Top 10 Cited Annals of Applied Biology Articles

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Nitrogen losses from the soil/plant system: a review

K.C. Cameron, H.J. Di, J.L. Moir, 

Abstract

Losses of nitrogen from the soil/plant system not only reduce soil fertility and plant yield but can also create adverse impacts on the environment. Ammonia emissions into the atmosphere contribute to acid rain and represent an indirect source of nitrous oxide greenhouse gas emissions. Nitrate leaching losses into rivers and lakes can cause eutrophication resulting in excessive growth of aquatic weeds and algae, which can reduce fish populations and the recreational value of the water. Nitrate contamination of drinking water supplies can cause health risks. Legislation that is designed to limit nitrate leaching losses from land has become a constraint on agricultural land use in many countries. Nitrous oxide emissions into the atmosphere contribute to the depletion of the ozone layer and also make a significant contribution to climate change. This review describes the nitrogen cycle in temperate soil/plant systems, the processes involved in each of the individual nitrogen loss pathways, the factors affecting the amounts of losses and the methods that are available to reduce these losses. The review has shown that careful management of temperate soil/plant systems using best management practices and newly developed technologies can increase the sustainability of agriculture and reduce its impact on the environment.

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THE TOXICITY OF POISONS APPLIED JOINTLY1

C. I. BLISS, 

Summary

A quantitative analysis of the toxicity of drugs or poisons applied jointly requires that they be administered at several dosages in mixtures containing fixed proportions of the ingredients. From a study of the dosage-mortality curves for several such mixtures, preferably in comparison with equivalent curves for the isolated active ingredients, most cases of combined action can be classified into one of three types:

(1) The first type is that in which the constituents act independently and diversely, so that the toxicity of any combination can be predicted from that of the isolated components and from the association of susceptibilities to the two components. The coefficient of association can be measured experimentally and should be constant at all proportions of the ingredients. When high, the toxicity of the mixture is reduced. The form of the dosage-mortality curve has been examined for several hypothetical mixtures. Whenever the curves for the two constituents were assumed to differ in slope, there was a relatively abrupt bend in the curve for the mixture, the rectilinear segments above and below the break approaching in slope the values for the original constituents. This observation indicates that in homogeneous populations the slope of a dosage-mortality curve is of toxicological significance. Since the same numerical relations would be expected if a single poison were to have two independent lethal effects within the animal, there is theoretical basis for fitting the linear segments of a dosage-mortality curve separately when a break occurs after transformation to probits and logarithms. This argument has been extended to time-mortality experiments to explain the smoothly concave curves characteristic of natural mortality.

(2) The second type of joint action is that in which the constituents act independently but similarly, so that one ingredient can be substituted at a constant ratio for any proportion of a second without altering the toxicity of the mixture. With homogeneous populations, dosage-mortality curves for the separate ingredients and for all mixtures should be parallel. Although by hypothesis the susceptibility to one ingredient is completely correlated with that to the other, mixtures in this category are more toxic than in the preceding class where association may vary from 0 to 1. The numerical relations have been illustrated by an experiment on the toxicity to the house-fly of solutions containing pyrethrin and rotenone. A mixture with a little less than four equitoxic units of pyrethrin to one of rotenone agreed closely with the definition but one in which the ingredients were about equally balanced showed a significantly greater toxicity than expected on the hypothesis of independent action, indicating the presence of synergism.

(3) Synergism forms the third type of joint action, characterized by a toxicity greater than that predicted from studies on the isolated constituents. It is the reverse of antagonism, which has not been considered directly. Two methods are proposed for the analysis of synergism. The more direct is to relate equitoxic dosages of mixture to its percentage composition in terms of the more active ingredient. When both are in logarithms the relation is linear over a useful range of compositions. This procedure preserves the original structure of the experiment, can be extended readily to three or more ingredients and leads to a convenient practical result. Theoretically it is less satisfactory than a second method in which for equitoxic dosages of each mixture the content of one ingredient (A) is related to the content of the other (B.) The equation which satisfies this relation most completely is (1 +k1A) Bi=k2, where the three constants are computed from the experimental data. When the exponent i is equal to 1, only two constants need be determined and their product, k1k2, is proposed as a measure of the intensity of synergism.

The synergism between a nitro-phenol and petroleum oil has been computed by both methods. For mixtures containing from 0·5 to 5% of the nitrophenol, the deposit of mixture (Dc) killing 98 % of the eggs of a plant bug could be expressed adequately in terms of the percentage of the phenol (Q) as log Dc= 0·687-0·307 log Q, for 98% of overwintering San Jose scale as log Dc= 0·472 -0·363 log Q. All observations, including those for a 0·1 % mixture and for oil alone which were omitted in the first method, could be fitted satisfactorily in terms of the separate ingredients. For plant bug eggs at LD50, (1+25·6A) B= 4·29 and for San Jose scale (1 + 66·7 A) B= 2·73: in both cases i= 1 and the intensity of synergism 110 and 182 respectively.

The full procedure has also been applied to the constituents of seven samples of Derris root. One sample gave an unaccountably low toxicity and was omitted. The log LD 50 of ether extract for the remaining six was related to the percentage composition of two components in the extract, rotenone (A) and dehydro mixture (B.) Since the toxicity of extract could be expressed almost entirely in terms of these particular two constituents, they were then related to each other by the second method. None of the samples contained a very small proportion of one ingredient, so that several equations were equally applicable, one of them being (1+0–714A) B= 56·1, from which the intensity of synergism was 40.

The problem of measuring synergism in fumigants has been discussed briefly.

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A uniform decimal code for growth stages of crops and weeds

PETER D. LANCASHIRE, H. BLEIHOLDER, T. VAN DEN BOOM, P. LANGELÜDDEKE, R. STAUSS, ELFRIEDE WEBER, A. WITZENBERGER, 

Summary

A universal scale (to be known as the BBCH scale) using a decimal code for the description of the growth stages of most agricultural crops and weeds is proposed. The scale and codes are based on the well-known Zadoks code for cereals.

Developmentally similar growth stages of different crops are given the same codes. The general scale provides a framework within which more specific scales for individual crops may be constructed. The uniformity of the scale makes it easy to remember and use in agricultural practice and simplifies storage and retrieval in a computer system.

A description of the general scale is given followed by specific scales for cereals, rice, maize, oilseed rape, field beans, peas and sunflower. Comparisons with scales currently in use are given where appropriate.

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A comparison of some quantitative methods of extracting small vermiform nematodes from soil

A. G. WHITEHEAD, J. R. HEMMING, 

SUMMARY

When 200 ml. dispersed soil is sedimented from an obliquely rising water current in a simple compartmented tank about three-quarters of the nematodes are extracted. About 95% of the nematodes in the concentrated suspension can be separated finally from mineral and heavy organic particles by centrifugal notation. When mobile nematodes were finally separated from soil particles by paper tissue, this sedimentation method extracted as many nematodes from sand and loam as Seinhorst's two-flask and elutriation methods, but in one test extracted fewer Tylenchorhynchus from clay and in another fewer Paratylenchus from clay than the elutriation method. The method is quicker (4 or 6 instead of 30–45 min.) and easier.

Mobile nematodes can be extracted from 300 ml. soil spread out on paper tissue in 23. 33 cm. trays of 8 mesh/cm. phosphor-bronze gauze, just resting on shallow water. The suspension obtained after 24 hr. at 16–18°C. was concentrated to 10–15 ml. without loss by sedimentation in two tapered cylinders, one of 8 cm. bore, the other of 2.6 cm. bore. This method usually extracted significantly more nematodes than the sedimentation, two-flask and elutriation methods.

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AN AUTOMATIC VOLUMETRIC SPORE TRAP

J. M. HIRST, 

Abstract

A suction trap has been made in which the spores entering a narrow orifice, directed into the wind, are impacted on a Vaseline-coated microscope slide moved across the orifice at 2 mm./hr. Estimates of spore content of the air can be made, with higher efficiency than by previous traps, at different times of day and thus be more closely correlated with variations in weather.

Wind-tunnel tests with spores of Lycopodium clavatum showed maximal and minimal efficiencies of 93.8 and 62.4% respectively, with a suction rate of 10.0 1./min., in the range of wind speeds from 1.5 to 9.3 m./sec.

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Silicon: its manifold roles in plants

E. Epstein, 

Abstract

The title of this essay declares that silicon does have roles in plants and all participants in this conference know that that is so. This knowledge, however, is not shared by the general community of plant biologists, who largely ignore the element. This baffling contrast is based on two sets of experience. First, higher plants can grow to maturity in nutrient solutions formulated without silicon. That has led to the conventional wisdom that silicon is not an essential element, or nutrient, and thus can be disregarded. Second, the world's plants do not grow in the benign environment of solution culture in plant biological research establishments. They grow in the field, under conditions that are often anything but benign. It is there, in the real world with its manifold stressful features, that the silicon status of plants can make a huge difference in their performance. The stresses that silicon alleviates range all the way from biotic, including diseases and pests, to abiotic such as gravity and metal toxicities. Silicon performs its functions in two ways: by the polymerization of silicic acid leading to the formation of solid amorphous, hydrated silica, and by being instrumental in the formation of organic defence compounds through alteration of gene expression. The silicon nutrition of plants is not only scientifically intriguing but also important in a world where more food will have to be wrung from a finite area of land, for that will put crops under stress.

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Asparagine in plants

P.J. Lea, L. Sodek, M.A.J. Parry, P.R. Shewry, N.G. Halford, 

Abstract

Interest in plant asparagine has rapidly taken off over the past 5 years following the report that acrylamide, a neurotoxin and potential carcinogen, is present in cooked foods, particularly carbohydrate-rich foods such as wheat and potatoes which are subjected to roasting, baking or frying at high temperatures. Subsequent studies showed that acrylamide could be formed in foods by the thermal degradation of free asparagine in the presence of sugars in the Maillard reaction. In this article, our current knowledge of asparagine in plants and in particular its occurrence in cereal seeds and potatoes is reviewed and discussed in relation to acrylamide formation. There is now clear evidence that soluble asparagine accumulates in most if not all plant organs during periods of low rates of protein synthesis and a plentiful supply of reduced nitrogen. The accumulation of asparagine occurs during normal physiological processes such as seed germination and nitrogen transport. However, in addition, stress-induced asparagine accumulation can be caused by mineral deficiencies, drought, salt, toxic metals and pathogen attack. The properties and gene regulation of the enzymes involved in asparagine synthesis and breakdown in plants are discussed in detail.

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Do plants need nitrate? The mechanisms by which nitrogen form affects plants

M. Andrews, J.A. Raven, P.J. Lea, 

Abstract

The literature on nitrogen (N) form effects on plants at different stages of their development has been critically reviewed, assessing the possible mechanisms of these effects. In particular, nitrate (NO3−) was compared with the other forms of N utilised by plants. It is concluded that the form of N available to plants can affect their time and rate of seed germination, leaf expansion and function, dry matter partitioning between shoot and root, and root architecture. The magnitude of these effects is dependent on environmental factors outside the supply of N. The mechanism of these effects is variable. Assessment of the importance of root or shoot NO3− assimilation under different environmental conditions is an important area for further study.

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Food security: the challenge of increasing wheat yield and the importance of not compromising food safety

T. Curtis, N. G. Halford, 

Abstract

Current wheat yield and consumption is considered in the context of the historical development of wheat, from early domestication through to modern plant breeding, the Green Revolution and wheat's place as one of the world's most productive and important crops in the 21st Century. The need for further improvement in the yield potential of wheat in order to meet current and impending challenges is discussed, including rising consumption and the demand for grain for fuel as well as food. Research on the complex genetics underlying wheat yield is described, including the identification of quantitative trait loci and individual genes, and the prospects of biotechnology playing a role in wheat improvement in the future are discussed. The challenge of preparing wheat to meet the problems of drought, high temperature and increasing carbon dioxide concentration that are anticipated to come about as a result of climate change is also reviewed. Wheat yield must be increased while not compromising food safety, and the emerging problem of processing contaminants is reviewed, focussing in particular on acrylamide, a contaminant that forms from free asparagine and reducing sugars during high temperature cooking and processing. Wheat breeders are strongly encouraged to consider the contaminant issue when breeding for yield.

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Rhizobacterial mediation of plant hormone status

I.C. Dodd, N.Y. Zinovkina, V.I. Safronova, A.A. Belimov, 

Abstract

Plant growth-promoting rhizobacteria are commonly found in the rhizosphere (adjacent to the root surface) and may promote plant growth via several diverse mechanisms, including the production or degradation of the major groups of plant hormones that regulate plant growth and development. Although rhizobacterial production of plant hormones seems relatively widespread (as judged from physico-chemical measurements of hormones in bacterial culture media), evidence continues to accumulate, particularly from seedlings grown under gnotobiotic conditions, that rhizobacteria can modify plant hormone status. Since many rhizobacteria can impact on more than one hormone group, bacterial mutants in hormone production/degradation and plant mutants in hormone sensitivity have been useful to establish the importance of particular signalling pathways. Although plant roots exude many potential substrates for rhizobacterial growth, including plant hormones or their precursors, limited progress has been made in determining whether root hormone efflux can select for particular rhizobacterial traits. Rhizobacterial mediation of plant hormone status not only has local effects on root elongation and architecture, thus mediating water and nutrient capture, but can also affect plant root-to-shoot hormonal signalling that regulates leaf growth and gas exchange. Renewed emphasis on providing sufficient food for a growing world population, while minimising environmental impacts of agriculture because of overuse of fertilisers and irrigation water, will stimulate the commercialisation of rhizobacterial inoculants (including those that alter plant hormone status) to sustain crop growth and yield. Combining rhizobacterial traits (or species) that impact on plant hormone status thereby modifying root architecture (to capture existing soil resources) with traits that make additional resources available (e.g. nitrogen fixation, phosphate solubilisation) may enhance the sustainability of agriculture.