Selection for low-temperature tolerance

Field survival trials have proven to be inefficient for selecting genotypes with low-temperature tolerance. Because low-temperature stress brings about many changes in plants, researchers are pursuing traits related to these changes in search of selection aids. Some factors that have shown promise in predicting low-temperature tolerance include plant erectness in winter, tissue water content, and cell size. Unfortunately, these tests are not effective in discriminating among small differences that are of practical breeding importance.

Researchers commonly use controlled-freeze tests conducted in artificial environments to measure lowtemperature tolerance. For example, the Field Survival

Index developed by Fowler and Gusta is often used. The researchers found that the crown and leaf water content of field acclimated plants was a good indication of field survivability. Molecular marker technology is being pursued in the quest for QTLs

associated with low-temperature tolerance. Other biotechnology tools are being explored to help transfer low-temperature tolerance genes into cultivars. In spite of not being as efficient and as desirable as controlled environment tests, field testing remains a widely used screening approach in low-temperature tolerance breeding. When other selection approaches are used, field testing is what is used as a final measure of plant winter survival. Researchers can take various precautions to improve the efficiency of field tests.

Breeding for tolerance to low-temperature stress

Whereas the genetics of low-temperature tolerance has been studied to a reasonable degree, breeders have only had minimal success in applying research

knowledge to practical breeding. Breeding superhardy cultivars remains a challenge. Several explanations for this lack of success have been proposed by D.B. Fowler and A.E. Limin. These are:

_ Exploitable genetic variability for low-temperature tolerance has been largely exhausted within the existing gene pools of most species.

_ A large number of genes with small effects and complex interaction is assumed to determine the phenotypic expansion of low-temperature tolerance,

making selection difficult.

_ Current methodologies for measuring lowtemperature tolerance give poor resolution of small phenotypic differences.

_ Measurements of low-temperature tolerance lack the precision for single plant analysis and many are destructive, making selection procedures complicated.

_ Poor expression of low-temperature tolerance in alien genetic background has prevented the expansion of gene pools through interspecific and intergeneric

transfers.

Salinity stress

Soil salinity constraints to crop production occur in an estimated 95% million hectares worldwide. Salinity is the accumulation of dissolved salts in the

soil solution to a degree that it inhibits plant growth and development.

Overview of salinity stress concepts

Soils with salinity problems are described as saltaffected. When the salt concentration measured in terms of electrical conductivity is more than 4 dS/m and the pH is less than 8.5, the soil is called a saline soil. When the ECe value is less than 4 dS/m and the pH is more than 8.5, the soil is a sodic soil.

Sodic soils are high in sodium but low in other soluble salts. Semi-arid regions have saline/sodic soils, whereby salts accumulate in subsoils because of the low permeability of the subsoil. Salinity may have natural origin as a result of weathering of parent materials that are rich in soluble salts. Human-aided salinity occurs as a result of agricultural activities, especially, irrigation with impure water. Salinity is often caused by a rising water table.

Plant growth is inhibited in salt-affected soils because the high salt concentration in the soil solution inhibits the process of water absorption by osmosis. In addition, the concentration of Na þ will increase intracellularly and interfere with essential biological processes, causing irreversible damage. When excessive amounts of salts enter the transpiration stream, plant cells may be injured. Plants that are tolerant to high soil salt concentration are called halophytes. Wheat is among the more salttolerant crops, while rice is one of the most saltsensitive crops. Maize is moderately sensitive to salts in the soil solution.