The Hertz Corporation

About: The Hertz Corporation is a based out in . It is known for research contribution in the topics: Population & Soil water. The organization has 9562 authors who have published 11044 publications receiving 447929 citations. The organization is also known as: Hertz Rental Car & Hertz Rent-a-Car.

Secondary metabolites in plant defence mechanisms

Abstract: SUMMARY Many secondary metabolites found in plants have a role in defence against herbivores, pests and pathogens. In this review, a few examples are described and discussed, and some of the problems in determining the precise role(s) of such metabolites highlighted. The role of secondary metabolites in defence may involve deterrence/anti-feedant activity, toxicity or acting as precursors to physical defence systems. Many specialist herbivores and pathogens do not merely circumvent the deterrent or toxic effects of secondary metabolites but actually utilize these compounds as either host recognition cues or nutrients (or both). This is true of both cyanogenic glucosides and glucosinolates, which art discussed in detail as examples of defensive compounds. Their biochemistry is compared and contrasted. An enormous variety of secondary metabolites are derived from shikimic acid or aromatic amino acids, many of which have important roles in defence mechanisms. Several classes of secondary products are ‘induced’ by infection, wounding or herbivory, and examples of these are given. Genetic variation in the speed and extent of such induction may account, at least in part, for the difference between resistant and susceptible varieties. Both salicylates and jasmonates have been implicated as signals in such responses and in many other physiological processes, though their prescise roles and interactions in signalling and development are not fully understood.

Measurement of microbial biomass phosphorus in soil

Abstract: A method for measuring the amount of P held in soil micro-organisms (biomass P) is described and the assumptions on which it is based are discussed. Biomass P is calculated from the difference between the amount of inorganic P (Pi) extracted by 0.5 (Spm) NaHCO3 (pH 8.5) from fresh soil fumigated with CHCl3 and the amount extracted from unfumigated soil. Some CHCl3-released Pi is sorbed by soil during fumigation and extraction: an approximate allowance for this is made by incorporating a known quantity of Pi during extraction and correcting for recovery. Most of the P released is in inorganic form and the proportion increases with duration of fumigation. Non-microbial P is little, if at all, affected by fumigation. Microbial biomass P is calculated from CHCl3-released Pi by dividing by 0.4, i.e. by assuming that 40% of the P in the biomass is rendered extractable as Pi by CHCl3. Measurements of biomass P must be done in fresh soil, CHCl3 releases much less P in air-dry soil.

Determination of nitrogen in soil by the Kjeldahl method

Abstract: 1 The reliability of the Kjeldahl method for the determination of nitrogen in soils has been investigated using a range of soils containing from 0·03 to 2·7% nitrogen2 The same result was obtained when soil was analysed by a variety of Kjeldahl procedures which included methods known to recover various forms of nitrogen not determined by Kjeldahl procedures commonly employed for soil analysis From this and other evidence presented it is concluded that very little, if any, of the nitrogen in the soils examined was in the form of highly refractory nitrogen compounds or of compounds containing N—N or N—O linkages3 Results by the method of determining nitrogen in soils recommended by the Association of Official Agricultural Chemists were 10–37% lower than those obtained by other methods tested Satisfactory results were obtained by this method when the period of digestion recommended was increased4 Ammonium-N fixed by clay minerals is determined by the Kjeldahl method5 Selenium and mercury are considerably more effective than copper for catalysis of Kjeldahl digestion of soil Conditions leading to loss of nitrogen using selenium are defined, and difficulties encountered using mercury are discussed6 The most important factor in Kjeldahl analysis is the temperature of digestion with sulphuric acid, which is controlled largely by the amount of potassium (or sodium) sulphate used for digestion7 The period of digestion required for Kjeldahl analysis of soil depends on the concentration of potassium sulphate in the digest When the concentration is low (eg 0·3 g/ml sulphuric acid) it is necessary to digest for several hours; when it is high (eg 1·0 g/ml sulphuric acid) short periods of digestion are adequate Catalysts greatly affect the rate of digestion when the salt concentration is low, but have little effect when the salt concentration is high8 Nitrogen is lost during Kjeldahl analysis when the temperature of digestion exceeds about 400° C9 Determinations of the amounts of sulphuric acid consumed by various mineral and organic soils during Kjeldahl digestion showed that there is little risk of loss of nitrogen under the conditions usually employed for Kjeldahl digestion of soil Acid consumption values for various soil constituents are given, from which the amounts of sulphuric acid likely to be consumed during Kjeldahl digestion of different types of soil can be calculated10 Semi-micro Kjeldahl methods of determining soil nitrogen gave the same results as macro-Kjeldahl methods11 The use of the Hoskins apparatus for the determination of ammonium is described12 It is concluded that the Kjeldahl method is satisfactory for the determination of nitrogen in soils provided a few simple precautions are observed The merits and defects of different Kjeldahl procedures are discussed

A mechanism for gene conversion in fungi.

Abstract: A mechanism for gene conversion is proposed which overcomes many of the difficulties that any copy choice model encounters. It is suggested that along with general genetic pairing of homologous genomes at meiosis, effective pairing over short regions of the genetic material occurs at the molecular level by the separation of the strands of the DNA double helices, followed by the annealing of strands from two homologous chromatids. If the annealed region happens to span a heterozygous site, mispairing of bases will occur. Such a situation may be analogous to that in DNA which is damaged by mutagens; the same or similar repair mechanisms may operate, and these, by adjusting the base sequences in order to restore normal base pairing, would bring about gene conversion in the absence of any genetic replication. The model indicates how precise breakage and rejoining of chromatids could occur in the vicinity of the conversion, so that conversion would frequently be accompanied by the recombination of outside markers. The model also proposes that the distance between two mutant sites on a fine structure map depends not so much on the frequency of a recombinational event occurring between them, but rather on the degree of inhibition of the processes of genetic pairing by the mutants themselves. The model will explain almost all the data in a formal way, and it has the advantage over copy choice mechanisms for gene conversion in (1) being compatible with semi-conservative replication of DNA, (2) not invoking DNA synthesis during or after genetic pairing, (3) providing a molecular mechanism for close specific pairing, (4) making it unnecessary to postulate sister strand exchange or a process akin to this, (5) suggesting why rates of gene conversion in opposite directions are sometimes unequal and (6) providing an explanation of the clustering of mutant sites, a basis for map expansion and for the apparently capricious departure of fine structure maps from additivity. Although the model proposed is a general rather than a specific one, it suggests that the process of conversion and intragenic recombination is more complex than is usually believed, since it depends on several interacting factors. Nevertheless, it is hoped that the introduction of a model with this complexity will help to stimulate specific experiments, and that these will provide definitive information which would never be obtained if simpler models of conversion and intragenic recombination were believed to explain the genetic data sufficiently well.