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Glossary: N

n/2 model
This is a model of the way that vertical subsystems and the gene-for-gene relationship most likely work in a wild plant pathosystem.
The fact that we only have a conceptual model shows how little research has been conducted on wild pathosystems. However, its mathematical basis is so convincing that we can use it quite confidently until we have more supporting evidence.
The model is based on the assumption that the gene-for-gene relationship acts as a system of locking. Its primary function is to reduce the population explosion of an r-strategist parasite.
It normally achieves this by reducing the frequency of allo-infections that are matching infections. But, when the vertical resistance is quantitative, it functions by reducing the reproduction, and the population growth rate of the parasite.
The system of locking is an emergent property that is observable only at the systems level of the pathosystem. That is, at the level of the two interacting populations of host and parasite.
If every individual in both the host and parasite populations has no vertical genes at all, every allo-infection will be a matching infection, and the vertical subsystem will not exist. Equally, if every individual has all the vertical genes, every allo-infection will be a matching infection, and the vertical subsystem will not function.
The midpoint between these two extremes is when every individual has exactly half of the vertical genes of the vertical subsystem. This is the n/2 situation, where n = the number of pairs of matching genes in the system.
For example, if there are twelve pairs of genes, n/2 = 6, and every individual will have a six-gene combination. That is, every host individual will have a six-gene vertical resistance, and every parasite individual will have a six-gene parasitic ability. Think of each vertical resistance as a biochemical lock with six tumblers. And think of each parasite ability as a biochemical key with six notches.
From Pascal’s triangle, we can see that, when n/2 = 6, there are 924 different locks and keys. If every lock and key occurs with equal frequency, and with a random distribution, the probability of an allo-infection being a matching infection will be 1/924. And, when there are twenty pairs of genes, n/2 = 184,756, and the probability of matching is 1/184,756. This is a remarkably economical effect produced from a few pairs of Mendelian genes.
A species that occurs naturally in an area, and has not been introduced, deliberately or accidentally, by people.
Natural cross-pollination
cross-pollination that occurs naturally, as opposed to artificial or hand-pollination.
Natural selection
The selection that occurs naturally within a wild population that is genetically diverse. The selection operates because the most fit individuals reproduce the most, while the least fit individuals reproduce the least. This is the mechanism of natural evolution by survival of the fittest.
Note that complexity theory now suggests that the mechanism of evolution is natural selection operating on emergents at all systems levels, and that the systems level of the individual is much less important than was previously thought.
See: Phaseolus vulgaris.
Necrosis, necrotic
Diseased plant tissue that is brown and dry.
A necrotrophic parasite is one that kills its host tissue before obtaining nutrients from it. Necrotrophic parasites are rather uncommon in plant pathology. See also: Biotrophic.
The sugary substance produced by plants, and made into honey by bees. The function of the nectar is to attract pollinating insects, as well as humming birds and other pollinating organisms.
Any organ of a flower or plant that secretes nectar. The usual function is to attract insects for the purpose of pollination.
Negative feedback
See: Feedback.
Negative screening
A screening technique designed to identify and eliminate the least desirable plants, as opposed to positive screening, which involves identifying and preserving the most desirable plants. This technique is often used with recurrent mass selection, in which the undesirables are weeded out, and the best plants are left to cross-pollinate.
It can also be used with genetically diverse populations of a tree crop in order to reduce parasite interference, and promote population immunity. See also: Cocoa.
A pesticide that kills nematodes, or round worms. These worms are often parasites of crops. They are usually soil-borne, and nematocides are usually applied as soil fumigants.
A class of worms called round worms. Plant parasitic nematodes are often called eelworms. They are invariably microscopic, and are usually soil inhabitants, which attack plant roots, often causing considerable damage to crops. However, leaf-invading nematodes are also known.
The scientific discipline concerned with the study of nematode worms, many of which are parasites of both plants and animals.
The new Stone Age. This was the last stage of the period when people depended solely on stone tools. It saw the start of agriculture, which depended on the domestication of plants. The period lasted 7000-4000BC and was followed by the Bronze Age.
The potatoes that resulted from an experimental breeding of Solanum andigena in order to confirm that it was the original parent of Solanum tuberosum. The change was complete after a mere five generations of recurrent mass selection, and the neo-tuberosum has provided a considerably widened genetic base for breeding purposes.
Nephelium litchi
The litchi fruit, which originated in China. An evergreen tree that is usually propagated vegetatively. Little scope for amateur breeders.
New encounter parasite
If a parasite and its host evolved independently, in different parts of the world, and were then brought together by people, the parasite is described as a new encounter parasite. The parasite would have evolved originally on a botanical relative of its new host. Potato blight (Phytophthora infestans) is a new encounter parasite, which evolved in Mexico, while its crop host evolved in South America. The new encounter occurred in Europe. See also: Old encounter, Re-encounter.
Non-governmental organisation.
Nicotiana spp.
N. tabacum produces commercial tobacco, used for smoking, chewing, and snuff. N. rustica has a much higher nicotine content than N. tabacum, and it is occasionally used for smoking, and for nicotine extraction for use as an insecticide.
Horizontal resistance is particularly important in tobacco intended for smoking, as these leaves must obviously be entirely free of chemical pesticides. Tobacco is an easy crop to breed but the now widespread dislike of smoking means that this is a crop in decline. It is not recommended to amateur breeders for this reason.
However, pyrethrum shows promise as a pesticide alternative for tobacco farmers, and this is a suitable crop for amateur breeders.
Nicotine is extracted from tobacco and can be used as an insecticide, often in the form of nicotine sulphate. It is apparently a stable insecticide. However, it is toxic to humans and is generally avoided for this reason.
Niederhauser, John S.
Working with potato blight (Phytophthora infestans) in Mexico, in the 1950s, John Niederhauser was the first scientist to deliberately avoid vertical resistance in favour of horizontal resistance. He was awarded the World Food Prize in 1991.
Night soil
Human excrement that is spread on crops as organic manure. As it can carry various intestinal parasites, it should be employed with caution.
Niloparvata lugens
The brown plant hopper of rice. This is one of the relatively few insect parasites of crops in which there is a vertical subsystem. Unfortunately, the miracle rices proved unusually susceptible, and vertical resistance proved unsuccessful in its control.
In the Philippines, it is being controlled with insecticides, but also with IPM, and farmer-participatory schemes breeding for horizontal resistance.
Nitrates are one of the commonest plant nutrients, applied either as artificial fertilisers or organic manures.
Nitrogen is an essential component of proteins and it is normally absorbed by plants as inorganic chemicals, usually as nitrates. Nitrogen deficiency shows as poor growth and a general light green colour. Nitrogen is mobile nutrient.
Nitrogen fixation
Although the atmosphere consists mainly of nitrogen, plants are unable to absorb it directly. Many micro-organisms fix atmospheric nitrogen in forms that plants can use, but the most important by far are the nitrogen-fixing nodules formed by Rhizobium on the roots of legumes.
Nitrogen-fixing root nodules
Nodules formed on the roots of plants of the botanical family Leguminoseae, by bacteria called Rhizobium. These nodules are able to convert atmospheric nitrogen into protein. This is a symbiotic association in which the bacteria provide protein, and the plant provides carbohydrates. This is one of the reasons why pulses and fodder legumes are such valuable crops.
It is possible to isolate the bacteria from these nodules, and culture them in order to inoculate the seed of leguminous crops. Commercial cultures of Rhizobium are known as legume inoculants. Some species of legume have Rhizobium strains in common, while other have their own special strains.
Nitrogenous fertilisers
All crops need nitrogenous fertilisers, either in the form of artificial fertilisers, organic manures, green manures, or by nitrogen-fixing micro-organisms and legumes.
During the hours of darkness, as opposed to diurnal.
A joint in a plant stem that bears one or more leaves, usually with an axillary bud between the petiole and the stem.
No-eye pea
See: Cajanus cajan.
Non-industrial country
Previously called ‘Third World’ or ‘less-developed’ countries, these are the poorer countries in which up to 80% of the population are engaged in agriculture, which is mostly subsistence agriculture.
They are provided with overseas aid by the rich industrial countries, and plant breeders associations, particularly in universities, show promise of becoming the most effective form of aid in agriculture.
Non-linear system
See: Linear system, and Self-organisation.
In terms of the gene-for-gene relationship, an infection is described as non-matching when the parasitism gene(s) of the parasite do not match the resistance gene(s) of the host (i.e., the biochemical key of the parasite does not fit the biochemical lock of the host). The vertical resistance then functions and the infection is unsuccessful. See also: Matching.
Non-target organisms
Organisms, particularly insects, that are unintentionally killed by crop protection chemicals. The most important are pollinating bees, and the agents of biological control.
Normal distribution
The mathematical characteristics of a population in which a quantitative variable, such as horizontal resistance, shows every degree of continuous difference between a minimum and a maximum.
The normal distribution is defined by two parameters: the mean or average, which locates the centre of the distribution, and the standard deviation, which determines the spread of the distribution. When plotted as a graph, the normal distribution is a bell-shaped or Gaussian curve. See also: Skewed distribution.
Noug oil
See: Brassica carinata.
Nucellar embryo
A plant embryo that has developed from the maternal tissue of the nucellus, without pollination. Such an embryo will produce a plant that is genetically identical to the maternal parent. This is a form of apomixis.
Nucellar seed
In most plants, seeds are produced as a result of fertilisation of an ovule by a pollen cell. In a few plants (e.g., citrus, mango), embryos can also be produced directly from maternal tissue (the nucellus), without any fertilisation by pollen.
Seeds with nucellar embryos are called nucellar seeds, and they have two agricultural advantages. First, like true seeds, they do not carry virus diseases, or any of the other parasites whose transmission is blocked by seed propagation. Second, they are genetically identical to the female parent, and they constitute a form of vegetative propagation.
Nucellar seeds can thus be used to produce clones, with few of the dangers of transmitting parasites that are normally associated with vegetative propagation.
See also: Apomixis.
The nutritive maternal tissue surrounding an ovule.
That part of a Eucaryote cell that contains the chromosomes.
See: Myristica fragrans.
While the nutrition of mammals requires organic chemicals, the nutrition of plants requires inorganic chemicals, the exception, in both cases, being iron. This is why artificial fertilisers have been so widely used in agriculture.
Organic farmers avoid the use of artificial fertilisers because they are destructive to the soil ecology, and can pollute ground-water systems. For plant nutrition, organic agriculture depends on a combination of good farming practices, soil genesis, and the addition of compost and manure.