The heritability of sodium and potassium transport to the xylem was measured by the regression of Fn+1, on Fn means in two segregating breeding populations of rice (Oryza sativa L.). The narrow‐sense heritabilities of shoot sodium concentration were 0.42 and 0.43 in the two populations, respectively, and the corresponding values for the heritability of shoot potassium concentration were 0.46 and 0.52. The sodium: potassium ratio was apparently heritable (0.36 and 0.40) because it was regressed positively on sodium concentration and negatively on potassium concentration. There was no significant relationship between the shoot sodium and potassium concentrations themselves. It is concluded that sodium and potassium uptake in rice are controlled by different genes which segregate independently. The magnitude of the transpirational bypass flow was estimated to be some 10 times greater in rice than in wheat (Triticum aestivum L.) and was found to be highly correlated with sodium uptake in rice but not in wheat. It is concluded that the bypass flow provides an additional pathway for sodium uptake in rice and that this accounts for the functional and genetic independence of sodium and potassium uptake in rice and consequently for the lesser prominence of potassium:sodium discrimination in rice than in wheat.

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 nat~are 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. S a l i n i t y in soil is n o t u n c o m m o n -about 7 % o f the w o r l d ' s l a n d s u r f a c e is s a l t -a f f e c t e d and a b o u t 5 % o f c u l t i v a t e d l a n d ( F l o w e r s and Y e t 1995, G h a ss e m i et al. 1995). M o s t i m p o r t a n t l y a b o u t 20 % o f i r r i g a t e d l a n d has s u f f e r e d f r o m s e c o n d a r y salinisation ( G h a s s e m i et al. 1995) and 50 % o f irrigation s c h e m e s are t h o u g h t to b e salt-affected ( S z a b o l c s 1992). In s o m e c o u n t r i e s s a l i n i s a t i o n is a k e y issue in their a g r i c u l t u r e . In P a k i s t a n , for e x a m p l e , a c o u n t r y that relies h e a v i l y on irrigation for f o o d p r o d u c t i o n and has a r a p i d l y g r o w i n g p o p u l a t i o n , a b o u t a q u a r t e r o f the i r r i g a t e d land is salinised (Ah-427

A multiple cross was constructed with the aim of combining component traits for the complex salinity resistance character. The aim was to combine donors for physiological traits with the agronomically desirable semidwarf/intermediate plant type and with the overall salinity resistance of the traditional tall land races. We report a study of selection strategies in the resulting breeding population. The effects of early selection for agronomic traits and early selection for low sodium transport were compared with a control population in which minimal selection was practised. Conventional selection for agronomic characters at early generations selected against low sodium-transporting (and thus potentially salt-tolerant) genotypes. In contrast, mild early selection for low sodium transport enriched the population in potentially salt-resistant genotypes but did not select against agronomic (semi-dwarf/intermediate) genotypes. It is concluded that selection for agronomic traits should be made after selection for salt resistance and, ideally, should be delayed until the population has reached near-homozygosity.