An improved variety of an outbreeding species , such as maize, sorghum, or alfalfa, which is a genetically diverse population. Although this population consists of a cross-pollinating, seed-propagated species, most of the plants within it are high yielding, high quality, with high resistances, and agronomic suitability. These qualities may be preserved, or even enhanced, by careful selection of the individual plants destined for seed. Alternatively, if this selection is not practised, the qualities may decline after a few generations, and the seed stocks must then be renewed. System of locking
In a wild plant pathosystem, the gene-for-gene relationship acts as a system of locking which controls allo-infections, and reduces the population explosion of an r-strategists parasite. This system of locking is an emergent, which can be observed only at the population level of the system. See also: Vertical subsystem, n/2 model. Systemic
In a wide sense, 'systemic' means involving the entire system. In the context of crop protection chemicals, it means an insecticide or fungicide that penetrates the entire plant, and which kills a parasite inside that plant. Systemic pesticide can thus cure parasitism, while protective pesticides merely prevent parasitism, but cannot cure it once the parasite has penetrated inside the host tissues. The practical difference is that protective pesticides must be applied before an epidemic or infestation starts, as a form of insurance premium. Systemic pesticides can be applied after the epidemic or infestation has started. There is also a toxicological difference in that protective chemicals can be washed off, but systemic chemicals can linger. Systems level
Most systems may be considered in terms of a hierarchical organisation, and each rank in the hierarchy is called a systems level. Thus, a system consists of various ranks of subsystems, and is itself part of a super-system. A pathosystem is a subsystem of an ecosystem, and it has subsystems such as the vertical subsystem, and the horizontal subsystem. Systems theory
See: General systems theory.
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