Glossary Plant Breeding

Canalised Character. A character whose phenotype is kept within narrow bounds
despite disturbing forces. Development is such that all the different genotypes have
the same constant phenotype over the range of environments that is usual for the
species. The genetic differences are revealed if the organisms are put in stress
environment or a severe mutation stresses the developmental system.
Candidate Gene. A sequenced gene of previously unknown function that, because of its
chromosomal position or some other property, becomes a candidate for a particular
function such as disease determination.
Carbohydrate. A macromolecule composed of carbon, hydrogen, and oxygen.
Carbohydrates are the main source of energy and are also important components of
cell wall and intercellular materials. Monosaccharides are simple sugars having the
general formula C n (H 2 O) n . Ribose, deoxyribose, glucose, fructose, galactose,
mannose, etc. are monosaccharides. Disaccharides are sugars formed by the
condensation of two hexose monomers with the loss of one molecule of water. They
are, therefore, C 12 H 22 O 11 . Sucrose and lactose fall in this category. Polysaccharides
result from the condensation of many hexose monomers, with a corresponding loss of
water molecules. Their formula is (C 6 H 10 O 5 ) n . The most important polysaccharides in
living organisms are starch (plants) and glycogen (liver and muscles), and cellulose,
the most important structural element of the plant cell wall.
Carbon Source. A nutrient (such as sugar) that provides carbon to organisms needed for
the synthesis of organic molecules.
Carboxydismutase. Also called ribulose-1,5-diphosphate carboxylase; the primary
photosynthetic enzyme that represents about 50% of the stromal proteins. It leads to
the integration of CO 2 and H 2 O with ribulose-1,5-diphosphate to produce two
molecules of 3-phosphoglycerate (the first stable product of the PCR cycle). The
enzyme is highly sensitive to O 2 , and it also brings about oxygenation of ribulose-1,5-
diphosphate to produce one molecule of 3-phophoglycerate and one molecule of
glycolate. The latter then enters photorespiratory cycle and ultimately oxidised to C0 2
and H 2 O. This loss is the characteristic of PCR cycle (C 3 plants). The enzyme
comprises 16 sub-units (L 8 S 8 ): 8 large sub-units of high molecular weight (L 8 ), and 8
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