Strengthened Assimilate Transport Improves Yield and Quality of Super Rice

Rice varieties with ultra-high yields play an important role in grain production and global food security. However, little information is available on the source–sink relationships that underpin the grain quality and ultra-high-yield properties. Photosynthesis, carbohydrate accumulation and allocation, vascular bundle morphology, and nutrient uptake and characteristics were, therefore, compared in two ‘super rice’ varieties: Yongyou9 (control) and Yongyou12 (ultra-high yield) that differ in grain production. Yongyou12 had a significantly higher (18.8–21.4%) grain yield than Yongyou9, together with a substantial improvement in appearance-related qualities. The total dry weight and the ratio of panicle dry weight to total dry weight were significantly higher in Yongyou12 than Yongyou9, suggesting that the improved seed traits were related to higher assimilate accumulation and allocation in the ultra-high-yield variety. Yongyou12 had larger vascular bundles and greater numbers of vascular bundles in the panicle-neck internode, as well as higher levels of SUT1, SUT2, and CIN2 transcripts in the grains than Yongyou9. The contents of nitrogen, phosphorous, and potassium were similar in Yongyou12 and Yongyou9. We concluded that assimilate transport and nutrient utilization efficiency are the main factors underlying the higher yield and quality traits of the super rice variety Yongyou12.