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Glossary: I

The International Board for Plant Genetic Resources, located in Rome, Italy.
The International Centre for Agricultural Research in the Dry Areas, located at Aleppo, Syria. This is one of the CGIAR research stations.
Ichneumon flies
Small wasps belonging to the Hymenoptera, that parasitise other insects by laying eggs in them, in their early instars. These are useful biological control agents.
International Crops Research Institute for the Semi-arid Tropics, located in Hyderabad, India. This is one of the CGIAR research stations.
The International Institute for Tropical Agriculture, located at Ibadan, Nigeria. This is one of the CGIAR research stations.
Ilex paraguariensis
Yerba maté. An infusion of the leaves is similar to tea, and is popular in the southern areas of South America.
Immobile nutrients
Immobile nutrients cannot move around in the plant and, consequently, their deficiency symptoms appear first in the young leaves. Immobile nutrients include: Calcium, Boron, Sulphur, Iron, and Copper.
Immunity means that a host cannot be parasitised by a particular species of parasite. Thus, coffee is immune to wheat rust, and wheat is immune to coffee rust. Immunity is a non-variable. The maximum level of horizontal resistance may be an apparent immunity, but it is not true immunity because it is variable, and it can be eroded. Vertical resistance has often been called immunity, but it too is an apparent immunity because it operates only against non-matching strains of the parasite.
Impartial resistance
See: Partial resistance.
Imperfect fungus
A fungus that has never been known to produce ascospores, basidiospores, or oospores, and which consequently cannot be classified among the Ascomycetes, Basidiomycetes, or Phycomycetes respectively. The imperfect fungi are also known as fungi imperfecti, the Deuteromycetes, or the asexual fungi, and their reproduction is apparently entirely asexual.
Inbred line
A genetic line of plants that has been self-pollinated for a sufficient number of generations (usually a minimum of six) to produce individuals that are more or less homozygous, and which ‘breed true’. See also: pure lines.
A species of plant that is autogamous (i.e., self-pollinating). See also: Outbreeder.
Inbreeding cereals
Cereals, such as wheat, rice, barley, and oats that are self-pollinating and are usually cultivated as pure lines.
Inbreeding crops
Many of the cereals and grain legumes are inbreeding and they require hand-pollination during the breeding process. See individual crops for details. Most tree crops are outbreeding, with the notable exceptions of arabica coffee and peach.
Traditionally, during the twentieth century, inbreeding crops have been subjected to Pedigree breeding and the gene-transfer techniques suitable for single-gene characters, rather than the recurrent mass selection that promotes manygene characters. Consequently, many of them exhibit a marked vertifolia effect, and they are mostly good candidates for breeding for horizontal resistance by amateur breeders.
Inbreeding depression
The converse of hybrid vigour, or heterosis. When an outbreeder is repeatedly selfed there is a steady loss of vigour. When two of these inbred lines are crossed, they exhibit heterosis.
Inbreeding grain legumes:
The following grain legumes are inbreeders:
All cultivated species of Phaseolus, Arachis, Cicer, Glycine, Lablab, Lens, Pisum, Psophscarpus, and Voandzeia. See also: Outbreeding legumes.
When both self and cross-pollination are unable to fertilise, the pollination is described as incompatible. See also: Selfincompatibility.
Incubation period
See: Latent period.
Some crops, such as haricot beans, can have either the determinate or the indeterminate habit. With the former, they are self-supporting, bushy plants. And with the latter, they grow as vines. Potatoes are determinate plants but, when grafted on to tomatoes, they become indeterminate, and this is a very useful technique when many flowers are needed for the production of true seed for breeding purposes.
This term means that a species is native to the area in question. The converse words are exotic and foreign.
See: Indigofera spp.
Indigofera spp.
Several species of this genus of the Leguminoseae are cultivated for a natural blue dye called indigo, or anil. This dye has been used for at least 4000 years, and it is superior to the European woad (Isatis tinctora). However, with the development of analine dyes, the world market for natural dyes collapsed.
Induced deficiencies
Occasionally, a nutrient deficiency can be induced, in spite of the fact that there is an adequate amount of that nutrient available. For example, water softeners replace calcium salts with sodium salts. An excess of sodium salts can induce a potassium deficiency. For this reason, house plants should never be watered with softened water.
Industrial country
The politically correct term for the rich countries of the world. The poor countries used to be called ‘Third World’ countries but are now referred to as non-industrial countries.
Industrial melanism
In Britain, during the industrial revolution, a species of moth, which had superb camouflage colouring when resting on the bark of a tree, became very visible to insect-eating birds when the tree bark turned black from soot pollution. It was shown by breeding experiments that light-coloured moths could easily be changed to black, and vice versa. This is an example of the ability of reversible micro-evolution to change ecotypes.
Infected seed
Infected seed has internal parasites that cannot be reached by surface chemicals which would control contaminated seed. Typically, covered smuts of cereals produce contaminated seed, while loose smuts of cereals produce infected seed.
In a plant pathological context, this term is defined quite strictly. It is the contact made by one parasite individual with one host individual for the purposes of parasitism. See also: allo-infections, Auto-infection.
This term is normally taken to mean that a disease is caused by a parasitic organism, and that it can be transmitted from one host individual to another. But, in common usage, a laugh or a yawn can also be described as infectious.
This term is usually used in relation to insects but, in a wide epidemiological context, the terms infection and epidemic can be applied to all categories of parasite, including the insects.
See: Genetic inflexibility.
A flowering structure that has more than one flower. For example, the Umbellifereae are so called because each inflorescence is made up of many florets in an arrangement that is reminiscent of an umbrella.
Inheritance is described as monogenic when the character in question is controlled by a single gene. Monogenic inheritance is qualitative in its effects and it leads to discontinuous variation in which a character is either present or absent, without any intermediates. Inheritance is described as polygenic if the character in question is controlled by many genes, called polygenes. Polygenic inheritance is quantitative in its effects, and it exhibits continuous variation with all degrees of difference between a minimum and a maximum. All polygenic resistance is horizontal resistance, but not all horizontal resistance is inherited polygenically.
Initial inoculum
The size of the parasite population at the beginning of the epidemic. Other things being equal, a high initial inoculum leads to a more rapid development of the epidemic, while a low initial inoculum leads to a slower or later development of the epidemic.
The injury from parasitism is the actual amount of damage done to an individual host, or the average amount done to a host population, by the parasite. The frequency of parasitism is the proportion of host individuals that are parasitised. In a wild plant pathosystem, the injury from parasitism is inversely proportional to the frequency of parasitism. That is, the higher the frequency, the lower the injury, and, conversely, the higher the injury, the lower the frequency. In this way, the total damage from parasitism never exceeds a tolerable level that does not impair the host's ability to compete ecologically and evolutionarily. Vertical resistance, with its system of locking, reduces the frequency of parasitism. horizontal resistance, as a second line of defence, reduces the injury from parasitism. Continuous plant pathosystems, that have horizontal resistance only, usually have a high frequency of parasitism, and a low injury from parasitism.
In a crop science context, this terms means to introduce a parasite to a plant individual or population. Thus a screening population may be inoculated (or artificially infested) with one or more species of parasite in order to exert selection pressure for resistance. See also: Designated pathotype.
The living culture of a parasite that is used to inoculate a host individual or population.
Inorganic chemicals
Any chemical compound that does not contain one or more carbon atoms. It is noteworthy that plants absorb all their nutrients as inorganic chemicals (e.g., nitrates, phosphates, potash) while the higher animals, and people, absorb all their nutrients as organic chemicals. The exception is iron; plants absorb it in organic form while animals absorb it in inorganic form. Animals also absorb water, oxygen, and common salt as inorganic chemicals.
Insect cages
Small cages, usually constructed of stiff wire covered in muslin or mosquito netting, and used to cover an individual plant in order to confine insects to that plant. The main use for insect cages in plant breeding is to multiply insects for purposes of inoculating a screening population. Alternatively, insect cages may be used to protect research plants from natural infestation, or to measure the population growth rate of an insect, as an indication of host resistance to that insect.
Insect culture
The multiplication of insects, usually in insect cages, for purposes of inoculating a screening population. This inoculation might involve screening for horizontal resistance to the insect in question, or for horizontal resistance to a virus disease of which the insect in question is a virus vector.
A pesticide that kills insects. An insecticide may provide a stable protection (e.g., natural pyrethrins, rotenone, nicotine, soap, oils, etc.) in which case it does not break down to new insecticide-resistant strains of the insect. Or it may provide an unstable protection (e.g., DDT, and most modern synthetic insecticides) and lead to a boom and bust cycle of insecticide production.
Insects are a Class of Arthropods that have three pairs of legs, and three body regions (head, thorax, and abdomen). In addition, they nearly always have a pair of antennae, and the adults often have one or two pairs of wings. Insects usually reproduce with eggs, but live birth also occurs (e.g., aphids). Insect growth involves a series of 4-8 moults, and the stages between moults are called instars. There is often a metamorphosis, usually at the time of the last instar (e.g., caterpillars turning into butterflies or moths). Most insect parasites of crops cause damage during the early instars, and the function of the final adult instar is often one of reproduction only, without any feeding. See also: Aphid, Beetle, Ladybird, Stem borer, Thrips, Whitefly.
A stage of growth of an insect that is concluded by the moulting or shedding of the exoskeleton, which is incapable of growth or expansion. Most insect species have 4-8 instars, often concluding with a metamorphosis.
Institutional plant breeding
Plant breeding conducted by a large institute. This kind of breeding is usually expensive and, consequently, it favours cultivars with a wide climatic adaptability. In practice, this means the use of vertical resistance if at all possible. Institutional breeding does not normally allow for farmer-participation schemes and it tends to be autocratic. See also: Corporate plant breeding, Democratic plant breeding, and Self-organising crop improvement.
Integrated pest management (IPM)
A system of pest management in which every important parasite in a crop is monitored and crop protection chemicals are used only when absolutely necessary. The idea is to minimise the use of crop protection chemicals in order to reduce biological anarchy and to stimulate biological control. IPM is used mainly against the insect parasites of crops, and it is greatly assisted by horizontal resistance.
Intellectual property protection
Legislation that provides the equivalent of a copyright on a breeder’s registered cultivar. The sale of all propagating material of that cultivar is then controlled, and the breeder earns royalties on those sales.
Intensive crop
A crop that has high profit margins and which consequently justifies considerable expense in its production. Horticultural crops are intensive crops, while cereals are usually extensive crops.
Inter-generic cross or hybrid
A hybrid between two different genera. Inter-generic hybrids are rare, and are usually difficult to make. Not recommended for amateur breeders.
International Agricultural Bureaux
International Research Centres
Agricultural research centres located in non-industrial countries, and financed by industrial countries through CGIAR. The principle centres working with crops are: Maize and wheat (CIMMYT) in Mexico; rice (IRRI), in the Philippines; potatoes (CIP), in Peru; wet tropical crops (IITA), in Nigeria; dry tropical crops (CIAT), in Colombia; semi-arid areas (ICRISAT), in India; and in arid areas (ICARDA).
The part of a stem that separates two nodes.
Interplot interference
See: Parasite interference.
Interspecific cross or hybrid
A hybrid between two species within the same genus. This type of plant breeding is not generally recommended for amateur breeders who are hoping to develop new cultivars with high levels of horizontal resistance. But attempts at inter-specific crossing can be fun.
See: CIP.
See: Integrated Pest Management.
Ipomea batatas
The sweet potato. This crop originated in tropical South America. It was taken by Polynesians to Fiji and New Zealand, where it is known by its Peruvian name ‘kumara’. The Portuguese took it to Africa and the Far East where it is known by its Caribbean name of ‘batatas’, which is the origin of the English word ‘potato’. And the Spanish took it from Acapulco to the Philippines where it is known by its Mexican name of ‘camote’. It is now one of the more important tropical food crops. Although it is cultivated as clones, the crop sets true seed freely, and farmers often keep self-sown seedlings as new cultivar. The harvestable product is a tuber which, in the USA, is often incorrectly called a yam. This is an excellent crop for farmer-participation schemes, and for amateur breeders.
The wild progenitors of sweet potato are extinct. Ipomea purpurea is the morning glory.
Irish famine
See: Hungry forties.
Iron is an important plant nutrient. It is a component of many enzymes. Iron is also an immobile nutrient and iron deficiency shows first in the young leaves which become pale green and then yellow, even necrotic, but the veins tend to remain green.
The International Rice Research Institute, located at Los Baños, Philippines. This is one of the CGIAR research stations.
The process of supplying a crop with water. Irrigation may be overhead irrigation with sprinklers, or furrow irrigation with water poured between the rows. Flood irrigation is used with rice paddies, and with the annual floods of a river such as the Nile. In areas where water is scarce, drip irrigation and subsurface irrigation are now used.
Isatis tinctora
This plant provides a natural blue dye called woad, which is inferior to indigo.
This word can be either a noun or a verb. The noun usually refers to a micro-organism that has been obtained as a pure culture from a mixture of organisms. The verb refers to the process of making an isolate.
Isolation from foreign pollen
When subjecting an open-pollinated crop to recurrent mass selection, it must be isolated from other compatible crops to ensure that no unwanted pollen from outside introduces unwanted characteristics, such as susceptibility, in the population breeding.
Isolation to protect neighbours
Plant breeders may choose to isolate their work, in either time or space, or both, in order to protect neighbours from crop parasites. For example, the screening plots might be located in the middle of a large field or farm growing a different species of crop. In general, however, the requirements of on-site screening restrict the possibilities of isolation in both time and space.