In our previous study, it was found that protein content was one of the most important factors affecting rice-eating quality. Because rice cultivars have different phenotypes, they may also have different protein concentrations and eating quality. Based on published phenotypic and measured protein content of 706 rice (Oryza sativa L.) cultivars [453 tropical indica, (O. sativa indica) 61 tropical japonica (O. sativa japonica), 22 temperate indica, and 169 temperate japonica], we studied the potential relationship between grain protein content and the rice phenotype (culm length, panicle length, thousand grain weight, and grain length) through cluster analysis and canonical correlation analysis. The results demonstrated significant differences between japonica and indica rice, as well as between landrace rice and modern rice varieties. The protein content of japonica (average = 8.76%) was higher than that of indica (average = 8.11%). The protein content of all japonica was negatively correlated with culm length (p = 0.007) and panicle length (p = 0.006). The protein content of all indica was positively correlated with grain length (p = 0.0005) and grain length/width ratio (p = 0.043). Canonical correlation analysis showed that panicle length and grain width had the highest canonical loadings and canonical cross-loadings on the external phenotype and grain characteristics in rice with different protein contents. Panicle length and grain width could better reflect rice phenotype traits under the identical grain protein content type.

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BACKGROUND: Improving rice quality is one of the main goals of global rice breeding programs, and rice protein content is one of the most important factors affecting rice eating quality. The protein content of rice is mainly determined by genetic factors and also affected by environmental factors. However, the differences and regional distribution of protein content during the evolution of different rice varieties are still unclear.RESULTS: The purpose of this study was to understand the differences in grain protein content of 706 rice accessions in different regions and different rice subtypes. The influencing factors of grain protein content differences were analyzed from the perspectives of genetic characteristics, environment, rice subtypes, and breeding process. The results showed that the grain protein content of indica rice in most countries and regions was higher than that of japonica rice. From the perspective of the rice breeding process, the protein content of modern varieties in japonica was lower than that of landrace varieties. In most countries, modern varieties of indica rice had higher protein content than landrace varieties did.CONCLUSION: The environment and genetic characteristics caused the change in the protein content of rice. This study has improved our understanding of the differences in protein content of rice from different rice varieties.