Mitochondrial growth and division during the cell cycle in HeLa cells

The growth and division of mitochondria during the cell cycle was investigated by a morphometric analysis of electron micrographs of synchronized HeLa cells. The ratio of total outer membrane contour length to cytoplasmic area did not vary significantly during the cell cycle, implying a continuous growth of the mitochondrial outer membrane. The mean fraction of cytoplasmic area occupied by mitochondrial profiles was likewise found to remain constant, indicating that the increase in total mitochondrial volume per cell occurs continuously during interphase, in such a way that the mitochondrial complement occupies a constant fraction( approximately 10-11(percent)) of the volume of the cytoplasm. The mean area, outer membrane contour length, and axis ratio of the mitochondrial profiles also did not vary appreciably during the cell cycle; furthermore, the close similarity of the frequency distributions of these parameters for the six experimental time-points suggested a stable mitochondrial shape distribution. The constancy of both the mean mitochondrial profile area and the number of mitochondrial profiles per unit of cytoplasmic area was interpreted to indicate the continuous division of mitochondria at the level of the cell population. Furthermore, no evidence was found for the occurrence of synchronous mitochondrial growth and division within individual cells. Thus, it appears that, in HeLa cells, there is no fixed temporal relationship between the growth and division of mitochondria and the events of the cell cycle. A number of statistical methods were developed for the purpose of making numerical estimates of certain three-dimensional cellular and mitochondrial parameters. Mean cellular and cytoplasmic volumes were calculated for the six time-points; both exhibited a nonlinear, approx. twofold increase. A comparison of the axis ratio distributions of the mitochondrial profiles with theoretical distributions expected from random sectioning of bodies of various three-dimensional shapes allowed the derivation of an "average" mitochondrial shape. This, in turn, permitted calculations to be made which expressed the two-dimensional results in three-dimensional terms. Thus, the estimated values for the number of mitochondria per unit of cytoplasmic volume and for the mean mitochondrial volume were found to remain constant during the cell cycle, while the estimated number of mitochondria per cell increase approx. twofold in an essentially continuous manner.     

Laboratory studies of non-preference resistance to cabbage root fly in radish

First generation progenies selected from four radish cultivars on the basis of their attractiveness to ovipositing cabbage root fly in the laboratory were compared in the same environment. There were significant differences between and within cultivars in their attractiveness for egg-laying, indicating that genetic variation existed and that selection had effected a heritable change. However, inconsistencies between experiments indicated that other, as yet unquantified, factors were compounded with the heritable component of non-preference resistance. There was no evidence to suggest that seed weight, leaf area, or hypocotyl size affected the attractiveness of the plants. The effects of eliminating any choice between selections was investigated in the laboratory by exposing 4-wk-old ‘high’ and ‘low’ preference selections of cv. Asmer Tip Top to the pest for 6 days. Three times as many eggs were laid on ‘high’ than on ‘low’ preference selections, indicating that the heritable non-preference resistance to cabbage root fly in Asmer Tip Top was maintained when no choice was offered to the flies, an essential requirement for ‘resistant’ cultivars in the field.     

An Investigation of the Influence of Constant and Alternating Temperature on the Germination of Cassava Seed using a Two-dimensional Temperature Gradient Plate

The germination of cassava seed in response to various constantand alternating temperature regimes within the range 19–40°C was investigated using a two-dimensional temperaturegradient plate. It was found that almost all seeds were incapableof germination unless the temperature for part of the day exceeded30 °C and the mean temperature was at least 24 °C. However,dormant seeds required environments where the temperature forpart of the day exceeded 36 °C, the mean temperature wasat least 33 °C, and the amplitude of the diurnal temperaturealteration was within the range 3–18 °C. Providingthese conditions were met, the times spent at the upper andlower temperatures within a diurnal cycle were not critical.Hermetic storage of the seed for 77 days at 40 °C with 7.9per cent moisture content did not influence the pattern of germinationin response to constant and alternating temperatures. It issuggested that an alternating temperature regime of 30 °Cfor 8 h/38 °C for 16 h applied for a minimum of 21 daysis appropriate for cassava seed viability tests. Manihot esculenta Crantz, cassava, germination, dormancy, temperature     

Ice premelting during differential scanning calorimetry

Premelting at the surface of ice crystals is caused by factors such as temperature, radius of curvature, and solute composition. When polycrystalline ice samples are warmed from well below the equilibrium melting point, surface melting may begin at temperatures as low as -15 degrees C. However, it has been reported (. Biophys. J. 65:1853-1865) that when polycrystalline ice was warmed in a differential scanning calorimetry (DSC) pan, melting began at about -50 degrees C, this extreme behavior being attributed to short-range forces. We show that there is no driving force for such premelting, and that for pure water samples in DSC pans curvature effects will cause premelting typically at just a few degrees below the equilibrium melting point. We also show that the rate of warming affects the slope of the DSC baseline and that this might be incorrectly interpreted as an endotherm. The work has consequences for DSC operators who use water as a standard in systems where subfreezing runs are important.     

Limits to the Negative Logarithmic Relationship Between Moisture Content and Longevity in Conidia ofMetarhizium flavoviride

Conidia ofMetarhizium flavoviridewere hermetically stored at50 °C and 14 moisture contents between 2.5 and 31.8% (freshweight basis) for up to 146 d, and tested for germination onSabouraud Dextrose Agar at 25 °C for 24 h. Survival of conidiaconformed to cumulative negative normal distributions and all14 survival curves could be constrained to a common origin.There was a negative logarithmic relation between longevityand conidia moisture content, but limits to the relation weredetected: the lower-moisture-content limit was 4.6% [in equilibriumwith 10.7% relative humidity (RH) at 20 °C], below whichvalue further reduction in moisture content did not increaseconidia longevity; and an upper-moisture-content limit betweenabout 21.2 and 31.8% moisture content (between 77 and 90.0%equilibrium RH at 20 °C) above which conidia longevity nolonger decreased. The observations could also be described bya negative semi-logarithmic relation between conidia longevityand equilibrium relative humidity. In this model, each reductionin equilibrium relative humidity by 11.2% within the range 10.7to 80% RH doubled conidia longevity. The similarities in theserelations, and the limits to these relations, between the conidiaof this entomopathogenic fungus and the orthodox seeds of higherplants are discussed.Copyright 1998 Annals of Botany Company Conidia longevity, equilibrium relative humidity,Metarhizium flavoviride, moisture content, hermetic storage, viability equation     

Development of a Model to Predict the Effect of Temperature and Moisture on Fungal Spore Longevity

A model was developed to quantify the effect of temperatureand moisture content on the longevity of conidia of the entomopathogenicfungusMetarhizium flavoviride . This model incorporated a negativesemi-logarithmic relation between longevity and temperatureand a negative logarithmic relation between longevity and moisturecontent. Replacing the latter with a negative semi-logarithmicrelation between longevity and the equilibrium relative humidityof the conidia was also effective. The latter model was appliedsuccessfully to observations on the survival of conidia of afurther four entomopathogenic fungi (Metarhizium anisopliae,Beauveria bassiana ,Beauveria brongniartii, andPaecilomycesfarinosus ) and ascospores, conidia or uredospores of four phytopathogenicfungi (Alternaria porri ,Helminthosporium oryzae ,Uromyces appendiculatus, andSclerotinia sclerotiorum ) across a wide range of differenttemperatures and relative humidities. The sensitivity of sporelongevity to both temperature and equilibrium relative humidity,and the upper and lower relative humidity limits to the applicationof the model, varied considerably between entomopathogenic andphytopathogenic fungi, among species within each group, andamong different strains within certain species. Metarhizium flavoviride W. Gams & J. Roszypal; entomopathogenic fungi; phytopathogenic fungi; spore survival; storage environment; conidia; uredospore; ascospore