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Glossary: L

Labelling
A feature of pedigree breeding is that every cross-pollination must be labelled, and this is very labour-intensive. With population breeding, labelling is unnecessary and this labour-saving can be devoted to more useful activities, such as the screening of larger numbers of crosses.
Labiate
A member of the botanical family Labiateae.
Lablab bean
See: Lablab niger.
Lablab niger
The bovanist bean, Egyptian bean, Indian bean, etc. This is a suitable crop for amateur breeders in warm countries working with recurrent mass selection and horizontal resistance.
Laboratory screening
When conducting recurrent mass selection, a laboratory screening can often enhance a field screening by determining aspects of quality that are not discernible in the field.
Labour-saving
Any plant breeding has a limit to the number of person-hours that can be devoted to it. Consequently, any labour-saving device will permit an increase in the number of crosses, and the number of plants in the screening population. When doing recurrent mass selection, there is no need to label crosses, or individual plants, or to keep detailed records of parasitism, etc. The only thing that matters is that the final selections must be the best plants of that generation and, the larger the screening population, the greater the genetic advance. Labour-saving is not laziness. It is increased productivity.
Lactuca sativa
Lettuce. This crop is a member of the Compositae family, and it is the main component of salads. There are four basic types known as ‘crisphead’, ‘butterhead’, ‘romaine’, and ‘leaf’. All types show great variation and there is considerable scope for increased horizontal resistance. The chief disease is downymildew caused by the fungus Bremia lactucae. Past resistance breeding has involved vertical resistance, and there is scope for horizontal resistance breeding by amateurs.
Ladybirds
Beetles of the family Coccinellidae. These beetles are distinctively oval, almost hemispherical, with a flat under-surface, and they are coloured red or orange, with conspicuous black spots. Both the adults and the larvae of many species of ladybird feed on other insects, particularly aphids, which are crop parasites, and the ladybirds are valuable agents of biological control.
Lagenaria siceraria
The bottle gourd. A monoecious member of the Cucurbitaceae, this is a very ancient crop that pre-dates pottery in many tropical areas. It is apparently the only crop that was common to both the Old World and New World before the development of trans-oceanic travel. Gourds are believed to have originated in Africa, and to have floated across the Atlantic Ocean to Brazil at a very early date. They also spread to India and China, and all parts of S.E. Asia. The dried hard shells of the fruit have a wide range of uses including bottles, bowls, spoons, ladles, tobacco pipes, musical instruments, and floats for fishing nets. The crop has a limited scope for local amateur breeders.
Landrace
A cultivated plant population which is genetically diverse and genetically flexible. A landrace can respond to selection pressures during cultivation. The maize crops of tropical Africa, which were so vulnerable to tropical rust, were landraces, and they responded to the selection pressure for resistance. Prior to the discovery of Johansen's pure lines in 1905, most crop varieties in the industrial world were landraces, and most subsistence crops in the non-industrial world are still landraces. See also: cultivar, Ecotype, Micro-evolution.
Larch
See: Larix spp.
Larix spp.
Larch, which is used as a plantation forest species of softwood. Not recommended for amateur breeders.
Larvae
The early instars of an insect are generally called larvae (singular, larva), particularly in insects that exhibit metamorphosis. Thus caterpillars are the larvae of butterflies and moths. It is often these juvenile stages that are voracious feeders, and that constitute some of the most serious insect parasites of crops. This term should not be confused with the molten rock that comes out of volcanoes, and is spelled lava. See also: Grub.
Late selection
Traditionally, selection is conducted on highly heterozygous individuals which then become the parents of the next screening generation. This is now called early selection. Late selection involves self-pollinating the variable progeny of a cross for 3-4 generations, using either the bulk breeding method or single seed descent, and producing a mixed population of relatively homozygous individuals. The late selection is made among these homozygous individuals.
Late selection is efficient because it produces plants with a reduced hybrid vigour, which can be misleading during the screening process, and it also produces a greater expression of recessive alleles, which are exhibited only in the homozygous state. The features of late selected plants thus have a higher heritability than those of early selected plants. However, this advantage must be equated with the longer breeding cycle required by late selection.
Latent period
In plant pathology, the period between infection and the start of pathogen reproduction. One of the many mechanisms of horizontal resistance is to increase the latent period, thus reducing the reproductive rate of the pathogen.
Lead
Before the days of DDT and synthetic insecticides, highly dangerous compounds of lead were often used to kill crop pests.
Leaf
The main site of photosynthesis, leaves are thin laminae of green tissue, and are typically carried by a stem-like petiole that emerges from a node of the stem. There is usually a leaf bud in the axil of each leaf.
Leaf hopper
Insects of the family Cicadellidae. Many are serious pests of crop plants.
Leaf miner
A plant parasitic insect that mines a tunnel between the upper and lower surfaces of a leaf. The tunnel has a white, translucent appearance, and it starts quite narrow but broadens as the insect larva increases in size.
Leaf spot
A spot, usually irregularly circular, and usually necrotic, caused by a pathogen.
Leek
See: Allium ampeloprasum.
Legume
A cultivated member of the botanical family Leguminoseae.
Leguminoseae
Legumes which are cultivated for their seeds, such as peas, beans, lentils, peanuts, soybeans, and grams, are known as grain legumes or pulses. Those that are cultivated for grazing, or hay, in order to feed farm animals are known as fodder legumes, and include clovers, alfalfa (lucerne), vetches, sainfoin, etc. Most of the pulses are self-pollinating, while the fodder legumes are mainly cross-pollinating.
Lemon
See: Citrus limon.
Lens esculenta
The lentil. A self-pollinated member of the family Leguminoseae, this is one of the oldest pulses and it has been cultivated in the wheat and barley lands of the Old World since the beginnings of agriculture. Most cultivars are pure lines and there is room for recurrent mass selection by amateur breeders. In addition to improved horizontal resistance, there is a need for higher yields, suitability for mechanical harvesting, and a reduction of flatulence factors.
Lentil
See: Lens esculenta.
Lepidium sativum
Garden cress, cultivated as a salad plant, and served as young seedlings.
Lepidoptera
The insect order that contains the butterflies and moths and is characterised by four large wings that are covered in scales. In the butterflies, the upper surfaces of the unfolded wings are usually brightly coloured sex attractants, while the lower surfaces of the wings have camouflage colours, which appear when the wings are folded together vertically. The moths fold their wings horizontally, and they then exhibit camouflage colours. Many of the early instars, known as caterpillars, or grubs, are serious crop pests. Most Lepidoptera are now rare because of the widespread use of insecticides.
Leptinotarsa decemlineata
The Colorado beetle of potatoes. Originally a parasite of the wild Solanum rostratum (buffalo burr, or prickly potato) in Colorado, USA, this beetle moved on to cultivated potatoes as a new encounter parasite, and became one of the worst insect pests in the whole of agriculture. It is a yellow and black striped beetle, the same shape as a ladybird, but much larger, being half an inch long. The larvae and beetles are voracious eaters of potato leaves and, if not controlled, they can destroy a potato crop.
Originally controlled with compounds of lead and arsenic, the beetles are now controlled with synthetic insecticides. Little breeding for resistance has been attempted, probably because no single-gene resistances could be found. An attractive project for amateur breeders working with horizontal resistance.
Lesion
Any visible damage or injury to a plant, usually caused by a parasite.
Lettuce
See: Lactuca sativa.
Lice
Plural of louse; the human louse is of interest in that it has never developed resistance to natural pyrethrins.
Life cycle
The complete cycle of events undergone by a living organism between birth (or hatching) and reproduction followed by death.
Lignin
The substance that is deposited in plant cell walls to make them woody. Lignin is thus the main constituent of timber.
Liliaceae
The lily family, which includes onions, leeks, garlic, shallots, etc. which are members of the genus Allium. Some taxonomists prefer to put this genus in a separate family called the Alliaceae.
Lima bean
See: Phaseolus lunatus.
Lime
This word has three quite distinct meaning in English. It can mean calcium oxide, as in quicklime; or the linden tree (Tilia europaea); or the citrus fruit.
Lime fruit
See: Citrus aurantifolia.
Lime tree
Tilia europaea. The lime tree, or linden tree.
Linden
See: Lime.
Line
In genetics, a line of descent. The term is used most frequently in the concept of a pure lines.
Linear system
The general systems theory originally concerned rather simple systems such as the solar system, and mechanical systems, such as clockwork. These are now called ‘linear’ systems, and they obey Newton’s laws. Modern complexity theory concerns more complex systems, which are non-linear.
Linear systems have parameters that are easy to measure, and outcomes that are easy to predict. Non-linear systems have parameters that are difficult to measure, and outcomes that are impossible to predict.
The solar system is a linear system. It obeys Newton’s laws of motion. Indeed, Newton formulated these laws to explain the behaviour of the solar system. We can predict the phases of the moon, and the tides, with great accuracy, for centuries ahead.
Weather systems, on the other hand, are non-linear and unpredictable. Weather forecasts of even a week ahead are famously unreliable.
In the context of complexity theory, ‘linear’ means that the parameters are fixed, while ‘non-linear’ means that the system parameters are likely to change. For example, a game of snooker is a linear system. But if the snooker table is on a ship in a rough sea, the game becomes a non-linear system.
The mathematics of non-linear systems is a very new, incomplete, and complex subdiscipline, and it originated in fluid dynamics.
In the context of complexity theory, linear also means that the output is proportional to the input, and the whole is equal to the sum of the parts. Non-linear means that the output is greater than the input, and the whole is greater than the sum of the parts. This ‘something extra’ consists of emergent properties.
All living systems are non-linear. Life itself is an emergent. So too are all the attributes of life, that used to be called ‘vital forces’.
See also: Self-organisation.
Linkage
Genetic linkage means that two genes are closely associated on one chromosome, and they tend to be inherited jointly. For example, a sex-linked gene will be expressed in one sex but not both.
Linnaeus
Carolus Linnaeus (1707-1778) was the father of taxonomy who introduced the binomial system of nomenclature.
Linseed
See: Linum usitatissimum.
Linum usitatissimum
Flax and linseed. This species is a self-pollinating annual. There are three categories of cultivar in this crop. Flax cultivars tend to be tall, with few branches and flowers. The stems are retted to produce linen. Linseed cultivars are relatively short, many-branched, and many-flowered, they produce seed used for the production of linseed oil. This oil was originally in great demand for the manufacture of paint, but it has now been almost totally supplanted by plastic latex paints. The dual cultivars can be used for both linen and oil production. A very new development comes from the discovery that newly ground flax seed is an excellent dietary source of Omega-3 polyunsaturated oils.
Flax is historically interesting in that H.H. Flor, in 1940, discovered the gene-for-gene relationship while working on flax rust (Melampsora lini) in Illinois, USA.
This crop is suitable for amateur breeders working with horizontal resistance and recurrent mass selection.
Liquorice
See: Glycyrrhiza glabra.
LISA
See: Low input sustainable agriculture.
Litchi
See: Nephelium litchi.
Local optimisation
A term from systems theory that concerns responses to variation within a system. In ecology, local optimisation is illustrated by the formation of ecotypes, which vary as a result of different selection pressures in various localities within the ecosystem. Each ecotype is locally optimised to its own locality. Similarly, genetically flexible landraces, or agro-ecotypes, are locally optimised to their own local agro-ecosystem, and they will invariably perform less well in a different agro-ecosystem. In plant breeding, the purpose of on-site selection is to achieve local optimisation of many quantitative variables such as horizontal resistances.
Locking
The system of locking that functions in the vertical subsystem of a wild plant pathosystem, controlled by the gene-for-gene relationship, apparently in accordance with the n/2 model, depends on a heterogenous mixture of locks and keys. A system of locking is ruined by uniformity (“What happens when every door in the town has the same lock, and every householder has the same key, which fits every lock?”). However, our use of vertical resistance genes in agriculture is based on uniformity, and this is why vertical resistance is temporary resistance in our crops.
Locks and keys
Every vertically resistant plant has one or more vertical resistance genes that collectively constitute a biochemical lock. And every vertically parasitic parasite has one or more vertical parasitism genes that collectively constitute a biochemical key. When a parasite is allo-infections a host, its key either does or does not fit the lock of the host. The allo-infection succeeds only if the key fits (i.e., a matching infection).
Lodging
Long-stemmed cereal plants are liable to be blown over when they are wet and heavy in a storm. This is called lodging. The basis of the Green Revolution was the development of short-strawed (i.e., dwarf) varieties of wheat and rice. These could be given high applications of fertiliser without risk of lodging, and the yields were increased accordingly.
Lolium spp.
Ryegrass. Two species are used in sown grass for fodder. Perennial ryegrass (L. perenne) and Italian ryegrass (L. multiflorum) are very important. They hybridise freely and offer scope for amateur breeders.
Long-day
Many temperate plants are photoperiod-sensitive, and depend on a long day to initiate flower production. For this reason, crops such as olives and hops cannot be cultivated in the tropics. Equally many tropical plants depend on a short day to initiate flower production and, possibly, other processes, such as tuber formation. See also: potatoes.
Loofah
See: Luffa spp.
Loose smuts
See: Ustilago.
Low-input sustainable agriculture (LISA)
A system of sustainable agriculture designed for farmers in non-industrial countries, which has low cash inputs, and minimum risks of soil erosion and other forms of damage to the farm. The crops are fertilised with farmyard manure and night soil.
Lucerne
See: Medicago sativa.
Luffa spp.
Monoecious vines cultivated for the production of the loofah-sponge which is prepared by retting. The fruit fibres were also used for a variety of filtering and shock absorbing functions. They have now been almost entirely replaced with plastics.
Lupin
See: Lupinus spp.
Lupinus spp.
Lupins have been cultivated since antiquity. They grow quickly on poor soil, they fix nitrogen, and they produce abundant seeds. However a perennial problem was the presence of toxic alkaloids. Modern breeding has eliminated these from a number of species, which show great promise as a source of protein for both humans and farm animals. Scope for amateur breeders.
Lycopersicon esculentum
The tomato. Although this is botanically a fruit, it is always considered to be a vegetable in culinary and horticultural terms. It is probably the second most important vegetable after potatoes.
The cultivated tomato is a self-pollinating, annual plant. It is plagued with parasites, largely because of low levels of horizontal resistance resulting from a century of the vertifolia effect. Much breeding has taken place in the past, but there has tended to be a very rapid turnover of cultivars because of the use of vertical resistance.
With the spread of the A2 mating type of blight (Phytophthora infestans) in the northern hemisphere, tomatoes have become more difficult to cultivate. When there was only the A1 blight, functional oospores could not be produced, and the only way in which blight could survive the winter was in potato tubers. This meant that tomatoes could get blight only from potatoes, and only rather late in the season. However, with functional oospores in the soil, tomatoes now get blight much earlier, and much more severely.
Organic gardeners can avoid blight be putting a temporary, transparent, plastic sheet roof over the tomatoes to ensure that the leaves and stems never get wet. Blight spores need free water on the leaves in order to infect. The plants must then be given furrow irrigation.
Tomatoes are a very promising crop for amateur breeders working for improved horizontal resistances by using recurrent mass selection.