Glossary Plant Breeding

antibiosis in its mechanism. The latter two require an active insect response or lack of
response. However, the former (tolerance) response requires a thorough
understanding of the ways in which plants are injured by insects as well as the ways in
which plants may repair the damage. Tolerance is more subject to variation as result
of environmental fluctuations more than non-preference and antibiosis. The age or
size of the plant and size of the insect population strongly influence the degree of
tolerance. Since tolerance does not involve prevention of insect infestation or injury to
the plant, some researchers prefer not to include it as a type of resistance.
Tolerance (Disease). An inherent or acquired ability of the host to endure a disease or
parasite. However, Virologists use the term to imply lack of symptom expression.
Tolerant cultivars show few or no disease symptoms although the virus is present.
Tolerance, like avoidance or resistance, helps the host to cope with the parasite. It
enables the host to suffer low biological damage compared to susceptible ones. To
evaluate tolerance, it is imperative to compare the damage inflicted by equal amounts
of parasite at the same developmental stage of each host genotypes. However, it is
difficult to measure because it is confounded with partial resistance and disease
escape (because of early maturity). A tolerant genotype should be biologically more
efficient and productive. When the quality of product depends on the absence of
disease symptoms, tolerance would be of little worth. For example, tolerance to smut
pathogens in cereals is of no value because they attack kernels.
Tonoplast. A unit membrane bordering the vacuole of plant cells. It has characteristics
similar to those of plasma membrane excepting greater permeability barrier.
Top Cross Test. A test for GCA of selected individuals/inbred lines or clones by
evaluating their progenies derived from crossing them with a single tester variety
(usually an open-pollinated variety).
Top Cross (Davis 1927). In corn, an inbred-variety cross; a cross between a selection,
line, clone, etc., and a common pollen parent which may be a variety, single cross, or
a number of elite inbred line. The common pollen parent is called the top cross or
tester parent. It is used to test general combining ability of inbreds or selections. Top
crosses to an open-pollinated variety, do in fact, reduce the burdensome labour of
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