Breeding for salt tolerance in crop plants - the role of molecular biology

Salinity in soil affects about 7 % of the land's surface and about 5 % of cultivated land. Most importantly, about 20 % of irrigated land has suffered from secondary salinisation and 50 % of irrigation schemes are affected by salts. In many hotter, drier countries of the world salinity is a concern in their agriculture and could become a key issue. Consequently, the development of salt resistant crops is seen as an important area of research. Although there has been considerable research into the effects of salts on crop plants, there has not, unfortunately, been a commensurate release of salt tolerant cultivars of crop plants. The reason is likely to be the complex nature of the effect of salts on plants. Given the rapid increase in molecular biological techniques, a key question is whether such techniques can aid the development of salt resistance in plants. Physiological and biochemical research has shown that salt tolerance depends on a range of adaptations embracing; many aspects of a plant's physiology: one of these the compartmentation of ions. Introducing genes for compatible solutes, a key part of ion compartmentation, in salt-sensitive species is, conceptually, a simple way of enhancing tolerance. However, analysis of the few data available suggests the consequences of transformation are not straightforward. This is not unexpected for a multigenic trait where the hierarchy of various aspects of tolerance may differ between and within species. The experimental evaluation of the response of transgenic plants to stress does not always match, in quality, the molecular biology. We have advocated the use of physiological traits in breeding programmes as a process that can be undertaken at the present while more knowledge of the genetic basis of salt tolerance is obtained. The use of molecular biological techniques might aid plant breeders through the development of marker aided selection.