Principles of Plant Genetics and Breeding

toxicities of economic importance to crop production is
that which occurs when aluminum concentrations are
greater than 2–3 ppm. At acid pH, Al 3+ ions predominate
in the soil. Aluminum is not an essential nutrient
for plants. At a pH of 5 or less, aluminum inhibits plant
growth by interfering with cell division in root tips and
lateral roots, increasing cell wall rigidity, reducing DNA
replication, decreasing respiration, and other effects. In
some cases, excess aluminum induces iron deficiency in
some crops (e.g., rice, sorghum, wheat). A visual symptom
of aluminum toxicity is so-called root pruning,
whereby root growth is severely inhibited. Stunting of
roots leads to chronic drought and nutrient stress in
afflicted plants.
Breeding for aluminum tolerance
Aluminum-tolerant genotypes have been identified.
Based on the patterns of aluminum accumulation in
plant tissue, three groups of aluminum-tolerant plants
may be identified: (i) those with an apparent exclusion
mechanism allowing lower accumulation of aluminum
in their roots than aluminum-sensitive plants (e.g.,
wheat, barley, soybean); (ii) those with less aluminum in
the shoot but more in the roots (e.g., wheat, barley,
potato); and (iii) those with high aluminum accumulation
in the shoot (e.g., pine trees). Research in wheat
suggests the possibility of more than one aluminumtolerance
gene and more than one aluminum-tolerance
mechanism. In one piece of research, two QTLs associated
with aluminum tolerance were identified in the F2 population of diploid alfalfa and confirmed in the backcross
population. Breeding aluminum tolerance helps to
expand crop productivity to acidic soils.
BREEDING FOR RESISTANCE TO ABIOTIC STRESSES 401
Table 21.3 Summary of selected essential mineral nutrients for plants and their roles.
Macronutrients
Nitrogen (N) Used in synthesis of amino acids and proteins; component of chlorophyll and enzymes
Phosphorus (P) Found in proteins and nucleic acids; critical in energy transfer process (adenosine triphosphate)
Potassium (K) Catalyst for enzyme reactions; important in protein synthesis, translocation, and storage of starch
Micronutrients
Calcium (Ca) Important in cell growth, cell division, and cell wall formation
Magnesium (Mg) Central atom of chlorophyll molecule; essential in formation of fats and sugars
Sulfur (S) An ingredient in vitamins and amino acids
Boron (B) Role in flowering, fruiting, cell division, and water relations
Iron (Fe) Component of many enzymes; catalyst in synthesis of chloroplyll
Molybdenum (Mo) Role in protein synthesis and some enzymes
Manganese (Mn) Role in phosphorylation, activation of enzymes, and carbohydrate metabolism
Zinc (Zn) Role in enzyme activation
Copper (Cu) Catalyst for respiration and carbohydrate and protein metabolism
Mineral deficiency stress
Concepts associated with mineral deficiency
Mineral deficiencies or toxicities are widespread. A
report from the Centro Internacional de Agricultura
Tropical (CIAT) in Peru estimates that about 60% of
the soils in the common bean production regions of
the world have some soil mineral problem. Soils that
are high in calcareous minerals tend to have high
amounts of basic elements (e.g., Ca, Mg, K) that tend to
raise soil pH. A high soil pH in turn causes mineral
deficiency problems (e.g., Fe, Zn, P). Common mineral
deficiency symptoms are summarized in Table 21.4.
Zinc deficiency in common bean has been reported in
production areas such as southern Idaho and Michigan.
Breeding efforts
Cultivars vary in their sensitivity to zinc deficiency.
Sensitive cultivars take up and store less zinc in various
plant parts and the seed than resistant cultivars.
Researchers in common bean identified a zinc
deficiency-resistant cultivar, “Matterhorn”, and subsequently
determined that a single dominant gene, Znd,
conditioned resistance to soil zinc deficiency.
Oxidative stress
Concepts associated with oxidative stress
Oxygen free radicals (or activated oxygen) have been
implicated in a variety of environmental stresses in