Principles of Plant Genetics and Breeding

402 CHAPTER 21
Table 21.4 Common deficiency symptoms of selected
essential nutrient elements for plants.
Mineral Deficiency symptom
Nitrogen (N) Chlorosis or yellowing of leaves;
stunted growth
Phosphorus (P) Dark green leaves; purpling of plant
parts
Potassium (K) Marginal necrosis; weak stem and
lodging; leaf curling
Calcium (Ca) Terminal bud growth ceases or is
defective
Magnesium (Mg) Chorosis of older leaves
Sulfur (S) Chlorosis; weak stems
Boron (B) Death of terminal bud producing
growth called witches’ broom
Iron (Fe) Interveinal chlorosis of young
leaves
Molybdenum (Mo) Whip tail growth in coniferous
species
Manganese (Mn) Interveinal chlorosis
Zinc (Zn) Mottled leaves
Copper (Cu) Stunting; interveinal chlorosis
plants. They are involved in many degenerative conditions
in eukaryotic cells (e.g., peroxidation of lipids,
cross-linking and inactivation of protein, and mutation
in DNA). However, the biosynthesis of some complex
organic molecules, detoxification of xenobiotic
chemicals, polymerization of cell wall constituents,
and defense against pathogens are examples of essential
cellular activities that depend on oxygen free radicals.
Hence, the issue is not preventing their formation but
how to control and manage the potential reactions
of activated oxygen. Plants have a system of complex
scavenging or activated oxygen that is highly conserved
among plants.
Numerous sites of oxygen activation occur in the
plant cell. These sites are highly controlled and coupled
to prevent the release of intermediate products. It is
presumed that such a control or coupling breaks down
when a plant is under stress, resulting in leaking of
activated oxygen. Injuries to the plant occur when the
production of activated oxygen exceeds the plant’s
capacity to detoxify it. Symptoms of oxidative stress
include loss of osmotic responsiveness, wilting, and
necrosis.
There are two forms of activated oxygen that are
produced via distinctly different mechanisms. Most
biological systems produce activated oxygen via reduction
of oxygen to form superoxide, hydrogen peroxide,
and hydroxyl radicals. In photosynthetic plants, a singlet
oxygen form is also produced by photosynthesis.
Applications and breeding efforts
Several herbicides are designed to function by the
involvement of activated oxygen. These herbicides promote
the accumulation of metabolic intermediates and
the energy used to create singlet oxygen, which kills
the plant. These herbicides are described as photobleaching
(e.g., p-nitrodiphenyl ethers). Other herbicides
that depend on light and chlorophyll are paraquat
and diquat (both bipyridylium herbicides). So far, few
plants have been selected for tolerance to oxygen free
radicals.
Flood stress (water logging)
Whereas some plants are adapted to water-logged conditions
(e.g., flooded rice culture), most plants need
well-drained soils to grow properly.
Concepts associated with water-logging stress
In soybean, stress due to water logging can reduce
crop yield by 17–43% when it occurs at the vegetative
stage, and by about 50–56% if the stress occurs at the
reproductive stage. Floods are often caused by excessive
rainfall due to a prolonged seasonal rainfall. The excessive
amount of water quickly creates anoxic (oxygendeficient)
soil conditions causing flood-sensitive plants
to suffer anoxia or hypoxia. Fermentation occurs in
plant roots under such conditions. The photosynthetic
capacity of plants is significantly inhibited. Floodtolerant
species have certain adaptive mechanisms, such
as the formation of aerenchyma and adventitious roots.
Some studies indicate that root tissue survival under
hypoxia depends on the fermentation rate and sufficient
sugar supply to maintain cell energy and membrane
function.
Breeding efforts
Tolerance to water-logging appears to be quantitatively
inherited. QTLs for tolerance to water-logging have
been reported in rice and soybean.