Glossary: G

Gaia hypothesis
The hypothesis developed by James Lovelock that postulates that the entire biosphere is a single, self-organising, non-linear system.
Gall
An abnormal plant growth, usually more or less spherical, and usually induced by a plant parasite.
Gamete
A sexual haploid cell that may be from either a male or a female, and which unites with another gamete from the opposite sex to produce sexual recombination in a zygote, which is diploid. In plants, the male gametes are called pollen, and the female gametes are ovules.
Gametic sterility
Sterility that results from the fact that either the male or the female gametes are infertile.
Garcinia mangostana
The mangosteen. A delicious fruit native to S.E. Asia. The plant is a dioecious tree, and the seeds are parthenogenetic. Not recommended for amateur breeders.
Garlic
See: Allium sativum.
Gaümannomyces graminis
Previously called Ophiobolus graminis, this fungus causes ‘Take-all’ disease of wheat and other cereals.
Gaussian curve
The bell-shaped curve of a normal distribution.
Gene
The unit of inheritance which is carried on a chromosome. An inherited character may be controlled by a single gene (i.e., a Mendelian gene), or it may be controlled by many genes (polygenes). See also: Allele.
Gene banks
The popular term for collections of plants made for purposes of genetic conservation. A gene bank may consist of collection of seeds, which have to be re-grown periodically, or of an arboretum of tree crops.
Gene frequencies
Mendelian breeding emphasises single genes, and gene-transfers by pedigree breeding.
Biometricians’ breeding emphasises polygenes, and changes in their gene frequencies by population breeding, recurrent mass selection and transgressive segregation.
For example, horizontal resistance is a polygenic character, and its level can be increased by increasing the frequency of its polygenes in a single individual.
Gene pool
The totality of genes possessed by a population of sexually reproducing organisms.
Gene-for-gene relationship
A gene-for-gene relationship exists when each gene for resistance in the host has a corresponding (or matching) gene for parasitic ability in the parasite. The gene-for-gene relationship was discovered by H.H. Flor in 1940.
This phenomenon is the definitive characteristic of the term vertical, and the concept of the vertical subsystem. When the host and parasite genes match, the vertical resistance does not operate, the infection is successful, and parasitism occurs. When the genes do not match, the vertical resistance functions, the infection is unsuccessful, and parasitism does not occur.
The sole function of the gene-for-gene relationship is to control the population explosion of an r-strategists parasite, which usually has an asexual reproduction that leads to a particularly rapid multiplication. This control is commonly achieved by reducing the proportion of allo-infections that are matching infections. But it can also function by reducing the growth, and hence the reproduction, of a non-matching parasite (see quantitative vertical resistance).
A gene-for-gene relationship will evolve only in a discontinuous pathosystem, and in seasonal host tissue (i.e., annual plants, or the leaves and fruit of deciduous trees or shrubs).
For mathematical reasons, it is thought that all individuals of both host and parasite in a wild plant pathosystem have half of the total genes available. This provides the maximum heterogeneity, and the maximum effectiveness, for a given number of pairs of genes in a system of biochemical locks and keys (see n/2 model).
General systems theory
The general systems theory concerns the properties that systems have in common. It is often helpful to study a system in terms of this theory, and in terms of other systems.
There are many different kinds of system, such as solar systems, political systems, ecological systems (ecosystems), mechanical systems, legal systems, electrical systems, and so on. The concept of the pathosystem is based on the general systems theory.
Systems theory is now divided into the general systems theory and complexity theory, which developed out of it. Systems theory is based on the concept of a pattern, and of systems levels, which are patterns of patterns. Thus a book is a pattern of chapters, each of which is a pattern of paragraphs, and so on down to words and letters.
In biology, a population is almost synonymous with ‘systems level’. Thus a forest is a population of trees, a tree is a population of leaves, a leaf is a population of cells, and so on.
Key aspects of systems theory involve the concepts of suboptimisation, emergent properties, and the holistic approach.
See also: Complexity theory, Linear systems, Non-linear systems.
Generation
A plant generation is generally considered to be the life span starting from seed and extending to the next production of seed.
Some annual plants produce up to five generations per year. At the other extreme, some trees require many decades to complete a single generation.
In plant breeding, one breeding cycle may embrace several plant generations, such as a multiplication generation, and selfing generations for single seed descent and late selection.
Genetic advance
The increase in the level of a quantitative variable that results from recurrent mass selection. For example, after one screening generation, there might be a 5% increase in the yield, or in the level of horizontal resistance to a particular species of parasite.
Genetic base
The totality of polygenes at the start of a population breeding program.
Consider a simplified model. Ten parents each possess 10% of the polygenes controlling horizontal resistance. Each parent is thus very susceptible, and the parent population is also very susceptible. But each parent possesses polygenes that no other parent possesses. This means that the genetic base contains 100% of the polygenes controlling horizontal resistance.
The purpose of the population breeding is to bring all these polygenes together in one individual by recurrent mass selection and transgressive segregation.
In practice, some 10-20 different parents, consisting of modern cultivars, preferably originating from independent breeding programs, will normally provide an adequate genetic base for a horizontal resistance breeding program. If the base proves to be inadequate, it can always be widened at a later stage by adding new parents to it.
Genetic code
The system of genetic information storage in DNA and RNA molecules in living organisms. The genetic code is analogous to writing, as a method of storing information.
Genetic conservation
The preservation of genetically controlled characters in gene banks, which consist either of stored seeds, or of living museums in botanic gardens and arboretums.
The concept of genetic conservation was first developed by Mendelians with respect to vertical resistance genes. It is of relatively minor importance for biometricians, and polygenically inherited characters such as horizontal resistance.
However, the conservation of old cultivars is of considerable importance to organic farmers, at least until such time as superior, new, horizontally resistant cultivars become available.
Genetic diversity
Genetic diversity means that the individuals within a population differ in their inherited attributes.
Wild plant populations are typically diverse. Most subsistence crops in tropical countries are also diverse. But modern commercial crops usually have genetic uniformity.
A genetically diverse population has genetic flexibility. A fundamental ecological principle states that diversity leads to stability.
Genetic engineering
A technique that makes it possible to change the genetic make‑up of an individual or species, by introducing a gene from a different species. Of necessity, this technique can work only with single-gene characters, and its scientific popularity is responsible for much of the regrettable neglect of the far more important many-gene (polygenic) characters.
Genetic flexibility
A genetically diverse population has genetic flexibility in the sense that it can respond to selection pressures.
For example, if a host population has too little horizontal resistance, it will gain resistance. This happens because resistant individuals, being less parasitised, have a reproductive advantage over susceptible individuals that are more heavily parasitised.
Both the proportion of resistant individuals, and the levels of resistance (see transgressive segregation), will be increased accordingly in the next generation.
See also: Genetic inflexibility.
Genetic homeostasis
The tendency of a population to maintain a genetic composition that provides an optimum balance with its environment. See also: Homeostasis.
Genetic inflexibility
Most modern cultivars are either pure lines or clones, and they are genetically inflexible in the sense that they do not respond to selection pressures during cultivation. This is a valuable characteristic because it ensures that useful agricultural properties are not lost.
See also: Genetic flexibility.
Genetic line
A line of descent in which each generation is descended from, and related to, the previous generation.
Genetic male sterility
See: Male sterility.
Genetic modification
A term used by molecular biologists to describe the results of genetic engineering. It is an unfortunate choice of words, which is best avoided, because any form of breeding constitutes genetic modification.
Genetic source of resistance
Mendelian breeders working with single-gene resistances must first find a genetic source of resistance, usually in a wild progenitor.
This concept has been so pervasive that many believed the only way to breed for horizontal resistance was to first find a source of resistance. This is incorrect, as polygenic characters cannot be transferred in the way that single-gene characters are transferred, either by back-crossing or by genetic engineering.
Breeding for horizontal resistance involves changing gene frequencies, and this can be achieved with susceptible parents using recurrent mass selection, provided that the genetic base is wide enough.
Genetic uniformity
A genetically uniform population in which all the individuals are identical.
Such a population lacks genetic flexibility in the sense that it cannot respond to selection pressures. For example, if a genetically uniform host population has too little horizontal resistance, it cannot gain more resistance, because all the individuals have an equal level of resistance, and are equally parasitised. No individual has a reproductive advantage over any other individual and, consequently, there will be no change in the level of resistance in the next generation. See also: Genetic flexibility.
Genetic uniformity means that all the individuals within a population are identical in one or more of their inherited attributes. Modern crops are typically uniform because they are cultivated as pure lines, hybrid varieties, or clones. A genetically uniform population has genetic inflexibility and this is desirable in agriculture because it ensures that valuable characteristics will not be lost.
See also: genetic diversity.
Genetically modified organism (GMO)
Any organism that had been modified by genetic engineering. The organisms involved range from micro-organisms modified to produce complex pharmaceuticals, to herbicide-resistant and parasite-resistant crops, and pigs intended to provide transplant organs for humans.
Genetically modified pharmaceuticals
Some rare pharmaceutical products can be produced in quantity by genetically modified micro-organisms, and this is a more acceptable application of genetic engineering that the use of GMOs in agriculture. This is because no alternatives to these often essential drugs exist, and the patient either takes them or does without. Even if these drugs did have long-term adverse effects, this is usually considered less damaging to the patient than having no drugs at all. In comparison, there is much less justification for the use of genetically modified crops in agriculture.
Geneticist
A scientist who studies genetics. Plant and animal breeders are often called applied geneticists.
Genetics
The study of biological inheritance. There are two branches of genetics called the Mendelian and the biometrical.
Mendelians study single-gene characters, which are either present or absent with no intermediates.
Biometricians study many-gene (polygenic) characters, which are continuously variable between a minimum and a maximum.
Some scientists recognise a third branch called population genetics, which studies the changing frequencies of Mendelian genes within a natural population. However, this term should not be confused with the very different population breeding.
Obviously, a good geneticist studies all aspects of genetics equally. Plant and animal breeding are sometimes described as applied genetics.
Gene-transfer breeding
See: Pedigree breeding.
Genome
The monoploid set of chromosomes which, in a homozygous plant, occurs in a gamete, and consists of all the genes. A term often used loosely to mean the complete set of genes in a plant.
Genotype
The genetic constitution of an organism, as opposed to its actual appearance, which is called the phenotype. The distinction allows for recessive genes and polygenes, which may be present but not expressed because of heterozygosity.
Genus
In the taxonomic hierarchy, a genus is a subdivision of a botanical family, and it normally constitutes a number of species. A genus is group of closely related species, which have clearly defined characteristics in common.
All plants have two Latin names; the first is the generic name, and the second is the specific name. The adjectival form is ‘generic’, as in inter-generic hybrid.
Geometric series
The series 20, 21, 22, 23, etc., with arithmetic values 1, 2, 4, 8, etc., is a geometric series. This series is relevant to the Habgood nomenclature, and the Person-Habgood differential interaction.
Geotropism
The response of a plant to gravity. A tap root exhibits positive geotropism, and grows downwards. An apical shoot exhibits negative geotropism, and grows upwards.
Germ tube
The microscopic tube, extruded by either a pollen grain or a fungal spore, that penetrates the stigma or the host tissues, as the case may be.
Germination
The first step in the growth of either a seed or a spore.
Germination percentage
A seed testing term which defines the viability of a seed lot.
Gherkins
See: Cucumis sativus.
Gibberellic acid
Also known as gibberellin, this compound was originally isolated from a fungus (Gibberella fujikuroi) but is now known to occur in all plants.
Many different gibberellins, called GA1, GA2, etc., have been identified. Gibberellins are plant growth substances that tend to affect the entire plant.
They stimulate growth and have many commercial applications such as breaking potato seed tuber dormancy, increasing celery stalk length, suppressing seed formation in grapes, increasing the size of ornamental flowers, and delaying fruit maturity.
Ginger
See: Zingerber officinale.
Globodera rostochiensis
A cyst-forming nematode, known as the ‘golden nematode’, that is a serious pest of potatoes and tomatoes.
Glomerella spp.
A poorly defined genus of fungi, which are often the perfect (i.e., sexual) stages of Colletotrichum.
Gluten
The main protein in wheat flour. Gluten allows wheat dough to stretch and this makes the production of bread possible, by allowing gas bubbles to develop in the dough from fermentation by yeast.
Glycine max
The soybean, now the most important grain legume in the world, with Brazil, USA, and China being the largest producers.
Soybean was domesticated in China about one millennium BC. However, the modern expansion in cultivation came only after breeding in the USA had produced types suitable for mechanical harvesting and with appropriate day-length responses, because photoperiod sensitivity limits a cultivar to a narrow belt of latitude.
Modern production is as an industrial crop for edible oil extraction (20-23% of the seed) and a high protein meal used mainly for animal feed, but with increasing prospects for human food. In the Far East, soybeans are utilised as soy sauce, soya milk, bean curd or tofu, and green beans. Soybeans can also be used as a pasture crop, and for hay and silage.
Soybean is self-pollinated. About 1% of natural cross-pollination occurs and can be utilised in a recurrent mass selection program if a suitable marker gene can be found. amateur breeders should aim primarily at horizontal resistance, with a view to producing cultivars for organic farmers.
Glycyrrhiza glabra
Liquorice. A pernnial herb of the family Leguminosae, which is of ancient cultivation in Central Asia and Southern Europe. It has sweet rhizomes and roots. The sweetness comes from glycyrrhizin.
GMO and GM
See: Genetically modified organism.
Golden gram
See: Phaseolus aureus.
Golden nematode
See: Globodera rostokiensis.
Gooseberry
See: Ribes grossularia.
Gossypium spp.
Cotton. This genus has about thirty species that are divided into linted and non-linted species.
The four linted species in cultivation are divided into Old World and New World cottons. The two Old World cottons are diploid and are Gossypium arboreum and G. herbaceum. The two New World cottons are tetraploid and are G. barbadense and G. hirsutum. The last of these is Upland cotton and is responsible for about 95% of world production. The long staple Sea Island and Egyptian cottons are G. barbadese and account for about 5% of world production.
Cotton is naturally cross-pollinated, but it is tolerant of inbreeding and inbred cultivar can be maintained. population breeding presents no difficulties.
Since the first use of DDT, cotton has suffered a major vertifolia effect with respect to its insect pests. The increased susceptibility has been aggravated by the boom and bust cycle of insecticide production, and the tendency for politicians and bankers to interfere in the cultivation of the crop.
Population breeding for horizontal resistance in cotton is likely to produce unexpectedly promising results. However, cotton breeding is somewhat technical, particularly in assessing fibre yield and quality, and it should perhaps be undertaken by university breeding clubs.
Gourds
See: Cucurbitaceae.
Gradient
See: Parasite gradient.
Grafting
The technique in which a scion is biologically joined to a stock.
The stock is usually a horizontally resistant rootstock, and this provides a means of controlling root and trunk diseases. The scion is usually a high quality but susceptible cultivar.
The classic example of this control method was the grafting of classic wine grapes on to American rootstocks in order to control Phylloxera.
Occasionally, a double graft is used, as with a susceptible rubber trunk being grafted to both resistant rootstocks and leaf blight-resistant crowns. Other uses of grafting include the grafting of potato parents on to tomatoes to produce a vine with many inflorescences for use in true seed production.
There are two general techniques of grafting. A bud graft involves inserting a bud of the scion under the bark of the stock, and is the usual method for tree crops. A wedge graft involves inserting a wedge of the scion into a V-slit cut into the stem of a decapitated stock, and is the usual method for herbaceous plants.
A third technique is the ‘approach’ graft in which the cut surfaces of the stems of two separately rooted plants are bound together, but it is rarely used.
Grain crops
This term covers all crops in which the harvestable product is a small seed. It includes all the cereals, the grain legumes, the pseudo-cereals, and mustard.
However, oil seed crops, such as sunflower, flax, and canola, are not normally considered to be grains.
Grain Legumes
See: Leguminoseae.
Gram
See: Phaseolus.
Gramineae
The grass family. Cultivated members of this Monocotyledonous family include the cereals, fodder grasses, and sugarcane. According to some taxonomists, the bamboos are also members of this family.
There are about 8,000 species of grass in some 700 genera. From the human point of view, this is quite the most important family of plants as it provides most of our food, either directly or indirectly (all beef is grass).
Gram-positive
A staining test used in the identification of bacteria. The bacteria are stained with crystal violet and then iodine. They are then washed in a solvent such as acetone or alcohol. Gram-positive bacteria retain the stain, while gram-negative bacteria lose it.
All plant pathogenic bacteria are gram-negative, except Corynebacterium spp.
Granadilla
See: Passiflora edulis.
Grapes
See: Vitis vinifera.
Grapefruit
See: Citrus paradisi.
Grassland
Most grasslands are natural, and sown grassland (pasture) is a relative new concept in agriculture. Natural grasslands are known variously as savannah, prairie, pampas, scrub, veldt, chaparral, and steppes.
Grass
Any member of the botanical family Gramineae. This family includes the cereals, the fodder grasses, sugarcane, and, according to some taxonomists, the bamboos.
Green manure
A crop grown specifically for improving the soil by being ploughed into the soil while still green. Green manures often consist of legumes which fix nitrogen.
Green pea
See: Pisum sativum.
Green Revolution
The Green Revolution resulted from the development of dwarf wheats and rices that could tolerate heavy applications of nitrogenous fertiliser without lodging. This led to major increases in the world production of food and earned Norman Borlaug a Nobel Peace Prize in 1970.
Greenhouse
Both glass and plastic film are transparent to light but opaque to radiant heat. A greenhouse absorbs sunlight which is re‑radiated internally as heat.
Temperate greenhouses have the problem of inadequate sunlight during winter and at night, and they have to be artificially heated.
Tropical greenhouses have the problem of excessive heat, and they have to be cooled. Refrigeration is prohibitively expensive, and the best cooling is by good ventilation that evaporates large amounts of water.
Greenhouses also protect plants from rain and hail, and they can be fumigated.
Commercial greenhouses are used for producing crops out of season. In plant breeding, research greenhouses are used mainly to reduce the length of the breeding cycle by increasing the number of plant generations in one year.
Greenhouse cooling
The cooling of a plant breeding greenhouse in hot seasons can be difficult. The external surface of the glass or plastic can be sprayed with a heavily diluted white plastic emulsion paint in order to reduce the light absorption. Rapid changes of air, by extraction fan if possible, and the evaporation of large amounts of water, offer the most efficient and the most economical cooling. Refrigeration is far too expensive.
Greenhouse effect
This is a geophysical effect, in which the so-called greenhouse gasses (i.e., carbon dioxide, methane, etc.) in the atmosphere are transparent to sunlight but opaque to radiant heat. This phenomenon is believed to lead to global warming.
Greenhouse gasses
See: Greenhouse effect.
Greenhouse screening
Screening a breeding population inside a greenhouse. Because of the requirements of on-site selection, greenhouse screening is inadvisable, except when breeding a crop that is to be cultivated in a commercial greenhouse. However, other components of the breeding cycle (e.g., multiplication, single seed descent, pollination) may be undertaken out of season in a greenhouse in order to reduce the total breeding time.
Gregarious
Growing or living in groups. Gregarious plants grow in closely-spaced clumps; gragarious insects tend to clump together and so do not cause evenly-distributed crop damage. This is important to note while screening for horizontal resistance.
See also: Sociability scale, Patchy distribution
Grid screening
Grid screening is a technique for overcoming parasite gradients and patchy distributions of parasites, in field screening. The entire screening population is divided into a grid of suitably sized squares, and relative measurements are used to select the best individual in each square. Squares that are totally free of a parasite should be eliminated from the screening process.
Gros Michel
The most popular cultivar of banana during the first half of the twentieth century. It eventually went out of production because of the new encounter Panama disease. Nevertheless, the continuous monoculture of a single clone, for half a century, over a huge area in the tropics, where there is no closed season, was a remarkable indication of the possibilities of horizontal resistance.
Groundnut
See: Arachis hypogea.
Groundnut, Bambara
See: Voandzeia subterranean.
Growth chamber
A special chamber with controlled light, heat, humidity, and atmosphere, for conducting research into plant growth. These chambers are often useful when studying crop parasites, but they are expensive, and are not normally necessary for amateur breeders.
Grub
The larval stage of many insects, including many crop parasites.
Guano
Semi-fossilised excrement of fish-eating birds. Guano was much prized as a source of natural phosphate but is now in short supply.
Guar
See: Cyamopsis tetragonolobus.
Guava
See: Psidium guajava.
Guignardia bidwelli
An Ascomycete fungus that causes black rot of grapes.
Guinea corn
See: Sorghum.
Gumbo
The West African name for okra, Abelmoschus esculentus.
Gum trees
See: Eucalyptus spp.
Guttation
The excretion of water by plants, usually at night, when atmospheric humidity is high, and transpiration is restricted. Typically, this makes lawns wet in the early morning. This wetness is often mistakenly called dew. A true dew is caused by condensation of water from a saturated atmosphere.
Gymnosperm
Seed forming plants whose seeds are not protected by a seed coat. This group includes the conifers, cycads, yews, and Ginkgo. Gymno- is Greek for naked, and the name Gymnosperm has the same root as gymnasium.
Gymnosporangium
A genus of rust fungi, in which most species are heteroecious, with the summer stage mainly on Cupressus spp., and the winter (sexual) stage on pome fruits.