Screening of rice (Oryza sativa L.) genotypes for physiological characters contributing to salinity resistance, and their relationship to overall performance

Abstract

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.

Sodium as nutrient and toxicant

“…In support of this idea, Jha et al (2010) have shown that there is a positive relationship between salt tolerance and expression levels of AtAVP1, which encodes an H + -pumping pyrophosphatase that is likely to be critical in the vacuolar sequestration of Na + . In contrast to such studies, however, the survey of rice cultivars by Yeo et al (1990;see above) indicated that there was no correlation between shoot Na + and tissue tolerance; moreover, tissue tolerance was negatively correlated with both plant survival and plant vigour. In any case, even when increased shoot Na + and increased salt tolerance occur together, the term "tissue tolerance" appears somewhat weak.…”

“…Until these accumulation issues are better resolved, the roles of Na + transporters in salt tolerance and toxicity will not be adequately understood. What is clear at this point is that salinity stress and tolerance is a complex, multi-faceted trait, and cannot be predicted by simple indicators such as Na + accumulation (Yeo et al 1990;Møller and Tester 2007;Rajendran et al 2009). …”

“…In the case of corn, increased influx and shoot accumulation of Na + has been linked to its toxicity in some studies (Schubert and Läuchli 1990;Cramer 1992), while Cramer et al (1994) later showed that Na + accumulation in the shoots of two varieties of hybrid corn did not reflect their relative salt tolerances, concluding that salinity-induced growth depression was primarily due to osmotic stress. Perhaps the most surprising study of this nature in the realm of cereal grasses (Yeo et al 1990) was conducted in rice, a particularly salt-sensitive species. In this survey of 150 rice genotypes, shoot sodium concentrations accounted for only a small degree of variation in survival under saline conditions.…”

Sodium and potassium transport to the xylem are inherited independently in rice, and the mechanism of sodium: potassium selectivity differs between rice and wheat

“…There is an overwhelming eotisensus in the literatute that exeessive concentration of salts in the cytoplastn is damaging to the functioning of eukaryotic cells (see e,g, reviews by Flowers et al 1977Flowers et al , 1986Greenway & Munns 1980;Munns & Tertnaat 1986), It is therefore difficult to see how sodium aeeumulalion, at least in excess of that which could be used for osmotic adjusttnent, can be of advantage to a higher plant, Sodiutn uptake in riee is exeessive by this definition and within a genotype is associated with (Yeo et al 1991), However, sodiutn uptake aecounted for only a stnall proportion of the vat iance in survival, as a tneasure of overall perfortnanee, when a large number of genotypes of rice were investigated (Yeo et al 1990), The reasons for this are exatnined in detail by Yeo et al (1990), Yeo (1994) and Flowers & Yeo (1995) and ean be sutntnarized as follows. Salt toleranee in riee is a eotnplex eharaeter eomprising a nutnber of cotnpotient traits (whieh inelude sodiutn uptake and eotnparttnetitalizatioti).…”

Screening, Inheritance and Linkage Marker Analyses of Salt Tolerance in Mutated Scented Japonica Rice (Oryza sativaL.)

“…To improve the growth and yield, we need to develop plants with tolerance to salinity in the soil. Evaluating tolerance to saline conditions in plants requires evaluation of a complex set of physiological traits measured by components such as: plant survival scores, plant vigor, green leaf area, ion concentration and osmosis (Yeo et al, 1990;Sabouri and Sabouri, 2008). The main mechanisms of salt tolerance in crops include: ion homeostasis, osmotic homeostasis, and control and repair of oxidative stress damage (Verslues et al, 2006).…”