Breeding aluminum tolerance

Aluminum-tolerant genotypes have been identified. Based on the patterns of aluminum accumulation in the plant tissue, three groups of aluminum-tolerant plants may be identified: those with apparent exclusion mechanisms allowing lower accumulation of aluminum in their roots than aluminum-sensitive plants; those with

less aluminum in shoot but more in roots; and those with high aluminum accumulation in the shoot. Research in wheat suggests the possibility of more than one aluminum-tolerance gene and more than one aluminum-tolerance mechanism. In one study, two QTLs associated with aluminum tolerance were identified in the F2 population of diploid alfalfa and confirmed in the backcross population. Breeding for aluminum tolerance helps to expand crop productivity to acidic soils.

Mineral deficiency stress

Concepts associated with mineral deficiency

Mineral deficiencies or toxicities are widespread. ACIAT report 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 that tend to raise soil pH. A high soil pH in turn causes mineral deficiency problems. Common mineral deficiency symptoms are summarized in. 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.

            Improving nitrogen use efficiency is an important breeding objective, being a vital component of yield and end-use quality. The price of nitrogen fertilizer is increasing, so are the associated environmental concerns. Classical and molecular techniques are being used in this search for genotypes with improved NUE. QTLs involved with NUE are being sought by researchers.

Oxidative stress

Concepts associated with oxidative stress

Oxygen free radicalshave been implicated in a variety of environmental stresses in plants. They are involved in many degenerative conditions in eukaryotic cells. 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. The plant has a system of complex scavenging of 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, the activated oxygen form is also produced by photosynthesis.