Glossary Plant Breeding

the incompatible allele in the style opposes the penetration of pollen tubes with the
same allele.
Order Effects. The effects associated with the sequence in which parents are crossed to
produce three-way (triallel) or double cross (quadriallel) hybrids. A critical analysis of
the combining ability effects of higher order provides evidence for the relative
significance of the order in which the parents have been arranged to produce double
cross hybrids. It results as a consequence of epistasis. For instance, if two inbred lines,
A and B, are derived from one source and lines C and D from another, the highestyielding
double cross is likely to be obtained from pairings of the type (A × B) ×(C ×
D).
Order. A systematic category that follows the class but precedes family.
ORF. Open Reading Frame. A section of sequenced piece of DNA that begins with a
start codon (AUG) and ends with a stop codon (any one of UAG, UGA and UAA). It is
presumed to be the coding sequence of a gene.
Organelle. A subcellular structure of characteristic size, shape, and specialized function,
for example, mitochondria, chloroplast, endoplasmic reticulum or nucleolus.
Organism. The unit of a continuous lineage with an individual evolutionary history.
Organogenesis. The process of the production of organ systems in animal
embryogenesis.
Osmoregulation. The adjustment of osmotic potential. Under water deficit condition,
there is reduction in the osmotic potential of crop plants. This reduction may stem
from either a net increase in the cell solute concentration or the loss of water from the
cell. Thus a genotype having capacity for osmoregulation can maintain turgor pressure
which is necessary for normal cell function and growth. Low osmotic potential can
also result in lowering of the level of leaf water potential, which triggers stomatal
closure in response to water deficit. Osmotic adjustment during seed-filling period
could help maintain photosynthesis under water stress, resulting in improved drought
resistance and yield. Considerable genotypic differences for osmotic adjustment have
been found in sorghum and other crops.
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