Accumulation of high levels of salts in the soil is characteristic of arid and semi-arid regions. Although different curative and management measures are being used to render salt-affected soils fit for agriculture, they are extremely expensive and do not provide permanent solutions to overcome the salinity problem. In contrast, a biotic approach for overcoming salinity stress has gained considerable recognition within the past few decades in view of the vast experimental evidence from what has happened in nature concerning the evolution of highly salt-tolerant ecotypes of different plant species, and also from the remarkable achievements that have been made in improveing different agronomic traits through artificial selection. Considerable improvements in salt tolerance of important crop species have been achieved in the past 2 decades using barley, rice, pearl millet, maize, sorghum, alfalfa, and many grass species. Such achievements relied solely on assessment of the phenotypic expression of t...

Breeding for Salinity Tolerance in Plants

Increased salt tolerance is needed for crops grown in areas at risk of salinisation. This requires new genetic sources of salt tolerance, and more efficient techniques for identifying salt-tolerant germplasm, so that new genes for tolerance can be introduced into crop cultivars. Screening a large number of genotypes for salt tolerance is not easy. Salt tolerance is achieved through the control of salt movement into and through the plant, and salt-specific effects on growth are seen only after long periods of time. Early effects on growth and metabolism are likely due to osmotic effects of the salt, that is to the salt in the soil solution. To avoid the necessity of growing plants for long periods of time to measure biomass or yield, practical selection techniques can be based on physiological traits. We illustrate this with current work on durum wheat, on selection for the trait of sodium exclusion. We have explored a wide range of genetic diversity, identified a new source of sodium exclusion, confirmed that the trait has a high heritability, checked for possible penalties associated with the trait, and are currently developing molecular markers. This illustrates the potential for marker-assisted selection based on sound physiological principles in producing salt-tolerant crop cultivars.

Avenues for increasing salt tolerance of crops, and the role of physiologically based selection traits

Plants acquire mineral nutrients from their native soil environments. Most crop plants are glycophytes and have evolved under conditions of low soil salinity. Consequently, they have developed mechanisms for absorbing, transporting, and utilizing mineral nutrients in nonsaline soils. Under saline conditions, which are characterized by low nutrient-ion activities and extreme ratios of Na / Ca , Na /K , Mg /Ca , and Cl /NO3 , nutritional disorders can develop and crop growth and quality may be reduced. This is not surprising, since under saline conditions, Na and/or Cl often exceed macronutrient concentrations by one or two orders of magnitude and even more in the case of micronutrients. Halophytes, native to saline environments, may also develop nutrient disorders despite their remarkable ability to absorb nutrients selectively from soil solutions dominated by Na and Cl . Nutrient imbalance can develop in salt-stressed plants in different ways. It may result from the effect of salinity on nutrient availability, uptake, transport, or partitioning within the plant or may be caused by physiological inactivation of a given nutrient resulting in an increase in the plant’s internal requirement for that essential element. It is likely that salinity may affect one or more of these processes at the same time, but whether or not this imbalance results in loss of crop yield or quality depends on its severity, which is influenced by a number of environmental factors. Nutrient availability and uptake by plants grown in saline environments is related to (a) the activity of the nutrient ion in the soil solution which depends on pH, pE (the negative log of

Mineral Nutrient Acquisition and Response by Plants Grown in Saline Environments

Mineral element acquisition and growth response of plants grown in saline environments

Nutrient Uptake by Plants Under Stress Conditions

Two experiments were conducted in saline culture solution to find out the causes of rolling and bleaching of young leaves of rice, observed previously in experiments on coastal saline-sodic soil. Symptoms similar to those methioned above were observed on young leaves of cv. KS282 in saline culture solution with Na/Ca ratios of 100 or greater. These symptoms were due to Ca deficiency but not of Cu, since Cu concentration was higher in saline shoots than in the control. The decreasing Na/K or Na/Ca ratio in the saline solution decreased Na and Cl concentration in the shoot. In addition to salinity, Na/Ca and Na/K ratios of the growth medium significantly influenced the shoot and root growth of rice.

Shah Muhammed

Papers

Effect of Na/Ca and Na/K ratios in saline culture solution on the growth and mineral nutrition of rice (Oryza sativa L.)