Rice is relatively sensitive to salinity and is classified as a silicon accumulator. There have been reports that silicon can reduce sodium uptake in crop grasses in saline conditions, but the mechanism by which silicon might alleviate salinity damage is unclear. We report on the effects of silicon on growth, gas exchange and sodium uptake in rice genotypes differing in salt tolerance. In non-saline media there were no effects of supplementary silicate upon shoot fresh or dry weight or upon root dry weight, indicating that the standard culture solution was not formally deficient with respect to silicon. Plants grown with supplementary silicate had slightly, but significantly, shorter leaves than plants grown in a standard culture solution. Salinity reduced growth and photosynthetic gas exchange. Silicate supplementation partly overcame the reduction in growth and net photosynthesis caused by salt. This amelioration was correlated with a reduction in sodium uptake. Silicate supplementation increased the stomatal conductance of salt-treated plants, showing that silicate was not acting to reduce sodium uptake via a reduction in the transpiration rate. Silicate reduced both sodium transport and the transport of the apoplastic tracer trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS). This implies that the mode of action of silicate was by partial blockage of the transpirational bypass flow, the pathway by which a large proportion of the uptake of sodium in rice occurs. Mechanisms by which silicate might reduce the transpirational bypass flow directly are discussed.

Phenotypic resistance of salinity is expressed as the ability to survive and grow in a salinised medium. Some subjective measure of overall performance has normally been used in plant breeding programmes aimed at increasing salinity resistance, not only to evaluate progeny, but to select parents. Salinity resistance has, at least implicitly, been treated as a single trait. Physiological studies of rice suggest that a range of characteristics (such as low shoot sodium concentration, compartmentation of salt in older rather than younger leaves, tolerance to salt within leaves and plant vigour) would increase the ability of the plant to cope with salinity. We describe the screening of a large number of rice genotypes for overall performance (using an objective measure based on survival) and for the aforementioned physiological traits. There was wide variation in all the characters studied, but only vigour was strongly correlated with survival. Shoot sodium concentration, which a priori is expected to be important, accounted for only a small proportion of the variability in the survival of salinity. Tissue tolerance (the cellular component of resistance reflecting the ability to compartmentalise salt within leaves) revealed a fivefold range between genotypes in the tolerance of their leaves to salt, but this was not correlated positively with survival. On the basis of such (lack of) correlation, these traits would be rejected in normal plant breeding practice, but we discuss the fallacies involved in attempting correlation between individual traits and the overall performance of a salt-sensitive species in saline conditions. We conclude that whilst overall performance (survival) can be used to evaluate the salt resistance of a genotype, it is not the basis on which parents should be selected to construct a complex character through breeding. It was the norm for varieties which had one good characteristic affecting salt resistance to be unexceptional or poor in the others. This constitutes experimental evidence that the potential for salt resistance present in the rice genome has not been realised in genotypes currently extant. The results are discussed in relation to the use of physiological traits in plant breeding, with particular reference to environmental stresses that do not affect a significant part of a species' ecological range.

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SUMMARYWhile members of the genus Oryza are very sensitive to salinity, salt concentrations as high as 20 "" of that of seawater had no adverse effect on the growth of the tropical wild rice (Porteresia coarctata Tateoka) in experiments undertaken in a greenhouse in the UK. P. coarctata plants accumulated sodium and chloride ions in the leaves, but maintained a Na:K ratio as low as 0-7, even after 6 weeks of growth in 25 °.o artificial seawater (.A.SW) where the Na: K ratio was 34. This ability to maintain a high K: Na ratio in the leaves is in part a consequence of the secretion of ions from the leaves. The ratio of Na: K in the secreted salt (more than 5 for plants growing in 25 % ASW) is similar to that measured by X-ray microanalysis in the vacuoles of hairs found in folds of the adaxial surface of the leaf lamina, suggesting that the secretions emanate from these hairs. The salt secreted by the hairs is an important factor in the salt-balance of the leaves: the consequences of these findings for the transfer of salttolerance from this species into cultivated rice are discussed.

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Secondary salinization and its relationship to irrigation are strong incentives to improve the tolerance of crops to salinity and to drought. Achieving this through the pyramiding of physiological traits (phenotypic selection without knowledge of genotype) is feasible. However, wide application of this approach is limited by the practicalities of assessing not only the parents, but also large numbers of individuals and families in segregating generations. Genotypic information is required in the form of markers for any quantitative trait loci involved (marker-assisted selection) or of direct knowledge of the genes. In the absence of adequate candidate genes for salt tolerance, a quantitative trait locus/marker-assisted selection approach has been used here. Putative markers for ion transport and selectivity, identified from analysis of amplified fragment length polymorphism, had been discovered within a custom-made mapping population of rice. Here it is reported that none of these markers showed any association with similar traits in a closely related population of recombinant inbred lines or in selections of a cultivar. Whilst markers will be of value in using élite lines from the mapping population in backcrossing, this has to be considered alongside the effort required to develop and map any given population. This result cautions against any expectation of a general applicability of markers for physiological traits. It is concluded that direct knowledge of the genes involved is needed. This cannot be achieved at present by positional cloning. The elucidation of candidate genes is required. Here the problem lies not in the analysis of gene expression but in devising protocols in which only those genes of interest are differentially affected by the experimental treatments.

Secondary salinization and its relationship to irrigation are strong incentives to improve the tolerance of crops to salinity and to drought. Achieving this through the pyramiding of physiological traits (phenotypic selection without knowledge of genotype) is feasible. However, wide application of this approach is limited by the practicalities of assessing not only the parents, but also large numbers of individuals and families in segregating generations. Genotypic information is required in the form of markers for any quantitative trait loci involved (marker-assisted selection) or of direct knowledge of the genes. In the absence of adequate candidate genes for salt tolerance, a quantitative trait locus/marker-assisted selection approach has been used here. Putative markers for ion transport and selectivity, identified from analysis of amplified fragment length polymorphism, had been discovered within a custom-made mapping population of rice. Here it is reported that none of these markers showed any association with similar traits in a closely related population of recombinant inbred lines or in selections of a cultivar. Whilst markers will be of value in using élite lines from the mapping population in backcrossing, this has to be considered alongside the effort required to develop and map any given population. This result cautions against any expectation of a general applicability of markers for physiological traits. It is concluded that direct knowledge of the genes involved is needed. This cannot be achieved at present by positional cloning. The elucidation of candidate genes is required. Here the problem lies not in the analysis of gene expression but in devising protocols in which only those genes of interest are differentially affected by the experimental treatments.

No abstract