lasted an average of 33.6 h to 24-27 degrees C, the average maximum rate of O2 consumption was 86.9 microl.g-1.h-1, a value of 6.7 times higher than the interpeak values. The respiratory peaks started abruptly in some cases while the decline was consistently gradual. During the periods between the peaks there were no discontinuous bursts of CO2 release, a feature common to diapause in many other insects. Diapause was characteristically terminated during a peak of the O2 consumption cycle. At diapause termination O2 consumption remained at the maximum values of the peak for many hours and then gradually increased to levels characteristic of nondiapause development.     

Characterization of fall armyworm mitochondrial DNA (Lepidoptera: Noctuidae)

Mitochondrial DNA from the fall armyworm, Spodoptera frugiperda (J.E. Smith), was cloned and characterized using restriction enzyme mapping. Genome size is approximately 16.3 kilobase (Kb), consistent with that of most animals. Three fragments, 8.1 Kb, 6.2 Kb, and 2.0 Kb, were produced by digestion with restriction enzyme Xbal and cloned into a pUC19 vector. Twenty-nine restriction enzymes were used to generate a detailed physical restriction enzyme map of the three cloned fragments. These data represent the first detailed characterization of a lepidopteran mitochondrial genome. © 1992 Wiley-Liss, Inc.     

The mechanism of extrachromosomal homologous DNA recombination in plant cells

Summary By cotransfecting plasmids carrying particular mutations in the -glucuronidase (GUS) gene into Nicotiana plumbaginifolia protoplasts and by monitoring the recombination rates using a recently developed transient assay, we were able to obtain insights into the mechanism of extrachromosomal recombination operating in plant cells. An exchange of flanking markers takes place in over 90% of the recombination events. In most of the remaining cases two consecutive, independent single crossover events occur. These events involve the same DNA substrate and lead to two successive exchanges of flanking markers, thus mimicking a presumed double crossover intermediate. A comparison of the outcome of our experiments with the predictions of two recombination models originally proposed for mammalian cells indicates that extrachromosomal recombination in plant cells is best described by the single strand annealing model. According to this model all recombination events result in an exchange of flanking markers. Our results rule out the double strand break repair model which predicts that flanking markers are exchanged in only half of all events.     

The effect of established plants on recruitment in the annual forb Sinapis arvensis

Summary The germination response of Sinapis arvensis to the presence of established plants was investigated in a greenhouse experiment. Established conspecific and heterospecific plants were found to inhibit germination and reduce the probability of recruitment of those seeds that germinate. Established plants have no effect on seed mortality in the soil. Using a simple recruitment model, it is demonstrated that the combination of variance in germination time coupled with the interaction between buried seeds and established plants can generate density dependence. The implications of these results for community processes, such as succession, are discussed.     

Studies of the relationship between isoprene emission rate and CO2 or photon-flux density using a real-time isoprene analyser

Abstract. Studies of the isoprene emission rate in response to changes in photon-flux density and CO₂ partial pressure were conducted using a recently developed on-line isoprene analyser combined with a gas exchange system and chlorophyll fluorometer. Upon darkening, the isoprene emission rate from leaves of aspen ( Populus tremuloides Michaux.) began to decline immediately, demonstrating that the internal pool of isoprene, or its precursors, is small and that the instantaneous emission rate is tightly coupled to the rate of synthesis. A post-illumination burst of isoprene was observed within 5 min after darkening and lasted for 15–20 min in four isoprene-emitting species that were examined. In leaves of eucalyptus ( Eucalyptus globulus Labill.), the magnitude of the post-illumination burst was dependent on the photon-flux density that existed before darkening, but not on ambient CO₂ partial pressure. The dependence of the post-illumination burst on photon-flux density paralleled that for the steady-state rate of isoprene emission. A step-wise increase in intercellular CO₂ partial pressure from 24.5 to 60 Pa resulted in an immediate decrease in isoprene emission rate and non-photochemical fluorescence quenching, but an increase in CO₂ assimilation rate. Given the several recent studies that link isoprene emission to chloroplastic processes, the results of this study indicate that the linkage is not dependent on the rate of CO₂ flux through the reductive pentose phosphate pathway, but rather on more complex relationships involving metabolites not appreciably influenced by CO₂ partial pressure.     

Hemoglobin Dallas (α97(G4)Asn→Lys):functional characterization of a high oxygen affinity natural mutant

Hemoglobin Dallas, an α-chain variant with a substitution of lysine for asparagine at position 97(G4), was found to have increased oxygen affinity ($\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 1}\text{mmHg at pH}\text{7.3}\text{and}\text{20°C), diminished cooperativity (0n,}\text{the Hill coefficient}\text{= 1.7}$) and reduced Bohr effect (about 50%). Addition of allosteric effectors (such as 2,3-diphosphoglycerate, inositol hexakisphosphate and bezafibrate) led to a decrease in oxygen affinity and increase in cooperative energy. Kinetic studies at pH 7.0 and 20°C revealed that (i), the overall rate of oxygen dissociation is 1.4-fold slower than that for HbA and (ii), the carbon monoxide dissociation rate is unaffected. The abnormal properties of this hemoglobin variant can be atttributed to a more ‘relaxed’ T-state.     

Nuclear genome characterization of the carrageenophyteAgardhiella subulata (Rhodophyta)

DNA reassociation kinetics were used to determine nuclear genome organization and complexity inAgardhiella subulata (Gigartinales, Rhodophyta). Results indicate the presence of three second-order components corresponding to fast (22%), intermediate (68%) and slow (10%) fractions. Thus, the genome consists of 90% repetitive sequences. Microspectrophotoometry with the DNA-localizing fluorochrome DAPI was used to confirm ploidy level differences in the gametophytic and tetrasporophytic phases. Results indicate that meiosis occurs during tetrasporogenesis. Comparison of mean nuclear DNA (I_f) values to chicken erythrocytes (RBC) resulted in an estimate of 0.9 pg/2C genome forAgardhiella. Karyological studies using aceto-orcein revealed a chromosome complement of 2N = 44 in carposporangia and the presence of 22 bivalents during diakinesis of tetraspore mother cells.     

Use of the Golden Section search method to estimate the parameters of the Monod model employing spread-sheets

An alternative procedure to obtain the parameters of Monod's growth model in batch culture is presented. It is based on the integral kinetic analysis methodology, employs a one-dimensional Golden Section search optimization method and is implemented on a spread-sheet programme. The procedure is discussed in detail and is illustrated by analysis of batch substrate consumption data by an aerobic bacterial consortium.     

Use of disease progress curves to study the effects of the biocontrol agent Sporidesmium sclerotivorum on lettuce drop

At the end of the spring 1987 growing season, the mycoparasite Sporidesmium sclerotivorum was applied at 0, 0.2, 2 or 20 kg ha^‐1 to lettuce plants infected with Sclerotinia minor. Disease incidence was monitored in the same plots for five subsequent crops (three fall and two spring crops) without additional application of either pathogen or mycoparasite. Logistic growth curves were fitted to the data to describe disease progression over time for each inoculum level within each of the five crops. Within each crop, increasing the quantity of mycoparasite inoculum resulted in positive horizontal displacement of the curve with respect to time. As quantities of inoculum of S. sclerotivorum increased, inflection points of the disease progress curves increased at a decreasing rate. Thus, additional mycoparasite inoculum resulted in ever‐smaller increases in inflection point, and after a certain threshold level of mycoparasite inoculum (< 0.2 kg ha^‐1), increases in inflection point did not result in meaningful increases in harvestable lettuce. Maximum rates of disease increase were not different among the treatments within each crop, but were different between crops. Maximum rates of disease increase averaged 3.4, 3.4, 2.1, 3.6 and 1.5% day^‐1 for the fall 1987, spring 1988, fall 1988, spring 1989, and fall 1989, respectively. At all inoculum levels, the fall epidemics began later after planting than the spring epidemics.     

The appearance of an enzyme activity catalysing the conversion of norsolorinic acid to averantin in Aspergillus parasiticus cultures

The activity of the enzyme responsible for the conversion of norsolorinic acid to averantin was studied in two strains of Aspergillus parasiticus. Cell-free extracts of the enzyme were purified from different aged mycelia and little activity was found prior to 24 hours after inoculation but this quickly reached a maximum at 48 hours and declined thereafter. Both strains of A. parasiticus, one in aflatoxin producing strain, the other a versicolorin A accumulating mutant, showed this trend. It was concluded that the enzyme responsible for this conversion was a secondary metabolic enzyme and was distinct from alcohol and mannitol dehydrogenases.