Post-anthesis moderate soil-drying facilitates source-to-sink remobilization of nitrogen via redistributing cytokinins in rice

This study investigated if and how cytokinins would mediate the effect of post-anthesis soil-drying on source-to-sink remobilization of nitrogen (N) in rice. Field experiments were conducted with normal (NN) and high amount of N (HN) applications. Two irrigation regimes, well-watered (WW) and moderate soil-drying (MD), were imposed from 8 days after full heading until maturity. The results showed that N use efficiency (NUE) including internal N use efficiency, N partial factor productivity and N harvest index, was substantially increased in MD relative to in WW. The MD promoted the reallocation and transfer of N, free amino acids (FAAs) and trans-zeatin (Z) + trans-zeatin riboside (ZR) from the source organs (stems and leaves) to the sink organ (panicles). Activities of protein degradation enzymes including endopeptidase (EP) and aminopeptidase (AP) in stems and leaves, and expression levels of FAAs transporter genes in panicles were substantially increased in MD than in WW, and were closely associated with Z + ZR content there. Application of kinetin to stems and leaves significantly decreased the activities of EP and AP, resulting in more N retention in stems and leaves and lower N harvest index. Application of kinetin to panicles significantly increased the expression levels of FAAs transporter genes in panicles, leading to less N retention in stems and leaves and higher N harvest index. Such facilitation in source-to-sink remobilization of N induced by the MD was greater at HN than at NN. Collectively, a post-anthesis MD can enhance source-to-sink remobilization of N and synergistically increase grain yield and NUE via redistributing cytokinins (Z + ZR) in rice.