Principles of Plant Genetics and Breeding

Figure 2 In vitro regenerated plants and seed-germinated
plants (control) growing in the greenhouse.
POLYPLOIDY IN PLANT BREEDING 225
Nursery stock clones for a breeding program
After in vitro plants were adapted to greenhouse conditions
for 2 weeks, they were used in a new, separate
study for 6 months. This study was designed to investigate
whether there could be late development of
somaclonal variants (off-types) amongst the in vitroproduced
plants. Some of the screenable variables that
were considered included the plant phenotype, plant
growth (as indicated by plant height, leaf size, and root
system development), and the number of shoots
formed by a single tuft arising from a single in vitro
plant. Once the plants were cleared of any detectable
abnormalities, they were then evaluated in
studies at Langston University, or at the USDA-ARS
Grazinglands Research Laboratory at El Reno, focusing
primarily on the screening of F 2 progenies for
potential segregation of genetically controlled variations.
No phenotypic reversal has so far been observed
in plants, which have been declared genetically
invariant following in vitro culturing. Generally, off-
types can be observed in new plants while they are still growing in test tubes prior to being transferred to the greenhouse. This was
the case in parallel studies on tall wheatgrass, which were conducted at the USDA-ARS Grazinglands Research Laboratory.
White stripe mutations were observed as soon as sizable new plants developed from callus (Kindiger 2002). The cause of those
mutations remains cryptic.
Conclusion
Classic breeding and clonal methods may be used to produce genetically uniform populations of plants. However, the purity of
such populations is more difficult to maintain through seed than clonal multiplication. Although the two crop improvement
methodologies may be used in complementary fashion in a single breeding program, the manner in which they are used may
depend on the breeding objectives and the species, among other factors. In the improvement program of tall wheatgrass, outlined
above, in vitro cloning was resorted to in the earlier stages of the improvement program. Seeds from agronomically selected individual
plants were used to initiate cloning by indirect regeneration. The selection of seed as the culture-initiating explant was
based upon our experience with the crop. Resulting clones were identical to their parents and were cataloged as germplasm for
the breeding program. A single treatment was used to initiate callus, shoots, and roots. All attempts to initiate callus from leaf and
root tissues were unsuccessful.
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