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 ha s suffered from secondary salinisation and 50 % of irrigation schemes are affected by salts. In many hotter, drier countries of the world sa linity is a concern in their agriculture and could become a key issue. Consequently, the development of salt resistant crops is seen as an i mportant area of research. Although there has been considerable resear ch into the effects of salts on crop plants, there has not, unfortunat ely, been a commensurate release of salt tolerant cultivars of crop pl ants. The reason is likely to be the complex nature of the effect of s alts on plants. Given the rapid increase in molecular biological techn iques, a key question is whether such techniques can aid the developme nt of salt resistance in plants. Physiological and biochemical researc h has shown that salt tolerance depends on a range of adaptations embr acing; many aspects of a plant's physiology: one of these the compartm entation 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 straigh tforward. This is not unexpected for a multigenic trait where the hier archy of various aspects of tolerance may differ between and within sp ecies. The experimental evaluation of the response of transgenic plant s to stress does not always match, in quality, the molecular biology. We have advocated the use of physiological traits in breeding programm es as a process that can be undertaken at the present while more knowl edge of the genetic basis of salt tolerance is obtained. The use of mo lecular biological techniques might aid plant breeders through the dev elopment of marker aided selection.