Grain yield and nitrogen use efficiency are increased by exogenous cytokinin application through the improvement in root physiological traits of rice

Cytokinins play crucial roles in plant growth and development. However, the mechanism underlying their regulation to the grain yield and nitrogen use efficiency (NUE) in rice (Oryza sativa L.) remains unclear. The purpose of this study was to elucidate the issue. Two types of rice varieties, the N-efficient variety and N-inefficient variety, were grown in a paddy field under 180 kg N hm− 2 with three chemicals applied to leaves at heading time including control (spraying distilled water), spraying 6-benzylaminopurine (6-BA), and spraying an inhibitor of cytokinins synthesis, lovastatin (Lov). Results showed that the 6-BA treatment significantly increased grain yield and internal NUE in both varieties when compared with the control, and the results were reversed under the Lov treatment. The N-efficient variety showed better potential and stability than the N-inefficient variety in yield performance under 6-BA and Lov treatments. The improved grain yield and internal NUE by applying 6-BA were mainly due to the increased filled grain percentage, greater canopy photosynthetic rate (CPR) and more N allocated from stems and leaves to panicles (i.e., greater panicle N accumulation at maturity, PNAM). Both CPR and PNAM were closely associated with root oxidation activity, root soluble sugars and cytokinin contents, activities of antioxidant enzymes and the expression level of genes related to nitrogen transport in roots during grain filling. The results suggested that cytokinins could synergistically increase grain yield and NUE of rice by enhancing photosynthetic production and nitrogen partitioning efficiency as a result of improved root physiological traits during grain filling.