Reproductive stage salinity tolerance in rice: a complex trait to phenotype

Rice is one of the major crops but rice productivity is adversely affected by salinity stress. The rice seedling and reproductive stages are the most sensitive growth stages against salinity stress with very poor association, suggesting that they are regulated by different processes and sets of genes/QTLs. Reproductive stage salinity tolerance is most important as it is translated into grain yield. There are hardly any studies on reproductive-stage salinity tolerance that exist mainly because of the lack of reliable reproductive stage-specific phenotyping techniques and incomplete knowledge of the stage-specific mechanisms of salinity tolerance. Establishing a precise and accurate phenotyping approach for reproductive stage is the utmost crucial step in identifying the genomic regions appropriate for marker-assisted breeding programs. Two major challenges for screening exclusively for the reproductive stage are (1) how to stress plants at the reproductive stage without stressing them at the seedling or late vegetative stages, and (2) how to impose the stress on different genotypes or mapping populations at equivalent growth stages of development because of the variability in the basic vegetative phase of the developmental stages. By cutting the old leaves, this study standardised a methodology that allows salt translocation to the reproductive organs as quickly as possible just at the initiation of booting which was growth stage-specific rather than growth duration-dependent. The results showed cutting the old leaves of the rice plant, leaving only the flag leaf and penultimate leaf, had no significant effect on yield components and that way salt reached to the reproductive parts very quickly after 2 or 3 days of stress treatment in the leaf pruning method. However, there is no high Na+ accumulation in the top two leaves even after 9 days of stress treatment in the untrimmed plants. The exclusive reproductive stage effect of salinity stress (10 dSm−1) to rice plants for 20 days from the day of ‘flag leaf appearance’ proved the efficacy of the methodology by clear discrimination between the salt-sensitive and salt-tolerant genotypes, which is being used to screen several mapping populations.