Principles of Plant Genetics and Breeding

Table 21.1 Relative salt tolerance in plants.
Plant growth is inhibited in salt-affected soils because
the high salt concentration in the soil solution inhibits
the process of water absorption by osmosis (osmotic
stress). When excessive amounts of salt enter the transpiration
stream, plant cells may be injured. Plants that
are tolerant of high soil salt concentration are called
halophytes. Wheat is one of the most salt-tolerant
crops, while rice is one of the most salt-sensitive crops,
although both are only moderately tolerant. Maize is
also moderately tolerant of salts in the soil solution
(Table 21.1).
Breeding for salt tolerance
A common approach to breeding salt tolerance starts
with assembling and screening germplasm for salinity
tolerance. The selected genotypes are used as parents
to transfer the trait to desired cultivars, followed by
selecting desirable recombinants from the segregating
population. This approach has yielded some success in
species such as rice, wheat, and lucerne. The challenge
in breeding for salt tolerance is how to measure salinity
tolerance. Screening is commonly based on the growth
of plants under salt stress. Two distinct mechanisms
exist for salinity tolerance:
1 Tolerance to the osmotic effect of the saline solution
(the osmotic effect makes it harder for plants to
extract water from the soil).
2 Tolerance to the salt-specific nature of the soil saline
solution (a high sodium concentration makes it difficult
for the plant to exclude NaCl while taking up
other ions).
Screening for salinity tolerance is a long process and
requires a large amount of space to screen progeny
from crosses. Screening for specific traits is quicker and
more effective (they are less influenced by the environment
than growth rates are). The most successful traits
for assessing the salt-specific effect in salt-tolerance
breeding is the rate of Na + or Cl − accumulation in leaves.
BREEDING FOR RESISTANCE TO ABIOTIC STRESSES 399
Tolerant Moderately tolerant Moderately sensitive Sensitive
Barley (grain) Barley (forage) Alfalfa Strawberry
Cotton Sugar beet Peanut Bean
Bermudagrass Wheat Corn Potato
Oat Rice (paddy) Tomato
Soybean Sweet clover Pineapple
Sorghum Sweet potato Onion
This is measured as the increase in salt in a given leaf
over a period of time. Selection for these ions was used
in breeding rice and lucerne cultivars with high salt tolerance.
Traits for osmotic effects are related to growth
(e.g., leaf elongation, root elongation, shoot biomass,
leaf area expansion), the latter two being the most effective
indices. Molecular maker technology and genetic
engineering techniques are being used in salt-tolerance
breeding efforts. Salinity tolerance has been found in the
wild species of crops such as tomato, pigeon pea, and
common bean.
Heat stress
Heat stress may be defined as the occurrence of temperatures
hot enough for a sufficient time to cause
irreversible damage to plant function or development.
A heat-resistant genotype is one that is more productive
than another genotype in environments where heat
stress occurs. Heat tolerance is the relative performance
of a plant or plant process under heat compared with the
performance under optimal temperatures.
Overview of heat stress concepts
Heat stress occurs to varying degrees in different climatic
zones. High temperatures can occur during the day or
night. Also, temperature effects can be atmospheric or
in the soil, with air temperature varying considerably
during the day and night. Annual crop species may be
classified into two categories according to maximum
threshold temperatures as either cool season annuals or
warm season annuals (Table 21.2). Cool season species
are more sensitive to hot weather than warm season
species.
High night temperatures have detrimental effects on
the reproductive function of plants. It has been shown
that there is a distinct period during the 24-hour day
cycle when pollen development is most sensitive to high