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. Some plant breeders use a direct measure of heat resistance 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

to species that can be efficiently yield tested in small pots. Breeders may also use this approach in environments where heat is the only major stress. When other stresses occur, evaluation of heat damage is less conclusive.

An approach to breeding for 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 and the trait amenable to effective measurement. This would involve screening large numbers of accessions from germplasm collections. Then these genotypes may be crossed with desirable cultivars, if they lack the yield and other desired plant attributes. The use of a controlled environment  has the advantage of providing a stable high nighttime temperature and stable air temperature from day to day and over a longer period. 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  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 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 nutrient elements and minor nutrient elements.  Each element has an optimal pH at which it is most available in the soil for plant up take. 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

and include iron and aluminum toxicities.

Aluminum toxicity

Aluminum is one of the most abundant elements in the earth’s crust. One of the important metal toxicities of economic importance to crop production is aluminum

toxicity that occurs when the aluminum concentration is greater than 2–3 ppm. At low pH, Al3 ions predominate in the soil. Aluminum is not a plant essential nutrient. At a pH of five 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. A visual symptom of aluminum toxicity is the so called root pruning, whereby root growth is severely inhibited. Stunting of roots leads to chronic drought and nutrient stress in afflicted plants.