Glossary Plant Breeding

C 306. A tall wheat variety developed through a complex cross in 1965. This variety still
continues to be dominant wheat for drought (soil moisture stress) and low-fertility
conditions in some parts of the plains of India.
C 4 Pathway. A pathway of CO 2 fixation in many angiosperms in which the first stable
product of photosynthesis is C 4 compounds such as aspartate, malate, and
oxaloacetate (instead of 3-phosphoglycerate of the Calvin cycle). In mesophyll cells,
the CO 2 is assimilated by carboxylation of PEP. C 4 acids so-produced are then
transferred to the bundle sheath cells, probably by diffusion. Within these cells, the
CO 2 released by decarboxylation enters the PCR cycle, giving origin to 3-
phosphoglycerate. It is interesting to note that the C 4 pathway and PCR cycle are
integrated in different cells of the same plant. From evolutionary point of view, this
system of CO 2 fixation is most efficient, as photorespiratory losses (a characteristic of
C 3 plants) do not occur in this case. The initial enzyme in C 4 pathway is PEP
carboxylase, which is insensitive to O 2 . The bundle sheath cells get only CO 2 through
decaboxylation of C 4 acids, and the enzyme ribulose-1,5-diphosphate carboxylase
produces only phosphoglyceric acid (in absence of O 2 ). There is thus no
photorespiration.
Callus. An undifferentiated mass of cells, originating from an anther, microspore, plant
tissue ex plant, or other cellular sources, when cultured in vitro.
CAM. A recently discovered photosynthetic mode especially adapted to deserts called
Crassulacean Acid Metabolism. In this mode, several desert succulent plants
including the cacti keep their stomata closed during the hot daytime and open them in
the cool of the night. CO 2 absorbed through leaf openings is stored in organic acids
and not fixed until the next day. This delayed photosynthesis greatly reduces water
loss during the day, thereby enhancing the succulent plant’s ability to maintain water
balance and water storage.
Canalisation. A developmental buffering system reducing potential variation due to
genotype or environment.
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