Relationship between rice grain protein content and key phenotypes in rice

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.