Principles of Plant Genetics and Breeding

400 CHAPTER 21
Table 21.2 Examples of warm and cool season crops.
Cool season plants Warm season plants
Sugar beet Okra
Cabbage Eggplant
Apple Corn
Wheat Cotton
Barley Sugarcane
Cauliflower Peanut
Sunflower
Sorghum
Note: some species have wide adaptation with varieties that are
adapted to both cool and warm growing regions.
night temperatures. In cowpea, plants that were
exposed to high temperature during the last 6 hours of
the night showed a significant decrease in pollen viability
and pod set. Further, this damage was more pronounced
in long days than short days. Other researchers
also show that the stage of floral development most sensitive
to high night temperature was between 7 and 9
days before anthesis.
Excessive heat in the soil affects the emergence of
seedlings of both cool and warm season crops causing
reduced crop stands. High temperatures tend to accelerate
reproductive development. This may be part of the
reason why the potential grain yields of warm season
crops (e.g., rice, cowpea) are usually higher in the subtropics
than tropics.
Breeding for resistance to heat stress
Breeding for resistance to heat stress has not been as
widely addressed as other environmental stresses that
plants face in crop production. Heat resistance is more
beneficial to the producer than heat tolerance. Some
plant breeders use a direct measure of heat resistance
in an approach to breeding whereby advanced lines
are grown in a hot target production environment.
Genotypes with greater yield than current cultivars are
selected as superior. This breeding approach is more
applicable for species that can be efficiently yield-tested
in small pots (e.g., wheat) than for those that require
larger plots or are more difficult to harvest. Breeders
may also use this approach in environments where heat
is the only major stress. When other stresses occur, the
evaluation of heat damage is less conclusive (e.g., insect
pests can cause damage to developing flower buds, similar
to that which would occur under heat stress).
An approach to breeding heat resistance that is
deemed by some to be more efficient is to select for
specific traits that confer heat tolerance during reproductive
development. To do this, genotypes with heat
tolerance have to be discovered. This involves screening
large accessions from germplasm collections. These genotypes
can then be crossed with desirable cultivars if they
lack the yield and other plant attributes desired.
The use of a controlled environment (hot greenhouse)
has the advantage of providing a stable high
night-time temperature and stable air temperature from
day to day and over a longer period of time. It is conducive
to screening for reproductive-stage heat tolerance.
However, the facility can handle only a limited
number of plants, compared to thousands of plants in
a field evaluation. Selection aids (e.g., leaf electrolyte
leakage) have been used by some researchers to identify
genotypes with heat tolerance.
Mineral toxicity stress
Plants obtain most of their nutrient requirements from
the soil, largely from the products of weathering of
mineral rocks or the decomposition of organic matter.
Uptake in improper amounts may lead to toxic consequences
to plants.
Soil nutrient elements
Metals occur naturally in soils, some of which are
beneficial and essential for plant growth and development,
while others are toxic. About 16–20 elements
have been identified as essential to plant nutrition.
These may be broadly classified into two groups based
on the amounts taken up by plants as major (macro)
nutrient elements (these are required in large amounts)
and minor (micro) nutrient elements (required in
very small amounts) (Table 21.3). Each element has an
optimal pH at which it is most available in the soil for
plant uptake. However, at extreme conditions of soil
reaction, excessive amounts of some elements become
available. Some micronutrients are required in only trace
amounts; their presence in large quantities in the soil
solution may be toxic to plants. Some of the known
toxicities of metallic elements occur at low pH (high
acidity) and include iron and aluminum toxicities.
Aluminum toxicity
Aluminum (Al) is one of the most abundant elements
in the earth’s crust. One of the most important metal