The maintenance of cultivar purity during seed production is achieved by the combination of several factors, some directly relate to agronomy while others result to satisfactory crop planning.
Isolation
One major factor during the course of seed production is to ensure that the possibility of cross-pollination between different cross-compatible plots or fields is minimized. This is achieved either by ensuring that the crops which are likely to cross-pollinate are not flowering at the same time (i.e. isolation by time) or that they are isolated by distance.
In addition to the question of undesirable cross-pollination, adequate isola- tion also assists in avoiding admixture during harvesting and the transmission of pests and pathogens from alternative host crops.
Tolerance limits to genetic contamination
The maximum permissible or acceptable contamination resulting from undesir- able cross-pollination will depend on the species and the class of seed to be produced. It follows that a cross-pollinated crop species will have a higher degree of variation than a self-pollinated crop. While it may be thought desirable to reduce pollen contamination to zero, the amount of permissible contamination Fig. 3.3. Lettuce seed crop flowering in an insect-proof structure.
will vary with the species and the purpose for which the seed stock is intended (i.e. the intended seed class). Even if it were possible to exclude pollen contami- nation completely, it would not be possible to have a tolerance limit lower than the species’ mutation rate (Bateman, 1946).
The higher the class (or category) of seed, the lower the acceptable number of off-types and these tolerated levels are specified by seed certification author- ities or seed quality control agencies according to the crop species and seed class.
Isolation in time
This type of isolation is possible within individual farms or multiplication sta- tions; with careful monitoring, it is also possible in ‘seed production villages’
where only one cultivar of a crop is cultivated. In the planning stage, it is arranged that the cross-compatible crops are grown in successive years or sea- sons. This principle is easier to achieve in those areas of the world where the climate allows two successive seed crops to be grown in a year. Seed produc- tion centres, or organizations, responsible for the multiplication of relatively few cultivars can plan production so that no two cross-compatible cultivars are multiplied simultaneously.
Despite the possibility of isolation in time, there still remains the need to ensure that cross-compatible crops are isolated by distance.
Isolation by distance
This type of isolation is based on the concept that if a seed crop is sufficiently distant from any other cross-compatible crop, then the adverse pollen contami- nation will be negligible. In practice, it is impossible to prevent completely
‘foreign’ pollen reaching a crop because the wind can carry pollen grains or pollinating insects over relatively long distances.
Regulations or recommendations for isolation distances of specific crops take into account the method of pollination (i.e. whether the species is predomi- nantly self- or cross-pollinated) and the pollen vector (i.e. insect or wind). In some countries (e.g. the Netherlands), the minimum isolation distance between different groups or types of cultivars of the same species is greater than for cultivars of a similar type. For example, the minimum isolation distance between runner bean cultivars with different flower colours is greater than for cultivars with the same colour. The specified distances are also greater for classes of seeds to be used for further multiplication than for those distributed to growers for production of a market crop. The generally accepted isolation distances for individual crops are given in Chapters 7–16, but these are only guidelines as the regulations vary between countries. Much of the experimental work investigat- ing optimal isolation distances has been done in temperate regions and factors such as topography, prevailing wind, insect species and insect populations can influence the efficiency of isolation distances.
In practice, there are several possible outside sources of contaminating pol- len during anthesis of a seed crop, examples include:
● Compatible crops in cultivar trials.
● Private gardens.
● Workers’ crops.
● Cross-compatible commercial crops.
● Volunteer plants.
● Wild or escape species.
Sources of contamination from genetically modified organisms (GMOs)
Other sources of potential contamination from GMOs during the course of vegetable seed production will increase as the number and types of GMOs released in the field increases.
Present examples of these sources of contamination from GMOs may include:
● Admixture with GMO material during harvesting, seed processing or packaging, usually referred to as ‘adventitious’ material.
● Genetically engineered male lines designed for hybrid production.
● Glyphosate-tolerant cultivars or lines.
● Any cultivar developed for increased shelf life, e.g. tomato cultivars (including those for marketing fresh and those developed for processing).
● Cultivars engineered for changes in starch composition, e.g. maize for agricultural outputs.
● Cross-compatible materials within Cruciferae which have been designed for agricultural or biofuel outlets.
● Undesirable characters which have entered or contaminated other seed crops, weeds or garden plants with a common period of anthesis.
Zoning
In addition to the primary isolation requirement for a seed crop there are, in some countries, zoning schemes for specified geographical areas which control the species to be grown either for market or as seed crops in specified areas.
In principle, the specification of what is allowed to be grown in a given zone ensures that cross-compatible species, subspecies or types of related crops do not freely cross-pollinate. For example, there are different types of Beta spe- cies which are all cross-compatible and largely wind-pollinated, and they include mangel, fodder beet, sugarbeet, Swiss chard and red beet. By allowing only one of these types to be grown in a specified area or zone, the chances of highly undesirable cross-pollination between any combinations of them will be greatly reduced. Other horticultural crop groups for which zoning arrangements are made in some countries include Brassica campestris (turnip), Brassica olera- cea (cole crops) and Allium species. Zoning arrangements may be made by voluntary agreement between seed producers and/or plant breeders or included in seed production legislation.
Zoning regulations may call for the registration of any seed or market crop in a specified area regardless of the purpose for which it is produced; this is especially likely for crop species which flower in the course of the development of the marketable vegetable crop. In the USA, sweetcorn seed is produced in Idaho where it is isolated from maize with which it freely cross-pollinates (Delouche, 1980).
The concept of zoning could be applied to the production of GMO seed crops although more care would have to be taken with all compatible plant species in the zone in addition to the seed crops.
Discard strip technique
According to Dark (1971), the pollen concentration in the air over a field of a wind-pollinated crop increases from the windward edge downwind with a ten- dency to decline again at the lee edge. Therefore, during the period of anthesis when the wind would have been blowing from every direction, a strip around the field’s perimeter would have received relatively little of the crop’s own pol- len and there would have been maximum concentration in the centre. The marginal strip is important in the production of genetically pure seeds. When a cloud of contaminant pollen passes over the field, a small number of pollen grains will drop out at random. Those falling over the centre of the plot will compete with the relatively high concentration of the crop’s own pollen and have a lower chance of fertilizing, whereas those which fall on marginal areas will not have so much competition and will therefore have a higher chance of fertilization. Thus, the seeds from a 5 m wide strip around the perimeter of the plot are harvested separately and, according to the genetic quality found when testing a sample by growing it on, they can either be destroyed or placed in a lower category. The bulk of the seed will come from the inner area of the plot and can be kept as a separate seed lot.
Field or plot shape
Most pollen contamination of either wind- or insect-pollinated crops occurs around the perimeter of the plot or field. Therefore, if the area of each crop is kept as near as possible to a square then fewer seeds are likely to have been produced as a result of undesirable pollen and a minimum amount of seed is involved, if the discard strip is applied.