Breeding for salt tolerance

A common approach to breeding for salt tolerance starts with assembling and screening germplasm for salinity tolerance. The selected genotypes are used as parents to transfer the trait to desired cultivars, followed by selecting desirable recombinants from the segregating population. This approach has yielded some success in species such as rice, wheat, and lucerne. The challenge in breeding for salt tolerance is how to measure salinity tolerance. Screening is commonly based on growth of plants under salt stress. Two distinct mechanisms exist for salinity

tolerance:

Tolerance to the osmotic effect of the saline solution

Tolerance to the cytotoxic effect of Na entering the cell. In addition, the salt-specific nature of the soil saline.

Screening for salinity tolerance, just like other stress factors, is a long process and requires a large amount of space to screen progeny from crosses. Screening for specific traits is quicker and more.One of the most successful traits to incorporate

in salt-tolerance breeding to combat salt-specific effects is the rate of Na accumulation in leaves. This is measured as the increase in salt in a given leaf over a period. Selection for these ions was used in breeding rice and lucerne cultivars with high salt tolerance. Traits for osmotic effects are related to growth, for example, leaf elongation, root elongation, shoot biomass, and leaf area expansion, the latter two being the most effective indices. Molecular marker technology and genetic engineering techniques are being used in salt-tolerance breeding efforts. Salinity tolerance has been found in the wild species of crops such as tomato, pigeon pea, and common bean.

Heat stress

Heat stress may be defined as the occurrence of temperatures high enough for sufficient time to cause irreversible damage to plant function or development. A heat resistant genotype is one that is more productive than most other genotypes in environments where heat stress occurs.

Overview of heat stress concepts

Heat stress occurs to varying degrees in different climatic zones. High temperatures can occur during the day or during the night. Also, temperature effects can be atmospheric or in the soil. Air temperature varies during the day and during the night. Annual crop species may be classified into two categories according to maximum threshold temperatures as either coolseason annuals or warm-season annuals. Cool-season species are more sensitive to hot weather than warm-season species.

High night temperatures have detrimental effects on the reproductive function of plants. It has been shown that there is a distinct period during the 24-hour day cycle when pollen development is most sensitive to high night temperatures. In cowpea, plants that were exposed to high temperature during the last six hours of the night show significant decrease in pollen viability and pod set. Further, this damage was more pronounced in long days than short days. Other researchers also show that the stage of floral development most sensitive to high night temperature was between 7 and 9 days before anthesis. Excessive heat in the soil affects emergence of seedlings of both cool-season and