Glossary: Sa-Sn

Saaz hops
See: Humulus lupulus.
Sabadilla
This natural insecticide has the lowest known mammalian toxicity. However, it is difficult to obtain commercially.
Saccharum officinarum
Sugarcane. This giant grass is of very ancient domestication in New Guinea and it is derived from a continuous pathosystem. It consequently has no vertical resistance and it provides many magnificent examples of the utility and durability of horizontal resistance.
There are about twenty-five cane breeding stations in the world and most of them still use pedigree breeding. The most notable exception is Hawaii, which uses a population breeding technique called the ‘melting pot’.
Safflower
See: Carthamus tinctoris.
Saffron
See: Crocus sativa.
Sago palm
See: Metroxylon spp.
Sainfoin
See: Onobrychis viciifolia.
Saline
Salty, with reference to sodium chloride. Soils can become saline from inappropriate irrigation which allows excessive surface evaporation and salt accumulation. The ancient civilisations of the Tigris-Euphrates Valley declined, in part, from soil salination, but also from soil erosion.
Sanitation
Plant sanitation is generally taken to mean the use of clean seed, equipment, and soil, all with a view to reducing plant disease.
Sapodilla
See: Manilkara zapota.
Saprophyte
Loosely, a saprophyte is an organism that derives its nutrients from dead material, as opposed to a parasite that derives its nutrients from living material. Strictly, a saprophyte is a plant, and any other organism living in this way is a saprotroph.
Saturation technique
This is a technique for ensuring that no vertical resistance are functioning during screening for horizontal resistance.
For example, it is no longer necessary to use the one-pathotype technique when breeding for resistance to potato blight in the Northern Hemisphere, because the presence of the second mating type (A2) ensures that all vertical resistances will be matched very quickly.
Equally, when screening for horizontal resistance to wheat stem rust, the presence of the alternate host, which is barberry, will produce a similar saturation of vertical pathotype. However, it is illegal in some countries to cultivate barberry in this way.
Scab
A plant disease that produces scablike symptoms. For example, potato common scab is caused by the bacterium Streptomyces scabies, and potato powdery scab is cause by the fungus Spongospora subterranea.
Scale insects
Members of the Homoptera (aphids, whiteflies, etc.), scale insects are often pests of crops. The first instar is an active insect with legs and antennae, but the subsequent instars are immobile and protected with a scalelike covering. Some species are valuable and are cultivated to produce shellac or cochineal.
Scarlet runner
See: Phaseolus coccineus.
Schistocerca gregaria
The desert locust. A species of grasshopper that has huge population explosions and very destructive migrations in Africa and the Middle East.
A large swarm may conrain a million tons of locusts, and each insect eats its own weight in green matter every day. This is the most destructive of many species of grasshopper with similar habits in various parts of the world.
Schlanstedt rye
See: Rimpau.
School children
It is entirely feasible to have a secondary school plant breeding clubs in which the children do the actual breeding. Ideally, such a club should be ‘twinned’ with a nearby university breeding club.
This approach to a combined education and plant breeding was first used in Nigeria when IITA scientists gave true seed of cassava to school children for this purpose. It is a highly recommended approach for foreign aid.
Schooling of fish
The schooling of fish provides an excellent example of systems levels, emergent properties, and shows the suboptimisation that can occur with reductionism (i.e., working at too low a systems level).
A scientist studying a single fish in an aquarium can't see or study schooling, which requires the holistic approach. In the same way, scientists who studied individual plants and not populations (i.e., pathosystems) ended up breeding for vertical resistance rather than the more holistic horizontal resistance.
See also: Flocking of birds, n/2 model.
Scientific monopolies
Science thrives on competition and a scientific monopoly is debilitating because it kills competition. The International Research Centres of the CGIAR have tended to become scientific monopolies that have favoured vertical resistance.
Scion
The piece of a plant that is used for grafting on to another plant, which is called the stock.
Sclerenchyma
Plant tissue in which the cell walls are thickened to provide mechanical strength. These cells have usually lost their living contents.
Sclerospora graminicola
A fungal plant pathogen belonging to the Order Peronosporales (i.e., downy mildews). It causes a mildew on many grasses in the tropics and subtropics and can be damaging on cereals such as millets, sorghum and maize.
Sclerotina sclerotiorum
An Ascomycete fungus that can cause severe disease on a wide range of crops, and is characterised by the formation of black sclerotia. The disease if often worse under wet conditions, such as waterlogging or continuing heavy rain.
Sclerotium
An over-wintering body of that is produced by some Ascomycetes. Sclerotia are usually visible to the naked eye and may be up to a centimetre long. They have a black surface but are usually white inside. They germinate to produce an apothecium lined with asci. (Plural: sclerotia).
Screening
An essential step in population breeding. A large heterogeneous population is screened to find the best individuals that are to become the parents of the next generation.
When breeding for horizontal resistance, the best approach is to let the locally important plant parasites do most of the screening, by spoiling or killing all the susceptible individuals.
The holistic approach is to screen for high yield, on the basis that only resistant plants can yield well.
All measurements should be relative. That is, only the highest yielding plants are kept, regardless of how poor their yield may be in commercial terms.
Screening overkill
When screening a large population for horizontal resistance, there is a danger, in the early breeding cycles, that every individual will be killed and the entire breeding population lost.
This overkill can be prevented by using natural crop protection late in the season, to ensure that the least susceptible plants produce at least a few seeds.
Seasonal tissue
The system of locks and keys of the gene-for-gene relationship and the vertical subsystem requires a discontinuous pathosystem in order to function.
In practice, this means that vertical resistance will occur only in seasonal tissues. That is, in all tissues of an annual plant, and in the leaf and fruit tissues of deciduous trees and shrubs.
Crops that are derived from continuous pathosystems (e.g., sugarcane, sweet potato, cassava, olives) will not have any vertical resistances. However some crops have perennial tissue that is functionally seasonal, as with coffee leaf rust.
Secale cereale
Rye, which differs from all the other temperate cereals in being open-pollinated (maize is technically a tropical cereal). For this reason, rye responds to selection pressures during cultivation and it generally has good levels of horizontal resistance to all locally important parasites.
Rye is the least important of the temperate cereals and it is used mainly in the manufacture of rye bread and rye whisky.
Historically, it was important in those areas of Europe that could not grow wheat and which suffered periodically from ergot poisoning. These areas, such as Ireland, eastern Germany, Poland, and western Russia later replaced rye with potatoes, and they suffered more than most from potato blight.
See also: Rimpau.
Seed
In the strict sense, seeds are the result of pollination and sexual fertilisation. Apomictic seeds are true seeds produced from maternal tissue only without sexual fertilisation. Farmers often refer to the units of vegetative propagation as ‘seed’. Thus seed tubers, seed setts, etc.
Seed certification
Seed can be officially certified in various ways. True seed can be certified for identity of cultivar, purity of cultivar, freedom from pests, weeds, and disease, cleanliness, and germination percentage.
Seed tubers, setts, and other units of vegetative propagation, can be certified for identity of cultivar, purity of cultivar, freedom from pests, weeds, and diseases, with special emphasis on diseases that are not carried by true seed, such as virus diseases.
Seed cleaning
The main reason for cleaning grains that are intended for planting is to eliminate weed seeds and various insects. However, grain that is intended for milling, or marketing as food, must be cleaned of all foreign matter, such as chaff, soil, stones, etc.
Seed colour
Seed colour is important in some grain crops. Haricot beans, for example, have a wide variety of colours and local preferences can be strong. Seed colour is also important in some cereals, such as maize, and in some pseudo-cereals, such as amaranth.
Some seed colours can be used as marker genes in a breeding program.
Seed counting
Seed counting is important when determining the ‘hundred seed weight’ or the ‘thousand seed weight’ to ensure that high yields of grain crops are due to many large seeds, rather than to very many small seeds. The manufacturers of seed testing equipment have various designs of equipment for counting and weighing seeds.
Seed disinfection
Infected seed (i.e., seed carrying an internal pathogen) can be disinfected. The most usual method is a hot water treatment. From the point of view of amateur plant breeding, seed disinfection should be avoided because the disease reveals and eliminates susceptible individuals.
Seed dormancy
Many plants produce dormant seeds, and these have various survival advantages. In annuals, dormant seeds will ensure long-term survival if some ecological disaster, such as a major drought, has entirely destroyed the current population. In perennials, dormant seeds often germinate during an exceptionally favourable situation, such as after a forest fire.
Seed dormancy can occasionally be a nuisance in plant breeding and the dormancy must be broken. There are various techniques for doing this, such as hot water treatment, fermentation, or physical damage to the seed coat with abrasives or acid.
Seed dressing
A pesticide that is applied to the external surface of seeds.
Seed industries
In many crop species, there are farmers specialised in seed production. In addition, there are specialised seed merchants, and there is often legislative control backed by seed inspectors and seed testing laboratories. These various groups are known collectively as the seed industry for that crop.
It is noteworthy that the importance of a seed industry is directly proportional to the pest and disease susceptibility of that crop. For example, the need for certified seed potatoes is absolute, while the need for certified seed setts of sugarcane is negligible.
Seed lot
A seed lot is usually a batch of seed that has all come from one farm or one crop. The whole of one seed lot can be covered by one seed certificate, and it can be expected to behave uniformly.
Seed production
See: Seed industries.
Seed testing
Seed offered for sale is usually tested in a seed testing laboratory. The main test is for germination percentage, but other tests can include seed health, freedom from weed seeds, identity and purity of cultivar, etc.
Seed-borne parasites
Some parasites are carried in the seed, both true seed and vegetative seed tubers etc. True seed can be contaminated, infected, or infested. Contaminated seed carries pathogen externally and these can be destroyed by seed dressings.
Infected seed carries pathogens internally and can be disinfected by heat treatment. Infested seed carries post-harvest insects. However, seed-borne parasites are the exception rather than the rule and the use of true seed eliminates most parasites.
In the cereals, loose smuts produce infected seed, while covered smuts produce contaminated seed.
Seed tubers, setts, etc that are used for vegetative propagation will carry any parasite that happened to invade the parent plant. This explains why the need for seed health certification is so much more important with crops that are vegetatively propagated, and which have a vertifolia effect because of a history of breeding without disease pressure.
Segregation
In plant genetics, the term ‘segregation’ refers to the separation of specified traits within the population of the next generation.
Selection
The selection of individuals within a plant population can be positive or negative. Positive selection identifies the individuals to be kept, usually as parents of the next breeding cycle. Negative selection identifies individuals that must be eliminated, or at least prevented from producing pollen, to ensure that they are not represented in future generations.
These terms can also be applied to selection within variable populations of tree crops. For example, a cocoa plantation might be suffering from witch’s broom disease (Crinpellis perniciosa). A positive selection would identify the most resistant trees for propagation in a new plantation. A negative selection would identify the most susceptible trees for elimination, on the grounds that they were causing severe parasite interference, and their removal would greatly reduce the overall disease incidence.
Selection coefficient
The proportion of plants selected in the screening population during recurrent mass selection. A selection coefficient of 10% means that the best 10% of the plants in the screening population are kept to become parents of the next generation.
Selection coefficients of 1% and 0.1% are often used, and they exert very strong selection pressures.
Selection pressure
Pressure (in the sense of coercion, persuasion, or bringing pressure to bear) that induces changes in the genetic composition of a mixed population.
The mechanism of selection pressure is that the fittest individuals have a reproduction advantage, while less fit individuals have a reproduction disadvantage.
Thus, in the face of parasitism, resistant individuals are advantaged, while susceptible individuals are disadvantaged. Selection pressures can function only in a population that is genetically diverse and genetically flexible.
Selection pressures can be either natural (i.e., in a wild ecosystem) or artificial (i.e., in a plant breeding program).
The term selection pressure usually refers to micro-evolution. Natural selection pressures produce new ecotypes, and artificial selection pressures produce new agro-ecotypes, or cultivars.
Selection pressures can be positive or negative. Positive selection pressure leads to the accumulation of a quantitative variable (e.g., horizontal resistance) that is deficient, while negative selection pressure leads to the decline of a variable that is excessive or otherwise unnecessary (e.g., horizontal resistance in the absence of a parasite).
See also: Vertifolia effect.
Selection, family
See: Family selection.
Self-compatible
Flowers, or plants, that are self-compatible are able to pollinate themselves. See also: Self-incompatible.
Self-fertile
See: Self-compatible.
Self-incompatible
Flowers, or plants, that are self-incompatible are unable to pollinate themselves. See also: Self-compatible.
Self-organisation
This is a crucially important property of non-linear systems in which the concept of ‘organisation’ must be elaborated to that of ‘self-organisation’.
Fritjof Capra has defined self-organisation as the “spontaneous emergence of new structures and new forms of behaviour in open systems far from equilibrium, characterised by internal feedback loops and described mathematically by non-linear equations”.
All living systems are non-linear systems, and have the property of self-organisation, which includes the property of reproduction and self-replication. Life itself is an emergent property of such non-linear systems and so too are all those characteristics of life that used to be called ‘vital forces’.
In political terms, self-organisation is democracy, while a denial of self-organisation by an authoritarian government is fascism or dictatorship.
The importance of this phenomenon of self-organisation was first recognised by Adam Smith (1723-1790) in his book The Wealth of Nations, published in 1776, although Immanuel Kant (1724-1804) was apparently the first to use the term ‘self-organisation’
Self-organising crop improvement
This concept is closely related to that of the self-organising system of food production.
There were no professional plant breeders before 1900, and all crop improvement was undertaken by farmers and amateur breeders. With the re-discovery of Mendel’s laws of inheritance, and the consequent emphasis on single-gene characters, plant breeding became difficult, technical, expensive, and professional. The range of cultivars was severely reduced, and farmers had much less choice in the cultivars that they could grow.
Once there are thousands of amateur breeders around the world, supported by OPBF and other plant breeders associations, who are working with population breeding and horizontal resistance, and each producing cultivars perfectly balanced for their own local agro-ecosystem, then crop improvement will become self-organising. In a political analogy, this represents democracy.
It will also produce a wide diversity of cultivars, and it is a fundamental ecological principle that diversity leads to stability.
Self-organising system of food production
If we consider the food production of a country, we find a self-organising system.
Many farmers, acting individually, choose what crops to grow, and what cultivar of those crops to grow. Their decisions are based mainly on their environment, and on market demand, which comes from the decisions of individual merchants who buy their produce.
Systems of transport and food processing convert raw materials into marketable products and retailers make these products available to consumers through stores and supermarkets. These consumers choose what they buy, usually on a basis of either cost or quality. The stores prefer to stock items that move the most quickly, according to customer preferences.
There must be some government control to ensure purity and hygiene, and to prevent monopolies and cornered markets. But, in general, too much government control is damaging. This was revealed dramatically by the failure of the Soviet system of State-controlled agriculture.
For self-organisation to work, government control must be kept to the essential minimum. In a political analogy, self-organisation represents democracy, while over-control by a single institute represents dictatorship.
Self-pollination
Fertilisation with pollen coming from the same flower, or the same plant. Repeated self-pollination leads to homozygosity, and the formation of a pure lines. Note that cross-pollination within a clone (e.g., potatoes) is equivalent to self-pollination. See also: autogamy, inbreeder, cross-pollination.
Self-sown seedlings
In some crops, such as sweet potato, self-sown seedlings can be a useful source of genetic variation, and they can provide material for screening.
Self-sterile
See: Self-incompatible.
Semi-permeable membrane
A membrane that allows the passage of some (usually small) molecules but not (usually large) others. See also: Osmotic pressure.
Senescence
The aging of plant tissues, as with the ripening of fruit, or the leaf-fall of a deciduous tree. A feature of senescent tissue is that it loses resistance to parasites. For this reason, it is not normally feasible to breed for resistance to fruit rots and similar problems.
Sepal
The outer covering of a flower is made up of sepals, which may be either free or united. Their primary function is to protect the unopened bud.
Septoria
A fungus belonging to the Sphaeropsidales which are imperfect fungi that are believed to be Ascomycetes although no asci have yet been found. Various species of Septoria cause disease on a number of vegetable and cereal crops.
Sesame
See: Sesamum indicum.
Sesamum indicum
This tropical crop is known variously as sesame, simsim, beniseed, gingelly, and till. It became famous in literature as the code word (“Open Sesame”) for entering the treasure cave in the Arabian Nights.
Its seeds are the source of sesame oil, which is often regarded as being second only to olive oil in quality. The crop is self-pollinated and it exhibits very great variation in many characteristics. A dehiscent strain is suitable for combine harvesters. There is scope for amateur breeders.
Setaria italica
Foxtail millet. This cereal is used as human food in sub-tropical Europe and northern Africa. It is also important in India, Japan, and China. In Russia, it is used for brewing beer. In Britain, it is used as birdseed, and in USA it is grown for hay and silage. Both self-pollination and cross-pollination occur. This crop offers scope for amateur breeders.
Setts
Pieces of stem of sugarcane used for vegetative propagation. Each sett usually has three nodes and the cut ends are often dipped in a mixture of insecticide and fungicide.
The first crop from these setts is called the ‘plant crop’ while all subsequent crops are called ratoon crops, until replanting becomes necessary.
The term ‘sett’ is occasionally used for vegetative propagation in other crops.
Sewage solids
Solids separated out from sewage and used as a low quality organic manure.
Sex attractant chemicals
See: Pheromones.
Sexual fertilisation
See: Fertilisation.
Sexual recombination
The recombining of genetic traits that occurs when a male gamete fuses with a female gamete.
Shallot
See: Allium cepa.
Shareware
Computer software that is copyrighted but that can be distributed free of charge to anyone. The author of shareware requests a voluntary donation from regular users who find the software useful.
Shifting cultivation
A system of agriculture in areas of low population density. New land is cleared each season, and this provides better plant nutrition, and an escape from parasites. This system is also known as ‘slash and burn’.
Short-day
Many tropical plants are photoperiod-sensitive, and depend on a short, twelve-hour day to initiate flower production and, possibly, other processes, such as tuber formation. Equally many temperate plants depend on a long day to initiate these processes. Some plants are photoperiod-insensitive, or day-neutral. See also: potatoes.
Shot-hole
A leaf disease in which the dead central portion of a roughly circular lesion falls out, leaving a hole. When a plant has many of these lesions, it looks as if it had been shot with a shotgun.
Shrub
A woody, perennial plant that is too small to be called a tree.
Sibling
In common usage, this term means brother or sister, without the gender being specified. In plant breeding, siblings are all the plants that come from one parent, and they are often referred to as ‘sibs’. Full-sibs have the same male and female parents. Half-sibs have the same female parent that was randomly cross-pollinated, and the male parents are thus unknown.
Sigmoid growth curve
The seasonal, S-shaped, population growth curve of an r-strategists organism such as a crop parasite. Typically, the curve shows an initial slow growth (the lag phase), followed by a population explosion with logarithmic growth (the log phase), followed by a rapid slowdown (the leg phase) as environmental factors become unfavourable.
This kind of population growth is usually followed by a population extinction.
Silage
Green fodder crop material, such as grass or clover, that has been stored in a silo, or in a large plastic film tube, and allowed to ferment to become food for cattle. This is a chemical fermentation and it produces heat that both stops the fermentation and sterilises the silage.
Silica gel
This substance absorbs water vapour and it is put into air-tight containers to keep the contents dry. It is particularly valuable for the long-term storage of seeds in genetic conservation.
Sinapis alba
See: Brassica alba.
Single seed descent (SSD)
A quick method of producing pure lines in crops that are inbreeding, and seed-propagated, such as many cereals and grain legumes.
A breeding population may contain many individuals that are both genetically diverse and heterozygous. A single self-pollinated seed is taken from each individual and is grown to maturity. This process is repeated up to six times.
Each individual becomes more or less homozygous, but the population is still diverse. The best individuals are selected and kept as new pure lines, or as the parents of the next generation of recurrent mass selection.
The idea behind SSD is to save time. There is no screening until the process is complete. With perhaps three generations of SSD each year, with the aid of hydroponics and a greenhouse, it is possible to produce homozygous lines in two years, or less.
The more traditional method would require screening under field conditions in each generation of selfing and, in a temperate climate, with only one screening season each year, this would require up to six years.
Single-gene character
Any genetic character whose inheritance is controlled by a single gene. The inheritance of a single-gene character follows Mendel’s laws of inheritance.
Single-gene resistances
This term covers both vertical resistance and genetically engineered resistance. Neither kind of resistance is likely to be stable.
Sisal
See: Agave sisalana.
Skewed distribution
A normal or Gaussian distribution that is slanted towards one extreme or the other.
Smuts
Plant parasitic fungi of the Order Ustilaginales, so called because they usually produce large quantities of black spores that resemble soot.