Principles of Plant Genetics and Breeding

398 CHAPTER 21
ice-nucleating proteins. The first field test of a bioengineered
organism was the testing of “ice minus”,
a microbe genetically engineered to be incapable
of producing the bacterial protein that causes ice
nucleation. This was intended to be an approach for
helping frost-sensitive plants survive frost.
3 Tolerance. Freezing tolerance occurs when a plant is
able to withstand both intracellular and extracellular
ice formation.
Selection for low-temperature tolerance
As previously stated, field survival trials have proven
to be inefficient for selecting genotypes with lowtemperature
tolerance. Because low-temperature stress
brings about many changes in plants (including morphological,
biochemical, and physiological changes),
researchers are pursuing these avenues in search of
selection aids. Some factors that have shown promise in
predicting low-temperature tolerance include plant
erectness in winter (for cereals), tissue water content,
and cell size. Unfortunately, these tests are not effective
in discriminating among small differences that are of
practical breeding importance.
Researchers commonly use controlled freeze tests
conducted in artificial environments to measure lowtemperature
tolerance. For example the field survival
index developed by Fowler and Gusta is often used.
The researchers found that the crown and leaf water
content of field-acclimated plants was a good indication
of field survivability. Molecular marker technology is
being pursued in the quest for QTLs associated with
low-temperature tolerance. Other biotechnology tools
are being explored to help transfer low-temperature
tolerance genes into cultivars. In spite of not being as
efficient and as desirable as controlled tests, field testing
remains a widely used screening approach in lowtemperature
tolerance breeding. When other selection
approaches are used, field testing is used as a final measure
of plant winter survival. Researchers can take various
precautions to improve the efficiency of field tests.
Breeding for tolerance to
low-temperature stress
Whereas the genetics of low-temperature tolerance have
been studied to a reasonable degree, breeders have only
had minimal success in applying research results to practical
breeding. Breeding superhardy cultivars remains a
challenge. Several reasons have been proposed by D. B.
Fowler and A. E. Limin to be causal:
1 Exploitable genetic variability for low-temperature
tolerance has been largely exhausted within the existing
gene pools of most species.
2 A large number of genes with small effects and complex
interaction are assumed to determine the phenotypic
expansion of low-temperature tolerance, making
selection difficult.
3 Current methodologies for measuring low-temperature
tolerance give poor resolution of small phenotypic
differences.
4 Measures of low-temperature tolerance lack the precision
for single-plant analysis and many are destructive,
making selection procedures complicated.
5 Poor expression of low-temperature tolerance in alien
genetic backgrounds has prevented the expansion
of gene pools through interspecific and intergeneric
transfers (e.g., the superior low-temperature tolerance
of rye is suppressed in the wheat background).
These researchers further observe that the ability to
acclimate or avoid low-temperature stress varies among
species and stages of crop growth. Hence, it is impossible
to develop a breeding approach that will be applicable
to all low-temperature tolerance breeding programs.
Each breeder would essentially have to design a unique
approach to breeding for cold stress.
Salinity stress
Soil salinity constraints to crop production occur in an
estimated 95% million hectares worldwide. Salinity is
the accumulation of dissolved salts in the soil solution to
a degree that inhibits plant growth and development.
Overview of salinity stress concepts
Soils with salinity problems are described as salt affected.
When the salt concentration measured in terms of electrical
conductivity (ECe ) is more than 4 dS/m and the
pH is less than 8.5, the soil is called a saline soil. When
the ECe value is less than 4 dS/m and the pH is more
than 8.5, the soil is a sodic soil. Sodic soils are high in
Na + but low in other soluble salts. Semiarid regions have
saline/sodic soils, whereby salts accumulate in subsoils
because of low permeability of the subsoil.
Salinity may have a natural origin (called primary
salinity) as a result of weathering of parent materials
that are rich in soluble salts. Human-aided salinity
(called secondary salinity) occurs as a result of agricultural
activities, especially irrigation with impure (salt-rich)
water. Salinity is often caused by a rising watertable.