Cytokinins redistributing drives nitrogen remobilization from source to sink in wheat under moderate water limitation during grain filling

This study examined the involvement of cytokinins in the process by which moderate water limitation (MWL) mediates nitrogen (N) remobilization from source to sink during the grain-filling phase in wheat. Field experiments were performed using N application rates of low (LN), medium (MN), and high (HN). Two soil moisture regimes were implemented for each N rate: conventional well-watered (CWW) and MWL post anthesis. The MWL application optimized N, total free amino acids (FAA), and trans-zeatin (Z)+trans-zeatin riboside (ZR) reallocation from the source organs (stems and leaves) to the sink organ (spikes) in wheat. Compared to those in the CWW regime, the activities of proteolytic enzymes, including endopeptidase, carboxypeptidase, and aminopeptidase within stems and leaves, and the expression levels of total FAA transporter genes in spikes were significantly elevated in the MWL regime, showing a close correlation with the Z+ZR levels in the spikes. Application of kinetin to stems and leaves significantly inhibited proteolytic enzyme activities, promoting N retention in stems and leaves, decreasing N accumulation in the sink organ, and reducing the N harvest index. In contrast, applying kinetin to spikes significantly upregulated expression levels of FAA transporter genes, reducing N retention in stems and leaves, increasing N accumulation in the sink organ, and raising the N harvest index. Such facilitation induced by the MWL in the remobilization of N from source to sink was greater at HN than at LN or MN. Results demonstrate that post-anthesis MWL can significantly intensify the remobilization of N from source to sink, while also synergistically enhancing grain yield and N use efficiency through strategically redistributing cytokinins (Z+ZR) between source and sink in wheat.