Glossary Plant Breeding

termed primary dormancy. However, if a germinable seed becomes dormant under
specific unfavourable conditions during storage (excessive drying) or germination
(high temperature), it is referred to as secondary dormancy. It is regulated by external
coverings (testa and pericarp) and tissues surrounding embryo. Sometimes,
underdeveloped embryo may be the real cause of dormancy. All these physiological
variables may be governed and/or influenced by genetic factors. Dormancy may be
released by heat and light treatments, treatments by growth hormones (e.g., GA 3
@100-1000 ppm with or without kinetin @ 50 ppm) or with chemicals such as KNO 3
(0.2%), thiourea (0.5 - 3%).
Dose Rate (Radiation). The amount of radiation received per unit time. The time over
which a given dose is received is a very important consideration. Large, single doses
delivered at short time intervals (minutes/hours) are known as acute doses, in contrast
to chronic doses that might be experienced continuously over the whole life cycle or
over a large period.
Double Cross (Jones 1918). The F 1 between two single crosses [(A x B) x (Cx D)]. The
number of possible double crosses using n inbreds would be [ n.(n-1).(n-2).(n-3) / 8 ].
Double cross hybrids have now been replaced by single cross hybrids in maize.
Double Helix (Watson and Crick 1953). The structure of DNA with two interlocking
helices joined by hydrogen bonds between paired bases.
Doubled Haploid. An otherwise diploid plant produced by doubling the chromosome
number of the haploid (2n =2x, 4x, 6x, etc).
Drift. The changes in the gene and genotypic frequencies in small populations due to
random processes.
Drought Avoidance. A mechanism of drought resistance. It is also called drought
tolerance with high tissue water potential. It consists of mechanisms to reduce water
loss from the aerial portion of plants, and of mechanisms to maintain water uptake.
The maintenance of water uptake under drought condition is related to several
properties concerning roots of plants such as root size and efficiency, root density,
size of xylem vessel, and the like. Genotypic differences have been noted for all these
attributes. The reduction of water loss from aerial portions of the plant includes
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